Wastewater from Olive Oil Production: Environmental Impacts, Treatment and Valorisation 9783031234484, 9783031234491

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Springer Water

Salah Souabi Abdelkader Anouzla   Editors

Wastewater from Olive Oil Production Environmental Impacts, Treatment and Valorisation

Springer Water Series Editor Andrey Kostianoy, Russian Academy of Sciences, P. P. Shirshov Institute of Oceanology, Moscow, Russia Editorial Board Angela Carpenter, School of Earth and Environment, University of Leeds, Leeds, West Yorkshire, UK Tamim Younos, Green Water-Infrastructure Academy, Blacksburg, VA, USA Andrea Scozzari, Institute of Information Science and Technologies (CNR-ISTI), National Research Council of Italy, Pisa, Italy Stefano Vignudelli, CNR—Istituto di Biofisica, Pisa, Italy Alexei Kouraev, LEGOS, Université de Toulouse, Toulouse Cedex 9, France

The book series Springer Water comprises a broad portfolio of multi- and interdisciplinary scientific books, aiming at researchers, students, and everyone interested in water-related science. The series includes peer-reviewed monographs, edited volumes, textbooks, and conference proceedings. Its volumes combine all kinds of water-related research areas, such as: the movement, distribution and quality of freshwater; water resources; the quality and pollution of water and its influence on health; the water industry including drinking water, wastewater, and desalination services and technologies; water history; as well as water management and the governmental, political, developmental, and ethical aspects of water.

Salah Souabi · Abdelkader Anouzla Editors

Wastewater from Olive Oil Production Environmental Impacts, Treatment and Valorisation

Editors Salah Souabi Faculty of Science and technology Mohammedia University of Hassan II Casablanca Casablanca, Morocco

Abdelkader Anouzla Faculty of Science and Technology Mohammedia University of Hassan II Casablanca Mohammedia, Morocco

ISSN 2364-6934 ISSN 2364-8198 (electronic) Springer Water ISBN 978-3-031-23448-4 ISBN 978-3-031-23449-1 (eBook) https://doi.org/10.1007/978-3-031-23449-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface I

This work constitutes a database on the management of liquid discharges produced during the production of olive oils. Indeed, the discharges of vegetable water pose serious problems for the environment all over the world. This, therefore, requires awareness and prevention within industrial units which use either clean technology such as double-phase or triple-phase machines on the one hand and conventional techniques which produce effluents too loaded with toxic pollutants having several impacts on the environment. This work provides insight into good upstream and forwards management practices through the application of clean technology to significantly minimize water consumption and, subsequently, wastewater production. Indeed, our book presents several methods used internationally, in particular natural flotation, which could have a fascinating effect in considerably reducing toxic pollution linked to phenols, detergents, etc., at low cost. In addition, valuation methods for olive oil discharges will be dealt with in this book, in particular, the recovery of discharges for the production of compost that can be recovered in agriculture, recovery of floating oils, etc. The book consists of ten chapters. Chapter 1 (Wafa Hassen et al.) was to highlight and draws attention to the problem of olive mill wastewater becoming a major concern in modern communities, namely the problem of waste. This brief chapter review will focus on olive oil extraction by-product processing, discharges, and valorization. Chapter 2 (Doaa A. El-Emam) discusses the various approaches used to mitigate wastewater contaminants produced by the olive oil industry. Recent research studies which focused on the valorization options for dealing with olive mill waste residues such as animal feed, biofuel, and biogas are also discussed. Chapter 3 (Sare Asli et al.) briefly suggests a proposal for sustainable treatment and valorization of olive mill wastewater and olive.

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Preface I

Chapter 4 (Antonella De Leonardis et al.) gives an overview of the possible utilization of OMW in food applications. OMW phenolic extract was assayed to fortify, preserve organoleptic properties, and enhance consumer acceptance of the meat and meat products. We report on the effective utilization of OMW extract in retaining or enhancing the nutritional value, stability, and bioavailability of starter cultures and delaying the production of browning due to Millard reaction in milk, functional, and fermented beverages. Chapter 5 (Roberta Imperatore et al.) will focus on the applications of olive mill wastewater (OMWW) bioactive molecules in terrestrial and aquatic animal husbandry. It will report: (1) molecules present in the OMWW; (2) extraction techniques, including green ones, most appropriate to recover various bioactive molecules, such as polyphenols, tocopherols, and sterols; (3) the in vitro actions of the bioactive molecules; (4) effects of OMWW bioactive molecules on terrestrial and aquatic animal species relevant to animal husbandry; and (5) critical issues and future prospects. Chapter 6 (Zakia Gueboudji et al.) was to highlight the OMW physicochemical and microbial characteristics when stored in evaporation ponds and to investigate first its disposal management considering the natural evaporation process and the effect of the environmental factors, the effluent constituents, their biodegradation, gas emission as well as its infiltration and soil impact. Secondly, the storage in open ponds was considered with different potential valorization/reuse of the storage effluent, providing specific awareness on circular economy operationalization in the agri-food system of the olive oil supply chain. The OMW impact on the soil in the long term was discussed compared to a control non-contaminated basin. Linear polluting economy migration to a circular economy by OMW storage in open ponds with economic incomes through recycling/reuse will be developed. Chapter 7 (Raja Jarboui et al.) is to compare the physicochemical characterization and pollution degree of OMW obtained from two different extraction systems, which was the cold extraction system and the traditional system, in Khenchela, east of Algeria. Chapter 8 discusses the different methods of managing vegetable waters produced during the olive growing period in Morocco, the impacts on the environment, and the treatment of rejecting the valuation of olive oil mill wastewater as raw materials for the production of several products. In Chapter 9 (Renan Tunalıoglu), it has been observed that there is a very serious change in Turkey. In recent years, it has been understood that olive oil factories prefer ecological processing systems and pollute the environment less. Because on the one hand, the enterprises that could not make the necessary investments for waste management before are supported by the state. On the other hand, plants used only as fuel (Olive Pomace + OMWW) are now used as fertilizer, feed for cattle, and cosmetics. Chapter 10 (Aysen Muezzinoglu) presents new trends for stand-alone or integrated methods of waste treatment/pretreatment in management schemes for olive agroindustry with waste minimization, bioconversion, and recovery/purification of beneficial chemicals that are evaluated. Among the options, biotechnology for producing

Preface I

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fuel, food, animal feed, pharmaceuticals, and fertilizers is especially important. These are being discussed for the development of a profile leading to future research interests in view of the large number of published scientific research related to the olive agro-industry. Finally, this book summarizes recent research on the wastewater from olive oil: characterization, environmental impact, valorization, and treatment; it eliminates the need to search through stacks of journals for critical information. An important feature of this book is that the author of each chapter has been given the freedom to present, as he/she sees fit, the spectrum of the relevant science, from pure to applied, in his/her particular topic. Any author has his own views on, and approach to, a specific topic molded by his own experience. This book will complement, very well, existing books on olive mill wastewater, which, in general, take the more traditional approach of reviewing systematically the fundamental (pure) aspects of the subject. I wish to thank all the authors of the chapters for their contributions and also for their patience in reviewing, despite their demanding agenda. We would like to thank Springer for publishing this title, and we sincerely welcome feedback from our valuable readers and critics. Casablanca, Morocco

Dr. Abdelkader Anouzla

Preface II

The book entitled Wastewater from Olive Oil Production: Environmental Impacts, Treatment and Valorisation presented by Abdelkader Anouzla and Souabi Salah concerns the grouping of ten chapters on managing discharges of olive oils produced during the olive growing period. The manuscript’s objectives are to give an overall idea of the methods for ensuring sound management of liquid discharges from olive oils. Olive mill wastewaters (OMW) are a significant source of environmental pollution, especially in important olive oil-producing countries such as Spain, Italy, Greece, Tunisia, Morocco, Turkey, Lebanon, Syria, and Portugal. When discharged into the environment, olive mill wastewaters create serious environmental problems, such as coloring of natural waters, alteration of soil quality, phytotoxicity, and nuisance odors. Under these conditions, the contamination of the environment by the wastewater of olive oil resulting from the release of pollutants such as phenols, polyphenols, and detergents poses serious problems for surface water, groundwater, and floor. In order to preserve the environment and water resources in particular, traditional treatment techniques have been already implemented. They involve several physicochemical processes, such as adsorption chemical oxidation, coagulation flocculation, as well as biological processes in aerobic and anaerobiosis; given the very heterogeneous composition of phenol, polyphenol, and surfactants, their elimination requires often a chain of physical–chemical and biological treatments ensuring the elimination of the different pollutants in successive stages. However, the investment and exploitation of these traditional treatment techniques required costs that exceed the financial capacities of the industrial units, especially in the developing countries. In parallel with downstream actions, upstream actions are needed in order to rationalize the consumption of water at the level of industrial units. Indeed, prevention and awareness in developed countries have shown their effectiveness which have led to the reduction of pollution.

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Preface II

In this context, the main objective of this book is to call on specialists in the treatment of wastewater from olive oils by various physical and chemical biological techniques or combined to suggest their contribution in chapters intended to enrich the Olive Oil Release Processing Database can help guide the appropriate choice of wastewater collection and treatment technology. Our book project will deal with: – – – –

The diagnosis, the impacts of rejections of olive oils, and regulations Valorization of wastewater from olive oil industries The treatment and recovery of sewage wastewater Assessment of investment and operating costs of treatment techniques.

The objective of our proposal is to associate different chapters on the management of olive oil releases published by several specialists in the field by simple techniques with the low cost in investment and exploitation. Finally, this book summarizes recent research in the wastewater from olive oil: characterization, environmental impact, valorization, and treatment; it eliminates the need to search through stacks of journals for critical information. An important feature of this book is that the author of each chapter has been given the freedom to present, as he/she sees fit, the spectrum of the relevant science, from pure to applied, in his/her particular topic. Any author has his own views on, and approach to, a specific topic, molded by his own experience. Casablanca, Morocco

Dr. Salah Souabi

Contents

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Processes of Valorization and Management of Olive By-Products: The Pomace and Olive Mill Wastewater . . . . . . . . . . . . Wafa Hassen, Bilel Hassen, Rim Werhani, Yassine Hidri, Naceur Jedidi, and Abdennaceur Hassen

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Olive Mill Wastewater: Treatment and Valorization . . . . . . . . . . . . . . Doaa A. El-Emam

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Valuable Materials from Olive Mill Wastewater . . . . . . . . . . . . . . . . . . Sare Asli, Mahmud Diab, and Manal Haj-Zaroubi

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Present and Future Perspectives on the Use of Olive-Oil Mill Wastewater in Food Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antonella De Leonardis, Vincenzo Macciola, and Ayesha Iftikhar

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Olive Mill Wastewater Bioactive Molecules: Applications in Animal Farming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Roberta Imperatore, Caterina Pagliarulo, Graziella Orso, Giuseppa Anna De Cristofaro, Daniela Sateriale, and Marina Paolucci

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Physicochemical Characterization and Estimation of the Pollution Degree of Olive Oil Mill Wastewaters from the Cold Extraction System and the Traditional System . . . . . . 143 Zakia Gueboudji and Kenza Kadi

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Open Ponds for Effluent Storage, a Pertinent Issue to Olive Mill Wastewater (OMW) Management in a Circular Economy Context: Benefits and Environmental Impact . . . . . . . . . . . . . . . . . . . . 153 Raja Jarboui, Salwa Magdich, and Emna Ammar

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Management of Olive Oil Mill Wastewater in Morocco . . . . . . . . . . . 183 Khattabi Rifi Safaa, Abdelkader Anouzla, Younes Abrouki, Hayat Loukili, Malika Kastali, and Salah Souabi

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Contents

Changes in Olive Mill Waste Water Management in Turkey . . . . . . . 213 Renan Tunalıoglu

10 Future Trends in Olive Industry Waste Management: A Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Aysen Muezzinoglu

Contributors

Younes Abrouki Mohammed V University, Rabat, Morocco Emna Ammar Laboratory of Environment Sciences and Sustainable Development, Preparatory Institute of Engineering Studies of Sfax, University of Sfax, Sfax, Tunisia; National Engineering School of Sfax, Sfax, Tunisia Abdelkader Anouzla Science and Technology Mohammadia, Université Hassan II Mohammedia, Casablanca, Morocco Sare Asli The Institute of Applied Research, The Galilee Society, Shefa-Amr, Israel; The Faculty of Science, Al-Qasemi Academic College, Baka EL-Garbiah, Israel; Institute of Evolution, University of Haifa, Haifa, Israel Giuseppa Anna De Cristofaro Department of Science and Technology, Univesity of Sannio, Benevento, Italy Antonella De Leonardis Department of Agricultural, Environmental and Food Sciences (DiAAA), University of Molise, Campobasso, Italy Mahmud Diab The Institute of Applied Research, The Galilee Society, Shefa-Amr, Israel; Institute of Evolution, University of Haifa, Haifa, Israel Doaa A. El-Emam Department of Environmental Sciences, Faculty of Science, Damietta University, Damietta, Egypt Zakia Gueboudji Biotechnology, Water, Environment and Health Laboratory, Department of Molecular and Cellular Biology, Faculty of Nature and Life Sciences, Abbes Laghrour University, Khenchela, Algeria

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Contributors

Manal Haj-Zaroubi The Institute of Applied Research, The Galilee Society, ShefaAmr, Israel; Department of Natural Resources and Environmental Management, Faculty of Management, University of Haifa, Haifa, Israel Abdennaceur Hassen Water Research and Technology Center (C.E.R.T.E.), BorjCédria Technology Park, Soliman, Tunisia Bilel Hassen Laboratory of Microbiology and Pathology of Aquatic Organisms, National Institute of Marine Sciences and Technologies (INSTM), Salammbô, Tunisia Wafa Hassen Research Unit of Analysis and Process, Applied to the Environmental (APAE), Higher Institute of Applied Sciences and Technology Mahdia, University of Monastir, Mahdia, Tunisia; Water Research and Technology Center (C.E.R.T.E.), Borj-Cédria Technology Park, Soliman, Tunisia Yassine Hidri Laboratory of Integrated Olive Production in the Humid, Sub-humid and Semi-arid Region (LR16IO3), Olive Tree Institute, Sousse, Tunisia Ayesha Iftikhar Department of Agricultural, Environmental and Food Sciences (DiAAA), University of Molise, Campobasso, Italy Roberta Imperatore Department of Science and Technology, Univesity of Sannio, Benevento, Italy Raja Jarboui Laboratory of Environment Sciences and Sustainable Development, Preparatory Institute of Engineering Studies of Sfax, University of Sfax, Sfax, Tunisia; Department of Biology, Colleges of Sciences, Jouf University, Sakaka, Saudi Arabia; Department of Biology, Faculty of Sciences of Sfax, University of Sfax, Sfax, Tunisia Naceur Jedidi Water Research and Technology Center (C.E.R.T.E.), Borj-Cédria Technology Park, Soliman, Tunisia Kenza Kadi Biotechnology, Water, Environment and Health Laboratory, Department of Molecular and Cellular Biology, Faculty of Nature and Life Sciences, Abbes Laghrour University, Khenchela, Algeria Malika Kastali The Water and Environmental Engineering Laboratory, Faculty of Sciences and Techniques of Mohammedia, Hassan II University, Mohammedia, Morocco Hayat Loukili The Water and Environmental Engineering Laboratory, Faculty of Sciences and Techniques of Mohammedia, Hassan II University, Mohammedia, Morocco Vincenzo Macciola Department of Agricultural, Environmental and Food Sciences (DiAAA), University of Molise, Campobasso, Italy

Contributors

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Salwa Magdich Laboratory of Environment Sciences and Sustainable Development, Preparatory Institute of Engineering Studies of Sfax, University of Sfax, Sfax, Tunisia Aysen Muezzinoglu Retired Professor, Department of Environmental Engineering, Dokuz Eylul University, Izmir, Turkey Graziella Orso Department of Science and Technology, Univesity of Sannio, Benevento, Italy Caterina Pagliarulo Department of Science and Technology, Univesity of Sannio, Benevento, Italy Marina Paolucci Department of Science and Technology, Univesity of Sannio, Benevento, Italy Khattabi Rifi Safaa The Water and Environmental Engineering Laboratory, Faculty of Sciences and Techniques of Mohammedia, Hassan II University, Mohammedia, Morocco Daniela Sateriale Department of Science and Technology, Univesity of Sannio, Benevento, Italy Salah Souabi Science and Technology Mohammadia, University of Hassan II Casablanca, Casablanca, Morocco Renan Tunalıoglu Department of Agricultural Economics, Faculty of Agriculture, Aydın Adnan Menderes University, Aydın, Turkey Rim Werhani Water Research and Technology Center (C.E.R.T.E.), Borj-Cédria Technology Park, Soliman, Tunisia

Chapter 1

Processes of Valorization and Management of Olive By-Products: The Pomace and Olive Mill Wastewater Wafa Hassen, Bilel Hassen, Rim Werhani, Yassine Hidri, Naceur Jedidi, and Abdennaceur Hassen Abstract The valorization of different organic residues like municipal solid wastes, sewage sludge, and olive tree by-products, specifically pruning products; pomace and olive mill wastewater, more commonly known as the vegetation water of olives or margins is becoming more and more required in the different modern communities and relevant and crucial in terms of environmental preservation. The treatment of these wastes should be associated and viewed under two simultaneous approaches: good disposal of these cumbersome and problematic residues and righteous valorization by considering economic profitability. Thus, pruning products and pomace could be an important and valuable source of food for animals, a second-grade oil source for cosmetic soap making, production of biogas, and electrical energy from all these problematic residues. Besides, the removal of polyphenols from olive mill wastewater would have a double interest: to solve a major environmental problem and to recover and valorize the olive mill wastewater for later applications in food animal W. Hassen Research Unit of Analysis and Process, Applied to the Environmental (APAE), Higher Institute of Applied Sciences and Technology Mahdia, University of Monastir, 5121 Mahdia, Tunisia e-mail: [email protected] B. Hassen Laboratory of Microbiology and Pathology of Aquatic Organisms, National Institute of Marine Sciences and Technologies (INSTM), Rue 2 Mars 1934, 2025 Salammbô, Tunisia e-mail: [email protected] W. Hassen · R. Werhani · N. Jedidi · A. Hassen (B) Water Research and Technology Center (C.E.R.T.E.), Borj-Cédria Technology Park, P. O. BOX 273, 8020 Soliman, Tunisia e-mail: [email protected] R. Werhani e-mail: [email protected] N. Jedidi e-mail: [email protected] Y. Hidri Laboratory of Integrated Olive Production in the Humid, Sub-humid and Semi-arid Region (LR16IO3), Olive Tree Institute, IBN Khaldoun, B.P. 14, 4061 Sousse, Tunisia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_1

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processing and soil amendments and fertilization. Using margins as process watering during composting or co-composting, stages of pre-fermentation-fermentation, and maturation are well recommended to obtain a compost of high soil fertilizing quality. The advantage of compost made from margins is the absence of pathogenic microorganisms and high levels of phosphorus and potassium content, unlike urban solid waste. About municipal solid wastes, it is recommended and interesting to associate two kinds of wastes by co-composting for example, vegetable gardens-pruning products, pomace-municipal solid waste, and green wastes-olive mill wastewater…, to get a mixed compost of good physical–chemical and biological qualities, safe and useful for agricultural soil fertilization. Keywords Organic residues · Olive tree by-products · Olive mill wastewater · Co-composting · Soil fertilization

1.1 Introduction Waste in modern societies has posed, is posing, and will extend to pose important problems, often considered remaining unresolved. Exploiting the workable or fermentable part by aerobic composting has long appeared, but achievements on this scale and in this sense remain very limited or unsatisfactory. This idea should have been strengthened following the emergence of two very crucial and closely related problems: the prodigious accumulation of municipal and food industry waste in modern cities and the flagrant lack of fertile soils coupled with the growing demand and need for agricultural products such as market gardening, fruit, and cereal crops (Esmaeilian et al. 2018). Domestic waste generated in some Mediterranean countries may vary on average from 0.5 to 1.2 kg/day/person. On a worldwide scale, the average rates (0.5–0.99 kg per person per day) of waste generation looked higher in some countries as compared to those (0.1–0.49 kg per person per day) in low-income countries and much lower than the developed economy ones with greater than 1.5 kg per day (Gupta et al. 1998; World Bank 2019; Parvez et al. 2019). This domestic waste is generally characterized by a substantial fraction of fermentable organic matter varying between 60 and 70% in some countries (Hassen et al. 2021; Saidi et al. 2008). These enormous quantities can therefore be transformed into stable compost that may correct the deficiency in fertilizer of agricultural soil. And in recent decades, most cultivated soils are becoming increasingly impoverished regarding organic matter. This organic matter affects to improve the soil structure by conditioning the aeration and water retention and ensuring microbial and plant root alimentation and nutrition, hence promoting growth vegetation and yield production. On the other side, olive cultivation is one of the oldest and most typically agricultural activities in the Mediterranean basins. For these countries, olive oil production is an economic asset handed down over several generations. However, it gets the disadvantage of generating enormous quantities of by-products with a complex organic

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fraction and high chemical oxygen demand (COD). Indeed, 100 kg of olives produces on average 20 kg of oil, 30 kg of pomace, and around 50 L of olive mill wastewater (OMW). These residues or by-products of the oil industries are therefore known as important waste pollutants (Fiorentino et al. 2003; Hachicha et al. 2012; Dakhli 2016). Annual production of olive mill wastewater and pomace is distributed over 2– 5 months and depends on the annual olive production size. Olive mill wastewater is principally characterized by low acidity, high electrical conductivity, a high organic matter load, and some polyphenolic compounds with a high-polluting power. The international legislation considered the olive mill wastewater as agro-industrial wastewater. Its average composition is 83% of water, 8–12% of organic compounds depending on the olive variety, the harvesting period, and the oil extraction method, and finally, 0.40–2.5% of mineral elements constituted mainly by 0.10–0.80% potassium and depending on the harvesting area and soil type (Morillo et al. 2009; Tsagaraki et al. 2006). These last years and with the promotion of the beneficial virtues of olive oil for human health, its demand continues to rise, and production is constantly growing at the expense of the natural environment. The discharge of olive mill wastewater showed a formidable ecological concern in the Mediterranean region (LesageMeessen et al. 2001; Dakhli 2016). Olive oil extraction plants equipped with modern apparatus and advanced processes promote olive oil by-product production with an average quantity of around 6–7 million tons/year that could reach 80–110% of the initial batch of olives; while with traditional equipment, the production of olive oil is reduced to around 50% (Mulinacci et al 2001). Given these excess volumes of wastewater released into the natural environment, treatment is critical to reducing their impact on this environment. The problem with this specific wastewater lies mainly in its high polyphenol content found at concentrations up to 24 g/L (Paraskeva and Diamadopoulos 2006) that may reach 1–10 g/L (Morillo et al. 2009; Di Caprio et al. 2018) with the most predominant sub-compounds as tyrosol, hydroxytyrosol, oleuropein, various phenolic acids and flavonoids, and these phenolic subcompounds showed toxic effects against the general microbial activity and specifically against gram-positive and gram-negative soil bacteria (Mekki et al. 2008). This mill wastewater polyphenol content varied according to the seasonal production and the olive variety (Aggoun et al. 2016). Besides, the general demographic development in the world has generated significant quantities of vegetable waste. In the Paris area, for example, about 217 tons/day of green waste are generated in 2017 (Lecrosnier 2017). These composted vegetable residues present a favorable substrate for biological treatment; over 90% of the volume of these residues contained organic matter with low water content. This required the mixing of the vegetable residues with other wastes, such as waste from agro-food origins mainly to correct the C/N ratio. The discharge of effluents from olive oil-producing industries, especially the olive oil mills, is a serious problem, especially in the Mediterranean Basin. The olive oil produced depends mainly on the chosen grinding process and fluctuations in annual olive production. The world agricultural area occupied by olive trees has reached

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11,193,000 hectares mainly in Spain, Italy, Turkey, and Tunisia corresponding to global olive oil production of approximately 2.89 million tons per year (AFIDOL 2012). Other countries are increasing their production, such as Australia, which has seen its capacity increase from 1000 tons in 2000/2001 to 12,000 tons in 2013/2014, Iran with 9000 tons, and Argentina with 6000 tons (IOC 2014). The world consumption of olive oil is between 2.8 and 3.05 million tons with an average annual growth of about 3.29% over the last 10 years. It has climbed from 1.85 million tons in 1991/1992 to around 3.08 million tons in 2011/2012 and down to around 2.72 million tons in 2016/2017, especially in Western countries like the United States (Mansour et al. 2018). The European Union consumes 56.37% of the world’s production. The majority of import flows are intra-European, which explains the concentration of consumption within these countries. However, in recent years there has been an opening of new markets in the United States, Brazil, Canada, Australia, Japan, and China (IOC 2014). Tunisia is known as one of the leading countries in olive production and marketing (Mansour et al. 2018). Spain came in first place with 1,311,000 tons, followed by Italy with 243,000 tons, Greece with 260,000 tons, followed Tunisia with 100,000 tons as the seventh-largest producer in the world after Spain, Italy, Greece, Turkey, Morocco, and Portugal; While Egypt came in tenth place with 27,000 tons (Mansour et al. 2018). At last and according to the recent estimates of the International Olive Council, although with provisional data, world production for the 2020/21 marketing year would reach 3,034,000 tons, i.e. a decrease of 6.9% compared to the previous marketing year, and consumption would be around 3,211,000 tons (− 0.2%). Imports are expected to reach 1,074,000 tons (− 9.3%), while exports are expected to decrease by 8.8% compared to the previous season, to 1,132,000 tons (IOC 2021). Thus, the problem of the valorization of the various organic residues of the Olives mill wastewater is worrying and is posed in terms of environmental preservation. The choice of the treatment technique must not be only from the viewpoint of monetary profitability, but above all must consider the efficiency of the treatment process. It is impossible to exhaustively develop all the techniques currently being tested (Khdair and Abu-Rumman 2020; Dermeche et al. 2013). From the foregoing, an attempt to rid polyphenols from olive mill wastewater would have a double interest: on the one hand, to solve a major environmental problem and to recover and valorize the olive mill wastewater for later applications in agro-food. It is likewise interesting to think of associating two harmful wastes to the environment: vegetable gardens and green wastes for an effort at retrieval in terms of co-composting to test the quality of the compost produced. The quality of organic matter windrow or biodegradable waste piling placed in compost can lead to different compost compositions. For this reason, different residual green wastes should be exploited to valorize them into compost. This process is commonly known as co-composting. The primary objectives of this chapter were to highlight and draw attention to a problem becoming a major concern in modern communities, namely the problem

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of waste. This brief chapter review will focus on olive oil extraction by-product processing, discharges, and valorization. We will identify and describe the specific problems related to each aspect of olive oil extraction by-products and the definite solutions to be envisaged to remedy these very intimate problems.

1.2 Importance of the Olive Tree on an International Scale 1.2.1 Main Activities in the Olive Sector 1.2.1.1

World Olive Production

According to recent figures from the International Olive Council (IOC), the annual production of olives amounts to 3,269,249 tons in 2010 and 3,135,000 tons in 2018 for the production of olive oil and 2,800,000 tons in 2010 and 2,751,000 tons in 2018 provided for consumption as table olives (Fig. 1.1). According to the production forecasts of the member countries established by the Executive Secretariat of the IOC, the world production of olive oil for the current campaign (2021/22) should reach almost 3,100,000 tons, a slight decrease compared to the two previous campaigns. However, olive oil consumption has grown at a faster rate than production over the last three seasons (2019/20, 2020/21, and 2021/22).

Fig. 1.1 Average annual world olives oil production and consumption over the last twenty years. Source http://www.huile-olive.org

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This consumption, although globally stable during the last two campaigns, has not been affected by this slight decrease in production. Figure 1.1 showed a continual increase in the average olive oil annual production beginning from 1990 until 2000, followed by a stabilization from this year 2000 that will develop in a sawtooth pattern until 2020 with an approximate amplitude of 30%. Some producer countries showed year by year a decline in annual production compared to previous years, ranging from 36 to 55%. This percentage of average decline appeared variable with the country. According to recent data from the International Olive Council at the end of 2018, the production of olives for olive oil represented 3,135,000 tons for the year 2018, and 2,751,000 tons of table olives (forecast for the year 2018–2019). This production is down from the previous year by 3,314,000 tons. The largest decreases were in Tunisia and Argentina, where the crops fall by 57% and 54% respectively. In Europe, production in Italy fall by 38% due to bad weather that damaged the olive trees, and by 35% in Greece. These drops in average annual production observed at the scale of major olive and olive oil-producing countries are inherent mainly in bad weather and periods of a prolonged drought that occurred in these different countries. Besides, the olive harvest varies according to the varieties, the production basin, and the climatic conditions. It takes place from October to December depending on the destination of the fruit (green, broken, black, or oil). But the world olive-growing heritage is estimated between 830 and 1000 million olive trees in the world mainly planted around the Mediterranean Basin, with 90% of total world production and with 3 main producing countries, Spain in the first place far before all others, Italy in second place and Greece in the third place. For countries like Turkey, Morocco, Portugal, and Tunisia, the production is variable according to the seasons and are often ranked respectively, in the 4th, 5th, 6th and 7th rank. But today, we find olive plantations in the Middle East, in the USA, in Latin America, and all North African countries, in other words, all over the world. As said, North Africa, especially Morocco, Algeria, and Tunisia, are major olive oil-producing countries; other countries scattered all over the world such as the Middle East like Palestine, Lebanon, the USA, and Latin America also contribute to sometimes non-negligible proportions in the total world production of olive oil. As reported by the website Conso Globe Planetoscope in 2018, olive oil production is concentrated in the Mediterranean region: Spain, Portugal, Italy, Greece, Turkey, Morocco, and Tunisia. These countries alone account for over 90% of world production. In 2018, this olive oil production worldwide was average as follows for Spain with 1,550,000 tons, Italy with 270,000 tons, Greece with 240,000 tons, Turkey with 183,000 tons, Morocco with 145,000 tons, Portugal with 130,000 tons, Tunisia with 120,000 tons, Algeria with 76,500 tons, Jordan and Lebanon with 24,000 tons and at last Argentina and Egypt with 20,000 tons. Olive oil production showed a net increase in the European Union during the 2020– 2021 crop year, reaching 2.3 million tons, according to a short-term agri-food outlook report. This net increase is promoted by the assuming climatic conditions in Spain as the main worldwide producer during spring rainfall and excellent flowering with 1.12 million tons of olive oil in 2019–2020. While, dry spring in some of Italy’s major

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producing regions and a heatwave in Greece are likely to lead to smaller harvests during the 2019–2020 crop year with 365,000 tons and 275,000 tons, respectively. Portugal also got a slightly smaller crop in 2020 than in 2019, when the country produced a record 140,500 tons (Dawson 2020). However, the commission reported that the country’s production continues to have an upward trend with an increase in overall consumption of olive oil in the EU of about 6% during the 2020–2021 crop year. While the statistics of the production of quantities of olive oil on a global scale are somewhat clear, the statistics for table olives remain poorly known since the field has not experienced a significant development from all points of view and has remained constrained using expensive artisanal harvesting.

1.2.1.2

World Olives Consumption

The main consumer countries are also the major producer countries. All the countries of the European Union represent 70% of world consumption and the countries around the Mediterranean show 77% of world consumption. The other consumer countries are the United States, Canada, Australia, and Japan. The evolution of production and consumption since 1970 shows a weak growth until the beginning of the ‘90 s, then a sudden increase in both production and consumption for the years 1996, 1997, and 1998 (Conso Globe Planetoscope 2018). The olive is an appreciated product, the taste and the conviviality are the key factors of consumption, its origin is a criterion of purchase, rather. There are several types of olives on the market. The different varieties of olives can be used as table olives (green olives, broken olives, black olives, according to many preparations) or as olives for oil. In the olives excluding fresh, green olives continue to hold the first place in the purchase of olives with over 9200 tons sold in 2009 (or 44% of the market). Aperitif olives represent over 6500 tons (31%) and black olives about 4800 tons (23%) (Conso Globe Planetoscope 2018). Finally, the olive as a tree, fruit, or oil knows a remarkable evolution from all points of view production, sale and consumption, and price. All activities and agricultural, consumer, and commercial aspects related to the olive have generated an intensive release of residual substances often with harmful characteristics and hostile to the natural environment that should be considered with great attention and awareness. The different agricultural, industrial, and commercial activities related to the olive are presented as activities generating nuisances to the natural environment and the living environment of the man. Thus, in recent years, various problems related to the margin and pomace main residues or by-products of the olive activity have emerged. The water olive oil and pomace are defined mainly as the liquid to gelatinous and/or solid waste from the extraction of olive oil in oil mills. The margin or pomace is a polluting material because of its high acidity (pH = 5.5) and its high content of polyphenols. The organic and mineral materials’ contents have a high chemical demand for oxygen, which

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explains the stench it emits during storage. The olive pomace is also an important solid by-product of the olive oil extraction process, consisting mainly of the skins, the pulp residues, and the fragments of the pits. But as a pertinent example, the olive oil production sector is one of Tunisia’s most vital. During the last three years 2015–2017, Tunisia has emerged as one of the leading olive oil-producing countries in the world. Thus, olive oil production reached 2019 a record of about 350,000 tons, exceeding the record set in 2015 of about 340,000 tons. With an average annual export value of US$0.5 billion, Tunisia exported most of its olive oil in bulk, and some of its bottled oils were successful internationally and among the winners of prestigious competitions. Thanks to the raising of its quality standards and the development of high-value-added products, including a growing production of organic olive oils, Tunisia has improved the competitiveness and recognition of its oils, both in the national market and export. A round table led by Tunisian and International producers of olive oil has deepened the theme of the importance of quality in a marketing strategy (FAO 2017). Besides the aspect of the quantity of production of olive oil, the value and the aspect quality of this oil began more and more to make its way these last five years on a world scale to impose itself commercially in the most important oil markets as those of the USA and Japan. This quality has resulted in creating real competition for olive oil exports at the international measure within the main producing leading countries (Bouazaa 2019).

1.2.1.3

Composition of the Olive

The olive is a drupe. It comprises three parts: the stone (or endocarp), the pulp (mesocarp), and the cuticle (epicarp). Its composition depends on its variety, the soil, and the climate. The content of the olive comprises the stone (17.3–23% m), the almond (2–5.5% m), the epicarp (2–2.5% m), and the pulp (71.5–80.5% m). The organo-mineral composition is organized: 48% water, 27% polysaccharides (hemicellulose, cellulose, and pectin), 21% triglycerides (the oil) are found mainly in the pulp and in the core, 3% mono and disaccharides and only 1% waxes, triterpenes, and phenols, with traces of alkanes, alkyl esters, methyl-phenyl esters, stearyl esters, aldehydes, alcohols, sterols, polycyclic triterpenoids and acids with long carbon chains … (Table 1.1). The nutritional interest of the olive lies in its excellent energy value compared to other fruits and vegetables. Its richness in calcium is considered equivalent to that of milk (100 g of olives = liter and a half of milk) and in vitamins A, B1, and E, in mineral salts and carotene makes it the food of high nutritional quality. The color of the olive would depend on the date of its harvest: green before ripening; pink when ripe and black after ripening. The color can also be a quality criterion. The more color becomes intense black to bright, the more the olive is rich in oils. The period from mid-November to the end of January corresponds to the ripening of the fruit and its strong enrichment in oil and the good period for the harvest.

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Table 1.1 The organic-mineral composition of the olive Components

Percentage (%)

Water

48

Oils

20

Mineral materials

1.5–2.5

Mono et disaccharides

3

Polysaccharides (hemicelluloses, celluloses, pectins)

27

Wax, triterpene, phenols

1

Various compounds: alkanes, alkyl esters, methyl-phenyl-esters, aldehydes, alcohols, sterols, triterpenoid polycyclic, fat acids with a long carbon chain

< 0.05

1.2.2 Olive Oil Extraction Processing To know the different by-products and residues of olive activity, we must cite and interpret the different processes of extraction of olive oil commonly adopted. There are nowadays, mainly, three types of processes of extraction of oil or oil mills available at the artisanal or industrial scale.

1.2.2.1

Artisanal, Classic, or Traditional Processes

In the artisanal, classic, or traditional extraction units, the process used for the extraction of oil includes two distinct stages: Crushing stage: This crushing is mainly carried out by granite stone millstones that rotate in a granite tank. This grinding can be done manually (case of small facilities) or through an animal (camel or mule case of craft facilities). The result of crushing is an oily and consistent paste that contains solid matter (debris of pits, epidermis, cell walls, etc.) and liquids (oil and vegetation water constitutive of the cells of the olive). Stage of separation of phases: The separation consists in putting the paste resulting from the stage of crushing on disks of vegetable fibers under a strong mechanical pressure to ensure the separation of the solid phase (pomace and solid residues) from the liquid phase (oil, water, water olive oil, etc.). The olive pomace is the solid residues or solid by-products recovered at the end of the second stage of phase separation. They are mainly composed of residues of the skin, pulp, starch, and fragments of olive stones. A separation by decantation of the liquid phases (oil and vegetation water) is carried out in the open air in cement, earthenware, or clay tanks. The liquid byproduct generated at the end of this stage is called water olive oil. These water olive oils are a liquid with a brown aqueous component that has separated from the oil by sedimentation after pressing or centrifugation. This liquid, which is rich in organic

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matter, often has a pleasant smell but a bitter taste and matters in the pollution of the natural environment and is the first major problem of the olive oil industry.

1.2.2.2

Batch Process or Super Press System

The olives received in the oil mills would pass directly through several stages: Crushing: The crushing is done by millstones characterized by a grinding slightly off-center from the axis of rotation, which would strengthen the possibility of crushing olives. Kneading: Kneading speeds up the release of maximum oil by strengthening the contribution of the paste under the wheels and its homogenization. Separation of phases: The paste obtained after the mixing operation will be placed in a layer of about 2 cm thick on vegetable or synthetic nylon fiber disks (scourtins), themselves stacked on top of each other around a central pivot (called needle) mounted on a small cart. The whole is placed on a hydraulic press piston, which allows the paste to be subjected to a pressure of about 100 kg cm−2 . The liquid phase flows into a tank. The pomace remains on the scourtins. This last operation lasts about 45 min. Then, each scourtins is removed from its nibs. Decantation: The oil of density 0.92 lower than that of water (1.00), rises to the surface and will undergo a natural separation. This property is better exploited by vertical centrifuges with plates to save time and especially improve the liquid–liquid separation.

1.2.2.3

Continuous Process

There are two types of continuous extraction processes: the three-phase centrifugal system and the two-phase centrifugal system. For the first system of extraction by three-phase centrifugation, the olives received, will undergo preliminary operations such as deleafing (removal of leaves), de-stoning (removal of stones), and washing to have the oil of good quality. The crushing operation occurs afterward by the action of continuous mechanical crushers with disks or hammers. Finally, operating mixing of the paste in a stainless-steel tank is moderately fluidized with warm water, which turns a spiral or an endless screw, also in stainless steel.

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1.3 Composition of Olive By-Products 1.3.1 Composition of Olive Pomace The main residue of the olive activity, as pomace, showed a variable composition depending on the varieties of olives crushed, climatic conditions, cultural practices, storage time (storage of olives), and the extraction process. Thus, its average composition is Dry matter with 75–80%; Mineral matter with 3–5%; Total nitrogenous matter with 5–10%; Gross cellulose with 35–50% and finally fatty matter with 8– 15% (Nefzaoui 1984, 1985; Dermeche et al. 2013). Globally, we can state that the pomace is composed by some fraction rich in lignin resulting from the fragments of nuclei, and another fraction containing mainly carbohydrates, such as cellulose and hemicellulose, and in a lesser measure, proteins and residual oil related to the extraction technique recommended.

1.3.2 Composition of Olive Mill Wastewater 1.3.2.1

Olive Mill Wastewater as Specific Waste

Olive mill wastewater is identified as semi-liquid effluents generated by olive oil manufacturers specifically during November and December, the season of olive crops. Olive mill wastewater is a very polluting semi-residual liquid, highly loaded with organic matter, and particularly its toxic propriety could affect the quality of water resources into which it is discharged. Also, it brings an immediate change to the watercolor, and its high organic load requires a high consumption of oxygen, leading to water eutrophication. When olive mill wastewater is spread as fertilizer in agricultural soil, it could decrease seriously the soil quality since it may contain various toxic substances and elements that settle and accumulate in the soil interstices.

1.3.2.2

Origin of Olive Mill Wastewater and Their Physical–Chemical Characteristics

As earlier said, the olive oil extraction process is based on these steps: crushing of olives, a cold pressing, and centrifugation separation to recover the oil. So, 1 ton of olives can produce on average 200 L of oil. Three essential products resulted from the extraction process of olive oil, olive oil, pomace or oil cake, and the olive mill wastewater. The extraction of oil by the commonly known processes, continuous or discontinuous, can generate, respectively, an average of 1.2–1.8 m3 of OMW/tons of olives processed, and 0.4–0.5 m3 of OMW/tons of olives processed. These data showed the superiority of the batch process over the continuous one (Tomati et Galli 1992). The volume of olive

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mill wastewater depends on the implemented pressing procedures and operational practices in the oil plant. Olive mill wastewater has a brown to reddish-brown color, with a cloudy appearance. These residual effluents are highly saline (electric conductivity of about 10 mS/cm), rich in potassium, chloride, calcium, and magnesium, and with an average pH acid (pH around 4.5–5), the high water content of 94%, the density of 1.04, high mineral and organic matter content of 15.8 g/L and 80.3 g/L, respectively, total organic carbon of 37 g/L and low biodegradable polyphenol content, 3.01 g/L (Hachicha et al. 2012). The COD is high and may range from 50 to 220 g/L according to the variety of olives and the processes of oil extraction adopted. The chemical composition of olive mill wastewater (OMWs) is highly variable, both qualitatively and quantitatively (Khatib et al. 2009). It depends on many factors, in particular, the variety of olives or cultivar type, fruit maturity linked to the period of production and climatic conditions (Bettazzi et al. 2006; Justino et al. 2010), and at last olive oil extraction method (Khdair and Abu-Rumman 2017). Table 1.2 summarized the overall chemical composition of olive mill wastewater (Khdair and Abu-Rumman 2017; Vlyssides et al. 1998; International Olive Oil Council 2016). These harvested olive crops could vary in the major olive-producing countries, according to the year, and olive harvested could be classified in the whole, on average 3–4 years. However, the increase recorded in the production of the olive mill wastewater is attributed not only to the evolution of the production but especially to the installation of the additional continuous kind of line treatment, whose specific production of olive mill wastewater is known as more important. Considering the average specific production coefficients of olive mill wastewater per crushing system, and the distribution of different crushing used, it is estimated that a ton of olives produces on average 0.35 tons of pomace and around 1 m3 (around 1.1 tons) of olive mill wastewater all olive crushing system combined, Table 1.3 (Rharrabti et al. 2018; Berndt et al. 1996; Borja et al. 2006; Chouchene 2010; McNamara et al. 2008).

1.3.3 Potential Techniques for the Treatment and Valorization of the Olive Mill Wastewater In technical publications about the olive mill wastewater treatment, up to over 20 processes or technologies applicable to the treatment of the olive mill wastewater could be cited (Paraskeva and Diamadopoulos 2006; Rharrabti and EI Yamani 2018). Mostly, these are basic or combined operations tested in a laboratory or pilot plant, with no subsequent industry projections. Therefore, these technologies have been identified as potentially applicable: (i) Natural evaporation in basins or lagoons (ii) Use of fertilizers (iii) Dehydration forced evaporation thermal concentration (iv) Incineration (v) Distillation (vi) Membrane processes: Ultrafiltration, reverse

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Table 1.2 Chemical composition of the olive mill wastewater Parameters

Units

Average values Batch system

Continuous system

Water

%

83

88

Temperature

°C

24

24

pH

Unit

4.5–5

4.7–5.2

Conductivity

Micro S/cm

16

8

SS

mg/L

1–2

6–8

OM content

%

10.5–15

–

COD

g/L

120–130

45–60

BOD5

g/L

90–110

35–48

Dry matter (DM)

g/L

120–170

50–60

Volatile matter (MV)

g/L

88–105

44–55

Total nitrogen content

g/L

5–20

2–8

NTK

mg/L

1–700

–

Fat content

g/L

0.5–1

3–10

Polyphenol content

g/L

10–24

5

Mucilage pectin and tannins

g/L

10–15

3–7

Mineral content

%

1.5–2

–

O-phosphate

mg/L

200–400

–

Calcium

mg/L

200–700

–

Magnesium

mg/L

130–290

–

Sodium

mg/L

10

–

Potassium

mg/L

5000–11,000

1200

Chlorides

mg/L

600–1200

–

Cadmium

ppm/DM

< 0.1

–

Chrome

ppm/DM

10

–

Copper

ppm/DM

68

–

Mercure

ppm/DM

< 0.01

–

Nickel

ppm/DM

8

–

Lead

ppm/DM

2)] to achieve a less dark (88%) permeate by rejecting 74% of polyphenols.

2.5.1.3

Thermal Processes

This processes includes several techniques and variants, but they all applied to concentrate OMWW using either a man-made heat source or a natural source of thermal energy (sun, air). Evaporation, pyrolysis, combustion, and distillation, lagooning (natural evaporation) are the most essential thermal processes Ghadraoui et al. (2020). Natural evaporation in ambient air using Evaporation ponds or storage lakes (lagoons) was one of the earliest procedures used for OMWW treatment. It is a simple, straightforward technique that depends on solar energy, using significantly less energy Khdair and Abu-Rumman (2020) and Tekerlekopoulou et al. (2017). In natural evaporation lagoons, COD removal rates range from 20 to 80%, although the waste spends 7–8 months in the lagoons, necessitating huge land surface areas (about 1 m3 for every 2.5 m3 of OMWW). Another environmental concern arising from these lagoons is the possibility of leakage, infiltration, and contamination of groundwater, methane emissions into the atmosphere as a result of anaerobic waste fermentation in lagoons and insects and noxious odors Siregar and Romaito (2020) and Kharbouch et al. (2020). Although thermal treatment techniques are reported to reduce waste volume (70– 75%), there are major discrepancies in the literature about their performance since it is dependent on several parameters such as olive ripening, extraction process, and notably waste storage period. The main disadvantage of these method is the post treatment and disposal of the created emissions: Apart from water, the distillate or condensate includes a significant amount of volatile chemicals such as volatile acids and alcohols. These substances cause the condensate to be overly acidic (pH 4–4.5) and have significant BOD more than 4 g/L and COD more than 3 g/L, necessitating extra treatment before discharge or reuse. Air pollution resulted from the combustion of the concentrated paste which has a high concentration of toxic organic load. These processes are also incredibly expensive because to the large amount of energy required and the equipment expenses that must be manufactured of corrosion-resistant materials Volpe et al. (2015), Christoforou and Fokaides (2016), Perazzini et al. (2016) and Barskov et al. (2019). Several researchers have proposed a combined thermal treatment of OMWW and olive husk in an effort to reduce the energy expenditures of thermal procedures. The needed heat for the evaporation of OMWW is created in these procedures by the burning of OMWW concentrated evaporation residue, olive husk, or a combination of these wastes. The degree of mixing of olive husk with OMWW is a significant component influencing the viability of this disposal method. Because such disposal

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systems have a relatively high technological level that requires significant capital expenditures and experienced employees, they are better suited to centralized treatment plants that service a large number of mills and benefit from economies of scale Khdair and Abu-Rumman (2020), Christoforou and Fokaides (2016), Souilem et al. (2017) and Vlyssides et al. (2017).

2.5.2 Oxidation and Advanced Oxidation Presses Ozone, hydrogen peroxide, chlorine, chlorinated derivatives, and combinations of these have all been investigated for OMWW treatment. Hydrogen peroxide and ozone systems are selected because of their strong oxidising potential and the ability to operate at atmospheric pressure and temperatures without producing unwanted breakdown products Auría Rasclosa (2014), Gholamzadeh et al. (2016) and Ramprakash and Karuppan (2019). Advanced oxidation techniques, which employ a mixture of oxidants as well as oxidants combined with UV light, have emerged in an attempt to boost oxidation rates. They are distinguished by the generation of the highly oxidative HO− radical at room temperature by a variety of photochemical and non-photochemical mechanisms. This potent radical has the ability to convert organic molecules to CO2 Al-Bsoul et al. (2020). Advanced oxidation using photocatalytic reactions showed high efficiency in the mineralization of organic compounds and the disinfection of pathogenic microorganisms in wastewater. Hajjouji et al. (2008) Reported that, UV/TiO2 treatment for 24 h resulted in the removal of the vast majority (94%) of the phenolic chemicals. Furthermore, the treatment destroyed 57% of the colored molecules and 22% of the chemicals responsible for the chemical oxygen demand (COD), which appear to be primarily pectins. Khani et al. (2020) reported, the removal efficiency of TOC and COD in a catalytic ozonation process (COP) reactor was 44 and 56%, respectively. While Moudden et al. (2020) confirmed, at low concentrations of TiO2 and vanillic acid, an efficiency of around 96% may be achieved. It might, however, be utilized as part of a multistep process or as a pre-treatment step to convert biorecalcitrant molecules to more easily biodegradable compounds. Hodaifa et al. (2020) also found that, the direct application of UV-light or the use of a UV/H2 O2 system reduced organic load by up to 50%. Ciggin et al. (2021b)studied the impact of ultrasonic pretreatment on Fenton-based oxidation of olive mill wastewater, the results demonstrated the value of Fenton-based oxidation, with an 80% TPh reduction and a toxicity reduction down to a tight margin of 13–17%, as well as a COD reduction of 20–26%. Most traditional oxidation procedures are ineffective due to either the expensive cost of antioxidants or the short COD period for which the system is suitable. It reduces COD significantly, but its running costs are significantly higher. So, chemical oxidation may emerge as a viable alternative when biological deterioration is not an option.

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2.5.3 Biological Processes Microorganisms are used in biological processes to break down the pollutants in OMWW removing inorganic nutrients and organic matter. According to the kind of microrganisms used, they are classified as aerobic or anaerobic procedures Elmansour et al. (2022).

2.5.3.1

Aerobic Activated Sludge

Microorganisms that arise naturally play an important role in olive mill wastewater treatment. Protozoa, fungus, bacteria, and other microorganisms are examples of these microorganisms. They eventually feed on a wide range of complex chemicals found in wastewater as it acts as a decomposer, oxidising complex organic molecules and converting them back to their simple forms of carbon that may be safely released to the environment Al Bawab et al. (2018). It can only function efficiently if the feed concentration is quite low. Higher concentrations can be tolerated only if the facility has a lengthy hydraulic retention period or a high recycling ratio; both of these options are uneconomical for a treatment plant. Furthermore, the aerobic treatment of concentrated wastewaters produces massive amounts of secondary sludge that must be removed from the system. Employing aerobic techniques to achieve the required removal efficiency of contaminants such as polyphenols and lipids is quite challenging. Aerobic procedures are inappropriate for direct and effective treatment of OMWW for all of the reasons stated above. They can be employed as pre- or post-treatment procedures to improve the efficiency of the primary treatment process Moussaoui et al. (2018). Fraij and Massadeh (2015) investigated one species of fungi, and he found that by the end of the treatment procedure, several phenolic and non-phenolic chemicals had been completely degraded, and the colour of OMW had been decreased by more than 60%. Ciggin et al. (2021a) reported in his investigation, where OMWW was treated under aerobic conditions, he noticed a COD removal rate of 87%. Particular interest has been exhibited in the N2-fixing microorganisms Azotobacter and numerous white rot basidiomycetes, such as Pleurotus, among all microorganisms used in biological treatment. These fungi have ligninolytic enzymes and can breakdown OMWW phenolic compounds that have structural connections with lignin. The majority of phenolic chemicals eliminated, however, are simple monomers, whereas polymerized molecules, such as tannins, breakdown more slowly. This is due to the fact that these chemicals adsorb heavily to mycelia and extracellular enzymes, making biodegradation impossible Zerva et al. (2017).

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Anaerobic Digestion

Anaerobic digestion is performed in the absence of molecular oxygen by a group of anaerobic microorganisms, primarily bacteria, with slower growth rates than aerobic microorganisms. It is a complicated process that involves the microbial conversion of organic molecules into methane and volatile fatty acids. The primary processes in the process include acidogenesis, hydrolysis, and methanogenesis, with the latter being the most important anaerobic stage. Anaerobic digestion is widely used method for treating olive mill wastewaters. During this processes biomass waste is transformed to biogas and compost. Although a pretreatment or post treatment step is required (gravity settling, sand filtration, waste dilution, membrane processes, centrifugation, physicochemical treatments, adsorption, and aerobic degradation), anaerobic treatment is thought to be the best option for OMWW detoxification Khdair and Abu-Rumman (2020) and Bampalioutas et al. (2019). The most important reasons for this choice are the low energy requirements, the ability to treat wastewaters with a high organic load, the production of significantly less waste sludge (than aerobic processes), the production of methane that can be used after appropriate treatment, and the ability to restart easily after several months of shut down Khdair and Abu-Rumman (2020), Rahmanian et al. (2014), Tsagaraki et al. (2007), Papadaki and Mantzouridou (2016) and Vavouraki et al. (2020). Several techniques have been investigated, including anaerobic filters (upstream and downstream), contact reactors, anaerobic baffled reactors (ABR), an upstream anaerobic sludge blanket reactor (UASB), and two-stage systems that separate acidogenesis and methanogenesis processes. The up-flow anaerobic sludge blanket reactor (UASB) is one of the most widely used bioreactors for treating OMWW. In this reactor, COD reductions of 70–80% were obtained with hydraulic retention durations of 2–5 days, and it was reduced by 87.9% when HRT was increased to 25 days Gunay and Karadag (2015) and Ahmad et al. (2020). The natural biological treatment of OMWW, as seen in its disposal in an evaporation pond, resulted in significant phenol and long-chain fatty acid breakdown. The microbial biomass composition was also influenced by the OMWW dumping in ponds. Both the shift in microbial and the removal of total aerobic mesophilic bacteria were caused by anaerobic digestion, which encouraged further oxidation of organic by-products, resulting in more reduced product and preventing the proliferation of anaerobic bacteria Rajhi et al. (2018). There are aerobic consortia that grow on undiluted or diluted OMWW and are capable of metabolising and eliminating its aromatic components, hence the combination of aerobic and anaerobic treatment is intensively investigated. In all conditions, some sort of pretreatment (thermal treatment, dilution, etc.) is required, and the produced effluent always requires extra treatment before it can be safely disposed of. As a result, aerobic or anaerobic processes alone are insufficient for OMWW detoxification Lee et al. (2019).

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2.5.4 Combined Processes Combined processes are used to maximise organic matter and phenol removal. As reported by Paraskeva and Diamadopoulos (2006), combining settling, centrifugation, filtering, and activated carbon adsorption, the highest amount of phenol (94%) and organic matter (83%) may be removed. To minimise COD, a combination of biological and sophisticated UV/O3 oxidation has been used Lafi et al. (2009). In another technique, catalytic wet oxidation and microbial technologies [(Al–Fe) PILC/H2 O2 ], the system operating at 50 C significantly decreased COD, colour, and total phenolic contents. Furthermore, oxidising agents like monosulfuric acid and MnO2 have been recommended as method to improve phenol removal from OMW Azabou et al. (2010). Duarte et al. (2012) presented a three method (adsorption, fungal biodegradation, and biodegradation product diffusion). Pleurotus sajor caju and Trametes versicolor were used, with the second biocomposite proving to be the most successful, resulting in colour reductions of up to 45%, COD reductions of up to 64%, and total phenol reductions of up to 89% after 29 days of treatment. Al-Qodah et al. (2014) employed a combination of ultrasonic irradiation and aerobic biodegradation to reduce hazardous phenolic chemicals in greenish black (GB) and dark brown (DB) OMW. Khani et al. (2020) hybrid processes of electrocoagulation/catalytic ozonation and biodegradation. The overall efficiency of the ec/cop/sbr system in removing cod and toc was 98.4% and 97.2%, respectively. COD was removed from OMW using a three-stage method that included advanced oxidation with ozone, aerobic biodegradation, and UV photodegradation. The COD remained high for both single-stage O3 therapy and two-stage O3 /UV treatment. However, with the OMW treatment, a combination of biological and UV/O3 processes resulted in a change in COD decrease. The greatest removal levels were seen in biodegradation of UV/O3 pretreated OMW, with a COD removal rate of around 91%. According to the kinetic analysis, the decrease of chemical oxygen demand follows first-order models for advanced oxidation and pseudo first-order models for biodegradation processes Lafi et al. (2009). Although these combined processes achieve better treatment, the increased number of processes can drastically raise the overall cost of the operation.

2.6 Applications of Olive Oil By-Products Hotspots of the olive oil industry include the valorization of olive oil by-products by the recovery of high-value active components while decreasing toxicity to the level set by legislation and regulatory systems. This strategy not only improves the environment but also provides economic advantages by making treatment more cost-effective and appealing to the industry (Fig. 2.2) Salomone et al. (2015).

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Fig. 2.2 Olive oil by-products valorization processes

In this respect, obtaining extracts rich in phenolic compounds that have effective benefits on human health for further inclusion in complex matrices production such as pharmaceutical and foods industry. Even though they have limited applicability in the food sector, as they are easily affected by external/environmental elements such as temperature, pH, and light. To maintain the integrity with the capability of a prolonged release of phenolic compounds without losing their physicochemical characteristics, these phenolic-rich extract compounds must be coated/encapsulated and finished in a final formulation. This valorization application includes extraction of valuable compounds, composting, land and agriculture application, bioplastics and biogas production, proteins production, and alternative uses as enzymes Hamimed et al. (2021) and Brandão et al. (2021).

2.6.1 Land and Agriculture Application Since traditional olive mill waste-treatment methods have been shown to be insufficient for completely removing OMWW pollutants, several studies have been

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conducted to turn this waste into a renewable resource and convert it to organic fertilizer and water source for agricultural. For many years, researchers have been studied the direct and indirect application of olive oil wastes to soil Marks et al. (2020), Bombino et al. (2021), Chaari et al. (2015) and Berbel and Posadillo (2018). Recent research has identified OMW as a source of nutrients, organic matter, and water for agricultural lands. It has been widely researched for agriculture irrigation, either as its raw form or after treatment. The effect of OMW irrigation on soils, plants, and the environment has shown contradictory findings. However, many studies have found that this approach promotes plant growth, agricultural output, and soil fertility Kapellakis et al. (2015), Belaqziz et al. (2016) and Chatzistathis and Koutsos (2017). Galliou et al. (2018) investigated the generation of organic fertilizer from olive mill effluent using a combination of two basic methods: solar greenhouse drying and composting. He found that an organic fertilizer rich in nutrients and low in phenol content has been produced. The researches also show that OMW is part of the group of natural compounds that can be used as a substitute for commercially available caustic disinfectants such as sodium hypochlorite. The disinfecting capabilities of olive mill waste concentrate were discovered to be beneficial against seed-borne pathogens. It had a substantial effect on the suppression of damping-off disease groups in the pre-germination stage of treated seeds Yangui et al. (2013) and El-Abbassi et al. (2017). OMWW has also been proposed for use as a biopesticide. The application of OMWW to soil and crops suppressed the development of most phytopathogenic bacteria, fungus, and weed species while having no influence on crop growth Foti et al. (2021) and El-Abbassi et al. (2017). However, when using OMWW as a biopesticide, some precautions should be taken, particularly in terms of dosage and application time.

2.6.2 Production of Dietary Fiber for Food and Animal Feed Dietary fiber is the edible plant cells that remains and include lignin, polysaccharides, and other compounds that are resistant to digestion by human enzymes. Olive cell wall polysaccharides recovered from olive mill byproducts have been offered as a source for a variety of goods, including powdered cellulose, fat substitutes or microcrystalline and gelling agents as well as a potential supply of fermentable sugars and particular saccharides Dermeche et al. (2013), Galanakis et al. (2010) and Padilla-Rascón et al. (2020). The feed and food industries appear to be promising sectors for the valorization of Dietary fiber. It is acknowledged, based on well-documented research, that it plays a significant role in numerous physiological processes as well as the prevention of various illnesses. Fiber-rich diets are beneficial to health, and its intake has been

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linked to a lower risk of numerous cancers. Pectic material is also found in OMWW and olive pomace may even be processed into a source of gelling material Dermeche et al. (2013). Olive cake has also been used as animal feed. In order to increase its nutritional qualities, fungal-treated depitted olive pomace was explored as an animal feed employing selective lignin-degrading fungus and solid-state fermentation. Despite its high fibre and protein content, it has a low nutritional value because its phenolic components function as digestive enzyme inhibitors. After 6 weeks, incubating depitted olive pomace (25%, w/w) with Pleurotus ostreatus and Pleurotus pulmonarius on barley grains, wheat middlings, wheat flour shorts, crimson clover, wheat bran, and field beans increased its crude protein content by 7–29% and removed 50–90% of its phenolic content. As a result, the use of olive by-products as a source of Dietary fiber or animal feed appears to be utopian and uncompetitive when the expense of treatments to remove their phenolic content is considered Brozzoli et al. (2010). The olive leaves used as a dietary supplement in chicken feed, with the goal of producing eggs rich in long-chain omega-3 fatty acids Gullón et al. (2018, 2020a, b). Once the component has been refined, it can be added into other foods to increase their functional characteristics or stability. Some studies in this field demonstrate that phenolic fractions (HT, OLE) from by-products can be included into oils to boost their antioxidant qualities. Previous research found that the product increased arylesterase activity while lowering oxidised LDL and sVCAM-1 levels, making it a functional food. In another study, hydroethanolic extracts from olive cake high in phenolic compounds were added to boost antioxidant capacity without increasing calorie intake Gullón et al. (2018, 2020a, b). The use of OLE extracts from leaves and olive fruits in sanitizing formulations has also been documented, as does the use of mannitol recovered from olive residues. Furthermore, due to their oxidative stability and antioxidant activity, vitamin E and monounsaturated fatty acids have been extracted from olive by-products and used as natural components Otero et al. (2018). According to recent research, OMWW is an environmentally acceptable alternative nutrient source for commercial edible mushroom growth on a wheat straw substrate with a mixture of tap water and OMWW Ahmed et al. (2019). Olive by-products were also used in the dairy, winery, and meat sectors, providing some novel applications. Because of its antibacterial qualities, HT has been utilised as a replacement for sulphur dioxide in the winemaking process. The potential protective impact of phenolics from olive by-products in the Maillard Reaction in dairy products was examined. Troise achieved this by incorporating phenolic powder content from OMWW in raw milk prior to ultra-pasteurization, which inhibited the production of off-flavor chemicals during the heat treatment, increasing both the nutritional and sensory characteristics Otero et al. (2021) and Caporaso et al. (2018). Furthermore, a phenolic extracted was added to milk as a novel functional component to boost its health characteristics Aliakbarian et al. (2015). In the case of meat products, collecting phenolics from agricultural byproducts was employed as an

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environmentally favourable food conservation approach. Indeed, a purified phenolic extract from olive vegetation water was added to fresh Italian sausages, and the growth of food-borne pathogens was clearly inhibited Fasolato et al. (2016) and Otmani et al. (2021).

2.6.3 Pharmaceutical Industry The most recent studies on the use of olive by-products in the development of new pharmaceutical products point to their positive use in gastrointestinal disorders, as antinociceptives, appetite suppressants, and modulators of human microbiota, among other things, owing to their high content in phenolic compounds, polysaccharides, and triterpenoid acids. One of its prospective use is the use of polyphenols derived from olive oil byproducts in the treatment of digestive disease Di et al. (2018). In this regard, the antiinflammatory activities of an aqueous extract of OP, as well as its influence on cell metabolome, were investigated in human intestinal Caco-2 cells. The primary proinflammatory cytokine, IL-8, was lowered by such supplementation, demonstrating the therapeutic potential of polyphenols from olive pomace in intestinal disease. Furthermore, the effects on the cell metabolome revealed a metabolic shift toward a glucose-saving strategy, which explains the appetite-suppressing effect observed after consuming polyphenol-rich foods Di et al. (2018). Furthermore, polar lipids derived from olive product and by-products inhibited platelet activating factor, which is implicated in inflammatory diseases such as atherosclerosis Galanakis et al. (2018). Biocompounds found in olive products, such as aliphatic aldehydes, have been demonstrated to inhibit or retard the growth of a variety of bacteria and yeasts, and might be used to prevent or cure infections. Furthermore, they have been studied for use to combat the spread of antibiotic resistant bacteria Bensid et al. (2020). At low concentrations, hydroxytyrosol has antimicrobial activity against respiratory and gastrointestinal infectious agents such as Vibrio parahaemolyticus, Vibrio cholerae, Salmonella Typhi, Haemophilus influenzae, Staphylococcus aureus, and Moraxella catarrhalis. OMWW contain tyrosol, hydroxytyrosol, oleocanthal, and oleuropeinare which have a wide range of biological impacts on physiological processes, including antidiabetic, anticancer, cardioprotective, antiatherogenic, neuroprotective, and anti-obesity properties Foti et al. (2021), Madureira et al. (2021) and Cavallo et al. (2020).

2.6.4 Cosmetic Industry Olive mill byproduct was developed for the skin care industry and is now widely used. It contains intriguing compounds (FAs, squalene, OLE, minerals) with biological activity (antibacterial, anticancer, antioxidant, and hydration) which could be

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used in cosmetic product design. Leaf ethanolic extracts containing this chemical inhibited Staphylococcus aureus growth and decreased melanin production in B16 melanoma cells, indicating that olive oil by-products containing OLE are promising candidates for skin therapy. In addition, the OMWW extract stopped RBL-2H3 cells from releasing granules. It is also an excellent squalene reservoir, with antioxidant effects against sun radiation at the epidermal level, operating as an active biological skin barrier Galanakis et al. (2018) and Kishikawa et al. (2015). Olive by-products are also high in minerals, which have been recommended as components for hydration-focused cosmetics.

2.6.5 Production of Bioenergy and Biofuels Because its content of volatile acids, polyalcohols, sugars, and lipids, OMWW is a potential substrate for biomethane, biohydrogen, and bioethanol synthesis. Furthermore, because high ammonium concentrations hinder nitrogenase synthesis and activity, its low nitrogen content makes it an excellent substrate for photo fermentative biohydrogen generation. The majority of research has focused on biomethane produced by anaerobic digestion of OMWW substrate; however, studies has also been done in the areas of bioethanol production and biohydrogen production via photo fermentative processes and dark fermentation Messineo et al. (2020). In contrast to lactate and ethanol, biohydrogen generation from organic substrates involves fermentative catabolic pathways, with acetate and butyrate generating the most hydrogen. Through the coordinated action of a variety of microbes, anaerobic digestion can also convert organic substrates to biogas. Effluents from a hydrogenproducing reactor with high fatty acid concentrations can be fed into an anaerobic digester, which produces methane as a secondary product. Thus, biohydrogen and biomethane have been created from olive oil by-products utilising both single-stage and two-stage fermentations, with two-stage systems proving to be a practical and helpful alternative for the treatment of non-diluted OMWW Ghimire et al. (2016). Olive oil by-products have a high organic matter content, making them a possible alternative resource for ethanol generation by bacteria or yeasts. The various forms of polysaccharides found in olive oil by-products can be bioconverted into ethanol using two unique procedures. Enzymatic hydrolysis or physicochemical pretreatment can be used to liberate reducing sugars, with yeasts or bacteria then converting them to ethanol in the second stage. Pleurotus sajor-caju bioconverted OMWW appears to be a promising substrate for bioethanol production, and pretreatment with Pleurotus sajor-caju has already been shown to improve ethanol yields. Researchers have focused their attention to olive oil by-product exploitation and bioconversion into biodiesel, discovering that a strain of Lipomyces starkeyi is capable of proliferating in OMWW and converting it into lipids that are a good feedstock for biodiesel production Ahmed et al. (2019) and Rashama et al. (2019).

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2.6.6 Biopolymers and Enzyme Production OMWW was also used in several studies as culture mediums in various bioprocesses. The generation of lipases, polyhydroxyalkanoates, lignin-modifying, algal biomass, and enzymes, among other things, has been reported as an important method of OMWW valorization Paz et al. (2021). Yeasts and filamentous fungi can manufacture industrial enzymes utilising olive oil by-products as a substrate. Lipases, laccases, Mn-dependent peroxidases, and pectinases are the primary enzymes derived from the fungal treatment of olive byproducts. Panus tigrinus produces laccases and Mn-dependent peroxidases from OMWW, while Cryptococcusalbidus var. albidus produces pectinases, while Candida cylindracea and Yarrowia lipolytica strains create lipases. Microbial lipases have been used in the dairy, pharmaceutical, and detergent sectors, among others. Using thermostable fungal cultures of Rhizomucor pusillus and Rhizopus rhizopodiformis, olive oil cake has also been successfully used for lipase production Ayadi et al. (2018) and Maicas and Mateo (2021). Paz et al. (2021) used OMWW to feed the strain Bacillus aryabhattai Bao3, which reduces L-ASNase. OMWW has been proposed as a low-cost substrate for Xanthomonas campestri xanthan production. Different strains of Paenibacillus jamilae have also been shown to grow and produce exopolysaccharides using OMWW as the sole nutrient and energy source, reducing the waste’s toxicity Foti et al. (2021).

2.6.7 Other Uses Meksi et al. (2012) described a high biosorption yield for olive cake for removing RR198 dye from real-world wastewater, and Akar et al. (2009) described a high biosorption yield for olive cake for removing RR198 dye from real-world wastewater. Another potential application for activated carbon derived from olive stone is the removal of dyes, odours, tastes, and even contaminants such as arsenic or aluminium for water purification and other decontamination processes. It also has long been used in the production of soaps such as Marseille soap. Other writers, on the other hand, have looked at non-biological uses for olive by-products, such as maximising their high adsorption capacity. Other research has looked into the usage of olive stone-based activated carbon as a heavy metal ion biosorbent Yakout and El-Deen (2016), Alshuiael and Al-Ghouti (2020) and Gul et al. (2021). As mentioned earlier, olive oil extraction produces a solid residue and a darkcolored OMWW, both of which can be bioprocessed and disposed of in parallel. Several types of research were conducted on olive oil byproducts to test their ability to be used in a multitude of situations. The uses found are only discussed in the scientific literature as olive oil by-products are not fully used. These strategies may

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be regarded as an integrated industrial system’s eco-design. Only regulations and integrated research based on well-funded studies will be able to achieve the goal of efficiency by constructing a global olive refinery that uses olive oil by-products in the most efficient possible way Zahi et al. (2022).

2.7 Conclusion Olive oil production contributes to the growth of the economy, especially in the Mediterranean countries. However, waste from this industry represents a serious environmental issue due to its heavy load of fats, phenolic compounds, and organic matter. Therefore, innovative, clean solutions and technologies must be found to value olive mill waste and identify it with the specific needs of local and global concerns. Although the multiplicity of the presented treatment and valorization methods deal with these pollutants, their by-products contain a large amount of heterogeneity in terms of composition, so they need more attention to deal with them. These treatment and valorization methods depend on two trends: first, the extraction of valuable phytochemical compounds with beneficial properties that are used in several other industries, such as the pharmaceutical, cosmetic, and food industries; and secondly, the biological transformation of the by-products of the olive mill without any environmental effects. regulations and integrated research must be developed to achieve the efficiency goal by setting up a global olive refinery that uses olive oil by-products in the most efficient way possible. Regulations and research must be developed to achieve the efficient goal of setting up a global olive refinery that uses olive oil by-products in the most efficient and eco-friendly way possible.

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Chapter 3

Valuable Materials from Olive Mill Wastewater Sare Asli , Mahmud Diab, and Manal Haj-Zaroubi

Abstract The olive oil industry represents one of the most economically essential agri-food sectors. The worldwide olive oil production goes beyond 2.5 million tons each year, with more than 95% of this production being generated in the Mediterranean countries. This industry produces unexploited agronomic residues of more than 20 million m3 of olive mill wastewater (OMW). OMW is typically acidic (pH ~ 4–5) and highly toxic to plants and microbes because of its high phenolic compounds (up to 30 g L−1 ) content. Hence, OMW is claimed to be one of the most polluting effluents. Several physical, biological, and physicochemical strategies have been proposed to treat OMW. Besides being costly, these techniques cause biodegradation or destruction of the phenolic fraction of OMW, thus resulting in a loss of valuable, functionally active, and exploitable compounds. These polyphenols are essential as natural antioxidants, antibacterial, antiviral, and antifungal compounds, and their potential as active components for increasing the tolerance of salt-stressed plants. Soil salinity is one of the global problems that affect approximately 20% of irrigated land, which can cause a fundamental reduction in crop yields. The severe result is translated into causing risks to food security for the escalating world population. Most crops species are not salt-tolerant in soil. The toxic effects, the concentration of salt, and the stress duration are the main factors determining the adverse plant responses to salinity stress. Plants respond to stress by modulating gene expression, which eventually restores cellular homeostasis, detoxifies toxins, and recovery of growth. Consequently, these diverse environmental stresses often activate similar cell signaling pathways and cellular responses, such as the creation of stress proteins, S. Asli (B) · M. Diab · M. Haj-Zaroubi The Institute of Applied Research, The Galilee Society, PO Box 437, 20200 Shefa-Amr, Israel e-mail: [email protected] S. Asli The Faculty of Science, Al-Qasemi Academic College, Baka EL-Garbiah, Israel S. Asli · M. Diab Institute of Evolution, University of Haifa, Haifa, Israel M. Haj-Zaroubi Department of Natural Resources and Environmental Management, Faculty of Management, University of Haifa, Haifa, Israel © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_3

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up-regulation of antioxidants, and accumulation of compatible solutes. In this regard, Ascorbic acid (ASA), Jasmonic, and the salicylic acid act as a primary substrate in the cyclic pathway for enzymatic detoxification of hydrogen peroxide. Additionally, they have emerged as an otent antioxidant and play a key role in plant stress signaling and growth, physiological and biochemical processes. In recent studies (Asli et al. in Sci Total Environ 630:728–737, 2022) have demonstrated that phenolic compounds from OMW (such as hydroxytyrosol) showed a significant increase in the tolerance of the salt-stressed plant. Therefore, OMW may be regarded as an inexpensive source of organic compounds to be recovered because of their ability to be transformed into valuable products in agricultural applications. Keywords Olive mill wastewater · Phenols · Antioxidants · Nanoparticles · Hydroxytyrosol · Salt stress · Salt tolerance · Plant water balance

3.1 Olive Oil Production Mediterranean countries have dominated the world’s olive oil production, which increased significantly within the last decades. Around 99.5% of olive oil production occurs in Spain, Italy, Greece, and Portugal, reaching 2.4 × 106 tons (Khdair et al. 2020). Olive oil production remains a high polluting activity despite its economic and social relevance due to the utilization of large amounts of water and the production of large quantities of waste (Azbar 2004; Roig et al. 2006). Olive milling and oil production is achieved by either three-phase system, which is used widely in Italy, Greece, Israel, and other Mediterranean countries, and the two-phase system is mainly used in Spain. In the three-phase system, the wastes generated are olive mill wastewater (OMW) and olive mill solid waste (OMSW); whereas, with the two-phase system, only a semisolid by-product is obtained that contains both water and solid residue. It is well known that phenols, polyphenols, and other natural organic compounds extracted from olives during oil extraction partially end up in the OMW. The milling process season is usually 2–3 months each year, where small mills are spread over tens of thousands of facilities in the Mediterranean and Middle Eastern countries, where the transportation of the OMW is the most economical and challenging solution so far in the site.

3.1.1 Production and Properties of OMW Annually, large amounts of OMW are produced worldwide, reaching up to 3 × 107 m3 in the short period of harvesting and olive oil extraction (Cabrera et al. 1996; El-Mekawy et al. (2014). In the three-phase system of olive oil production, Fresh water is added to the process in order to separate better the two phases of oil and water. From each ton (1000 kg) of olives 0.2 ton (200 kg) of olive oil and up to 1.6 m3

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OMW are obtained (Khdair et al. 2020; Azaizeh et al. 2020). Each year 12 × 106 tones of olives are used to produce ca 2.4 × 106 tones olive oil and ca 3 × 107 m3 OMW (ca 10 times more OMW than oil production). As mentioned previously, in the three-phase centrifugation process of extracting olive oil, 1.6 m3 of OMW is produced from each ton of raw olives (Azaizeh et al. 2020). This waste consists of many organic and inorganic pollutants, such as polysaccharides, sugars, phenolic compounds, polyols, proteins, organic acids and lipids, and high levels of potassium and phosphorus (Mekki et al. 2007; Michael et al. 2014). Although OMW is a natural product, it pollutes water bodies and the environment because of its composition: High Biological Oxygen Demand (BOD) (up to 50 g L−1 ) and Chemical Oxygen Demand (COD) (up to 200 g L−1 ); Low pH (≤5); High EC (7–11 dS m−1 ) and ion content (mostly K); very high content of phenols; and above all, toxicity to living organisms, especially in water bodies (Mekki et al. 2007). Although OMW is characterized by a seasonal production of moderate extent, if compared to other agricultural wastes, its environmental impact is very high. The high organic load of OMW makes it hard to manage and difficult to meet the legal thresholds for their discharge to the environment. The presence of high concentration of phenols (toxic compounds) in their composition makes the common biological treatments inefficient. Due to these characteristics of OMW, the pollution effect of 1 m3 of OMW has been estimated to be equivalent to 200 m3 of domestic sewage (Paraskeva and Diamadopoulos 2006). The phenolic compounds in OMW (Fig. 3.1) are mainly responsible for toxicity (Mekki et al. 2007) and phytotoxicity (Belaid et al. 2013, they are characterized by their large varieties and complexity (De Marco et al. 2007; Belaid et al. 2013). The varieties and concentrations of these compounds depend on many factors, including maturation stage, soil, climatic conditions, olive cultivation, and the treatment to which it is subjected (Kallel et al. 2009; Niaounakis and Halvadakis 2006). In addition, each olive variety has a different composition of phenolic compounds. These unique characteristics prevent OMW from being directly discarded into environmental media, like land and water, as well as into domestic wastewater treatment plants (WTPs), and it must be correctly treated before being released into the environment.

3.1.2 Environmental Impacts of OMW Olive mills dispose of their OMW in settling ponds that are normally under-sized and sometimes get overloaded, causing spillage to nearby valleys (Khdair et al. 2017, 2019). There are no proper facilities for treating OMW in individual mills, so their minimization, prevention, and treatment have long been investigated to reduce the environmental impacts caused by their uncontrolled disposal (Galanakis 2012). The wastewater is also a major source of odor and can be harmful to plants if irrigated at a high OMW concentration.

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Fig. 3.1 Structures of some phenols in OMW

The uncontrolled disposal of OMWs on soil results in decreases in water retention and infiltration rate, increases soil hydrophobicity, and causes strong phytotoxic and antimicrobial effects, in addition to affecting soil acidity, salinity, nitrogen immobilization, nutrient leakage, organic acids, lipids concentration, naturally occurred phenols and microbial activity (Sierra et al. 2007). Land disposal without treatment can result in the contamination of soil and water, which threatens the quality of the water resources in the arid Mediterranean countries representing a severe environmental concern. Furthermore, in the European countries, the cost of land disposal is about 5–10% of the turnover of the olive oil industries and can achieve values of 15–20% in some regions. Notably, OMW cannot be merely applied to soil and crops because of its phytotoxic properties, and adverse smells are particularly undesirable in touristic Mediterranean hot points. In other words, OMW is a significant health, ecological, and economic hazard. Several physical, physical–chemical and even biological strategies have been proposed to treat OMW (Vavouraki et al. 2019; Elkacmi and Bennajah 2019, Ochando-Pulido et al. 2017).

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3.1.3 Agricultural Impacts OMW affects the environment (Sayadi et al. 2000; Selcuk Kuscu and Eke 2014) and causes harmful consequences to soil (clogging, asphyxiation of organisms) (Chartzoulakis et al. 2010), plants (Muscolo et al. 2001; Sampedro et al. 2004; Asli et al. 2022), superficial and underground waters (Justino et al. 2012) not to mention the unpleasant odors. OMW is rich in polyphenols, has low pH, various heavy metals and minerals, and needs high chemical and biological requirements for its dissolution. If the OMW spreads to the soil without pretreatment and in high amounts, it both affects the physical, chemical, and biological characterization of the soil in a negative way and damages the plant (Komnitsas and Zaharaki 2012). Even after ten years since the OMW was thrown into the environment, it causes development disorders in the vegetation and heavy metal pollution (Komnitsas and Zaharaki 2012). Therefore, different concentrations of OMW in chickpea, durum wheat, tomatoes, and corn prevented germination (Andreozzi et al. 2008). Even 1/8 dilution of OMW significantly reduces the germination rate (El-Hadrami et al. 2004). In addition, OMW changes the soil structure with a high salt concentration in its contents and carries potential damage to plant growth (Gigliotti et al. 2012). There are different studies on the cytotoxic effects of OMW in plants. For example, In Vicia faba, OMW causes chromosomal abnormalities, root tip darkening, micronuclei, as well as rootstock and mitotic inhibition (El-Hajjouji et al. 2007). In a study conducted by Aybeke et al. (2000), numerical or structural chromosomal mutations, mitotic abnormalities, and increased mitotic frequency, as well as highly nuclear or broken nucleated cells in wheat root tips, were emphasized. Also, It was detected that the amount of protein decreased with increasing concentration and duration of treatment. Also, OMW caused wall and nuclear damage, cytoplasmic membrane, and cellular organization disorders in wheat stem meristem cells (Aybeke et al. 2008). Furthermore, OMW caused damage to the DNA genomic structure and DNA structure. It was also stated that all OMW applications had a free radical threat and that there was more damage, especially in the 5-day OMW tests (Aybeke 2018).

3.1.4 The Effect of OMW on Gene Expression Activity Despite all of these morphological, cytological, agronomic, genotoxic, and cytotoxic findings mentioned above, no information was available on how the OMW actually affected the metabolism of plant hormones and secondary metabolites (phenols) during the application to plants. How does this waste, which is so toxic indeed, affect the mechanisms of plant hormones and secondary metabolite (phenol)? Therefore, Aybeke (2018, 2020) recently investigated the effects of OMW on plant hormones and phenol metabolism in sunflowers. As a result of hormonal and phenolic analyzes, especially in the 5-day experiments, it was understood that the

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hormonal and phenolic changes are more so that the damage is greater. Damages and hormonal/phenolic changes are comparatively less in 3- and 10-day treatments. The most effective hormone against OMW stress is salicylic acid. Especially in 5-day experiments, jasmonic acid also was active together with salicylic acid. Likewise, most phenolic synthesis has been done in 5-day experiments. OMW reduced auxin (IAA), Gibberellic acid (GA), and Abscisic acid (ABA) hormones in sunflower plants. In short, OMW degrades hormonal balance and phenol metabolism in the plant.

3.2 Solutions The valorization of OMW is considered challenging due to its high organic content, complexity, and phenolic compounds that can inhibit their biodegradation (Tafesh et al. 2011). This is why several physical, physicochemical, biological, and even integrated chemical-biological strategies have been proposed to treat OMW (Vavouraki et al. 2019; Elkacmi and Bennajah 2019; Ochando-Pulido et al. 2017; Kontos et al. 2014). Biological processes including aerobic treatments, composting, or anaerobic digestion for biogas production have been developed, in addition to chemical oxidation processes. Evaporation ponds and co-generation are the conventional OMW treatment methods due to their simple constructions (Alfano et al. 2009). However, the failure in the basin insulation can easily contaminate the groundwater and produce a robust stinky smell that eventually attracts insects during the decomposition processes. Meanwhile, the co-generation process would stimulate the nitrogen oxides production and emission of suspended ashes which would further pollute the environment (Rincón et al. 2012).

3.2.1 Biological Treatment The biological treatment is based on the utilization of microorganisms in reducing the OMW’s organic load. However, most studies on this way focused on anaerobic digestion (Anastasiou et al. 2011). There is a need for enough removal of polyphenols or making significant multiple dilutions for the OMW to receive efficient treatment in biodegradation (Speltini et al. 2015). El-Hajjouji et al. (2014) evaluated the use of aerobic treatment to remove toxicity from OMW by removing toxic components from the OMW. The authors have not measured COD and BOD levels, and most of the concern was put on the toxicity of the OMW. To improve the treatment’s efficiency in mitigating the wastewater’s toxicity, the authors suggested that aerobic treatment should be applied before the anaerobic process.

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On the other hand, several researchers reported that the aerobic processes of OMW could be collapsed with organic-rich wastewater (Mantzavinos and Kalogerakis 2005). So there is a need to dispose of organic-rich wastewater accumulation in the reactor. This may be implemented by diluting the OMW several times before the aerobic process (70–100 times in standard practice). The dilution is a reasonable pretreatment that can reduce the feed wastewater’s organic load up to a COD content of 1 g L−1 before biological treatment. However, this would likely result in higher costs. Polyphenols are the primary toxic material in the OMW. The best way to eliminate polyphenol toxicity from OMW is using co-composting with sesame bark (Hachicha et al. 2009). Consequently, the total organic matter was reduced by 52.72%, while the degradation of hydrophilic phenols decreased by 72%. The polyphenolic compounds disappeared after 7 months of processing. However, the demonstrated ability to remove phenols from the co-composting process still needs more time than anaerobic digestion; moreover, potential energy recovery is lost. Several researchers investigated the possibility of utilizing OMW as a biopesticide. For example, El-Abbassi et al. (2017) reported that OMW with its phenolic components might play a serious role in controlling pests and plants. Thus OMW could help protect plants from plant diseases, consequently eliminating the need for synthetic insecticides minimizing environmental problems. It was found that the bio-prepared OMW has excellent potential to inhibit the germination of major bacterial and fungal phytopathogens and weed species without affecting plant growth. However, the dose and timing of OMW biopesticide application should be carefully measured.

3.2.2 Physicochemical Treatment In general, the physicochemical treatment mainly uses natural resources as adsorbents for the purification process of water. For example, the physiochemical treatment was able to produce clean water for the agricultural water source. The ultrafiltration membrane is also one of the physicochemical processes mainly used in molecule disposals. Previously, the use of physicochemical processes was less focused on OMW treatment. Aly et al. (2014) investigated the efficiency of several mediums in the purification of OMW. For this goal, three successive columns of gravel, fine sand, and a mixture of acidified cotton and natural zeolite (clinoptilolite) were used for a purportedly low-cost treatment before polishing steps with activated carbon (AC) and lime (Aly et al. 2014). In this case, many OMW components were removed in all three columns. While the organic particles were removed with AC, and the pH was raised from 2.9 to 5.1 with lime. Azzam et al. (2015) investigated the possibility of adsorbing OMW on the local natural Jordanian clay. The process of adsorbing components from the crude OMW exposed to the Jordanian clay was reduced, with a total reduction of 10–20% of COD and phenols recorded in a batch experiment. A better result of 50% was obtained in a

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continuous fixed-bed experiment. However, further studies are needed to process the wastewater further and achieve an acceptable discharge level. The authors suggest integrated or multiple treatment stages to effectively treat the wastewater, as a single treatment method is insufficient. The membrane ultrafiltration process could be an acceptable method for treating OMW if the severe fouling problems could be overcome. Operating conditions below the limiting flux reduce fouling, making the approach economically infeasible. Photocatalysis has been proposed as a pretreatment to increase the limiting flux (Stoller et al. 2015, 2017). However, the economic evaluation shows that high costs are required for commercialization. Photocatalytic hydrogen gas evolution has been studied using OMW as a sacrificial agent (Speltini et al. 2015). The author highlighted that OMW is a cost-effective and worthwhile sacrificial agent, as 30 mL of OMW was used to generate 280 μmol of H2 . No CO2 was produced in this process compared to production from natural gas, which released a large amount of CO2 . However, the overall efficiency of the photocatalytic system still needs to be increased by improving the photoreactor properties.

3.2.3 Thermochemical Treatment The attention to releasing the energy potential of biomass waste has led to the initiation of research in the hydrothermal treatment of biomass waste. The hydrothermal process includes several methods, such as carbonization, which provides its main product as biochar; liquefaction which provides bio-oil; gasification to produce syngas. Today, hydrothermal gasification has become a worth investigating method to mitigate the waste problem. The basic benefit of this method is its ability to generate waste from damage to a resource by extracting energy from valuable biodegradable organic compounds and biomass waste (Kıpçak et al. 2011). In hydrothermal gasification, water in biomass waste acts as a reaction solvent and contributes to higher thermal and energy efficiency, with no pre-drying process (Taylor et al. 2009). Several researchers have investigated the hydrothermal gasification of OMW. For example, Kıpçak et al. (2011) investigated the practicability and performance of hydrothermal gasification. The OMW was filtered without further dilution, then pumped into a coiled tubular reactor system. The hydrothermal reaction was carried out at a temperature of 673–873 K and a reaction time of 30–150 s at a constant pressure of 250 bar. The formation of CO decreased dramatically with temperature and reaction time, as the conversion to CO2 occurred at a higher temperature and longer reaction time. The optimum gas yield was obtained at 823 K and an equivalent reaction time of 30 s. The gas product consisted of CH4 at 34.84%, H2 at 9.23%, and CO2 at 49.34%. Reducing the system pressure from 300 to 100 bar resulted in decreased gas efflux, most likely due to coking and soot forming, which clogged

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the system over time. The CH4 gas volume decreased while the CO2 gas volume increased as the system pressure decreased. Similarly, Yan et al. (2006) reported similar observations regarding CH4 , H2 , and CO2 . They have used glucose as a model in the thermodynamic analysis of hydrogen production from biomass gasification. The application of catalytic gasification of OMW was examined (Kıpçak and Akgün 2013). The experiment was conducted under supercritical conditions. They used the commercial catalyst Ru/Al2 O3 at a constant pressure of 250 bar to investigate the effects of temperature (673–873 K) and reaction times (30–150 s) on the gasification yield and gas composition and purification efficiency of the effluent. This catalytic reaction exhibited an above-average yield of gas products (9.3 L kg−1 OMW), including CO2 , CH4 , and singly bonded hydrocarbons. The author reported that energy recovery could achieve even better results with increasing temperature and reaction time, increasing efficiency to 98% under supercritical water conditions. Production was measured in a preheated sand bath reactor during hydrothermal treatment of OMW under supercritical water (803 K and 250 bar). Commercial homogeneous catalysts KOH, CO3 , NaOH, and Na2 CO, were used, and the COD removal was 75–89% for all catalysts, especially for the carbonate salt. An optimum result, 76.73 mol kg−1 OMW, was obtained in the hydrothermal treatment at 803 K and 230 bar for 20 min using 58 wt% KOH catalyst and a COD value of 23 ± 2.56 g L−1 OMW. A higher concentration of hydroxide catalyst resulted in a more significant reduction of CO and thus higher production. This treatment method was economically and energetically efficient (Casademont et al. 2018). Hydrothermal carbonization (HTC) has also been used to convert OMW into biochar material, commonly known as hydrochar (Poerschmann et al. 2013a, b). The HTC experiments were performed in a laboratory-scale 200-mL autoclave. The HTC experiment was performed at 493 K for 14 h (Poerschmann et al. 2013a). 100 mg L−1 citric acid was used as a catalyst in the reaction with a temperature range of 453– 493 K and a reaction time of 14 h under autogenous pressure (Poerschmann et al. 2013b). The authors did not indicate the use of HTC in OMW treatment because the yield of hydrochar was low, which could be related to the low carbohydrate content in OMW. Attempts were made to impregnate OMW with sawdust to produce green biofuels and biochar by pyrolysis (Haddad et al. 2017). The produced biochar showed higher K, N, and P content than sawdust not loaded with OMW. The authors also confirmed the function of biochar for plant growth and demonstrated that it contributed to the increase in leaf size and total mass. These nutrients can improve soil fertility, increasing plant growth rate (Zhao et al. 2016). Although the authors reported that the evaporated water could be recovered for irrigation purposes, the drying time was too long to prepare the feedstock for pyrolysis. The minimum drying time required was 107 min at 333 K to remove moisture from 20 g of raw cypress sawdust in 400 mL of OMW. Erkonak et al. (2008) studied the subcritical and supercritical water oxidation using H2 O2 as the oxygen source in a tubular reactor. The OMW was merely filtered without being subjected to dilution. It was assumed that this method of feedstock preparation would preserve the organic matter in the sample for oxidation purposes.

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The experimental results showed that oxidation with supercritical water reduced the total organic content (TOC) by up to 99.96% for very short residence times ranging from 5 to 30 s. Among the parameters studied, the temperature effect was the most important parameter for reducing TOC compared to pressure, residence time, and oxygen concentration. The prolonged reaction time and higher oxygen concentration also increased the TOC conversion. Treatment efficiency was increased by reducing the system pressure from 300 to 100 bar. Nevertheless, with the increased carbonation rate, the low-pressure system suffered from soot formation, which eventually settled on the inner reactor wall and caused blockages, especially in the flow-through tube reactor.

3.2.4 Integrated Treatment The integrated treatment of OMW was tested using several types of treating methods. Combining two types of techniques or more revealed new encouraging results. For example, the effect of combined treatment of domestic waste with Fenton’s reagent, a catalyst composed of hydrogen peroxide (H2 O2 ) and ferrous sulfate (FeSO4 ) used to oxidize organic compounds in wastewater, was studied together with anaerobic digestion (Amor et al. 2015). OMW treatment by anaerobic digestion alone was not sufficient due to the toxicity and biodegradability of the wastewater. Therefore, catalytic oxidation with Fenton’s reagent was proposed for the wastewater to degrade complex organic acids and recalcitrant components. The COD removal increased overall, and a higher methane yield was obtained (306 mL CH4 g−1 degraded from COD) than simple anaerobic digestion (288 mL CH4 g−1 degraded from COD). However, the authors pointed out the high capital and operating costs associated with the chemical reactor and reagents, significantly limiting scaling up the process to industrial size. Combined chemical-biological treatment was used to solve the OMW problem (Karahan Özgün et al. 2016). The problem of organic complexity leading to ineffective biodegradation of OMW was highlighted. The experimental results showed that filtration and chemical pretreatment with Fenton and iron electrodes were ineffective in removing COD. However, chemical treatment minimized the sensitive particles, which improved the biodegradability of OMW. It can be inferred that biological treatment of wastewater with high organic content is challenging; therefore, complementary methods are essential for better treatment outcomes. Table 3.1 summarizes the individual results and observations of the above treatment approaches, listing the drawbacks of each method. The biological treatment study is unfavorable for OMW treatment, which is probably due to the considerable content of phenolic compounds. A significant amount of phenol can be recovered by combining different physicochemical processes. At the same time, biological processes help to remove phenolic content, although they are usually used as a pretreatment method (Rahmanian et al. 2014). Membrane filtration and biological

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treatment were recommended for larger-scale production, while oxidation technologies such as photo-Fenton were suitable for industries with lower annual production capacity (Ioannou et al. 2017). However, low efficiency in phytotoxicity removal is the major limitation of the membrane process, while unavoidable pre-filtration is the weak point of oxidation technologies. Supercritical hydrothermal gasification has efficiently converted OMW into valuable gasses (Casademont et al. 2018). Hydrothermal treatment could be a novel strategy for treating agricultural wastewater because it is a common reaction medium, and the pre-drying process can be bypassed. Further research is needed to improve the treatment system to meet discharge requirements. Besides the techniques mentioned above being costly, they cause biodegradation or destruction of the phenolic fraction of OMW, thus resulting in a loss of valuable, functionally active, and exploitable compounds.

3.3 Valorization of Valuable Materials from OMW The olive fruit consists of pulp (75–85% by weight), stones (13–23% by weight), and seeds (2–3% by weight). Hydroxytyrosol and tyrosol (Fig. 3.1) are phenol compounds in olive oil that contribute to bitter taste, astringency, and oxidation resistance. They are now being reported in the popular press as a desirable health component of olive oil. The flavonoid polyphenols are natural antioxidants in olive oil that have been shown to have a host of beneficial impacts, from healing sunburn to lowering cholesterol, blood pressure, and risk of coronary disease. The OMW waste is rich in various exploitable phenols. Typically, 2% of total phenols move to olive oil, and 98% remain in OMW. For a long period, OMW has been regarded as a hazardous waste with a negative impact on the environment and an economic load on the olive oil industry. However, in the last decade, this view has changed, and OMW is now being recognized as a potential low-cost starting material rich in bioactive compounds, particularly phenols that can be extracted and utilized as natural antioxidants for the food and pharmaceutical industries (Obied et al. 2005; Azaizeh et al. 2012).

3.4 Antioxidant Activity The production of biologically active materials from OMW creates a viable alternative for valorizing this problematic waste. According to recognition of the antioxidant activity of OMW (Azaizeh et al. 2012; Visioli et al. 1999, 2002) and the association of oxidative stress with many diseases, it was natural to consider OMW as a potential source of bio-phenolic antioxidants. For example, Azaizeh et al. (2012) obtained five different OMW fractions and determined their phenolic content and

The polyphenols toxicity of OMW was Time-consuming and low energy recovery reduced via co-compositing with sesame bark

Co-composting

Clean wastewater treatment output could be produced with the aid of photocatalytic pre-treatment H2 output with no CO2 emission

Membrane ultrafiltration

Photocatalysis

Catalytic hydrothermal supercritical gasification using Ru/Al2 O3

Hydrothermal supercritical gasification

Overall photocatalytic system feature upgrade is demanded

Membrane fouling. costly

The sole adsorption method is insufficient

It is challenging to be scaled up

Dosage and timing of the usage of OMW biopesticide need to be well measured

High gaseous output (9.3 L kg−1 OMW) comprises H2 , CO2 , CH4 , and C2 –C4 hydrocarbons. The rise in temperature and reaction time led to an increase in the efficiency up to 98%

Severe operating conditions as the supercritical phase was involved

The optimum result of CH4 (34.84 mol%), H2 Severe operating conditions as the supercritical phase was involved (9.23 mol%), and CO2 (49.34 mol%) was obtained at 823 K and 30 s

A 50% reduction of COD and polyphenols

Adsorption (Jordanian clay)

Thermochemical treatment

High efficiency in removing contaminants The outcomes are effluents for agricultural purposes

Adsorption (gravel, fine sand, and a mixture of acidified cotton and natural zeolite)

Physicochemical treatment

OMW was bio-processed to be an organic pesticide

Biopesticide

Multiple OMW dilutions or proper polyphenols removal are necessary

A standard agricultural wastewater treatment method

Anaerobic digestion

High-cost implication

Difficulties

Reducing the phytotoxicity of raw OMW

The main findings

Aerobic digestion

Biological treatment

Treatment technique

Table 3.1 Literature findings of OMW treatment

(continued)

Kıpçak and Akgün (2013)

Kıpçak et al. (2011)

Speltini et al. (2015)

Stoller et al. (2015, 2017)

Azzam et al. (2015)

Aly et al. (2014)

Hachicha et al. (2009)

El-Abbassi et al. (2017)

Anastasiou et al. (2011) Peltini et al. (2015)

El-Hajjouji et al. (2014)

References

72 S. Asli et al.

Impregnated OMW on the sawdust produced biochar with more elements K, N, and P than non-OMW-loaded sawdust TOC reduction up to 99.96% at short residence times of 5–30 s

Pyrolysis

Subcritical and supercritical water oxidation using H2 O2 The degradation of COD and yield of CH4 were higher than single anaerobic digestion The chemical treatment minimized the delicate particulate matter and improved the OMW biodegradability

Fenton’s reagent process with Anaerobic digestion

Chemical–biological method

Integrated treatment

Production of beneficial biochar with the minimal degradation of simple bio-phenols

HTC

References

Erkonak et al. (2008)

Haddad et al. (2017)

Poerschmann et al. (2013a, b)

Casademont et al. (2016)

Casademont et al. (2018)

High cost

Karahan Özgün et al. (2016)

High capital and operational expenses to be Amor et al. (2015) applied to the industry

Smut deposits at the reactor’s inner wall and causes blockage

The drying time was too long

Low yield of biochar output

Costly noble metal catalyst and energy-intensive

The highest gaseous output with minimum solid residue was achieved using Au–Pd catalyst at 803 K and 250 bar for less than 3 min. 90% COD removal was achieved for both catalytic and non-catalytic runs

Catalytic hydrothermal reaction

Difficulties

The main findings COD removal recorded 75–89% in all the Severe operating conditions as the catalysts such as KOH, K2 CO3 , NaOH, and supercritical phase was involved Na2 CO3 . The optimum result was achieved at 803 K and 230 bar for 20 min, with 58 wt% KOH catalyst

Treatment technique

Catalytic hydrothermal supercritical gasification using homogenous catalysts

Table 3.1 (continued)

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Fig. 3.2 Chromatographic profile of an extract of OMW obtained by HPLC-PAD detected at 280 nm. (1) Hydroxytyrosol; (2) 3,4-dihydroxyphenylacetic acid; (3) tyrosol; (4) protocatechuic acids; (5) verbascoside; (6) vanillic acid; (7) caffeic acid; (8) ferulic acid; (9) p-coumaric acid (Tafesh et al. 2011)

antioxidant potential. They showed that the main phenolic compounds were: hydroxytyrosol tyrosol, caffeic acid, vanillic acid, verbascoside, oleuropein, ferulic acid, and p-coumaric acid (Figs. 3.1 and 3.2). These five fractions showed critical antioxidant activity, mainly attributed to total phenol content and hydroxytyrosol level, where some of them were better than commercial antioxidant products. This rich source of bioactive compounds and natural phenols like hydroxytyrosol and others (Galanakis 2014; Rahmanian et al. 2014) calls for its utilization in various commercial products.

3.5 Antibacterial and Antifungal Activity The potential of OMW, if treated correctly, to become an available material of polyphenols is substantial. Besides being essential as natural antioxidants (Azaizeh et al. 2012; Obied et al. 2005), they are antibacterial compounds. Tafesh et al. (2011) evaluated in vitro the antimicrobial activity of the OMW extracts against Grampositive (Streptococcus pyogenes and Staphylococcus aureus) and Gram-negative (Escherichia coli and Klebsiella pneumonia) bacteria. Hydroxytyrosol is a single phenol at 400 μg mL−1 that caused complete growth inhibition of the four strains, while Gallic acid was effective at 200 μg mL−1 against S. aureus and at 400 μg mL−1 against S. pyogenes but not against the gram-negative bacteria. Other phenols were not effective at all when tested as single compounds. In the same study, an extract from OMW contained mainly hydroxytyrosol (10.3%), verbascoside (7.4%), and tyrosol

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(2.6%) was combined with Gallic acid. It was tested using minimal inhibitory concentrations, revealing that 50/100–100/100 μg mL−1 caused complete growth inhibition of the four strains. These results suggest that specific extracts from OMW combined with natural phenols can be utilized to control pathogenic bacteria. Other studies showed that OMW is a rich source of antifungal agents and has merit as an alternative to synthetic fungicides (Yangui et al. 2010; Chaves-Lopez et al. 2015). Yangui et al. have proved that hydroxytyrosol-rich extracts (HRE) from OMW have strong fungicidal activity against Botrytis cinerea in tomatoes with minimal fungicidal concentrations (MFCs) of 14.36–28.72 and 28.72–57.44 mg L−1 (dry weight). This study revealed that HRE offers a natural and effective extract to control grey mold. In a recent study, Asli et al. (2022) have shown that individual natural phenols and extracts from OMW are active components of increasing the tolerance of salt-stressed plants.

3.6 Utilization of OMW in Agricultural Applications Plants are sensitive to various abiotic stresses like drought, salinity, temperature, waterlogging, pollution, etc. Extreme growth conditions are expected due to global warming effects that led to climate change. Consequently, a substantial economic loss has been caused because of their adverse impact on crop productivity and plant growth. Most plant species are glycophytes, which are not salt-tolerant (Jajarmi 2012). Thus, soil salinity is one of the global problems that affect approximately 20% of irrigated land, which can cause a fundamental reduction in crop yields. Salinity is more evident in semi-arid, coastal agricultural lands, particularly in arid regions of the world (Kasim et al. 2013; Qadir et al. 2014; Hashem et al. 2018). Changes in weather patterns have resulted in the frequency of recurrent drought or rain falling above the average value for more than a decade (Ayanlade et al. 2018). The upward movement of water in shallow water tables and coastal areas with seawater intrusion resulted in root zone salinity. Changes in precipitation and temperature have a more significant influence on soil salinity. Bannari and Al-Ali (2020) reported that the long-term effects of increased temperatures and decreased precipitation for 30 years positively correlated with increased soil salinity in the arid landscape. Hence, the salinity has increased from 1 to 33% in coastal agricultural lands in the last 25 years (Rahman et al. 2018). The high salinity environment causes the formation of reactive oxygen species (ROS), which negatively affects the ability of the plant’s defense system to deal with it. Researchers found that antioxidant molecules, such as salicylic acid (SA) (Shaki et al. 2018), jasmonic acid (JA) (Mimouni et al. 2016), and ascorbic acid (ASA) (Alves et al. 2021), can be promising scavengers molecules of the formed ROS and reduce plants’ environmental stresses. For example, salicylic, ascorbic, and jasmonic acids enhanced salt tolerance in Safflower, tomato, and wheat, respectively (Asli et al. 2022).

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Interestingly, the OMW is rich in antioxidant molecules, such as hydroxytyrosol (HT), oleuropein, tyrosol, caffeic acid, vanillic acid, verbascoside, p-coumaric acid, catechol, and rutin compounds. Asli et al. (2022) were the first to examine the influence of extracted fraction of OMW on the tolerance of Zea mays (as porotype) seedlings to salt stress. Adding an OMW fraction rich in phenolic compounds to the hydroponic growth medium of a stressed plant (with 80 mM NaCl) increased the root growth rate by 36–41%. It is worth mentioning that separating the phenolic compounds from the OMW is still challenging. Therefore, the authors decided to study commercial phenolic compounds like HT compound (a major and one of the significant phenols in OMW) and Caffeic acid. Using 2 ppm of commercial HT enhances the maize plant’s shoot growth rate by 204% (Fig. 3.3a) and root growth rate by 284% (Fig. 3.3b). A

2.0

260%

1.5

179%

196% 204%

a c

1.0

Growth rate (Cmd-1)

100%

c c

b

0.5

0.0 B

4

284%

297% 259%

3

a'

209%

d' d'

c'

2

100% 1

b'

T H

m ss M m on ic ac id C af fa ic ac id

on tr

ja

N

C

aC l8 0

ol

0

Fig. 3.3 Additive materials can enhance the growth rate of salt-stressed plants. Zea mays L. seedlings were grown hydroponic with 0.1 Hoagland solution for 3 days as control plants, and NaCl 80 mM was added to the treatment plants. Additive materials at a concentration of 2 ppm were added to treated plants for 3 days following. A Describes the shoot growth rate, and B represents the root growth rate. Different letters indicate significantly different according to Tukey’s multiple comparisons test one-way ANOVA (α = 0.05), (Means ± Se, n = 10). Adapted from Asli et al. (2022)

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The authors attributed this enhancement to the ability of the HT to inhibit reducing the pore’s diameter. It was revealed that in maize seedling roots, the diameter of the pores (root cell) was reduced from 6 nm to less than 2 nm, apparently, due to 80 mM NaCl. However, the addition of the HT expands the pore’s diameter to more than 4 nm, thus increasing the root’s ability to absorb water and nutrients. Interestingly, the addition of HT showed better results than using ASA, rising by 87% (not shown).

3.7 Green Synthesis of Noble Metal Nanoparticles (NPs) from OMW In recent years, nanotechnology has been gaining the interest of researchers in different fields. Nanoparticles have tiny sizes, due to which they have acquired some unique characteristics, which make them different from their bulk complements. They have more solubility, surface area, and reactivity than bulk material. Therefore, they have gained a promising position to lessen the damaging effects of abiotic and biotic stress to reach the goal of sustainable agriculture globally. Today, the research related to the application of nanoparticles is increasing because of their impact on stress tolerance and the nutritional quality of crops. Different types of nanoparticles have been examined for their potential role in protection from biotic and abiotic stresses. These nanoparticles were reported to overcome nutritional deficiencies, increase enzymatic activities, and help grow plant growth-promoting bacteria in the rhizosphere under abiotic stresses; as a result, they improved the tolerance of crops to stresses. The OMW is rich in phenolic compounds, which have either OH and COOH or both as a functional group. Those groups can reduce metal ions such as Ag+ (aq) to Ag(s) or Au3+ (aq) to Au(s). Moreover, they can behave as ligands that attach and passivate the surface of the metal. Recently, De Matteis et al. (2021) and Qi et al. (2021) have demonstrated the usage of OMW as an efficient reactive agent to prepare well-controlled Au and Ag NPs, as shown in Fig. 3.4. The OMW solution was mixed with a one mM aqueous HAuCl4 (AgNO3 ) solution at room temperature to prepare the Au (Ag) NPs. Furthermore, the formation of the NPs using the OMW reveals a significant reduction of the COD and the amount of the total polyphenol in the OMW. The results showed that the used OMW was with 42 ± 15 g O2 L−1 COD and then reduced to 3.2 ± 1.5 g O2 L−1 and 3.8 ± 1.3 g O2 L−1 when the Au and the Ag NPs were formed, respectively. The COD reduction can be attributed to the formation of the polyphenol shell on the NPs, as shown in the TEM images. The phenolic compounds were consumed to form the polyphenol coating on the NPs surface.

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Fig. 3.4 Transmission electron microscopy (TEM) images of Au (a) and Ag (b) NPs prepared using OMW. Adapted from De Matteis et al. (2021)

3.8 Summary and Future Perspectives The inconsistent seasonality of the OMW quality is often a problem in yielding valuable products and effluent treatment. Different alternative treatment methods were proposed for OMW, especially biological or chemical pathways. Direct biological processing of OMW is challenging due to polyphenols, which contribute inhibitory effects on the bacterial strains (Speltini et al. 2015). Removing phenolic compounds or a large dilution factor for raw OMW is often essential as its pre-treatment (Speltini et al. 2015). Single operations such as flocculation, chemical treatments, ultrafiltration, or its integrated operations, for instance, centrifugation–ultrafiltration, direct discharge on lands, and compositing, are some common treatments in handling OMW (Haddad et al. 2017). Low cost and energy-generating wastewater treatment systems are the attributes of the system’s superiority (Fujihira et al. 2018). Treatment for the agricultural effluents is demanding for an industrially effective method; at the same time, the capital and operation costs could remain at the minimum. It is possible and practical to achieve the transformation of OMW into wealth. The cellulose and starch-containing waste can be treated to produce H2, which has a high energy density, higher conversion efficiency of useable power, and minimum generation of pollutants (Mamimin et al. 2015). A comparison of biological, physicochemical, and thermochemical treatment methods has been summarised in Table 3.1. In general, biological treatment is too time-consuming and land-dependent for agricultural treatment. Physiochemical alone were proven insufficient to reduce the levels of COD and BOD to a safe discharge limit as this group is basically capable only of filtering the total solids, solvent extractable compounds, phosphates, and sulfates. The thermochemical method emerges to be a potential approach in biomass and effluent treatments. Among all the available thermochemical technologies, the hydrothermal treatment appears to be an outstanding candidate for effluent treatment as the pre-drying is circumvented in addition to higher-value output. An assessment in the context of energy usage and economic analysis is recommended to measure the profitability of the current and alternative wastewater treatment systems. An adequate treatment technology selection is highly reliant on the

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demand and supply of inhabitants, geographical condition, meteorological situation, economic profitability, industrialization, development rate, and the regulatory discharge limit of that country. These factors require a ‘scenario-based’ decisionmaking computation to decide the most suitable treatment based on the factors revolving around social, environmental, and economic clusters (Kalbar et al. 2016; Molinos-Senante et al. 2015). Besides, the expensive materials and long-term stable operation are the concerns in commercializing the treatment research from lab-scale to the industrial level (Krishnan et al. 2019). In the future perspective, the wastewater treatment systems should consume the least time and be able to meet the discharge standard at the end-of-pipe of the treatment process. Beyond that, the conversion of OMW into value-added products, such as syngas, activated carbon solid, and organic compounds like residual oil, is highly appreciated. The cost of materials and operation stability should be taken into consideration in designing the alternative OMW treatment systems. The utilization of a safer, more convenient, and profitable system could be the favored treatment option in the future. We believe that in the near future, OMW will be involved in the nanomaterial formation as a ligand reducing agent. Acknowledgements This work was supported by the Israeli Ministry of Environmental Protection (project agreement 142-4-1 and 192-3-1). The authors gratefully acknowledge the financial support of the ministry. They also thank Dr. Sobhi Basheer for his help. The authors declare that they have no conflict of interest. We would also like to thank the Israeli Ministry of Innovation, Science and Technology for their continuous support of the authors.

References Alfano G et al (2009) Present and future perspectives of olive residues composting in the mediterranean basin (CompMed) In: Dynamic soil, dynamic plant. Global Science Books, Isleworth, UK, pp 39–56 Alves RDC, Rossatto DR, da Silva JDS, Checchio MV, de Oliveira KR, Oliveira FDA, Gratão PL et al (2021) Seed priming with ascorbic acid enhances salt tolerance in micro-tom tomato plants by modifying the antioxidant defense system components. Biocatal Agric Biotechnol 31:101927 Aly AA, Hasan YNY, Al-Farraj AS (2014) Olive mill wastewater treatment using a simple zeolitebased low-cost method. J Environ Manag 145:341–348 Amor C et al (2015) Combined treatment of olive mill wastewater by Fenton’s reagent and anaerobic biological process. J Environ Sci Health Part A 50(2):161–168 Anastasiou CC et al (2011) Approaches to olive mill wastewater treatment and disposal in cyprus. Environ Res J 5(2):49–58 Andreozzi R, Canterino M, Di Somma I, Lo GR, Marotta R, Pinto G, Pollio A (2008) Effect of combined physico-chemical processes on the phytotoxicity of olive mill wastewaters. Water Res 42:1684–1692 Asli S, Diab M, Hugerat M, Haj-Zaroubi M (2022) Investigating the physiological effects of hydroxytyrosol on the recovery of the growth, transpiration, and the root cell-wall pore size of the salt-stressed plant submitted to plant growth regulation

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Chapter 4

Present and Future Perspectives on the Use of Olive-Oil Mill Wastewater in Food Applications Antonella De Leonardis, Vincenzo Macciola, and Ayesha Iftikhar

Abstract Olive mill wastewater (OMW) are one of the main by-products or pollutants of virgin olive oil production. Otherwise, OMW show to have high recycling potential. Especially, the phenolic compounds extracted from OMW exhibit profound antioxidant, antimicrobial, antiviral, antibacterial, and anti-inflammatory activities. The present paper gives an overview of the possible utilization of OMW in food applications. OMW phenolic extract was assayed to fortify, to preserve the organoleptic properties and to enhance consumer acceptance of the meat and meat products. We report on effective utilization of OMW extract in retaining or enhancing the nutritional value, stability and bioavailability of starter cultures, and delaying the production of browning due to Millard reaction in milk, functional and fermented beverages. When added in oil and oil/water emulsion, OMW polyphenols effect on retarding the lipid oxidation improves the oxidative status of the product. OMW extract is also used in bakery products to preserve their technological properties and slow down the chances of fungal spoilage. OMW is a source of various beneficial bioactive compounds, such as fiber, prebiotics, enzymes and natural ingredients in packaging film material. Finally, from a biotechnological point of view, OMW could be used at the industrial level to produce economically convenient microbial media. All studied applications evidence that OMW extracts have the suitable potential to improve functional properties and shelf life of the food products. However, there are still several hurdles to overcome for the realistic utilization of OMW in food applications. Keywords Olive mill wastewater (OMW) · Phenolic extract · Food application · Antioxidant · Antibacterial · Bioactive ingredient · Natural preservatives A. De Leonardis (B) · V. Macciola · A. Iftikhar Department of Agricultural, Environmental and Food Sciences (DiAAA), University of Molise, Via De Sanctis, 86100 Campobasso, Italy e-mail: [email protected] V. Macciola e-mail: [email protected] A. Iftikhar e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_4

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4.1 Introduction Olea europaea L. is the principal specie in the Oleaceae plant family that produce edible oily fruits, the olives, which from ancient time the Mediterranean populations used to extract the virgin olive oil or consume (directly or cured) (Vossen 2007). Olive structure and composition is given in Fig. 4.1. Olive fruit is a drupe consisting of the following anatomic parts (Pantziaros et al. 2021): . epicarp (external cuticular layer)—up to the 1.0–3.0% of fruit’s weight . mesocarp (flesh or pulp)—up to the 70–80% of fruit’s weight—containing tissues rich in oil and water . endocarp (kernel or stone)—up to the 18–22% of fruit’s weight—composed externally by a woody kernel and, internally by endosperm or almond rich in protein and oil. Indicatively, olive fruits are composed of water, oil, minor soluble lipid compounds, proteins, carbohydrates, cellulose, and minerals, and others, enclose polyphenols (up to 10 g/L). The chemical composition of the olives depends on several factors, such as genetic origin (variety, ecotypes); final use (for oil or table olives); environmental conditions of the olive grove (altitude, latitude, exposure, soil nature); cultivation techniques (planting density, cultivation practices, irrigation); seasonal climatic trend; ripening stage (Seifi et al. 2015). It is well known that olive oil is produced by a mechanical process, generally carried out at cold, in which olives are preliminary grounded or crushed and then, oil is extracted from the olive paste by using the press (traditional or classic systems) or centrifugal (continuous systems) force. The extracted oil is called ‘virgin’ because it is directly edible without further refinement.

Fig. 4.1 Olive fruit structure and chemical composition

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Until the last century, there was exclusive interest in the olive oil industry to produce virgin oil, without giving any particular attention to recycling the waste considered only a pollutant or hazardous residue. Olive oil mill wastes are composed of the residual parts of the olive fruits, such as skin, pulp, seeds, pieces of stone, and mainly water. In the functioning of the oil extraction method, the above-mentioned residual materials are separated in a solid or semisolid waste (pomace) and/or in thick liquid effluents (olive oil mill wastewater). Effectively, solid and/or liquid olive oil mill wastes have an important environmental issue due to their organic load that generates high COD (chemical oxygen demand) and BOD (biological oxygen demand) values (Dermeche et al. 2013). Currently, depollution is one of the principal management approaches for olive oil mill wastes; otherwise, renewable energy, feed and compost are also frequent utilization (Akay et al. 2015). Recently, according to the principles of circular economy aiming at “zero waste” industrial process, new systems to recycle and valorize olive oil mill wastes are strongly encouraged (Sánchez-Sánchez et al. 2020; Klisovic et al. 2021). In this perspective, recent innovations are also introduced in the olive-oil processing (i.e. stone removal, pulp dehydration, cold pressing) to obtain by-products with multifunctional uses in food, cosmetics and pharmaceutical sector (Olmo-García et al. 2019). The olive oil mill wastes should be considered ‘virgin’ second raw materials, because they are obtained mechanically without any chemical manipulation. In other words, theoretically, the virgin olive oil mill wastes should be considered edible, just like the oil, for exempla, appropriately transformed or as a component of new food formulation (Obied et al. 2007; Faustino et al. 2019; Foti et al. 2021). In the specific case of the olive oil mill wastewater (OMW), due to their residual oil, sugars, proteins, minerals, fiber and phenol content, a wide range of recycling and application opportunities are experienced. Specifically, potential use of the OMW in the food industry, as valuable ingredients, natural food antioxidants, antimicrobial and antifungal, oxidative stabilizer, and others, was frequently studied (Mekki et al. 2006; Fasolato et al. 2016; Silvan et al. 2019). For this purpose, OMW can be used whole (such as or properly pre-treated) or as a source of bioactive extracts. Certainly, polyphenols are the most attractive bioactive compounds having a wide range of proven biological activities and beneficial effects (ElMekawy et al. 2014). Specifically, there is copious pieces of evidence that olive polyphenols can prevent several pathologies, including heart, atherogenic, microbial and viral disease and cancer (Obied et al. 2005; Araújo, et al. 2015; Mateos et al. 2019). In Europe, polyphenols of olive oil have the following approved health claim (Reg EU No 432/2012): ‘consumption of olive oil polyphenols contributes to the protection of blood lipids from oxidative damage’ with the uptake of 5 mg/day of HT and its derivatives. In addition, there are evidence that OMW polyphenols (OMWPPh) exhibit antioxidant, antimicrobial, antifungal, activities also in food application (De Leonardis et al. 2009; Caporaso et al. 2018). The OMW’s food applications discussed in this chapter are summarizes in Fig. 4.2.

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Fig. 4.2 Potential application of olive-oil mill wastewater in food

4.2 Meat and Meat Products Preservation The quality of meat and fish food is judged in terms of color and flavor. In the case of meat, consumers make the decision to purchase by looking at color surface and freshness. The factors that directly affect the color and flavor of meat are lipid oxidation and rancidity, due exposure to light and air, and spoilage and deterioration, due to microbiological growth (Wang et al. 2021). Mainly the phenol from OMW (secoiridoids, hydroxytyrosol, flavonols, flavonoids, and others) are declared by food industries and researchers as a bioactive ingredient and natural preservative for meat products (Novelli et al. 2014). The hydrophilic character of polyphenols separated into the water phase during olive processing is considered very important in prevention of heart diseases and cancer. Several analytical testing of OMW declared its richness in phenolic constituents and high value-added powerful natural antioxidant properties (El-Abbasi et al. 2012). Italian fresh sausage is considered as an ideal example for a case study due to its high level of pH and water activity (aw), which cannot control microbial growth and undergoes rapid spoilage (Cocolin et al. 2004). Purified phenol OMW extract showed a powerful in vitro effect with some bacteria applied as starter culture against some food-borne pathogens. Specifically, a dosage of 0.15% phenolic extract may affect the taxonomic composition of culture use and delayed the growth of spoilers (Fasolato et al. 2016). Modernization and changing lifestyle allow consumers to prefer food that is easy to cook, pleasant in taste and of high nutritional profile, so hamburgers are frequently consumed. Ground meat is more prone to lipid oxidation and spoilage as it has more surfaces for exposure to light, air and microbes. OMW-PPh addition in hamburgers

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has reduced primary and secondary oxidation, prove effective in red color retention and reduce browning in storage (Barbieri et al. 2021). After storage of 9 days 56% phenol was retained in a fresh hamburger, while for one grilled at 200 °C the concentration of phenols was more than 20%. The bitter taste of burger that could arise due to concentration of bioactive compounds i.e. polyphenols can be masked by hydration/extraction of dry olive paste flour with milk or water before using in the formulation of burger (Cedola et al. 2017). Most of the food-born pathogenic bacteria produce biofilm around themselves which provides them inhabitant and favorable environmental conditions to increase their viability by protecting from environmental stress and oxygen inactivation (Buswell et al. 1998). This microbial characteristic resistance compromises the quality and safety of meat commodities (Shi and Khu 2009). The growth of Campylobacter coli and Campylobacter jejuni associated with the consumption of uncooked, unhygienic and cross-contaminated meat was retarded by an extract of olive mill wastewater (Corry and Atabay 2001). Antimicrobial and anti-film effect of polyphenols present in spray-dried olive mill wastewater (OMW-SD) against isolated Campylobacter strains from chicken meat was determined by minimum bactericidal (MBC), minimum inhibitory concentration (MIC), crystal violet assay on polystyrene plates methods (Roila et al. 2019). Therefore, a promising inhibitory effect of OMWSD is obtained as an alternative to chemical additives and preservatives at extract dilutions over 80%. All over the world, the commercial value of crustaceans is high. The most consumed and commercialized species in Mediterranean European countries is deep-water pink shrimp, Parapenaeus longirostris (Sae-Leaw and Benjakul 2019). Shrimps are highly preferred for their meat consistency, higher nutritional value and a very delicate taste. But due to their physic-chemical properties (high pH, humidity, poor collagen and non-protein nitrogen), the shelf life of shrimps is very short and just refrigeration temperature is not enough to store and preserve its organoleptic properties for a while (López-Caballero et al. 2019). The study of Miraglia et al. (2021) reveals the effectiveness of OMW for shrimps, both from hygienic point of view and in terms of quality aspects. Specifically, assayed OMW phenolic extracts exhibited bactericidal and antioxidant activity and a pleasant taste (proportional to the concentration of phenolic compound) without changing the characteristic sensory attributes of enriched shrimps. Also, it was derived that sulphites in combination with the above-mentioned OMW extract appear to be protective for phenolic compounds from oxidation during cooking and storage, even though prove least effective in antibacterial action. In another study by Miraglia et al. (2016), the antioxidant and antimicrobial effect of OMW-PPh is determined on fresh salmon steaks. In the diluted solution of phenolic extract at 1.5 and 3.0 g/L salmon steaks was immersed for 6 days storage at 4 °C in a modified atmosphere of 70% carbon dioxide 25% nitrogen and 5% oxygen. Numerous analyses, such as total viable count, color, pH, α tocopherol, phenolic composition, antioxidant activity through 2,2 diphenyl-1-picrylhydrzyle (DPPH)

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assay, thiobarbituric acid reactive substances (TBRAS), of prescribed samples are performed during specified intervals. The results showed that 3 g/L phenolic extracts have positively controlled the microbiological growth during storage resulting in contributing to the hygienic control of salmon fish. OMW has been explored also as a feed supplement to increase the nutritional value of some meat food. Balzan et al. (2021) have studied extensively the dietary supplementation of OMW-PPh in feed formulation and its effect on the carcass, breast quality, shelf life, lipid peroxidation and gut microbiota. The outreach of an intensive breeding system is the main cause of stressors in broiler chickens. These stressors are main reasons for the production of reactive oxygen species (ROS) and lead to negative consequences for immune response, growth performance due to injured cellular components such as protein, DNA and lipids (Farahat et al. 2017). The enrichment of the diet of chicken with phenols is not associated with differences in animal growth and the carcass yield. The study of microbiota showed a significant effect of feeding time but not of alteration microbial groups. In the samples of chicken that consumed the feed with added phenols, an increase in the level of hydroxytyrosol is observed in muscle tissues showing intestinal absorption. Further studies are needed to better understand the distribution of hydroxytyrosol in cellular and extracellular compartments as oxidative processes mainly occurred in the membrane structure of the muscle cells. The availability of crude concentrate phenols from OMW in livestock nutrition could be an alternative to improve the shelf life and quality of fresh meat (Novelli et al. 2017).

4.3 Milk and Milk Products People from different age groups like to consume a wide range of dairy products, from milk to a large variety of derived products, such as cheese and other fermented products. The main purposes of milk processing are to inactivate the pathogens or their induced toxins and to improve the digestibility, bioavailability, and shelf life of the final products. Generally, several thermal processing guarantees the safety and prolong the shelf life of milk products and promote nutritional and beneficial aspects with minimum thermal damages (Awuah et al. 2007). Conversely, these thermal processing generate main side effects, enclose textural and color loss, production of undesirable compounds with bad sensorial properties, loss of certain essential nutrients, and potential toxic effect on human health (Van Boekel et al. 2010). Especially, the Maillard reaction resulted in the production of brown compounds by reaction between sugar and amino group (Nursten 2005). Maillard’s reaction is not desirable because of nutritional and sensory reasons (Van Boekel 1998). Main Maillard reaction products (MRPs) in milk are lactose derived Amadori compounds, their concentration varies according to thermal treatments and

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water activity in this order milk powder < sterilization < ultra-high-temperature (UHT) < pasteurization, with consequent blockage of a lysine residue and ultimately, decreasing the nutritional value (Monti et al. 1999). A very severe thermal condition degrades the Amadori products causing the formation of pro-oxidant and flavor compounds, polymerization of protein, and brown color induction due to melanoidin generation. Spray-dried ultra-filtered OMW, used as an ingredient in milk, has shown the ability to retard the Millard reaction (Troise et al. 2014). The reduction of the early stage MRPs (such as APs, protein bound MRPs; furosine, carboxymethyl lysine) and the trapping of reactive carbonyl species (RCSs) are the main results. A functional milk product in form of yogurt is developed by mixing OMW into pasteurized cow’s milk and then concentrated to achieve a final phenol concentration of 100–200 mg/L (expressed as hydroxytyrosol equivalent) (Servili et al. 2011). The same was inoculated with Lactobacillus delbrueckii subsp. bulgaricus, Staphylococcus thermophiles, Lb. plantarum, Lb. paracasei selected starter cultures. The Addition of OMW-PPh has resulted compatible with the microbial growth, activities and survival of functional lactic acid bacteria showing only a slight phenol concentration effect. Moreover, an increment of hydroxytyrosol was also observed during the storage. Hydroxytyrosol intake deriving from the consumption of 100 ml of this functional milk product is found to be comparable to that of 20 g of virgin olive oil (Servili et al. 2011).

4.4 Functional and Fermented Beverage In the world, there are numerous beverages consumed regularly for different reasons, such as pleasure, thirst-quenching, refreshing, soothing or stimulating. Recently, beverages are also consumed for health reasons being formulated as a natural or supplemented source of vitamins, minerals, proteins, amino acids, and other bioactive components. Application of OMW in the beverage sector principally are: . addition of OMW functional extracts to prepare new formulations . production of fermented beverage. The production of commercial beverages fortified with olive oil by-products (especially leaves or their extracts) is already a reality. Conversely, there are not many studies about the effect of processing and storage conditions on the quality of OMW-fortified beverages (i.e., OMW formulation and concentration, pH, temperature, oxygen and light exposition). Furthermore, OMW-PPh stability and their effect on browning and taste must be considered (Zbakh and El Abbassi 2012). High bioavailability of the soluble phenols has been repeatedly demonstrated, especially that of hydroxytyrosol that is absorbed 100% by humans (Manna et al. 2000; Caruso et al. 2001; Vissers et al. 2002; Visioli et al. 2003; Cardinali et al.

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2011). Furthermore, olive phenolic compounds show low toxicity up to 2000 mg/L concentration, neither any undesirable effect if compared to synthetic antioxidants, like the BHT (Farag et al. 2003; Christian et al. 2004). Generally, commercial functional beverages also include antioxidant additives, such as ascorbic acid, to prevent the oxidation process; this procedure is no need for the beverages prepared with OMW-PPh which may act also as antioxidants. In the literature, fermentation of OMW has been considered more for energy purposes than for food applications (El Asli and Qatibi 2009; Tayeh et al. 2014). In every case, sugar addition in the OMW is needed because they have sugar natural content insufficient to obtain a satisfactory level of alcohol. In the food sector, an ‘olive wine’ was produced in China, evidencing in vitro an excellent power in clearing free radicals and in vivo beneficial effects on mouse malondialdehyde (MDA) level in liver and protein–carbonyl concentration in plasma (Yao et al. 2016). The combination of alcohol and OMW-PPh shows enhanced antimicrobial, antioxidant and anti-inflammatory activities of the fermented solution (Tayeh et al. 2014). OMW has been used as raw material to produce an innovative ‘olive vinegar’ which was permanently clear, vinous-red colored and without any abnormal smell (De Leonardis et al. 2018, 2019). Acetic acid level up to 4% is performed by applying both the alcoholic-acetous double fermentation with addition of Saccharomyces cerevisiae starter or the spontaneous acetification without the addition of any other starter. Olive vinegar was characterized by the huge presence of mineral compounds and phenol substances, especially hydroxytyrosol.

4.5 Oil/Fat/Emulsion Stability OMW, with also the other olive oil by-products, are considered as antioxidants to improve the oxidative stability of vegetable oils and animal fats. Günal-Köro˘glu et al. (2019) investigated the antioxidant activity of an OMW phenolic extract on sunflower oil (SO) that is characterized by a high content of unsaturated fatty acids. Measurement of antioxidant activity was performed through Trolox Equivalent Antioxidant Capacity (TEAC) and Linoleic Acid Oxidation system along with Differential Scanning Calorimetry (DSC) at a very higher temperature of 180 °C. Results showed that OMW extracts in SO lowered the p-ansidine value, preserved the tocopherol content and delayed the oxidative induction period more than the synthetic BHT. Furthermore, the antioxidant activity of natural OMW at 0.2 mg/mL dosage in SO was found higher than that of BHA, BHT and α-tocopherol (Omar 2010). In another work, the addition of 50 mg/L phenolic extracts in sunflower oil resulted in enhancing oxidative stability up to 50% (Romeo et al. 2020). Lecithin extract has a synergistic effect enhancing the antioxidant activity of OMW for the following reasons: (i) addition of lecithin regulates slowly degradation of tocopherols (Takeungwongtrakul and Benjakul 2013); (ii) lecithin increase the

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solubility of antioxidant in reward increasing the efficiency of it (Lee et al. 2009); (iii) phospholipids located at air interface working as barrier against oxygen (Cui and Decker 2016); (iv) oxidation products and amine groups of phospholipids undergo through amino-carbonyl reactions which form compounds with profound antioxidant characteristics (Zamora and Hidalgo 2011). OMW-PPh was added in different concentrations into refined olive oil with the aim to limit the negative effects of frying. At the 400 mg/kg concentration, OMW phenolic extract has reduced the emission of low-molecular-weight aldehydes and oxidation of tocopherols (Esposto et al. 2015). Oxidative stability of lard is increased significantly by the addition of OMW-PPh, by demonstrating also (through cytotoxicity assay) that OMW-PPh did not inhibit cell growth at the usual dose of 100–200 mg/kg (De Leonardis et al. 2007). Mayonnaise is most probably one of the most consumed sauces in the world and its trend goes on increasing. The main composition of mayonnaise included fat 70–80%, pH range of 3.2–4.2, a water activity of 0.95 and acetic acid ranging 0.8–6.0%. The composition of mayonnaise i.e., acidic nature, higher fat content and acetic acidic made the chances of microbiological spoilage minimize up to a certain level (Cambero et al. 2011). The chances of microbial contamination could arise due to ingredients, preparation practices, storage temperature and preservation methods. There is also the possibility of infectious outbreaks due to Salmonella spp. commonly attributed to improperly prepared eggs (Chousalkar et al. 2018). Afterward changes in the recipes of mayonnaise preparation such as reduction in the level of sodium and caloric content, along diversifying the product also make it commercially more attractive. These changes having good concerns also change the final acidity range and other factors which might be involved in microbiological control; some of the ingredients that are used as antimicrobial or preservatives such as spices itself could be a major source of contamination (Keerthirathne et al. 2016). Finally, mayonnaise is used as savory to other food items, for example, salads; thus, if it is contaminated with a microbial pathogen, it could be a source of contamination for the other food. Some of these above-said mayonnaise issues could be resolved by using OMWPPh extract. Olive vinegar, obtained by the fermentation of olive-oil-mill wastewaters, exhibited clear antioxidant activity in vinegar dressing-based formulation (De Leonardis et al. 2022). In the research work of Menchetti et al. 2020, a proliferation of Salmonella enteritidis is studied in a homemade mayonnaise prepared with and without the addition of OMW-PPh extracts (250 mg/kg phenol). The enumeration of Salmonella was taken place at 4 and 22 °C for 48 h. In the control mayonnaise sample population of Salmonella remain constant at 4 °C whereas proliferation takes place at 22 °C. Instead, a considerable antibacterial effect of OMW phenolic extract was observed just after 1 h of Salmonella inoculation dropping the growth rate by 9.5% at 22 °C and 5.8% at 4 °C per h. Olive polyphenols from OMW, despite even though due to their hydrophilic nature, proved to be more effective to control all stages of oxidation if compared to liposoluble tocopherols. Also, compared to ascorbic acid, OMW-PPh has a remarkable capability to inhibit the first lipid peroxidation. Therefore, to increase the efficacy

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at the first oxidation stage, the ascorbic acid and OMW-PPh could be used in the emulsification process between the water and oil phase (Galanakis et al. 2018a, b). Finally, the fish oil, having good repute to impart health benefits for its long-chain polyunsaturated fatty acids (LC-PUFA) higher contents, is tremendously used to enrich the food products (in emulsion form). Main parameter that limits fish oil usage is its low oxidative stability. OMW is identified as a promising source of antioxidants to retard lipid oxidation in fish-oil-enriched food products (Jimenez-Alvarez et al. 2008).

4.6 Bakery Products Bakery products are a wide variety of foods, such as bread, biscuits, crackers, snacks, bread-sticks, and others. Some bakery products are consumed worldwide, while others are specialties typical of limited geographical areas prepared according to ancient recipes. The extreme variability of bakery products is due to ingredients and baking technology. Commonly, basic ingredients are wheat flour and water, but other ingredients may be employed in specific recipes, such as no-wheat-cereal flours, oil/fat, salt, eggs and others. As regard baking technology, the main differences are the leavening conditions, leavening agents, kneading method, cooking temperature and moisture of the final product. Bakery products represent the basis of daily nutrition for people of all ages being an important source of energy and nutrients, such as carbohydrates, protein, vitamins, and minerals. For this reason, attention to bakery products is constantly high, as well as the search for new strategies to improve their nutritional quality and shelf life. In this sense, one of these new strategies can be the addition of olive oil mill by-products. As regards improving nutritional-quality of bakery products, especially the OMWPPh have been assayed to transform them into potential carriers of natural bioactive substances. Generally, bakery products have a poor supply of phenolic compounds, especially those produced with refined flour. As already said before, bioavailability measures the amount of the ingested phenolic compounds that are effectively absorbed and utilized by the human body (Angelino et al. 2017). In the case of bakery products enriched with OMW-PPh, the bioavailability of phenols appears to be strongly conditioned by both the type of OMW by-product added and the baking process conditions (Di Nunzio et al. 2020). Indeed, different phenols may act synergistically or antagonistically with each other, while the fermentation process and cooking temperatures may modify the bioaccessibility of phenolic compounds (Katina et al. 2012; Wang et al. 2014). Regarding the shelf life, like many other foods, also bakery products are affected by physical, microbial, and chemical spoilage (Smith et al. 2004). Physical spoilage is mainly due to the loss of original colour, crunchiness and consistency, while the growth of Penicillium, Aspergillus, Cladosporium and Neurospora molds is the most common fungal spoilage. Some bacteria, such as

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Bacillus subtilis and Bacillus licheniformis, may also cause bakery deterioration (Pateras 2007). Finally, the most common chemical spoilage is lipid oxidation which occurs especially in bakery products prepared with oil/fat. Lipid oxidation can start in a different stage of the baking process (i.e., oxygenation during kneading and high cooking temperatures) with the initial formation of peroxides and secondary development of rancidity and off-flavor. OMW-PPh has given encouraging results in the protection of bakery products by lipid oxidation and rancidity and by microbial spoilage. For instance, it was found that OMW-PPh (contained in an ethyl acetate extract) delays lipid oxidation in glutenfree-breadsticks in a concentration-dependent manner (Conte et al. 2021). Similar outcomes are obtained also in bread enriched with OMW-PPh powder (Galanakis et al. 2018a, b). Conversely, the antimicrobial activity of OMW-PPh appears to be higher at lower phenol concentrations (Galanakis et al. 2018a, b). At this regard, it must be remembered that lipid oxidation occurs in the oil phase, while microbial spoilage in the water or solid phase. Higher efficient antimicrobial activity may be attributed to the favorable interaction between the phenolic compounds and the other bread components, especially carbohydrates and wheat proteins (Sivam et al. 2011). Also, emulsification of OMW-PPh enhances both the antioxidant and antimicrobial effects (Galanakis et al. 2018a, b). The impact of OMW extracts on the sensory quality of bakery products has been also investigated finding that the OMW extracts may decrease hardness and brittleness in a concentration-dependent manner (Conte et al. 2021). Moreover, the enrichment of bakery products with OMW may determine hardness, color variation, extraneous flavors formation, marked bitter and spicy taste, which can affect the acceptance of the consumers (Cedola et al. 2020). Finally, it was found that OMWPPh interferes with the Maillard Reaction, at different stages, during the preparation of cookies (Troise et al. 2020).

4.7 Active Ingredients as Pectin and Food Packaging Materials Several studies reported that OMW could be a potential source of dietary fibers (Lama-Muñoz et al. 2012; Rubio-Senent et al. 2015). Pectic materials, extracted through ethanol precipitation from OMW, were characterized by high molecular weight and low degree of methyl-esterification and acetylation, and by emulsifying and antioxidant activity higher than citrus pectin (Rubio-Senent et al. 2015). Galanakis et al. (2010a) have developed a method to recover dietary fiber from OMW by using a thermal treatment with mixtures of ethanol and acids prior to isolating the alcohol insoluble residue (AIR). Recovered AIR was up to 60/100 g of OMW dried matter and showed satisfactory gelling ability. The heat-time extraction

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processing conditions (ranging from 50 to 80 °C up to 180 min) and the presence of endogenous active enzymes may affect the functional and rheological properties of OMW pectin (Galanakis et al. 2010b). OMW dietary fibers are used as an additive to improve the water-holding capability of meatballs (Galanakis et al. 2010c). In addition, an enhancement of palatability is found when OMW dietary fibers are used as a fat replacement. Today, it is growing awareness that synthetic polymers, used as food packaging materials, pose serious environmental issues, despite their favorable economic and technological benefits. Consequently, biodegradable films and coating are proposed as an eco-friendly alternative to synthetic packaging. Moreover, many studies also report the efficacy of active films and coatings to preserve for a long time the quality by extending the shelf-life extend of many food products, e.g., vegetables, fruits, animals, and dairy (Díaz-Montes and Castro-Muñoz 2021). Therefore, the development of biodegradable bio-polymers incorporating bioactive compounds, may be obtained from low-cost materials, like waste, is growing intensively. To this end, the high potential of bioactive compounds extracted from olive by-products, mainly olive leaves and pomace, in improving barrier and mechanical biofilm properties and in extending food shelf-life has been more time investigated, as evidenced in the recent overview of Khwaldia et al. (2022). Especially, hydroxytyrosol and its derivatives resulted in high performance due to the ability of their hydroxyl groups to interact with the polymer chains forming a stronger, cohesive, and stretchable film structure, such as it is tested on pectin-fish gelatin (Bermúdez-Oria et al. 2018), fresh strawberries (Khalifa et al. 2016), alginate bilayer (Ng and Tan 2015), and others (Khwaldia et al. 2022). Although less studied than other olive-oil by-products, also the OMW-PPh evidenced good potential as a component of active film and coating. Apicella et al. (2019) formulated multi-layer active films incorporating OMW different phenolic extract by demonstrating that: (i) oxygen and water vapor barrier properties of films are not affected by the addition OMW extract; (ii) OMW extract has the highest affinity of the film matrix with 95% ethanol; (iii) multi-layer films with 3% of OMW extract effectively release antioxidant compounds.

4.8 High Value-Added Microbial Products Generally, biological treatment of OMW has been studied to reduce their pollution and phenolic load. For this aim, several species of molds (i.e., Phanerochaete spp., Pleurotus spp., Panus tigrinus, Geotrichum spp., Lentinula edodes, Trametes versicolor or Aspergillus spp.) and bacteria (i.e., Pseudomonas putida, Klebsiella oxytoca, Lactobacillus plantarum, Citrobacter diversus) would have the potential to reduce the OMW’s COD value (Barr and Aust 1994; Millan et al. 2000). From another point of view, microbial depollution of OMW may be paired with biotechnological applications to produce valuable biomass or high-added molecules (Ruiz et al. 2002; Fernández-Bolaños et al. 2006; Dourou et al. 2016). The inhibitory

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effect of phenols on the microbial growth could be resolved through a simple dilution or appropriate chemical, physical or biological pre-treatments of the OMW, such as the supplement with low-cost sugars and other nutrients to optimize the microbial growth and the selection of single or mixed culture of endogenous or inoculated microorganisms. However, exclusively focusing on the food applications of the OMW use as biotechnological media, many yeast and bacterial species have given encouraging outcomes, as highlighted in Table 4.1. Ethanol is certainly one of the compounds more sought after by the biotechnological treatment of OMW due to its versatility of use in several sectors (i.e., energetic, food, pharmaceutics) (Bambalov et al. 1989). Table 4.1 High value-added microbial products obtained by bioconversion of olive oil wastewater Exogenous or endogenous microbiota

Microbial products

References

Torulopsis sp. MK-1, Saccharomyces norbensis MC-1, Saccharomyces oleaceus MC-2, Saccharomyces oleaginosus

Ethanol

Bambalov et al. (1989)

Lipomyces starkeyi, Yarrowia lipolytica, Candida tropicalis, Saccharomyces cerevisiae

Single cell oil (SCO), mannitol, citric acid, ethanol

Dourou et al. (2016)

Yarrowia lipolytica

Citric acid

Papanikolaou et al. (2008)

Xanthomonas campestris

Xanthan gum

Lopez et al. (2001) and Lopez and Ramos-Cormenzana (1996)

Botryosphaeria rhodian, Lentinula edodes

β-glucans

Crognale et al. (2003) and Tomati et al. (2004)

Candida and Saccharomyces spp.

Single cell protein (SCP)

Gharsallah (1993) and Arous et al. (2016)

Cryptococcus albidus

Pectic enzymes

Federici et al. (1988)

White rot fungi spp.

Lignin peroxidase, manganese peroxidase, Laccase

Barr and Aust (1994), Strong and Claus (2011) and Mann et al. (2015)

Aspergillus oryzae, Aspergillus Lipase niger, Candida cylindracea, Geotrichum candidum, Penicillium citrinum, Rhizopus arrhizus, Rhizopus oryzae Rhodotorula glutinis

D’Annibale et al. (2006), Muralidhar et al. (2001)

Catalase, superoxide dismutase De˘girmenba¸sı and Takaç (2018)

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Selected yeast strains of Lipomyces starkeyi, Yarrowia lipolytica, Candida tropicalis and Saccharomyces cerevisiae cultivated on OMW substrates, potentially enriched with mineral, glucose or glycerol, have successfully produced single cell oil (SCO), mannitol, citric acid and ethanol, with concurrent degradation of phenolic compounds (Dourou et al. 2016). Strains of Yarrowia lipolytica is able both to reduce pollution of OMW (Scioli and Vollaro 1997) and to produce citric acid (Papanikolaou et al. 2008); nevertheless, the obtained low amounts of citric acid (28 g/L) result noncompetitive with other biotechnological process and strongly affected by sugar supplementary addition as carbon source. Production of functional exopolysaccharides (ESP) is also explored. Xanthan gum is a microbial extracellular polysaccharide polymer used for food and nonfood applications as a thickener or emulsifying agent. Microbial production of xanthan gum was attempted by using Xanthomonas campestris spp. on OMW based medium. However, OMW showed a clear inhibitory concentration-dependent effect on the X. campestris microbial growth and xanthan production (Lopez and RamosCormenzana 1996) especially due to its high phenolic content (Lopez et al. 2001). Other explored ESP was the beta-glucans that are well proved anti-tumor, antibacterial, antiviral, anti-coagulator activities. In this case, strains of Botryosphaeria rhodian (Crognale et al. 2003) and Lentinula edodes (Tomati et al. 2004) on OMW growth media were tested with satisfactory results. Single-cell protein (SCP) production is a very interesting opportunity for the OMW due to the recent increasing worldwide demand for alternative protein sources. Currently, SCP is produced from several microorganisms, including algae, fungi, and bacteria. Studies with Candida and Saccharomyces spp. yeasts evidenced a significant and promising microbial biomass protein production from OMW-based medium (Gharsallah 1993; Arous et al. 2016). Another very attractive challenge is the production of commercial enzymes by using low-cost raw materials, such as the OMW. Species of Cryptococcus are assayed to produce pectic enzymes (Federici et al. 1988), while the white-rot fungi are considered as the producer of a mix of lignin and manganese peroxidase and laccase (Barr and Aust 1994). Laccase in OMW is of considerable interest for its ability to bioremediate phytotoxicity of a wide range of phenols (Strong and Claus 2011). Furthermore, it is proven that the production of laccase is positively induced by the presence of phenols in the substrate. Significant laccase amounts are obtained by Cerrena consors cultivated in OMW medium supplemented with 50% minimally sugar (Mann et al. 2015). OMW is found to be a valuable liquid growth medium also to produce lipase enzymes (D’Annibale et al. 2006). Commercially lipase (glycerol ester hydrolase E.C. 3.1.1.3) is produced through well-established fermentation processes and has a lot of the number of applications. Several lipolytic fungi species, such as Aspergillus oryzae, Aspergillus niger, Candida cylindracea, Geotrichum candidum, Penicillium citrinum, Rhizopus arrhizus, Rhizopus oryzae are demonstrated to grow on OMW and to produce significant amounts of extracellular lipase (D’Annibale et al. 2006). Lipase has been positively produced in OMW based media characterized by initial

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sugar contents (no sugar added) and yeast extract as a nitrogen source. However, it has emerged that a key factor in lipase yield is the fortification of OMW with vegetable oils, especially olive oil, whose turn both as growth and inducer substrate (Muralidhar et al. 2001). Finally, satisfactory production of catalase (E.C.1.11.1.6) and superoxide dismutase (E.C.1.15.1.1) was performed by using the oleaginous red yeast Rhodotorula glutinis and OMW based media (De˘girmenba¸sı and Takaç 2018). Catalase is an important enzyme that decomposes hydrogen peroxide into oxygen and water utilized in the food industry for sterilization or bleaching processes. Superoxide dismutase (E.C.1.15.1.1) is another important enzyme used as an antioxidant in the food industry. Furthermore, both these enzymes protect the cell from oxidative damage done by ROS. In conclusion, OMW appears to be a very competitive and low-cost liquid growth medium, on the condition that the chosen microorganisms could effectively survive and grow in a so high phenol content substrate. However, the yield and the cost of isolation, purification or recovery of the products could affect the economic convenience of the industrial use of OMW as microbial media.

4.9 Conclusion and Perspectives Numerous studies in the literature demonstrate a wide range of OMW in food applications at an experimental scale. Generally, OMW, used whole or in form of bioactive extracts, exhibit the suitable potential to improve functional properties and shelf life of the food products. In addition, the recycling of OMW in the food industry promotes the sustainability of the olive-oil chain and circular economy. However, there are still several hurdles to overcome for the realistic utilization of OMW in food applications. Some of the critical aspects must be considered carefully and resolved when OMW is prospected for food application are summarized below: . excess of the oleuropein phenolic compound, exhibiting a bitter taste, could alter the sensory characteristic of the food; . regarding the safety of human health, the OMW toxicity, presence of pesticide residue, contamination with mycotoxins and metals must be evaluated; . management conditions of OMW (collection, storage, transportation) are not defined clearly; . economic feasible and legal implications of OMW treatments for application at an industrial scale are not appropriately calculated; . present studies on the environmental, social, and economic impact of OMW’s food applications are still too few.

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Acknowledgements This paper was supported by Italian MIUR, project “Profood IV”, ARS01_00755, CUP: B64E20000180005. We kindly acknowledge the Consorzio per lo Sviluppo dei Sistemi a Grande Interfase (CSGI).

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Chapter 5

Olive Mill Wastewater Bioactive Molecules: Applications in Animal Farming Roberta Imperatore, Caterina Pagliarulo, Graziella Orso, Giuseppa Anna De Cristofaro, Daniela Sateriale, and Marina Paolucci Abstract For many years, olive by-products have been considered environmental pollutants because of their high organic content and phytotoxic activity. However, in the last decades, an increased interest in olive mill wastewater (OMWW) has been registered since the polyphenols contained in it have shown, in vitro and in vivo, antioxidant, anti-inflammatory, and antimicrobial activities. Both in farmed fish and terrestrial animals, oxidative stress induces pathological conditions, such as enteritis, peripheral and central infections, and cancer. The administration of OMWW seems capable to prevent these diseases or reduce their symptoms, promoting animals’ growth performance and health. Such beneficial effects have been observed in several food-producing animal species, such as lambs, pigs, chickens, and fishes, and are largely attributed to the contained polyphenols (mainly Hidrossityrosol, Tyrosol, Caffeic Acid, Oleuropein). In conclusion, being OMWW one of the most abundant agro-industrial by-products rich in toxic pollutants with disposal still exceedingly difficult, the recovery and valorization of OMWW is a priority for the implementation of a circular economy, although numerous efforts are still necessary to valorize this waste reducing its environmental impact.

R. Imperatore · C. Pagliarulo · G. Orso · G. A. De Cristofaro · D. Sateriale · M. Paolucci (B) Department of Science and Technology, Univesity of Sannio, Via De Sanctis, snc, 82100 Benevento, Italy e-mail: [email protected] R. Imperatore e-mail: [email protected] C. Pagliarulo e-mail: [email protected] G. Orso e-mail: [email protected] G. A. De Cristofaro e-mail: [email protected] D. Sateriale e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_5

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Keywords Olive mill waste water · Antioxidant · Anti-inflammatory · Antimicrobial · Animal husbandry · Aquaculture

Abbreviations AA AGER APX ATP BAX BCL2 CA CARB CAT COX-2 DHAR DNA EGF FA FAs GCS GPx GR GSH GST hECs HTyr H2 O2 IL iNOS LDL LPS MAP MDA MRSA MSSA NF-κB NHEKs Ole OMWW PUFA RNS ROS

Ascorbic acid Advanced glycosylation end-product specific receptor Ascorbate peroxidase Nucleoside triphosphate B-cell lymphoma 2 associated X apoptosis regulator B-cell lymphoma 2 Caffeic acid Carbonylated proteins Catalase Cyclooxygenase-2 Dehydroascorbate reductase Deoxyribonucleic acid Epidermal growth factor Ferulic acid Fatty acids Glutamilcistein sintentase Glutathione peroxidase Glutathione reductase Reduced glutathione Glutathione-S-transferase Human endothelial cells Hydroxytyrosol Hydrogen peroxide Interleukin Nitric oxide synthase Low-density lipoproteins Lipoxygenase Mitogen-activated protein kinase Malondialdehyde Methicillin-resistant S. aureus strain Methicillin-sensitive S. aureus strain Nuclear Factor kB Human epidermal keratinocytes Oleuropein Olive mill wastewater Polyunsaturated fatty acids Reactive nitrogen species Reactive oxygen species

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SIRT SOD TBARS TNF-α Tyr Vbs

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Sirtuins Superoxide dismutase Thiobarbituric acid–reactive substances Tumor necrosis factor-α Tyrosol Verbascoside

5.1 Introduction The olive mill wastewater (OMWW) is the main liquid effluent of the olive oil production process. It derives from the combination of (i) the liquid separated by centrifugation from the oily must, (ii) the water derived for washing the olives, and (iii) the water used to facilitate the oil extraction. OMWW differs in quantity, density, and concentration of dissolved substances, as a result of the characteristics of the extraction equipment used, the olive cultivar, the harvest season, and the degree of ripeness of the fruit. The OMWW is a dark and acidic liquid, mainly composed of water, organic substance that includes carbohydrates, pectin, mucilage, lignin, and tannins (responsible for the color), lipids and inorganic salts such as Na+ , K+ , P, N, Mg2+ , Ca2+ . Free sugars include glucose, fructose, galactose, mannose, and sucrose (Rahmanian et al. 2014). However, due to the high content of salts, organic matter, and phenolic compounds, as well as the high acidity, the OMWW is highly polluting and toxic to microorganisms and plants. Indeed, OMWW is considered one of the most polluting effluents produced by the agri-food industry due to its high load of substances of various kinds such as phenolic compounds, capable of discoloring streams and rivers and inhibiting the germination of plants. Furthermore, the high concentration of reduced sugars can stimulate microbial respiration, lowering the concentrations of dissolved oxygen (Barbera et al. 2014). Therefore, it is widely accepted that OMWW treatment to reduce the environmental impact is necessary. The systems currently used for the treatment of OMWW can be classified as biological, physicochemical, and combined processes aimed at the recovery or removal of phenolic compounds and other substances from the waste effluents. For the reader interested in learning more about the treatment of OMWW, please refer to Chap. 2 of this book. Although OMWW is characterized by high polluting power, it represents a source of compounds with high biological value such as natural pigments, antioxidants, sugars, and nutrients. Some of these compounds can be recovered and reused from the perspective of OMWW valorization and reduction of its environmental impact, ensuring sustainable use of the resource itself. The main bioactive molecules of olive oil and by-products are represented by phenolic compounds or polyphenols, characterized by high antioxidant activity and responsible for the relationship between

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oil consumption and the prevention of chronic-degenerative events, cardio-cerebralvascular diseases, and tumors. Thus, this chapter focuses on the in vitro and in vivo activities of OMWW polyphenols. Polyphenols are secondary metabolites produced by plants as a defense against UV radiation, pollution, aggression by parasites, viruses, bacteria, and the attack of herbivorous animals. They are structurally heterogeneous substances, characterized by the presence of an aromatic ring with one or more hydroxyl groups with strong antioxidant action. Today more than 8000 different polyphenolic compounds are known, present in different concentrations and relative proportions in different plants. The resulting complex mixture of polyphenols is defined as a phytocomplex, which maximizes the protective effect of the individual molecules thanks to synergistic and complementary activities. These natural molecules, extracted from plants, or from the waste of the agro-food industry, have attracted great economic interest for their antioxidant properties that can be used in the food, nutraceutical, pharmaceutical, and cosmetic industries, as well as in other sectors such as additives for plastics, and lubricants. Polyphenols exist in free form (aglycones), or glycosylated, or esterified with organic acids. Over 8000 aglycones have been identified and approximately 4000 are present in the form of glycosides, esters, and other combinations. Polyphenols have a strong antibacterial and antifungal activity, due to the combined action of the antioxidant power and the chelating capacity of phenolic hydroxyl groups. The antioxidant power is the main and most studied characteristic of polyphenols. It enables them to counteract a dangerous phenomenon known as oxidative stress, induced by an imbalance between free radicals and antioxidant molecules in the body, in favor of free radicals. Free radicals are unstable atoms containing unpaired electrons seeking other electrons to stabilize themselves. Thus, free radicals react with other molecules and if this process becomes continuous it can induce damage to DNA (DeoxyriboNucleic Acid), and/or biomolecules (proteins, lipids), and subcellular structures (cell membranes) causing several diseases and accelerating aging. Numerous natural biological processes, such as defense against pathogens and aerobic metabolism (oxidation processes), or external factors, such as radiation, pollutants, and diets, can induce the production of free radicals. There are many subsets of free radicals in biological systems, however, the most significant are those derived from oxygen or reactive oxygen species (ROS). The ROS family includes not only free radicals (superoxide, hydroxyl radical), but also non-radicals (hydrogen peroxide, lipid hydroperoxide) that occur naturally in the body and can attack the fatty acids (FAs) of the cellular membrane with consequent membrane destruction and release of lipid peroxidation products affecting cells viability and tissues. The oxidative stress, induced by the free radical increase, plays a crucial role in the development of inflammation, a dangerous condition, as it anticipates serious diseases such as cancer. Indeed, high ROS formation can induce the increase in pro-inflammatory cytokines production, such as cyclooxygenase-2 (COX-2), tumor necrosis factor-α (TNF-α), and interleukins (IL), by the activation of Nuclear Factor κB (NF-κB) pathway (Reuter et al. 2010).

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It is clear that molecules capable of regulating free radicals’ production are essential to maintain the health status at cellular and tissue levels. Antioxidant molecules play this role by being good at neutralizing the instability of free radicals reducing the risk of cell and tissue damage. Antioxidant molecules can act at different levels. In particular, they can: (i) scavenge ROS already formed; (ii) inhibit ROS formation; (iii) repair the damage or modifications induced by ROS; and finally, (iv) bring about the increase in the activity and gene expression of antioxidant enzymes which represent the first cell line of defense. Such enzymes and/or non-enzymatic cellular antioxidants like ascorbic acid (AA), reduced glutathione (GSH), and glutathione peroxidase (GPx) are involved in ROS neutralization. The main antioxidant enzymes are superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and dehydroascorbate reductase (DHAR). In this contest, the OMWW bioactive molecules have shown high antioxidant activity being capable of decomposing peroxides and acting as radical scavenger. Specifically, OMWW polyphenols regulate SOD, CAT, GR, and GPx activity in numerous tissues (intestine, muscle) of different animals, from fish to humans. Although the phenolic content may differ, the main phenolic compounds in OMWW reported in the literature are hydroxytyrosol (HTyr) and tyrosol (Tyr), along with lower concentrations of Caffeic (CA) and Ferulic (FA) acids, Verbascoside (Vbs) and Oleuropein (Ole), the last one being more abundant in olive leaves (Di Meo et al. 2022). For a thorough description of polyphenols and other bioactive substances present in OMWW the reader is referred to Chaps. 4 and 10 of this book. The large amounts of OMWW remnant from olive oil production is considered a low-cost starting source rich in polyphenols for potential use in agro-food and pharmaceutical industries.

5.2 The “In Vitro” Actions of the OMWW Bioactive Molecules 5.2.1 Antioxidant and Anti-inflammatory Activities The antioxidant activity of polyphenols from OMWW has been reported in numerous in vitro studies and has been attributed to several mechanisms mediated by OMWW polyphenols, such as decomposition of peroxides, binding of transition metal ion catalysts, reductive capacity, and radical scavenging (Carrara et al. 2021). HTyr, Tyr and phenolic acids (CA and FA) participate in OMWW antioxidant activities by their electron-donating properties, neutralizing the ROS and producing more stable radicals. The antioxidant activity of CA, FA, Ole and Vbs also depends on their ability to chelate metal generated free radicals. Moreover, it has been demonstrated that the HTyr provides additional antioxidant protection through the

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activation of cell signaling pathways that increase the endogenous defense system against oxidative stress. In addition, Tyr has appeared to be an effective cellular antioxidant, due to intracellular accumulation. Thanks to its chemical characteristics, Tyr from OMWW has been utilized for the synthesis of effective antioxidants (Namazifar et al. 2019). The antioxidant effects of OMWW polyphenols are considered the basis of their widely demonstrated anti-inflammatory properties. Particularly, HTyr shows in vitro evidence of the attenuation of pro-inflammatory inducible agents such as nitric oxide synthase (iNOS), COX-2, TNF-α, and interleukin (IL)-1β, and inhibition of granulocytes and monocytes. Tyr and Tyr metabolites show remarkable properties on oxidative stress and inflammation in TNF-α-treated human endothelial cells (hECs). Furthermore, Tyr significantly reduces the expression of the proinflammatory cytokines TNF-α, IL-1β, and IL-6 in macrophages stimulated by lipoxygenase (LPS). Several studies demonstrate the implication of CA in both NF-κB and mitogen-activated protein kinase (MAP) pathways. OMWW extracts, containing 30– 50% of Ole and 5–35% of Vbs, inhibit the interaction of NF-κB with DNA in vitro, in human epidermal keratinocytes (NHEKs), thus preventing the transcription of genes involved in inflammation. In murine macrophages, Ole reduces COX-2 and iNOS rates. Moreover, it significantly lowers TNF-α in human fibroblasts stimulated by IL-1β. It has also been shown that Vbs reduces IL-8 levels in TNF-α stimulatedkeratinocytes cell cultures. The in vitro biological activities of OMWW phenolic compounds are reported in Table 5.1.

5.2.2 Antimicrobial/Prebiotic Activities OMWW polyphenol-rich extract possesses a broad spectrum of in vitro antibacterial activities against Gram-positive and Gram-negative bacteria, as well as a significant antifungal activity. A wide range of both human and animal pathogens have been shown to be sensitive to OMWW extracts and to the single bioactive phenolic constituents. OMWW extract, containing high concentrations of HTyr, Tyr, FA, CA, and Ole, is effective against several pathogenic species. Particularly, OMWW extract exhibits strong antimicrobial activity against Staphylococcus aureus and Staphylococcus epidermidis (aerobic Gram-positive), Escherichia coli, and Pseudomonas aeruginosa (aerobic Gram-negative), and Propionibacterium acnes (anaerobic Gram-positive). The antibacterial activity could be ascribed to the high content of HTyr and Ole in the OMWW extracts (Schlupp et al. 2019). The extract compounds tested individually were less effective, highlighting the synergistic effect of the bioactive phenolic constituents (Obied et al. 2007). In addition, OMWW polyphenols show in vitro efficacy as inhibitors on a wide range of microbial gastrointestinal pathogens, such as Helicobacter pylori, inhibit the adhesion ability of two important gut pathogens,

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Table 5.1 In vitro biological activities of major phenolic compounds from olive mill wastewater (OMWW) Bioactive phenolic compound

Biological activities

References

Hydroxytyrosol (HTyr)

• Antioxidant • Anti-inflammatory

(Son and Lewis 2002; Bayram et al. 2012; Kitsati et al. 2016; Frascari et al. 2019)

• Antimicrobial

(Bisignano et al. 1999; Medina et al. 2006; Tafesh et al. 2011; Wu et al. 2017; Diallinas et al. 2018)

• Anti-atherosclerosis • Neuroprotective • Anti-cancer

(Fabiani et al. 2012; Lopez-de las Hazas et al. 2017; Hornedo-Ortega et al. 2018; Imran et al. 2018)

• Antioxidant • Anti-inflammatory

(Son and Lewis 2002; Di Benedetto et al. 2006; Lu et al. 2013)

• Antimicrobial

(Puel et al. 2008; Tafesh et al. 2011; Hassan Abdel-Rhman et al. 2015; Monteiro et al. 2015)

• Anti-atherosclerosis • Neuroprotective • Anti-diabetes

(Perona et al. 2006; St-Laurent-Thibault et al. 2011)

• Antioxidant • Anti-inflammatory

(Son and Lewis 2002; Khan et al. 2012; Balupillai et al. 2015; Kitsati et al. 2016)

• Antimicrobial

(Cheah et al. 2014; Costa et al. 2019)

• Cardioprotective • Anti-cancer

(Yang et al. 2014; Zdu´nska et al. 2018)

• Antioxidant • Anti-inflammatory

(Son and Lewis 2002; Nile and Park 2014; Kitsati et al. 2016)

• Antimicrobial

(Patzke and Schieber 2018; Kot et al. 2019; Hernández et al. 2021; Pinheiro et al. 2021)

• Cardioprotective • Anti-cancer

(Ambothi and Nagarajan 2014; Zdu´nska et al. 2018)

• Antioxidant • Anti-inflammatory

(Lampronti et al. 2013; Ryu et al. 2015; Kitsati et al. 2016; Castejón et al. 2017)

• Antimicrobial

(Fleming et al. 1973; Tassou et al. 1991; Pereira et al. 2007; Sudjana et al. 2009; Lee and Lee 2010)

• • • •

(Carluccio et al. 2003; Achour et al. 2016; Rigacci and Stefani 2016; Shamshoum et al. 2017; Imran et al. 2018)

Tyrosol (Tyr)

Caffeic acid (CA)

Ferulic acid (FA)

Oleuropein (Ole)

Anti-atherosclerosis Neuroprotective Anti-cancer Anti-diabetes

(continued)

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Table 5.1 (continued) Bioactive phenolic compound

Biological activities

References

Verbascoside (Vbs)

• Antioxidant • Anti-inflammatory

(Korkina et al. 2007; Lampronti et al. 2013; Kitsati et al. 2016)

• Antimicrobial

(Pereira et al. 2007; Ali et al. 2011; Ahmed et al. 2014; Khalil et al. 2014; Liu et al. 2017)

• • • •

(Zhang et al. 2002; Sheng et al. 2002; Korkina 2007; Kostyuk et al. 2011)

Anti-atherosclerosis Neuroprotective Anti-cancer Anti-diabetes

Bacillus cereus and Listeria monocytogenes (Ribeiro et al. 2021), and show antifungal effects against clinically important fungal pathogens, such as Candida spp. and Aspergillus spp. (Ali et al. 2011). Furthermore, it is important to mention the important effect exerted by OMWW polyphenols on gut microbiota of mammals and other animals. As aforementioned, phenolic compounds isolated from OMWW can inhibit the growth of gastrointestinal pathogenic bacteria, thus influencing the microbiota composition. In vitro studies showed that they can also act as prebiotics stimulating the growth of probiotic bacteria, such as Lactobacillus spp. and Bifidobacterium spp. (Marcelino et al. 2019). An interesting in vitro gastrointestinal digestion model, followed by in vitro fermentation model, shows a positive effect of the OMWW phenolic compounds on the Firmicutes/Bacteroidetes and Prevotella spp./Bacteroides spp. ratios, demonstrating their prebiotic effects on gut microbiota (Ribeiro et al. 2021). In vivo studies (later reported) are more numerous. Through the integration of animal diets with polyphenolic compounds, it has been demonstrated how OMWW polyphenols could generate a beneficial microbial balance in the gut. Regarding individual compounds, HTyr has been demonstrated to possess in vitro antimicrobial properties against respiratory and gastro-intestinal tract infectious agents, such as Vibrio parahaemolyticus, V. cholerae, Salmonella typhi, Haemophilus influenzae and S. aureus, and also against foodborne pathogens as L. monocytogenes, S. aureus, Salmonella enterica, Yersinia spp., Streptococcus pyogenes, E. coli and Klebsiella pneumoniae. HTyr antibacterial activity has also been shown against bovine isolates of S. aureus with a protective effect on S. aureus-induced mastitis. In vitro results indicate that Htyr also shows a considerable antifungal activity against medically important yeasts, like Candida albicans, in which it causes significant cell wall damages as well as inhibition of germ-tube formation. In addition, Htyr and some Htyr analogs have a broad antifungal activity against major fungal pathogens, such as Aspergillus fumigatus, Aspergillus flavus, Aspergillus nidulans, Fusarium oxysporum and C. albicans. Importantly, this antifungal activity is directly related to its rapid destructive effect on fungal plasma membranes (Diallinas et al. 2018).

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Tyr is active against S. pyogenes and S. aureus isolated from infectious wounds and against P. aeruginosa and S. aureus mixed-species biofilms. Antibacterial properties of Tyr can be linked to the binding and inhibition of bacterial ATP synthase. Tyr exhibits an inhibitory effect on C. albicans and Candida glabrata adhesion to abiotic surfaces, showing the potential anti-biofilm activity against Candida infections (Monteiro et al. 2015). Antibacterial activity has also been noted for CA, which shows interesting antibacterial properties against E. coli, P. aeruginosa and S. aureus and against several strains of S. aureus, including both methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) strains. Some studies investigated also on the antifungal activity of CA, demonstrating its inhibitory effect against planktonic cells and biofilms of C. albicans and against planktonic cells of Candida parapsilosis. FA antimicrobial effects have been demonstrated not only against human pathogens, such as S. aureus, but also against fish isolated Aeromonas species (A. salmonicida, A. hydrophila, A. sobria) and against a porcine fecal isolate of E. coli. Regarding antifungal properties, FA inhibits the growth of Botrytis cinerea and other relevant fungal pathogens responsible for post-harvest fruit decay, such as Monilinia fructicola and Alternaria alternate. Antimicrobial activity has also been described for Ole. Ole has been demonstrated to exert a strong inhibitory effect against Salmonella enteritidis, even if better effects were achieved using a combination of polyphenols from olive leaf extract (Lee and Lee 2010). Ole has been found to inhibit the growth of S. aureus, B. subtilis, Psudomonas solanecearum, and K. pneumoniae. It was also shown that it inhibits the germination of B. cereus. More recent studies demonstrated that Ole can be a promising source for the development of new anti-candida therapy, considering its significant antifungal effects against C. albicans, C. glabrata, and C. parapsilosis (Teodoro et al. 2015). Vbs shows significant antibacterial activity against S. enteritidis, E. coli, and B. cereus, alone and in synergy with Ole. Some reports have also shown that Vbs is a suitable choice against S. aureus infections (Ahmed et al. 2014). In addition, Vbs has protective effects against infections caused by Cryptococcus neoformans, a basidiomycetous yeast ubiquitous in the environment, a model for fungal pathogenesis, and an opportunistic human pathogen of global importance (Pereira et al. 2007). Finally, Vbs significantly reduces the inhibitory concentration of amphotericin B (AmB), antifungal used against Candida spp. and Aspergillus spp. (Ali et al. 2011). In Table 5.2 are reported the main in vitro antimicrobial/prebiotic activity of olive mill waste water (OMWW) whole extracts and of single phenolic constituents. Literature data regarding antiviral activity of OMWW compounds are still few, but some preliminary studies investigated the HTyr and Ole antiviral effects against HIV-1 infection and replication by inhibiting viral entry and cell-to-cell transmission (Lee-Huang et al. 2007). In conlusion, the antimicrobial effects of OMWW whole extract and single molecules are promising tools to enhance the fight against microbial infections, also the resistant ones.

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Table 5.2 In vitro antimicrobial/prebiotic activity of olive mill waste water (OMWW) whole extracts and single phenolic constituents Antibacterial activity

OMWW whole extracts

HTyr

Tyr

Antifungal activity

Prebiotic activity

Candida spp. Aspergillus spp.

Bifidobacterium spp. Lactobacillus spp.

Gram-positive bacteria

Gram-negative bacteria

Staphylococcus aureus Bacillus subtilis Staphylococcus epidermidis Listeria monocytogenes

Escherichia coli Pseudomonas aeruginosa Helicobacter pylori

(Medina et al. 2006) (Obied et al. 2007) (Belaqziz et al. 2017) (Schlupp et al. 2019) (Ribeiro et al. 2021)

(Medina et al. (Ali et al. 2011) 2007) (Romero et al. 2007) (Obied et al. 2007) (Schlupp et al. 2019)

Staphylococcus aureus Streptococcus pyogenes Listeria monocytogenes

Vibrio parahaemolyticus Vibrio cholerae Salmonella typhi Haemophilus influenzae Salmonella enterica Yersinia spp. Escherichia coli Klebsiella pneumoniae

Candida albicans – Aspergillus fumigatus Aspergillus flavus, Aspergillus nidulans Fusarium oxysporum

(Bisignano et al. 1999) (Medina et al. 2006) (Sogawa et al. 2018) (Tafesh et al. 2011) (Wu et al. 2017)

(Bisignano et al. 1999) (Medina et al. 2006) (Tafesh et al. 2011)

(Zoric et al. 2013) (Diallinas et al. 2018)

Staphylococcus aureus Streptococcus pyogenes

Pseudomonas aeruginosa

Candida albicans – Candida glabrata

(Tafesh et al. 2011) (Hassan Abdel-Rhman et al. 2015)

(Hassan Abdel-Rhman et al. 2015)

(Monteiro et al. 2015)

(Marcelino et al. 2019)

(continued)

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Table 5.2 (continued) Antibacterial activity

CA

FA

Ole

Vbs

Antifungal activity

Gram-positive bacteria

Gram-negative bacteria

Staphylococcus aureus Methicillin-resistant S. aureus

Escherichia coli Pseudomonas aeruginosa

(Costa et al. 2019)

(Costa et al. 2019) (Özçelik et al. 2011) (Cheah et al. 2014)

Staphylococcus aureus

Aeromonas salmonicida Aeromonas hydrophila Aeromonas sobria

Prebiotic activity

Candida albicans – Candida parapsilosis

Botrytis cinerea Monilinia fructicola Alternaria alternate

–

(Pinheiro et al. 2021) (Kot et al. 2019)

(Patzke and Schieber 2018) (Hernández et al. 2021)

Staphylococcus aureus Bacillus subtilis Bacillus cereus

Candida albicans – Candida glabrata Candida parapsilosis

Salmonella enteritidis Klebsiella pneumoniae Psudomonas solanecearum

(Fleming et al. 1973) (Fleming et al. (Tassou et al. 1991) 1973) (Pereira et al. 2007) (Lee and Lee 2010)

(Sudjana et al. 2009) (Teodoro et al. 2015)

Staphylococcus aureus Bacillus cereus

Salmonella enteritidis Escherichia coli

Cryptococcus neoformans Candida spp. Aspergillus spp.

(Ahmed et al. 2014) (Khalil et al. 2014)

(Liu et al. 2017)

(Pereira et al. 2007) (Ali et al. 2011)

–

5.2.3 Other In Vitro Activities OMWW polyphenols have great importance when it comes to health benefits for both humans and animals, and the interest in their heterogeneous bioactivities is huge. A growing body of in vitro studies has revealed new aspects of already known biological activities and several new specific health effects of these compounds.

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Antioxidant and anti-inflammatory properties are the basis of cardioprotective, anti-cancer, neuroprotective, and endocrine effects of bioactive compounds from OMWW. HTyr alleviates the inflammatory response leading to the development of atherosclerosis in cells treated with highly toxic air pollutant acrolein. The neuroprotective effects of HTyr in Alzheimer’s and Parkinson’s diseases are demonstrated. They are linked to the reduction of oxidative stress in neuronal cells. HTyr is likely to have anti-cancer effects on human colon adenocarcinoma cells. The most common anti-cancer mechanism of HTyr seems to be its auto-oxidation which is induced by the accumulation of hydrogen peroxide (H2 O2 ). However, various studies have highlighted that HTyr exerts different anti-proliferative and pro-apoptotic mechanisms depending on the cancer cell type studied. Until now, the beneficial HTyr anti-cancer effects have been investigated in colorectal, breast, bladder, blood, gastric, hepatic, skin, prostate, cervical, brain, lung, and thyroid types of cancer suggesting its possible broad use in cancer prevention (Imran et al. 2018). Several studies confirmed that Tyr could exert its beneficial effects against atherosclerosis, due to inhibition of monocytes adhesion to endothelial cells. Tyr also possesses neuroprotective effect in Alzheimer’s disease, as shown by Neuro2a cells protection against amyloid-β-induced toxicity. It was reported that CA and FA possess, among their biological functions, cardio protective effects. In addition, CA owns an inhibitory effect in human skin cancer cells, influencing epidermal growth factor (EGF)-induced neoplastic transformation of HaCaT cells. FA has also been found to be a promising anti-cancer substance, preventing ultraviolet-B radiation induced oxidative DNA damage in human dermal fibroblasts. Among OMWW polyphenols, Ole is considered to have the greatest potency as anti-atherosclerotic factor. Ole inhibits cell adhesion on endothelial surfaces in vitro (Carluccio et al. 2003). Furthermore, Ole may prevent neuronal degeneration in a cellular dopaminergic model of Parkinson’s diseases, with a significant decrease in neuronal death and reduced mitochondrial production of ROS. The anti-cancer effects of Ole are associated with its ability to modulate gene expression of a variety of different signaling proteins that play a role in the proliferation and apoptosis of cancer cells. Finally, Ole is able to interfere with amylin, a peptide hormone preventing post-prandial spikes in blood glucose by preventing the cytotoxicity of amylin in pancreatic cells in vitro, with beneficial effects in type 2 diabetes (Rigacci and Stefani 2016). Large amounts of data exist in support of the regulatory role of Vbs in vascular inflammation, thanks to its action in selective prevention of the low-density lipoproteins (LDL) oxidation, neutralizing their atherosclerosis effect. Additionally, it is assumed that Vbs could exert anti-cancer, cytotoxic, and anti-metastatic properties due to its estrogenic and anti-estrogenic actions (Korkina 2007). Particularly, Vbs has been reported to induce in vitro apoptosis, by telomere–telomerase-cell cycle-dependent modulation of human gastric carcinoma cells. The neuroprotective effect of Vbs has been investigated in neuronal PC12 cells where Vbs significantly decreases apoptotic death, with potential beneficial effects on oxidative stress-induced neurodegenerative diseases.

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In conclusion, the results of in vitro studies on the biological properties of OMWW polyphenols demonstrate their great pharmacological potential, with promising practical application in animals and humans.

5.3 Effects of OMWW Bioactive Molecules in Animal Husbandry 5.3.1 Effects of OMWW Bioactive Molecules on Terrestrial Species of Zootechnical Interest According to the World Organization for Animal Health, morbidity and lethality due to diseases, cause the loss of at least 20% of livestock production worldwide, hampering the efforts to improve productivity and reduce poverty. In farmed animals, pathological conditions, directly linked to animal welfare, are often related to oxidative stress. Free radicals such as ROS and reactive Nitrogen species (RNS) are produced in the living organisms from the normal essential metabolic processes or from external sources (exposure to X-ray, air pollutants, industrial chemicals). Moreover, several factors such as inflammation, dietary imbalances, thermal stress, high metabolic load, respiratory diseases, and parasites can lead to the formation of free radicals, resulting in the manifestation of various pathological conditions (Halliwell 2001; Birben et al. 2012). To enhance animal’s antioxidant defense system and to prevent oxidative stress, antioxidant supplementation throughout nutrition is a useful strategy. In Mediterranean areas, farmers often use olive by-products as feed integrators for livestock, but there are some limitations regarding their use as non-conventional feed. These limitations are due to the low percentage of protein (6.6–9.9%), high ether content (10– 30%), high neutral detergent fiber (23–73%), acid detergent lignin (12–37%), and others compounds such as phytic acid, polyphenols, and tannins. These substances, including the phenolic compounds, seem to be involved in the reduction of fiber degradability and methane emission in rumen cellulolytic bacteria and protozoa, and negatively influence the palatability and digestibility of the olive by-products (Molina-Alcaidea and Yáñez-Ruiz 2008; Vasta et al. 2019). OMWW contains about 50% of the total polyphenols found in olive fruits and if it has been proved that polyphenols at high concentrations can interfere with the regular physiological processes or reduce the palatability of food/feed, it has been also proved that they possess numerous and healthy promoting effects that could be exploited through their extraction and addition to the diet in due concentrations. Currently, representing the olive waste a problem from an ecological point of view and in relation to disposal costs, the recovery of polyphenols from OMWW provides them with an added value. In particular, OMWW polyphenols supplement in animal diets could be a strategy to enhance the health of farmed animals and the quality of the products of animal origin, while reducing the adverse environmental effects of these by-products.

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Antioxidant Activity of OMWW

At an early age animals have a low antioxidant system compared to adults, so the administration of antioxidant molecules, such as polyphenols, to protect against diseases would be very helpful. In the monogastric animals the metabolism of phenolic compounds begins in the epithelium of the upper intestine and continues in the lower intestine and liver, to reach peripheral organs such as kidneys and adipose tissue. The absorption of phenolic compounds can happen either via the stomach and small intestine or by the colon after chemical modification. It has been shown that only 5–10% of the total polyphenols taken with the diet is absorbed in the small intestine, while 90–95% of absorption occurs in the large intestine where polyphenols are broken into smaller molecules to facilitate absorption by intestinal enzymes and colon microbiota. The microbiota present in the colon allows polyphenols to be absorbed into the bloodstream and subsequently to be excreted either in the urine or via the bile The smaller molecules undergo biotransformations within enterocytes that make them water-soluble and therefore capable to reach the liver throughout the circulatory system. Dietary polyphenols show prebiotic and selective antimicrobial properties enhancing the growth of certain beneficial bacteria (Bacillus spp., Lactobacillus spp.), while inhibiting specific pathogenic bacteria in the intestinal tract. Such activity tends to establish a competitive relationship between beneficial and pathogenic bacteria and determines a balance of the intestinal flora which, indirectly, improves the host’s immune system and determines the state of well-being of the animal (Hashemi and Davoodi 2011). In Fig. 5.1 is reported a diagram of the metabolism of polyphenols in monogastric animals. The literature presents different studies reporting the effects of the in vivo administration of OMWW polyphenols. The main animals studied are pigs, sheeps, lambs, rabbits, and chickens. Many of these studies evaluate the presence of markers of oxidative stress in the blood or in different organs, some studies focus on the study of the inflammatory signaling pathways involved in oxidative stress, some others on polyunsaturated fatty acids (PUFA) of biological membrane damage initiated by lipid peroxidation and leading to the deterioration of lipids and so to the destruction of the cell. Furthermore, oxidized lipids tend to steal electrons to nearby molecules (proteins and DNA) propagating the damage. Pigs Beneficial effects of OMWW polyphenols administration on farmed pigs have been investigated in the last years. The weaning period is a stressful situation for pigs, causing gastrointestinal disorders directly associated with reduced function of the antioxidant mechanisms (Zhu et al. 2012). GSH is one of the most important antioxidant molecules in a living organism, but its concentration is very low during weaning, so pigs are more vulnerable to oxidative stress. The administration of feed enriched with OMWW polyphenols improves the oxidative status of piglets, the morphological and histological status of the intestine, and preserves the intestinal barrier (Aquilano et al. 2014).

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Fig. 5.1 Diagram of polyphenols metabolism in monogastric animals. 5–10% of dietary polyphenols are absorbed in the small intestine after the hydrolyses. Most polyphenols are bio-transformed in the enterocytes and liver. The hydrophilic metabolites reach blood and tissues or can be excreted via urine and bile. From 90 to 95% of ingested polyphenols, which are not absorbed, enter the colon and are bio-transformed by the microbiota into various polyphenol metabolites. The remaining polyphenol metabolites are excreted via the feces

Several studies carried out on farmed animals evaluated the activity of OMWW polyphenols in the modulation of lipid peroxidation. In particular, feed supplemented with OMWW polyphenols determined the reduction of lipid peroxidation and oxidative stress markers in blood and other tissues in piglets (Gerasopoulos et al 2015). Moreover, OMWW polyphenols not only enhance the redox status of piglets, but also reduce the oxidative damage of biological molecules, boost the antioxidant mechanisms, and regulate the FAs level and ratio in blood and tissues (Gerasopoulos et al. 2016). FAs play a fundamental role in the metabolism, storage, and transport of energy, and can act as essential components and gene modulators. Hydrolysis of triglycerides, performed by lipases, results in energy release. Indeed, FAs are important energy sources because, when metabolized, they produce large quantities of ATP (nucleoside triphosphate), the main chemical energy carrier within the cell. Omega-3 (ω-3) and omega-6 (ω-6) FAs are the precursors of the components of many cell membranes and are precursors of many other substances in the body involved in the inflammatory responses. Their content within the body depends on the nutritional intake and the quantitative relationship established between omega-3 (ω-3) and omega-6 (ω-6) is fundamental for health. These two FAs must be present in the right ratio, which must be in favor of ω-3. An inversion of this relationship causes adverse

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phenomena including inflammation. Piglets fed with OMWW polyphenols showed a significant reduction in ω3/ω6 ratio in different tissues, including the muscle, with negative effects on the meat quality. Poultry OMWW polyphenols are involved in promoting the chicken performance and quality of derived products. OMWW polyphenols in the diet improve the redox status of broiler chicken, with a reduction in protein oxidation promoted by ROS, and thus a reduction in the formation of carbonylated proteins or CARB, especially in the muscle. The prevention of protein oxidation is crucial since high levels of CARB in poultry meat affect digestibility and nutritional values. At the same time, the addiction of OMWW polyphenols to the chicken diet determines the decrease of thiobarbituric acid–reactive substances (TBARS), a marker of lipid peroxidation, whose reduction improves meat quality by preventing off-flavor production. OMWW polyphenols in the diet positively affect the gastrointestinal system of chickens. In chickens treated with OMWW polyphenols digestion and intestinal adsorption are improved. Furthermore, OMWW up regulates genes involved in defense mechanisms against viral genome replication (Sabino et al. 2018) as also reported in pigs and lamb. On the other hand, OMWW polyphenols down-regulate gene involved in cholesterol and lipid metabolism, with down-regulation of sterol biosynthesis leading to the reduction of fat deposition in chicken meat. Finally, OMWW polyphenols show also antimicrobial activity, reducing fecal shadding of Camphylobacter spp. in broils (Branciari et al. 2016). Ruminants Ruminants are a large group of herbivorous mammals including bovines, sheep, goats, deers, and antelopes. Several studies indicate that feed supplemented with raw OMWW improved the redox status of sheep (Kerasioti et al. 2017). The mechanisms seem to involve both the activity and gene expression of liver and spleen antioxidant enzymes SOD, Glutathione-S-transferase (GST), GR, and Glutamilcistein sintentase (GCS), a key enzyme for the synthesis of the antioxidant molecule GSH. The liver is a vital organ with several functions such as detoxification and protein synthesis and is one of the primary organs affected by ROS. Indeed, the liver cells: parenchymal cells, Kupffer cells, hepatic asteroid cells, and endothelial cells, are quite sensitive to oxidative stress and when ROS production increases and alterates the redox homeostasis, an increase in cytokines occurs, leading to inflammation and/or apoptosis, with the consequent establishment of liver diseases and degenerative disorders. Spleen is a lymphoid organ and the largest lymphatic tissue in animals. It constitutes a hematopoietic organ during fetal life, acts as a filter that purifies blood from abnormal cells, antigens, and microorganisms and plays an important role in non-specific and specific immunity, other than in mediating oxidative damage in response to stress factors. A study carried out on lambs fed with a diet supplemented with 7.5% OMWW in the post-lactation period showed that the addiction of OMWW to the diet improved the redox status and was paralleled by an increase in the growth rate (Makri et al. 2018). It is likely that ROS generation during oxidative stress oxidizes and damages

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the cellular biological molecules and the intestinal membrane integrity, with consequent impairment of nutrient absorption. Such a situation is counteracted by the addition of polyphenols, whose antioxidant properties result in ROS scavenging, reducing intestinal membrane damage, with consequent improvement of gut functionality and nutrient absorbance. The increase in growth performance parameters is a common trait in farmed animals treated with polyphenols and phytochemicals in general, as long as the concentration is adequate. Too little concentration does not elicit any effect, but when the concentration is too high adverse effects can occur. Depending on concentration and source, polyphenols can behave either as antioxidants (in dietary physiological doses) or as pro-oxidants. Indeed, animal studies show that high-dose polyphenol supplements may cause kidney damage, tumors, and an imbalance in thyroid hormone levels (Granato et al. 2020).

5.3.1.2

Anti-inflammatory Activity of OMWW

Several studies in the last decades report that the therapeutic effects of phytochemicals are not exclusively linked to the antibacterial and antioxidant activities, but also to the immunomodulatory and anti-inflammatory activities. The antioxidant activity of OMWW polyphenols has been widely demonstrated in farmed animals, while few studies analyzed the anti-inflammatory activity of these compounds. Evidence has emerged that oxidative stress plays a crucial role in the development and perpetuation of inflammation. Oxidative stress is directly linked with inflammation since ROS are activators of NF-κB, a key regulator of inflammation. The NF-κB protein complex is present in almost all animal cell types and its inactive state is bound to inhibitory proteins in the cytosol. ROS formation and many other stimuli such as cytokines, bacteria, viruses, and ultraviolet radiation, determine the release of the inhibitory proteins from NF-κB. This facilitates the translocation of active NF-κB into the nucleus where it induces the transcription of a large set of genes involved in inflammation (mainly cytokines and chemokines). NF-κB is involved in several farmed animal inflammatory diseases such as pneumonia, enteritis, sepsis, mastitis, pneumonia, and airways obstruction (Lykkesfeldt and Svendsen 2007). OMWW polyphenols are able to inhibit the production of various inflammatory molecules in vitro and in vivo. Studies on alveolar macrophages and leukocytes have shown an inhibition in the production of pro-inflammatory factors and the expression of genes such as COX-2 and iNOS. The administration of OMWW polyphenols in adult pigs has an anti-inflammatory effect on the intestinal mucosa (Varricchio et al. 2019). After 120 days of feeding, the authors report a significant increase in epithelial leukocytes in the lamina propria of stomach, duodenum, jejunum, ileus, cecum, and colon of the treated animals compared to controls. During the intestinal inflammation leukocytes are recruited to the site of infection or inflammation where they induce other immune cells to release mediators for the inflammatory response. It has been commonly accepted that leukocytes directly contribute to disease pathology when excessive recruitment and activation led to the release of toxic products and

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massive trans-epithelial migration, eventually resulting in morphological alterations of villi and crypts and extensive mucosal injury (Fournier and Parkos 2012). It is apparent that leukocytes can act as double-edged sword in that contribute to intestinal homeostasis through the elimination of unwanted pathogens, participate in harmful inflammatory processes, and exacerbate the inflammation owing to the release of toxic granule contents and pro-inflammatory molecules. In rabbit OMWW polyphenols act as anti-inflammatory, metabolic, and apoptotic effectors on gene expression in the liver (Cappelli et al. 2021). NF-κB regulates the expression of various effectors involved in inflammation such as TNF-α, advanced glycosylation end-product specific receptor (AGER), and BCL2 (B-cell lymphoma 2) associated X apoptosis regulator (BAX) genes. After 60 days of feeding, these genes involved in apoptosis and proinflammatory pathway, were downregulated in the group fed with OMWW polyphenols. The data demonstrate the beneficial effect of OMWW polyphenols in the diet as they regulate oxidative stress and liver inflammation, also affecting the defense system and apoptosis. Liver also showed a reduction of Sirtuins (SIRT), the DNA-dependent type-class III histone deacetylases, in the group fed with OMWW polyphenols. SIRT have a catalytic activity regulated by the metabolic state of the cells. In general, SIRT are involved in lifespan and metabolism regulation by deacetylating the histones, thus making the link between these proteins and the DNA tighter with consequent impeding the transcription of some transcriptional regulator genes including NF-κB. New studies are required regarding the use of OMWW polyphenols for the nutrition of farmed animals considering the positive effects both in animal welfare and in the quality of food products.

5.3.1.3

OMWW Bioactive Molecules and Final Quality Products

OMWW polyphenols influence not only the well-being of the animals, but also the quality of the products that derive from them (meat, milk and derivates). Animal meat is an essential part of food and the primary protein source for the human population. In recent years, there has been an increased global demand for meat products. A set of parameters, attributes, and characteristics are used to define the suitability for consumption of fresh or stored meat and determine the meat quality. The meat industry development and the increasing demand for high-quality meat have brought about new challenges, including the efficient assessment of meat quality and new sustainable approaches to improve it. During meat storage, several processes may occur, affecting the meat quality parameters such as color, freshness, and odor, which impact on the consumer acceptability. In this regard, lipid and protein oxidation results in the production of several off-flavors. Lipid peroxidation is one of the primary causes leading to meat’s quality deterioration, while it may also result in the production of toxic compounds. Besides, during lipid oxidation there may be an increased risk of spoilage by different microorganisms. Meat oxidation is a natural process occurring postmortem. It determines the deterioration of lipid and proteins in smaller molecules. Lipid peroxidation and

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protein oxidation are interrelated in meat and probably the first induces the other. The byproducts obtained by lipid deterioration are hydroperoxides, prostaglandins, carbonyls, and malondialdehyde (MDA) (Rocchetti et al. 2020). On the other hand, protein oxidation causes the production of free amino acids that influence the meat organoleptic properties. In particular, poultry meat during the storage is subjected to protein deterioration by microorganisms and enzymes. Such degrading processes affect the meat stability and quality during the storage (Wen et al 2020). The use of polyphenols can reduce the oxidation reactions and improve the shelf life of meat products. Furthermore, the use of polyphenols could replace chemical preservatives which are carcinogenic. At the same time, their use as bio-preservatives could reduce the production of environmental pollutants by agri-food companies and open new perspectives for the packaging industry. A reduced content of lipids and protein oxidation products determines a food product that is certainly richer in proteins and healthier for the consumer. Studies have shown that ROS-induced oxidation of muscle proteins results in the loss of essential amino acids and affects water-holding capacity of meat proteins, color and texture of processed meat products, and digestibility, leading to the reduction of meat nutritional values (Lund et al 2011). Oxidation of porcine myofibrillar proteins has been shown to reduce their gelation, which is important for the textural and structural characteristics of the meat products (Zhou et al. 2014). Moreover, a reduction in protein oxidation was found in piglets fed with a diet supplemented with OMWW polyphenols. Thanks to the protective effect of polyphenols on lipid peroxidation and protein oxidation in muscle, the use of OMWW polyphenols as feed supplement in animal diets can be regarded as a useful strategy to enhance meat quality. OMWW polyphenols can reduce lipid peroxidation resulting in an improvement of animal health and meat quality. In fact, the administration of OMWW polyphenols in lambs determines the decrease in lipid peroxidation in all tissues, especially in the muscle (Arshad et al. 2013). As reported, in monogastric animals the monomeric flavonoids and polyphenols are mainly absorbed in the small intestine, while it is not yet clear whether in ruminants they are absorbed by the rumen epithelium. In ruminants, plant polyphenols are hydrolyzed and bio-transformed by enzymatic activities of the rumen microbiota to obtain polyphenol metabolites and aglycones (phenolic compounds remaining after the glycosyl group on a glycoside is removed) that are then absorbed, at least in part, through the rumen epithelium. The remaining unabsorbed polyphenols pass into the small intestine where they are metabolized as in monogastric animals. Nutritional addition of polyphenols in ruminants seems to preserve the PUFA, rumenic acid and vaccenic acid from the complete biohydrogenation, resulting in an enrichment of healthy fatty acids in meat and milk, at the expenses of saturated acids fat. Studies on the effects of OMWW polyphenols addition to a typical sheep diet have shown that polyphenols can modify the chemical characteristics, antioxidant capacity, oxidative status, and sensory properties of milk products. A study carried out on dairy sheep speculates that the addiction of polyphenols from OMWW could improve milk nutritional quality and its by-products (Branciari et al. 2020). However, regarding milk

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production, in literature there are conflicting results, as in most cases, no effect was reported independently from the type of polyphenol source in dairy cow and sheep.

5.3.2 OMWW Bioactive Molecules: Aquacultural Potential According to the latest Food and Agriculture Organization (FAO) reports, aquaculture is the fastest growing food producing sector, with an average annual increase of about 6% (FAO Report 2018, 2019, 2020). The need to increase the supply and the production of both salt and fresh-water fish has led to the use of intensive and super intensive systems in aquaculture, that however, impose environmental conditions extremely stressful to the aquatic species. If on the one hand, the intensive farming conditions are useful to obtain a higher production in line with the growing demand of seafood, on the other hand. The increase of stock density and organic matter coming from the formulated diets and fish excretions spoils water quality and imposes stressful conditions to fish, accelerating the onset of diseases and infections, and, consequently, leading to heavy economic losses. Thus, diseases and infections are easily and quickly spread among fish in intensive farming conditions and often the preventive use of antibiotics and medications is a necessary practice to avoid losses and economic damage. To prevent bacterial infections and improve fish immune systems, antibiotics and synthetic chemicals have been commonly used as chemoadditives in aquafeed for many years, however, their extensive and improper use has caused resistance against pathogenic microorganisms, negatively affecting the development and the sustainability of aquaculture (Watts et al. 2017) and posing a possible threat to human health. Since the ban or at least the reduction of the use of chemicals in animal farming, including aquaculture (2015/C 299/04 h), there has been a growing scientific interest to find and evaluate safe, natural, and potentially beneficial substances with health promoting effects on farmed fish. The use of natural feed additives, such as polyphenols extracted from plants and agri-food chain byproducts, has been applied to aquaculture activity with positive effects on fish health (Leyva-López et al. 2020). A possible strategy to preserve fish health is to intervene on the feeding of farmed fish in intensive culture systems, considering that the composition of the diet can influence the growth rate, immune response, disease resistance and health of aquatic animals. In recent years, great attention has been paid on the inclusion of natural feed additives, such as bioactive molecules extracted and recovered from plants and agri-food by-products (Leyva-López et al. 2020). In fact, a large body of literature demonstrates that a wide range of natural active compounds such as polyphenols, terpenoids, alkaloids, and pigments, have antioxidant, antimicrobial, and immunomodulatory properties, making them effective functional feed ingredients in farmed aquatic species. Fish feed accounts for about 40–60% of the total cost of aquaculture operations, consequently it is necessary to look for new low-cost feed additives to decrease the overall cost of fish production. In this regard, agrifood processing residues can be considered as a largely available low-cost source of

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bioactive compounds, offering a range of opportunities for eco-friendly and sustainable production of functional feeds. Nutritional manipulation is a reliable method to increase fish health and among dietary supplements the bioactive molecules extracted from OMWW have gained great attention due to their natural origin and wide range of positive effects. Into freshwater systems, OMWW is harmful and deadly to both plants and aquatic organisms because reduces the transparency of natural water, prevents plants from receiving adequate light, and causes the deoxygenation of the waters. Moreover, the presence of simple sugars in OMWW increases the number of microorganisms, leading to oxygen consumption increase with consequent alteration of the ecosystem. Considering that millions of tons of OMWW are disposed without any treatment every year, it is obvious that OMWW is becoming a potential environmental issue due to the polluting effects on the quality of soil, surface, and groundwater, representing a threat for freshwater and marine organisms (Pavlidou et al. 2014; Leris et al 2019). In this regard, numerous authors have shown that untreated OMWW added to water is highly toxic to fish and other aquatic species. Barakat and Saad (2020) report that different concentrations of untreated-OMWW (2.5, 5 and 10%) provoke acute toxicity in common carp (Cyprinus carpio), leading to an increase of the mortality rate. Moreover, Koca and Koca (2016) report genotoxic and histopathological effects of OMWW in Lepomis gibbosus, with severe damage of gills, liver, and muscle tissue. The results of the study of Babi´c et al. (2019) underline the negative and toxic impact of crude OMWW on different model organisms at different levels of biological complexity, including bacteria, algae, plants, crustaceans, and fish embryos. Considering the detrimental effects on aquatic species, to prevent damages to the aquatic organisms and the environment, OMWW should not be released untreated into water systems. However, as reported above, it has been demonstrated that the compounds contained in OMWW, such as polyphenols, mainly represented by HTyr, Tyr and Ole, have antioxidant, anti-inflammatory and antimicrobial activities. Several studies demonstrate beneficial effects of olive by-products and/or extracts on in vitro intestinal metabolism of rainbow trout (Coccia et al. 2019), and health and growth in different farmed aquatic species (Parrillo et al. 2017; Hoseinifar et al. 2020a; Van Doan et al. 2020; Safari et al. 2020; Jahazi et al. 2020; Ahmadi et al. 2022). In this frame, the reutilization of OMWW polyphenols as functional feed ingredients in farmed animal nutrition has received considerable attention, while only limited information is available on their use in farmed fish feeding. Few available literature studies evaluated the effects of dietary supplementation of OMWW in aquaculture and, based on what is reported, it is possible to state that the resulting effects change according to the diet manufacturing, whether OMWW is used raw or as an extract, the doses employed, and the time of administration.

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Effect of OMWW Bioactive Molecules on Fish Growth Performance

Polyphenols are a very heterogeneous category of substances containing different chemical classes such as phenolic acids, flavonoids, tannins, lignans, stilbesterols. Although the overall action of polyphenols is essentially antioxidant, they also act as antimicrobial agents and stimulate the innate and acquired immunity of fish. Polyphenols indeed, modulate the functions of immune cells, such as T cells, B cells, NK cells, and macrophages by increasing the production of cytokines and antibodies, and the expression of genes related to the immune system (Mileo et al. 2019). The antioxidant and immunostimulating properties of polyphenols promote health, lead to disease resistance and faster growth in fish. However, the in vivo mechanisms of action of polyphenols are much more complex and it has been shown that polyphenols perform activities and roles independent of the actions on the redox state or inflammation, interacting with cellular functions at different levels (Ahmadifar et al. 2021). In fact, polyphenols can exert various health benefits such as chelating heavy metals, providing cofactors and inhibitors of enzymatic reactions, absorbing, and neutralizing toxic substances by eliminating them, improving the absorption and/or stability of essential nutrients (Holst and Williamson 2008). One of the issues affecting the aquaculture sector is related to intensive farming, now a very common technique in much of the world. Intensive farming is very convenient from an economic point of view but has negative effects on fish health due to the high density, poor water quality, and the need to use formulated feed, often rich in vegetable-derived nutrients such as soybean, the main cause of intestinal inflammation, thus negatively affecting the state of fish health and aquaculture profitability (Bravo-Tello et al. 2017). Intensive farming brings about a generalized stress condition that favors the onset of diseases, infections transmitted by pathogens and intestinal inflammation resulting in poor growth. Gut health plays a vital role in digestive and absorption functions of the digestive tract, thus exerting a significant effect on fish nutrition and growth. The use of polyphenols as dietary additives has been proved to be an important tool for the partial resolution of intestinal inflammation (Orso et al. 2021). The administration of diets enriched with phytochemicals is characterized by an increase in fish growth performance, although the dose administered is crucial and varies with the fish species, as well as the type of phytochemical used, if it is a raw material or an extract. Sometimes the use of raw material can cause growth inhibition, due to the presence of anti-nutritional molecules and aromatic molecules, such as hydroxylated-benzoic and phenyl propenoic acid, which can interact with the absorption and digestion processes of proteins, resulting in anti-nutritional effects (Gatlin et al. 2007). Growth inhibition of gilthead sea bream (Sparus aurata) and rainbow trout (Onchorynchus mykiss) has been reported after administration of diets containing 1 and 5% of crude OMWW (Sicuro et al. 2010a, b). However, no inflammatory processes and histological modifications in the digestive organs were recorded among the fish experimental groups. Moreover, the OMWW inclusion on rainbow trout fish diets did not negatively affect the fish digestive processes, as evidenced by the absence of alterations of the digestive enzymes such as lipase, amylase, and total alkaline protease. Considering that

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in fish the intestinal health is pivotal for growth, general health, and ability to resist infections and environmental stressors, and that the enzymatic profile appears to be a good physiological indicator of feed effects, the results of Sicuro et al. (2010c) allow to exclude that OMWW can have damaging effects both structural and physiological levels of fish intestine, but the amount employed is crucial for proper growth. Similarly, Ighwela (2015) report that there were no differences in growth performance parameters between the experimental and control fish Nile tilapia (Oreochromis niloticus) fingerlings fed with diets enriched with solid matter extracted from OMWW. Moreover, these results are in agreement with the findings of Nasser et al. (2011), which evaluated the development of high-performance artificial compounds for aquafeed to be used for efficient growth in intensive production systems. Nasser et al. (2011) study indicates that the substitution of wheat bran with less than 25% of olive waste in experimental tilapia diet did not induce any significant differences in the growth performances compared to the control group, while a progressive decrease in the weight gain and growth performances were observed in fish fed with higher levels of olive waste. Probably, with the increase of OMWW dietary concentration there is a higher content of crude fiber that can affect the digestibility and reduce the efficiency of feed utilization, causing the reduction in growth performance. In light of what has been reported, it is evident that it is necessary to establish the optimal concentration of polyphenolic extract for each farmed species, as well as the type of extract. Furthermore, the standardization of the extract is very complicated due to the high number of bioactive molecules present. Many authors study the phenolic content using HPLC, considered the most reliable analytical technique, however, this technique is not available in many labs for routine analysis, due to the instrumental costs and the running expenses. Spectrophotometric methods are more affordable with little expenses, low reagent consumption, and rapid measurement, although the precision is limited. The most commonly used methods are the Folin–Ciocalteu for the determination of the total phenolics, the colorimetric assay with aluminium chloride for the total flavonoid content, and the pH differential method according to AOAC (2007) for the determination of the total anthocyanins. However, even if affordable, these assays are not widespread and most of the experiments conducted on farmed aquatic species do not report any indications in this regard. Recently, some experiments conducted on different fish species treated with polyphenols extracted from OMWW and chestnut, evaluating the amount of polyphenols employed, indicated that the optimal percentage of polyphenols ranged between 1 and 2% (Jahazi et al. 2020; Van Doan et al. 2020; Hoseinifar et al. 2020a; Safari et al. 2020; Ahmadi et al. 2022). Similar results were obtained in the narrow clawed Astacus leptodactylus fed with OMWW polyphenols for 24 weeks (Parrillo et al. 2017), where the best growth performances and nutritional indices were obtained with 0.5% polyphenols. With all limitations due to the small number of experimental studies, the emerging picture indicates that diets enriched with polyphenols extracted from OMWW have beneficial effects on fish growth performance and hence could be successfully

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employed in aquaculture and considered as a novel strategy of development to the feed industry sector. However, a better valorization and extraction of polyphenols is necessary to make the OMWW a potential natural feed additive in farmed fish.

5.3.2.2

OMWW Bioactive Compounds and Fish Antioxidant Defense

Oxidative stress is considered an important biomarker in modern aquaculture, being the antioxidant system vital and protective for the health and wellbeing of fish. In fish, the first antioxidant enzyme that protects the cells against hydrogen peroxide damage is CAT, which, together with peroxidase, convert superoxide ion into water. Therefore, an enhancement in the activity of CAT is regarded as a stimulation of antioxidant defense. In the last few years, polyphenols of OMWW have been studied for their antioxidant properties in aquatic species. Parrillo et al. (2017) evaluated the activities of the oxidative enzymes, including CAT, GR and GPx, in the hepatopancreas of narrow clawed Astacus leptodactylus fed with polyphenols OMWW-enriched diets, showing that the feed additive used positively affected the oxidative status enhancing the activity of these enzymes. Also, Jahazi et al. (2020) report that polyphenols from chestnut and OMWW have strong antioxidant activity in common carp, increasing the serum CAT and peroxidase activities in fish fed with supplemented diet. Interestingly, polyphenols present in OMWW are potent inducers of the antioxidant defense also in ornamental fish convict cichlid (Amatitlania nigrofasciata) by increasing the activation of stress enzymes such as CAT (Hoseinifar et al. 2020b). Considering what has been reported, if on the one hand the phenolic compounds of OMWW are the most responsible for their polluting load, on the other hand they are characterized by a strong antioxidant activity (Long et al. 2017). These results lead to the possibility to regard OMWW as functional feed additives in aquaculture to protect fish against oxidative stress.

5.3.2.3

OMW Bioactive Compounds and Fish Immune System and Infection Resistance

The fish immune system shares many similarities with higher vertebrates, including both innate and adaptive components (Brugman 2016). Innate immunity (or nonspecific, or constitutive, or natural) represents the first line of defense against pathogens such as viruses and bacteria and is an essential defense mechanism for fish survival and infection resistance. It is the first response to pathogens and does not retain memory of previous exposures to infectious agents. Innate immunity is divided into physical barriers, cellular and humoral components. Physical barriers include the skin, gills, and the intestinal mucosa. The skin mucus contains numerous components with high antimicrobial activity, such as lysozyme, complement proteins, antibacterial peptides, and immunoglobulins M (IgM) that inhibit the entry of pathogens. The intestinal mucosa plays a fundamental role in fish that are exposed to the external

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environment (water) much more than terrestrial animals. In fact, the water in fish penetrates through all pores, including the anal one, deeply influencing the health status. The cellular component acts through the mechanisms of endocytosis and phagocytosis to capture and destroy extracellular material. Lymphocytes, granulocytes, basophils, macrophages, and dendritic cells are part of the innate immune system of fish. These cells have different functioning mechanisms, but they are all capable of identifying and eliminating pathogens. The humoral component is mediated by chemical signals (complement system, lysozyme, antimicrobial peptides, cytokines, etc.) produced by cells and released into the extracellular environment following infection by a pathogen. Innate immunity is activated when the organism comes into contact with a virus or bacterium, before the more specific response, typical of acquired immunity (also adaptive or specific), can develop. Acquired immunity is able to recognize a certain foreign agent and remember the pathogens or vaccines with which it comes into contact. It acts through cells (lymphocytes) or molecules (antibodies). However, it is worth remembering that innate and acquired immunity although taking different tasks work in synergy. Fish cellular and humoral immune components include lytic enzymes, the complement pathways, antibodies, or immunoglobulins (Ig), cytokines, chemokines, and antibacterial peptides. The acquired immune response, in fish is characterized by some limitations with respect to mammals, such as little repertoire of antibodies and slow proliferation and maturation of lymphocytes (Magnadottir 2010). Since the enhancement of the immune response of fish farmed under intensive conditions seems to be an advantageous strategy to control the spreading of diseases and infections and the consequent mortality, the scientific world is looking for natural alternatives to stimulate the immune system of farmed fish species. Diet integration with bioactive natural molecules capable of stimulating the immune system seems to be a strategic move in fish, where vaccination is a cost-effective practice that limits the development of commercial vaccines. Moreover, fish generally need an antigen dose larger than terrestrial animals and therefore inactivated viral vaccines are difficult to develop. Some fish species are quite vulnerable, and the handling practice unavoidable during the vaccination is highly stressful and prone to develop severe side effects post vaccination. In other species, larval or fry stages, which have not yet developed a functional immune system, and which have dimensions that do not yet allow vaccination, can more easily manifest disease problems. Furthermore, it appears that such fish cannot be protected by vaccinating the mother as they apparently lack maternal immunity (Ma et al. 2019). While the employment of natural bioactive substances, among which polyphenols, is the focus of many research programs aiming at the improvement of health and immune system in farmed fish, the employment of OMWW polyphenols appears to be quite limited. However, the evidence points at the efficacy of OMWW polyphenols alone or in combination, to boost the immune response in fish.

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Recent studies have shown that a dietary mix of chestnut wood and OMWW significantly stimulated immune functions increasing the serum and skin mucus total Ig, and lysozyme activity in common carp, convict cichlid, Nile tilapia, beluga sturgeon (Huso huso), and Asian sea bass (Lates calcifer) (Jahazi et al. 2020; Van Doan et al. 2020; Hoseinifar et al. 2020a; Safari et al. 2020; Ahmadi et al. 2022). Besides ameliorating the immune status, OMWW-enriched diets affect the microbiota, significantly stimulating the growth of anaerobic bacteria at the expense of other microbial groups, except for yeasts, and hence playing an important role in preventing disease outbreaks. In particular, the most abundant phenolic compounds present in OMWW (HTyr, Tyr and Ole) have shown potent antimicrobial and cytotoxic activity against various strains of bacteria responsible for intestinal and respiratory infections (Cicerale et al. 2010). In Astacus leptodactylus, OMWW polyphenols-enriched diets enhance the immune status, increasing the phenoloxidase activity and total hemocytes. In Crustacean the hemocytes play a key role in the immune response (i.e.: recognition, phagocytosis and melanization) by releasing numerous molecules including antimicrobial peptides and enzymes such as phenoloxidase after pathogen stimulation (Lee and Söderhäll 2002), thus acting as important components for increased resistance against the infection. In conclusion, the feed inclusion of OMWW could be a natural strategy capable of conferring disease resistance enhancing the immune status of farmed species, in agreement with the immunostimulant, antimicrobial, and microbiota regulator effects of OMWW polyphenols.

5.3.2.4

OMWW Bioactive Molecules and Final Quality of the Fish Products

Fish are among the most perishable food commodities. One of the main reasons of fish quick decline of quality resides in the fact that fish muscle contains relatively low concentrations of glycogen and, consequently, post-mortem pH values are high, facilitating microbial growth. On the contrary, in the muscle of terrestrial animals, the degradation is caused by endogenous enzymes, with the demolition of amino acids, while bacterial activity establishes only after death. At first, bacterial activity leads to a decomposition phenomenon which makes the fish smell like “fruit”. Thereafter, with the degradation of nitrogenous compounds, amino acids, fats and sugars, unpleasant odors develop due to the presence of new chemical molecules, such as trimethylamine, ammonia and hydrogen sulphide responsible for the repellent odor called “Fecaloid-ammoniacal”. Along with the main causes of the deterioration of fish products, that is microbial growth and production of catabolic products, the post-mortem metabolism leads to the production of biogenic amines, including putrescine, histamine and cadaverine, responsible for unpleasant flavors. Histamine is responsible for foodborne intoxications, although the other biogenic amines play a synergistic and enhancing role. In general, the bacteria responsible

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for the production of high levels of histamine are Morganella morganii, Klebsiella pneumoniae and Hafnia alvei. Another phenomenon of particular importance during the conservation of fish is lipid oxidation. Fish muscle is in fact characterized by the presence of long-chain polyunsaturated fatty acids (C20 and C22), that are much more susceptible to autooxidation phenomena than monounsaturated and saturated ones. Lipid oxidation leads to the production of undesirable off-odors and flavors, and this makes the fish muscle particularly prone to spoilage. Since the seafood product demand has been increasing around the world, seeking methods that help preserve quality and duration (shelf-life) is a top priority of the sector. The traditional methods of fish preservation are freezing, canning, smoking, and pickling. More recent technologies have been finding application such as vacuum packaging or modified atmosphere packaging, although not always applicable especially in less developed countries. The addition of edible, non-toxic natural substances, alone or dispersed in an edible film or coating, seem to hold a possible future, due to the simple technological procedures required (Volpe et al 2015a, b, 2019). In the study conducted by Kuley et al. (2017) the in vitro antimicrobial activities of crude OMWW on foodborne pathogens and fish spoilage bacteria were evaluated in order to determine the potential use of this olive by-products in the shelf-life and preservation of fish products. The results suggest that OMWW might be used as a source of bioactive compounds to control pathogenic bacteria such as Streptococcus pyogenes, Staphylococcus aureus, Escherichia coil, and Klebsiella pneumoniae, which represent the primary causes of human foodborne diseases. Therefore, OMWW can be considered as a good food additive to improve fish products safety. Sicuro et al. (2010b) investigated the effect of crude OMWW enriched diet in the fillets of gilthead sea bream (Sparua aurata L.) and rainbow trout. The results were encouraging, showing that OMWW could confer a ‘freshness’ odor to the flesh, while slightly darkened the color. Moreover, OMWW polyphenols administered in in vivo feeding trials ensure the organoleptic and nutritional improvement of the quality of the goldfish (Carassius auratus) fillet (Alesci et al. 2014). The results point at the possible utilization of OMWW as natural antioxidant in fish feed with positive effects on fillet quality and as a preservative to increase the shelf-life. Both strategies could represent promising directions for future aquaculture.

5.4 Conclusions and Future Perspectives “The future of Food and agriculture: trends and challenges” states that the reuse of agri-food chain by-products is crucial to both trigger and sustain the virtuous circle of the sustainable economy reducing what has been perceived so far as the inevitable food waste along the food chain (FAO 2017). Most of the agri-food chain waste is rich in phytochemicals, such as polyphenols, well known for their antibacterial, antifungal, antioxidant, and anti-inflammatory

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properties on in vitro cell lines and in vivo animal models. The reuse and valorization of OMWW polyphenols are eco-friendly, economic, and useful strategies for several applications, such as animal farming, representing an optimal feed additive with numerous benefits, from health-promoting to improving the quality of the final animal products, and their shelf-life. Therefore, OMWW can be considered a waste with considerable environmental implications, characterized by high value components, whose recovery represents a valuable opportunity for environmental sustainability and a key point of action of the circular economy. However, the composition of OMWW bioactive molecules changes according to the method of production, isolation, and purification, that affect the biological properties. Therefore, the standardization of effective techniques for the isolation and purification of the bioactive compounds is crucial for the development of feed additives with replicable properties. Moreover, more studies are needed to investigate the molecular mechanisms of OMWW polyphenols, in order to optimize the dosage and unveil the cell target.

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Chapter 6

Physicochemical Characterization and Estimation of the Pollution Degree of Olive Oil Mill Wastewaters from the Cold Extraction System and the Traditional System Zakia Gueboudji and Kenza Kadi Abstract Olive oil production is widespread in Mediterranean countries such as Algeria. One of the main by-products from olive oil extraction is olive oil mill wastewater (OMW). It considers as an environmental problem for the producing countries. This study aimed to compare the physicochemical characterization and the pollution degree of OMW obtained from two different extraction systems, cold extraction system and traditional system, in Khenchela, eastern Algeria. The results of the physicochemical analyzes have shown that olive mill wastewaters from the two systems were very acidic and very rich in organic and mineral matter. There was a significant effect on electrical conductivity (EC), biological oxygen demand (BOD5 ), chemical oxygen demand (COD), fatty matter (FM), organic matter (OM), and polyphenols (PP). While there was no significant difference for the parameters pH, humidity (H%), dry matter (DM), total suspended solids (TSS%), and mineral matter (MM). For the two pollution indices studied, BOD5 /COD and BI (biodegradability index), there was not a significant effect. It was recorded approximately similar values. In conclusion, the cold extraction system was the least polluting compared to the traditional extraction system. In addition, the pollution indicators clearly demonstrated the biodegradable nature of these wastewaters, for which biological remediation is appropriate. Keywords Cold extraction system · Olive oil · Olive oil mill wastewater · Physicochemical characterization · Pollution · Traditional system

Z. Gueboudji (B) · K. Kadi Biotechnology, Water, Environment and Health Laboratory, Department of Molecular and Cellular Biology, Faculty of Nature and Life Sciences, Abbes Laghrour University, BP 1252 Road of Batna Khenchela, 40004 Khenchela, Algeria e-mail: [email protected]; [email protected] K. Kadi e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_6

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6.1 Introduction Mediterranean countries have become the largest consumers, producers, and exporters of olives and olive oil in the world. The olive oil production of the countries of the Mediterranean basin represents approximately 98% of the world’s production. The production is concentrated in Spain, followed by Italy, Greece, Turkey, Tunisia, Morocco, Portugal and Algeria (Manzanares et al. 2020). Nowadays, with the promotion of the beneficial virtues of olive oil on human health, Olive oil production has risen in recent decades as a great source of antioxidants and vital fatty acids in the human diet, and it is now one of the world’s most powerful diet trends (Souilem et al. 2017). The demand of olive oil does not cease increasing and consequently the production increases. The olive industry generates, in addition to oil as the main product, large amounts of solid by-products called olive mill pomace (OMP) and the other liquid called vegetable water or olive oil mill wastewater (OMW) (Gueboudji et al. 2021a). The world production of olive oil mill wastewater represents a volume of more than 30 million m3 /year (Nunes et al. 2018). For the production of olive oil, the fruits are harvested from the tree from the month of November, crushed and then kneaded to increase the yield of released oil. The extraction of the oil from the olive paste can be carried out according to different processes: (i) batch press, (ii) threephase continuous and (iii) two-phase continuous (Klen and Vodopivec 2012). Three different products and by-products are generated for the first two processes (olive oil, pomace and vegetable water) and only two (olive oil and wet pomace) with the two-phase system. Even if they are less environmentally friendly and generate large quantities of olive oil mill wastewater, traditional pressing processes and three-phase centrifugation processes are still used in Algerian oil mills (Yakhlef 2019). Olive oil mill wastewaters or vegetable waters are water from vegetation that is generated during the extraction of virgin olive oil. These are effluents rich in organic matter (phenolic compounds, lipids, sugars, proteins, etc.) and in mineral salts (potassium, sodium, magnesium, etc.). These vegetable waters are often spread uncontrollably on agricultural soils or stored in evaporation ponds near oil mills, thus exposing water–soil–plant systems to inevitable pollution. The physicochemical and biological treatments of olive oil mill wastewaters, which consist in reducing their impact on water resources, are still insufficient and costly (La Scalia et al. 2017; Cedolaa et al. 2020; Gueboudji et al. 2022c). Algeria, a country with approximately 6.2 × 106 olive trees spread over an area of 471,657 ha according to provisional figures from the Directorate for the Regulation of Agricultural Production, is among the countries of the Mediterranean basin where the Olivier finds its area of extension. It is the ninth olive oil producer country in the world with a production of 80,000 tonnes in 2017/2018 of Mediterranean production. Thus, like all Mediterranean olive oil-producing countries, Algeria is faced with the problem of eliminating OMW with a production of 200,000 tonnes of OMW per year. In order to reduce the costs of the various treatments applied to OMW and to rationalize the management of their waste, research is focused on their recovery in various fields: composting, agriculture,

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cosmetics and even in the pharmaceutical industry (Senani-Oularbi 2018; Gueboudji 2022). The objective of this study is to compare the physicochemical characterization and pollution degree of OMW obtained from two different extraction systems, which was the cold extraction system and the traditional system, in Khenchela, east of Algeria.

6.2 Olive Oil Extraction Processes The processing of olives for extraction of oil can be done mechanically (by pressure or by centrifugation). Various extraction systems are used to extract olive oil (Baccioni and Peri 2014).

6.2.1 Basic Process or Press System (Most Common System) Press system is the most common and well-known traditional olive oil extraction system in ancient times. Milling takes place with granite stone stones, which are converted into a tank whose floor is also made of stone. The grinders used for grinding are then slightly unbalanced with respect to the axis of rotation. So they slide a little on the surface when you turn, allowing you to knead the dough. The dough is obtained in about 30 min. The kneaded dough is placed in a thin 2 cm thick layer on a nylon fiberboard’s called scourt. They are stacked on top of each other around a center pin mounted on a small cart. The set is placed on a hydraulic grass piston that allows a dough pressure of 400 kg/cm2 . The liquid phase flows into the receiving tank. This operation takes 45 min. Finally, centrifuges allow the separation of olive oil from vegetable waters. The solid and liquid phases are separated by simple pressure, while the oil is separated from the plant water by natural decantation or centrifugation in vertical centrifuges. The productivity of the extract is 86–90% compared to the oil in the fruit (El-Abbassi et al. 2012; Ben-Hassine et al. 2013).

6.2.2 Continuous Process (Centrifugation System) This modern extraction design replaces the traditional pressing. It uses horizontal centrifuges called decanters, which improve the yield and productivity of the oil mills. There are two types: the two-phase system and the three-phase system.

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Three-Phase Centrifugation System

This system allows the separation of three substances: olive oil, press residues and vegetable water (Zbakh and El Abbassi 2012). It requires the addition of hot water to the olive paste from the kneading before entering the centrifugal edge. So the different steps are separated according to their density by means of high centrifugal forces in machines called horizontal centrifuges to separate solid–liquid phases or vertical centrifuges to separate liquid–liquid phases that rotate at high speeds of about 3500 rpm (El-Hajjouji 2007; Zbakh and El Abbassi 2012).

6.2.2.2

Two-Phase Centrifugal System

This two-phase system (oil and ore), also called the ecological system, allows you to produce olive oil without the need for water in the carafe. So that the latter separates the oil and pomace-water, mixture from the vegetation into a single phase of pasty consistency called wet vinegar or two-phase vinegar, which limits the production of vegetable water (Rahmanian et al. 2014).

6.2.3 Refining There is also another industrial process, which is refining. Refining is a relatively recent technology; it is the set of operations that transform crude oil into an industrial product by removing impurities that make it unfit for consumption. The oil contains desirable and useful elements (vitamins, unsaponifiable, etc.) and other undesirable elements such as (waxes, peroxide, free fatty acids, etc.). The purpose of refining is to maintain or improve organoleptic characteristics: (taste, neutral odor, clarity, light youthful color), nutritional characteristics and the stability of fatty substances in general. To do this, it implements several steps to eliminate, for example; waxes, free fatty acids, pigments, metallic traces, etc.), which are compounds that are harmful to the quality (Peri 2014).

6.3 Olive Oil Mill Wastewater (OMW) The olive oil extraction industries generate a large quantity of by-products and residues (pomace and wastewater) calling for specific management, in order to minimize or attenuate its nuisances, and thus enhance and exploit their wealth. The pomace represents the solid fraction, coming from the pulp and the stone of the olive. This waste is generally recycled in different areas, including composting, energy production, animal feed, etc. Olive oil mill wastewater represent the liquid fraction from vegetation water and water added to extraction processes. These waters

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remain difficult to treat, given their acidity and their high pollution load. Olive oil mill wastewater (OMW) is a reddish-brown colored liquid, which turns black, foul smelling, cloudy in appearance and has a specific smell of olive oil (Rahmanian et al. 2014; La Scalia et al. 2017).

6.4 Physicochemical Properties of OMW The amount and physicochemical properties of waste created are determined by the utilized oil extraction technology, processed fruits, and operation circumstances (added water, temperature, etc.). During the washing of the olives prior to milling, a minor quantity of solid waste (leaves and tiny twigs) is created. Nonetheless, these by-products do not provide a management challenge. The primary waste from threephase extraction systems and typical mills is OMW. It is made up of fruit vegetable water and water utilized at various phases of oil extraction. Furthermore, the threephase extraction techniques produce solid waste, which is utilized to extract olive kern. Table 6.1 summarizes the physicochemical properties of OMW as reported by various publications. The composition of OMW varies greatly depending on the olive type, the maturity of the fruit, the volume of added water, and the extraction technique (press or centrifuge). The typical weight content of OMW is 83–94% water, 4–16% organic molecules, and 0.4–2.5% mineral salts. Among other things, the organic fraction contains 2–15% phenolic compounds, which are divided into low molecular weight (tyrosol, hydroxytyrosol, p-coumaric acid, ferulic acid, syringic acid, protocatechuic acid, etc.), and high-molecular-weight compounds (tannins, anthocyanins, etc.) and catechol-melaninic polymers. It has a dark hue (due to lignin polymerization with phenolic chemicals), a high acidity (pH about 5), and strong electrical conductivity. The most recent characteristic varies according to the salt content of OMW, which is determined by the procedures utilized for olive fruit conservation before to milling (Souilem et al. 2017). Table 6.1 Physicochemical characteristics of olive mill wastewaters (Souilem et al. 2017) Parameters

Unit

Range of values

pH

–

4.8–5.7

Conductivity

mS/cm

5–81

COD

g/L

16.5–156

BOD

g/L

13.4–37.5

Dry residue

g/L

11.5–90

Lipids

g/L

7

Phenols

g/L

0.8–8.9

Sugars

g/L

1.3–4.3

Total nitrogen

g/L

0.06–0.9

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6.5 Comparison Between Physicochemical Characterizations of Olive Oil Mill Wastewaters from the Cold Extraction System and the Traditional System The treatment of wastewater is a thorny problem because it is rich in organic matter but also in pollutants and dangerous substances. Several parameters are indicators of the presence of pollutants in the water and their quantity. Among these parameters, four are very often used: pH, TSS, COD, and BOD5 . In this study, 13 characteristics are measured. OMW samples were taken from two different mill, traditional and cold extraction system located in Khenchela eastern Algeria. Potential of Hydrogen (pH) and electrical conductivity (EC) are measured directly in samples using pH meter and conductivity meter. Fatty matter (FM) was determined by the chloroform/methanol method. Total suspended solid (TSS) is obtained by filtration after centrifuging samples. Dry matter content (DM) and humidity (H%) was measured by drying at 105 °C for 24 h. Organic matter (OM) was calculated by the difference between the dry weight of the OMW and its weight after the calcination. Mineral matter (MM) was determined by weighing after ignition in a muffle furnace type (Nabertherm) at 550 °C, for 24 h. The chemical oxygen demand (COD) was determined using potassium dichromate, as described by BOD5 (biological oxygen demand) is determined by the respirometric method. Analyzes were carried out in triplicate (Gueboudji 2022). Polyphenols (PP) were determined by the reagent method of Folin Ciocalteau (Müller et al. 2010), after an extraction with maceration according to the method described by Gueboudji et al. (2022d). BOD5 /COD is an indicator, which determines the degree of pollutants produced by liquid effluents. If, BOD/COD > 0.5, effluent treatable by biological processes. If, 0.2 < BOD/COD < 0.6, feasibility of treatment using selected microbial strains. If, BOD/COD < 0.2, biological treatment impossible (Radoux and Cadelli 1995). Biodegradability Index (BI) is found by calculating the COD/BOD5 ratio. It indicates the significance of pollutants with little or no biological degradation (Rodier 1996), if, BI > 6, hardly biodegradable substrate. If, 3 < BI < 6, partially (or less easily) biodegradable substrates. If, BI < 3, Very easily biodegradable substrate. The results obtained of physicochemical parameters are showed in Table 6.2. The results of the physicochemical analyzes have shown that olive mill wastewaters from the two systems were very acidic and very rich in organic and mineral matter. There was a significant effect on electrical conductivity (EC), biological oxygen demand (BOD5 ), chemical oxygen demand (COD), fatty matter (FM), organic matter (OM), and polyphenols (PP). While there was no significant difference for the parameters pH, humidity (H%), dry matter (DM), total suspended solids (TSS%), and mineral matter (MM). For the two pollution indices studied, BOD5 /COD and BI (biodegradability index), there was not a significant effect. It was recorded approximately similar values.

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Table 6.2 Physicochemical characterizations Parameters

Unit

pH

–

Traditional system 5.06 ± 0.02

Cold extraction system 5.11 ± 0.02

EC

mS/cm

4.10 ± 0.07

13.55 ± 0.05

FM

%

3.87 ± 0.10

1.98 ± 0.12

TSS

%

0.89 ± 0.03

0.98 ± 0.04

H%

%

94 ± 1.15

83.5 ± 1.02

DM

%

06 ± 0.22

16.5 ± 0.51

OM

g/L

13.05 ± 0.71

34.94 ± 1.25

MM

g/L

12.2 ± 0.08

24.6 ± 0.05

PP

g/L

7.3 ± 0.15

0.98 ± 0.09

BOD

g/L

51.1 ± 2.25

77.4 ± 4.15

COD

g/L

220 ± 4.00

250 ± 4.5

BOD5 /COD

–

0.23 ± 0.01

0.31 ± 0.01

BI

–

4.31 ± 0.03

3.23 ± 0.03

The comparative analysis of the physicochemical characteristics of the vegetable waters in the two different methods studied showed that most of the time, OMW are characterized by an acid pH (between 5.06 and 5.11), in due to the presence of organic acids (phenolic acids, fatty acids, etc.) (Zaier et al. 2017; Gueboudji et al. 2021b). Thus, the values recorded in our study are within the limit of the range quoted in the literature (4.2–5.9). However, the acidity of the vegetable waters is linked to the duration of their storage in the storage basins. This can also be explained by auto-oxidation and polymerization reactions that transform phenolic alcohols into phenolic acids. These reactions are manifested by a change in the initial coloring of the vegetable waters towards a very dark black color. Indeed, OMW samples studied are characterized by a very dark brown color. The mineral composition of the vegetable waters studied has shown that these wastewaters have a high saline load due particularly to sodium chlorides, probably linked to the salting practiced to preserve the olives until they are crushed, in addition to the natural richness of the olives in salts minerals. This could be explained by the high values of the electrical conductivity found in the various oil mills studied (between 4.10 and 13.55 mS/cm). The average contents in dry matter is of the order of 6 g/L and 16.50 g/L respectively. OMW studied are very rich in suspended solids, their average content is between 0.89 and 0.98 g/L. However, the average values recorded during this study were high, this could be explained by the fact that in the basins, the suspended solids of OMW drop under the effect of settling and this is probably due to the effect of the wind and/or the commotion caused when unloading the vegetable waters. In the same way, vegetable waters are very loaded with organic matter expressed in terms of BOD5 and COD. Thus, according to Table 6.2, the values obtained can reach 51.1 g/L (BOD5 ), 220 g/L (COD) and 13.05 g/L organic matter in OMW obtained from the traditional extraction system. For the cold extraction system, it found 77.4 g/L (BOD5 ), 250 g/L

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(COD) and 34.94 g/L organic matter. This shows the high oxygen demand for the complete oxidation of the organic matter contained in these effluents, which reflects their very important polluting powers. Thus, this COD content is very high compared to that recorded in other types of discharges: These values are 200–400 times higher than those of municipal waters are. Indeed, COD does not exceed 4.02 g of O2 /L in slaughterhouse effluents, which are considered the main discharges of a dominant organic nature (Aissam 2003; Gueboudji et al. 2022a). The biodegradability index (BI) expressed by the COD/BOD5 ratio is important for the definition of the purification chain of an effluent. The results of this report are an indication of the importance of pollutants with little or no biodegradability. Indeed, a low value of the COD/BOD5 ratio implies the presence of a large proportion of biodegradable materials and makes it possible to consider biological treatment. Conversely, a high value of this ratio indicates that a large part of the organic matter is not biodegradable and, in this case, it is preferable to consider a physicochemical treatment. Thus, the average values of the COD/BOD5 ratio of OMW studied is greater than 3, which is the limit threshold for biodegradability. Therefore, it can conclude that even if OMW studied have a high organic load, they are partially biodegradable. Examination of this report underlines the biodegradable nature of OMW of the two oil mills for which a biological treatment seems quite suitable. These discharges are also characterized by the predominance of toxic substances, in particular phenolic compounds. Values of 0.98 g/L in that of cold extraction system and can reach 7.3 g/L have been measured in OMW of traditional system. This high concentration could limit any natural biodegradation, and therefore could lead to a more or less profound disturbance of the entire ecosystem. Thus, phenolic compounds are very varied cyclic organic substances (Babi´c et al. 2019). According to Jail et al. (2010), their solubilization in oil is lower than that in vegetable water, which explains their high concentration in vegetable waters. According to Justino et al. (2012), these compounds have a variable structure, they come from the enzymatic hydrolysis of glucosides and esters from the olive pulp during the extraction process. According to the results, in OMW of traditional system, the highest concentrations of phenolic compounds were noted compared to the cold extraction system, this could limit any natural biodegradation, and therefore could lead to more disturbance, or shallower of the entire ecosystem. This variability in the phenolic composition of OMW between the two extraction methods depends not only on the technological processes used to separate the phase aqueous (OMW) of the oily phase, but on the variety, the maturity of the fruit and the climatic conditions, but also on the duration of storage of OMW and (Gueboudji et al. 2022b).

6.6 Conclusion The discharge of OMW from olive oil producing industries is a major problem, especially for the countries of the Mediterranean basin, because they contain a large organic fraction and cause several types of pollution. The variability of the results

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obtained from the physicochemical properties of OMW samples is mainly due to a difference in the olive oil extraction process. They play an important role in the process of biological treatment of these effluents, the richness of their impact on the growth and development of microbes, as well as its treatment capacity. This study showed that the cold extraction system was the least polluting compared to the traditional extraction system, and made it possible to show that OMW studied have a high organic load and they are partially biodegradable according to the COD/BOD5 ratio and subsequently the biological treatment processes for these OMW are more effective than the physicochemical ones. Acknowledgements Many thanks to the leader editors Professor Salah Souabi and Anouzla Abdelkader, Hassan II University Casablanca, Faculty of Sciences and Techniques of Mohammedia, Morocco, for the suggestion to contribute to this book.

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Gueboudji Z, Kadi K et al (2022d) Maceration and liquid–liquid extractions of phenolic compounds and antioxidants from Algerian olive oil mill wastewater. Environ Sci Pollut Res (ESPR) 1–9 Jail A, Boukhoubza F et al (2010) Traitement des effluents d’huileries par un procédé combinant un traitement intensif (Jet-Loop Reactor) suivi d’un traitement extensif (bassins de stabilisation). Environ Technol 31(5):533–543 Justino CI, Pereira R et al (2012) Olive oil mill wastewaters before and after treatment: a critical review from the ecotoxicological point of view. Ecotoxicology 21(2):615–629 Klen TJ, Vodopivec BM (2012) The fate of olive fruit phenols during commercial olive oil processing: traditional press versus continuous two- and three-phase centrifuge. LWT—Food Sci Technol 49(2):267–274 La Scalia G, Micale R et al (2017) A sustainable phenolic compound extraction system from olive oil mill wastewater. J Clean Prod 142:3782–3788 Manzanares P, Ballesteros I et al (2020) Processing of extracted olive oil pomace residue by hydrothermal or dilute acid pretreatment and enzymatic hydrolysis in a biorefinery context. Renew Energy 145:1235–1245 Müller L, Gnoyke S et al (2010) Antioxidant capacity and related parameters of different fruit formulations. LWT—Food Sci Technol 43(6):992–999 Nunes MA, Costa AS et al (2018) Olive pomace as a valuable source of bioactive compounds: a study regarding its lipid-and water-soluble components. Sci Total Environ 644:229–236 Peri C (2014) The olive oil refining process. In: Peri C (ed) The extra-virgin olive oil handbook. Wiley, pp 201–210 Radoux M, Cadelli D (1995) Epuration extensive des eaux usées des petites collectivités en zones rurales. Smmarcelli-Biguglia 7:45–52 Rahmanian N, Jafari S et al (2014) Recovery and removal of phenolic compounds from olive mill wastewater. J Am Oil Chem Soc 91(1):1–18 Rodier J (1996) L’analyse de l’eau naturelle, eaux résiduaires, eau de mer. Denod Paris 1:1383 Senani-Oularbi N (2018) Etude des activités anti-oxydante, antifongique et hypoglycémiante des margines d’olives et de leur extrait phénolique. Applications sur matrice alimentaire et sur modèle murin et cellulaire. Doctoral dissertation, Universite Mouloud Mammeri, Tizi Ouzou, Algérie Souilem S, El-Abbassi A et al (2017) Olive oil production sector: environmental effects and sustainability challenges. In: Olive mill waste. Academic, pp 1–28 Yakhlef W (2019) Caractérisation des profils phénoliques et évaluation de l’activité antibactérienne du contenu phénolique des margines monovariétales. Thèse de doctorat en science, spécialité microbiologie appliquée, Université Larbi Ben M’Hidi, Faculté des sciences exactes et sciences de la nature et de la vie, Oum El-Bouaghi, Algérie Zaier H, Chmingui W et al (2017) Physico-chemical and microbiological characterization of olive mill wastewater (OMW) of different regions of Tunisia (North, Sahel, South). J N Sci Agric Biotechnol 48(2):2897–2906 Zbakh H, El Abbassi A (2012) Potential use of olive mill wastewater in the preparation of functional beverages: a review. J Funct Foods 4(1):53–65

Chapter 7

Open Ponds for Effluent Storage, a Pertinent Issue to Olive Mill Wastewater (OMW) Management in a Circular Economy Context: Benefits and Environmental Impact Raja Jarboui, Salwa Magdich, and Emna Ammar Abstract The olive oil production is focused in the Mediterranean countries, accounting around 98% of total olive oil production worldwide. The olive mill wastewater (OMW), an olive oil processing by-product, is considered as one of the major polluting effluent in the world, produced in a relatively short period with high polluting load. Among OMW valorizing-treatments, the effluent storage in evaporation ponds is one of the practiced solutions reducing its environmental negative impact. Therefore, the aim of the present work was to highlight the OMW physicochemical and microbial characteristics when stored in evaporation ponds, and to investigate first its disposal management considering the natural evaporation process and the environmental factors effect’, the effluent constituents, their biodegradation, gas emission as well as its infiltration and soil impact. Secondly, the storage in open ponds was considered with different potential valorization/reuse of the storage effluent, providing specific awareness on circular economy operationalization in the agri-food system of olive oil supply chain. The OMW impact on soil at long term was discussed compared to a control non-contaminated basin. Linear polluting economy migration to a circular economy by OMW storage in open ponds with economical incomes through the recycling/reuse will be developed. Keywords Olive mill wastewater · Effluent management · Evaporation ponds · Physicochemical · Microbial characteristic · Soil impact · Circular economy

R. Jarboui · S. Magdich · E. Ammar (B) Laboratory of Environment Sciences and Sustainable Development, Preparatory Institute of Engineering Studies of Sfax, University of Sfax, Route Menzel Chaker km 0.5, 3018 Sfax, Tunisia e-mail: [email protected] R. Jarboui Department of Biology, Colleges of Sciences, Jouf University, Sakaka, Saudi Arabia Department of Biology, Faculty of Sciences of Sfax, University of Sfax, Sfax, Tunisia E. Ammar National Engineering School of Sfax, B.P. 1173, 3038 Sfax, Tunisia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_7

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7.1 Introduction In the academic world, the term “circular economy” (CE) becomes more frequently mentioned. Circular economy can be understood as “an idea and ideal” (Gregson et al. 2015) for facing the increasing limitations of Earth’s natural resources (Martin et al. 2017), in front of the limitations as a new transitioning path to production and consumption for more sustainable future (Cooper 2005; Stempfle et al. 2021). The industrial structure and the industrial policies reform must be adjusted to promote new technologies development in order to reach a solution by changing the waste recycling focus (Yuan et al. 2008; Tu et al. 2011; Khounani et al. 2021). Indeed, it is recognized as a sustainable alternative to the linear traditional model, contributing to sustainability, lowering resources consumed and reducing environmental pollution impact (Khounani et al. 2021; Qu et al. 2021). CE in the olive oil supply chain can play an important role in the agro-ecological systems of the Mediterranean region where the oil industry has an important economic, environmental as well as social impact, since these countries produce about 98% of the world olive-oil estimated to around 3.1 MTons in 2019–2020, with 93.6% produced by the International Olive Council member countries (Gullón et al. 2020). Therefore, the valorization and the recycling of this by-product can represent a successful strategy for the implementation of CE models in the agri-food industry. Olive oil production represents a major sector in Mediterranean countries, which despite of the economic benefits and the nutritional olive-oil attributes, generates many environmental problems because of the huge residual by-products amounts generated along the process used to extract the oil. These are olive cake (pomace), olive stones and high amount of liquid residue of olive mill wastewater (OMW), known also as Zebar in Middle East countries (Al Baweb et al. 2017; Khdair and Abu-Rumman 2020; Delgado et al. 2022). These residual materials include highadded value compounds and their extraction would present a bioactive sources of low cost, such as antioxidants, fibre, carbohydrates, pigments … (Rodrigues et al. 2015; Gullón et al. 2020; Khounani et al. 2021; Qu et al. 2021; Delgado et al. 2022). Actually, these bioresources could be valorized in food, therapeutic, cosmetic industries and for biofuels production. These residues are not only undesirable in terms of sustainability and environmental impact, but they also need high costs for management and disposal. Approximately 11 mille hectares of olive trees are cultivated in worldwide. Around 50% of these trees are cultivated in European Union countries. The annual olive oil production is around 3.32 million, with 72% produced in Europe (FAOSTAT 2020). According to the International Olive Council, Spain is the first olive oil producer with 42% of production; Italy is the second with17% and the third Greece with11%. Tunisia is the fourth olive oil producer in the world with 6% of world production (FAOSTAT 2020; Khdair and Abu-Rumman 2020; IOC 2021).

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The CE approach can contribute to reducing the amount of waste generated by olive oil industries, by valuing and recycling waste, and by shifting to more sustainable and efficient patterns of production and consumption (Jurgilevich et al. 2016; Khounani et al. 2021). This may contribute to the minimization of waste produced from olive mills (EMF 2013; Murray et al. 2017; Delgado et al. 2022). The management of olive wastes, especially that of OMW has always been challenging to find an effective strategy transferring the polluting effluent into sustainable materials. The OMW disposal in open evaporation ponds consists of a low coast and effective method reducing the environmental problems related to the important effluent volume generated annually. This CE approach could be a relevant management system by valorizing, recycling and reusing this effluent in order to its reutilization in crops fertilization, materials construction, and industrial applications (Bouaich et al. 2021). These issues could make the olive oil production process environmental friendly by transforming the production system to reach a closed zero waste system by recovering bioresources.

7.2 Olive Mill Wastewater: The Main Olive Oil Industry Polluting By-Product The olive oil world production is increasing annually by 5% (FAOSTAT 2019). The worldwide OMW production is estimated around 3 × 106 to 6 × 106 m3 with 98% produced in the Mediterranean basin (Kavvadiasa et al. 2010; FAOSTAT 2020; Kurtz et al. 2021). In Tunisia, a total of 600,000 tons of OMW is produced yearly, and can reach 1 Mm3 (Jarboui et al. 2010; Dahmen-Ben Moussa et al. 2020). It is estimated that one ton of olives may generate up to 1.6 m3 of OMW using a three-phase extraction system (Souilem et al. 2017; Antonio et al. 2020). OMW is carried out by three main extraction processes (Fig. 7.1): the traditional press-cake process, the three-phase decanter process, and the modern two-phase centrifugation process (Boskou 1996; Ammar et al. 2005; Hamimed et al. 2021). Nowadays, two-phase and three-phase centrifugation systems are the most widely used. These processes generate huge volume of OMW. The three-phase extraction and traditional systems discharge, respectively, 170% and 70% (Ammar et al. 2005; Roig et al. 2006; Souilem et al. 2017), whereas the two-phase extraction is distinguished by its lower wastewater volume production (McNamara et al. 2008; Morillo et al. 2009) (Fig. 7.2).

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Fig. 7.1 Olive oil processing production. Source the authors

Fig. 7.2 Schematic plan of the OMW evaporation ponds in Sfax (Tunisia), showing 16 storage ponds, 7 extended parts with 28 drying ponds (total storage capacity: 523 m3 )

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7.3 OMW: Physicochemical Composition and Characterization The OMW chemical properties strongly depend on olive fruits maturity, climatic, agronomic, and genetic factors (Ammar et al. 2005; Ghanbari et al. 2012; Zrnˇcevi´c 2018). Consequently, the OMW composition may vary according to the authors (Table 7.1). This by-product generated at large quantity contains generally 60–90% of water, 4–16% organic compounds, and 0.4–2.5% minerals (La Cara et al. 2012; Roulia et al. 2021; Delgado et al. 2022). In addition, OMW includes 13–15% lignin, 18–20% cellulose and hemicellulose, 3% residual olive oil, 1% volatile fatty acids, 0.2% polyphenols, 0.2% polyalcohols, 1.5% proteins and other pigments (0.5%) (Borja et al. 2006; Jarboui et al. 2008, 2010; Kavvadiasa et al. 2010; Magdich et al. 2016; Delgado et al. 2022). OMW is an acidic waste with pH ranging between 3 and 6, characterized by high organic matter content, its COD and BOD vary from 40 to 220 g l−1 and from 35 to 110 g l−1 , respectively. It is characterized by a dark color, a strong olive-oil smell and high electrical conductivity (EC) (JICA 1993; Jarboui et al. 2008, 2010). OMW contains relatively significant concentrations of aromatic compounds, responsible for its phytotoxic and antimicrobial effects (Perez et al. 1992; Aliotta et al. 2000; Chtourou et al. 2004; Quaratino et al. 2007; Jarboui et al. 2012; Delgado et al. 2022). However, the most noted pollutants are the monomeric phenolic compounds constituting 2–15% of the organic fraction, containing low molecular weight compounds (i.e., protocatechuic acid, tyrosol, caffeic, hydroxytyrosol acid, p-coumaric acid, ferulic acid, and syringic acid), high molecular weight compounds (i.e. tannins and Table 7.1 OMW physico-chemical composition Parameters

Hachicha et al. (2009) Jarboui et al. (2012) Dakhli et al. (2021)

pH

5.30 ± 0.21

Electrical conductivity (mS 20.01 ± 1.55 cm−1 )

4.87 ± 0.10

4.80 ± 0.20

14.15 ± 0.20

10.00 ± 0.52

COD (g l−1 )

178.40 ± 8.95

125.00 ± 0.80

98.00 ± 2.10

BOD (g l−1 )

25.60 ± 5.25

31.80 ± 0.66

66.00 ± 2.40

Organic matter (g l−1 )

78.60 ± 0.50

56.46 ± 1.98

44.20 ± 1.10

l−1 )

0.975 ± 0.1204

1.40 ± 0.08

1.60 ± 0.10

Total phenols (g l−1 )

10.50 ± 1.43

4.00 ± 0.09

8.80 ± 0.30

P (g l−1 )

0.260 ± 0.015

1.02 ± 0.02

0.35 ± 0.02

N (g

l−1 )

0.917 ± 0.035

4.27 ± 0.32

6.10 ± 0.20

Na (g l−1 )

0.280 ± 0.018

1.20 ± 0.12

1.57 ± 0.01

Cl− (g l−1 )

0.32 ± 0.02

0.47 ± 0.13

0.65 ± 0.04

K (g

l−1 )

0.724 ± 0.014

1.36 ± 0.05

1.10 ± 0.10

Mg (g l−1 )

0.298 ± 0.024

0.58 ± 0.03

0.42 ± 0.01

Ca (g

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anthocyanins), and catecholmelaninic polymers (Chtourou et al. 2004; Bertin et al. 2011). Due to their low oil-per-water partition coefficients, the phenolic compounds are rather concentrating into the water instead of oil, a fact explaining the increased concentrations of polyphenols in OMW (Galanakis et al. 2010). Standard deviations represent analytical procedure variability. The OMW environmental problems stemming from its composition as well as its potential hazards have led to the establishment of their discharge limitation and the investigation of new technologies for reducing the pollution load issued from olive oil extraction. Consequently, OMW treatment and management strategies were considerably investigated. Physical treatment including filtration, sedimentation, centrifugation and adsorption are applied as pretreatment steps for OMW solid contents removal. Chemical methods (oxidation, electrocoagulation, neutralization, precipitation etc.) which are less expensive but cannot reduce the OMW pollution were also developed. High coast thermal methods (combustion, distillation and pyrolysis) are mostly used to concentrate OMW in order to its reuse for agricultural purpose. Biological aerobic and anaerobic treatments and combined biological processes with physical treatment or composting have being already investigated (Paraskeva and Diamadopoulos 2006; Milanovi´c et al. 2019; Delgado et al. 2022). OMW bioremediation by composting/co-composting is an integrated olive oil waste management technique, and one of the main technologies for OMW recycling (Hachicha et al. 2006; Sellami et al. 2008; Rigane et al. 2014; Milanovi´c et al. 2019; Azzaz et al. 2020; Masmoudi et al. 2020). Meanfully, the most common method for two-phase OMW management and disposal consists of its storage in open evaporation ponds (Kavvadiasa et al. 2010).

7.4 Disposal and Management of OMW in Open-Ponds Storage The olive milling concentration from both spatial and temporal points of view and the low biodegradability characterizing the OMW limit its disposal on agricultural lands, according to the available regulations in the majority of the olive-oil producing countries, except in Italy and Tunisia, and therefore arise management problems as well as environmental impacts. Different treatment and bioremediation methods suggested solving the problems associated to OMW (physico-chemical and aerobic and anaerobic treatments, composting etc.) present some benefits related to the potential utilization of the issued products. However, beside their relatively high cost, some of these treatments are not efficient enough to solve definitely the OMW environmental problem. The current OMW cost effective management practice is its storage in evaporation ponds. Indeed, the evaporation ponds system is one of the simplest methods used for wastewater treatment: it is widely practiced in rural communities and at some industries throughout the world for wastewaters treatment. It has been

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developed to produce a purified effluent for discharge and for reclamation at low cost, and with unskilled labor. This technique can be considered as a natural biological treatment method because of the high evaporation rates, prevailing during the summer period, especially in the Mediterranean countries (Fig. 7.3a). According to Kapellakis et al. (2015), the OMW disposal in evaporation ponds is the best method for reducing environmental pollution, especially river contamination. Indeed, the biodegradable matter contained in the wastewater is stabilized by the natural process favorable conditions for such purification (WHO 1987). This treatment does not impose economic burdens, but serious environmental problems may arise, such as overflow, neighboring systems contamination with phenolic compounds and other toxic organic compounds, organic load accumulation, low dissolved oxygen, anaerobic conditions induction, and odor nuisance and infiltration through soil layers (Jarboui et al. 2009; Kavvadiasa et al. 2010; Roulia et al. 2021). During the storage period, different processes take place including the water evaporation which leads to the OMW concentration. Under these conditions, volatile compounds are generated through the organic matter degradation and hydrolysis by the different microorganisms’ metabolism, and the chemical reactions occurring structure under the solar radiation effect (Benitz et al. 2001; Rana et al. 2003; Jarboui et al. 2009; Kavvadiasa et al. 2010). These processes held produced compounds that will be valorized in different fields, starting with soap manufacturing, then agriculture (soil irrigation and amendment, co-composting and biochar production) and finally health domain by the recovery of some bioactive molecules (Delgado et al. 2022). In Sfax (Tunisia), the most important city producing olive oil in Tunisia, a unique OMW discharge-system based on huge evaporation ponds is available (Fig. 7.1). Progress studies on the OMW stored are scarcely investigated. At the authors’ knowledge, no published work describing the time-dependent changes of OMW characteristics in evaporation ponds is available, with the exception of previous studies conducted on the liquid phase (Jarboui et al. 2008, 2009, 2010). After olive oil production, the OMW generated is collected in the olive mills and then transported to uncovered evaporation ponds system. All of the ponds are connected together by underground channels. The connection allows OMW drainage from one pond to another. In the last pond, the effluent is dried and transformed into a pasty-sludge (Fig. 7.3).

7.4.1 Processes and Environmental Impact The OMW drying is the consequence of different processes held in the ponds during the storage period (Fig. 7.3). Firstly, OMW evaporation is considered as the main phenomena in all the storage ponds and induces volatile compounds emission such as phenolic compounds, CH4 , NH3 , CO2 and SO2 . Secondly, the organic matter decantation at the bottom of the basins generates three different successive layers: an aerobic one at the surface, an anaerobic zone in the bottom and in the pond middle

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Fig. 7.3 OMW storage in open evaporation ponds and its transformation after drying (Magdich and Ammar 2022)

(a) Open evaporation ponds

(b) Emission gas in evaporation ponds

(c) Drying beds of OMW

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an aero-anaerobic layer. These different layers may select three microflora groups, insuring the organic and mineral matters biodegradation along the open-ponds. The OMW infiltration through the soil layer is considered as one of the major impact in relation with storage in open-ponds. Generally, a clay-soil area is selected to construct the evaporation ponds system.

7.4.1.1

Natural Evaporation

In evaporation ponds, the storage period affects the OMW properties and different physical, chemical and microbial processes may occur. In open storage-ponds, the water evaporation effect seemed to be tightly dependent on metrological conditions. Furthermore, seasonal temperature, wind speed and direction, sunshine and precipitation are determining such evaporation yield. Under solar temperature effect, the OMW organic matter is concentrated in the bottom of the ponds. The solar radiation is intensively absorbed from the dark OMW surface layer in ponds, the photons affect OMW color, which is intensified and varied from red-brown to black, through phenolic compounds auto-oxidation (Jarboui et al. 2010). Also, the water quality parameters fluctuations would be related to meteorological variations. Consequently, intense evaporation may disturb the ecological balance in stabilization ponds and increase the OMW solids’ concentration (Kavvadiasa et al. 2010). In OMW evaporation clay-sandy pond, the sun radiation intensity is an important factor contributing to the organic constituents’ degradation, especially phenolic compounds (Jarboui et al. 2008). Miranda et al. (2001) mentioned the photodegradation of 20–40% of benzoic acid derivatives after 6 h of solar exposition. The use of serial connected ponds would improve the removal of OMW organic and mineral matters. The case study of evaporation ponds in Sfax (Tunisia) reported the removal of COD, BOD5 , total solids and total suspended solids by 40%, 50%, 50% and 75% respectively, in the last pond compared to the raw OMW reception pond (Jarboui et al. 2010). After harvesting period, the OMW stored in evaporation ponds flows to drying beds where its water content decreases progressively to be totally evaporated, forming a pasty sludge mass which may be integrated with organic wastes to be co composted or used in furnace to produce energy (Fig. 7.3b) (Hachicha et al. 2008b; Jarboui et al. 2010).

7.4.1.2

Microbial Biodegradation

The natural biodegradation is the second process occurring in OMW open storage ponds, where the organic matter is degraded by the microorganism metabolisms (involving oxidation, reduction, hydrolysis, deamination and decarboxylation) and the solar radiation (Millán et al. 2000; Jarboui et al. 2009). Indeed, in open-air ponds, the OMW has a residence time of 7–8 months, and large land surface

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areas are required. In Sfax region (Tunisia), the huge evaporation pond covers more than 50 ha with a storage capacity of 523,335 m3 (Jarboui et al. 2008, 2010). This long storage period can be considered as a natural biological treatment method, where active microbial community such as heterogeneous aerobic and anaerobic microflora, including both bacteria and fungi, characterize the storage-pond environment (Chtourou et al. 2004). The registered temperature range exhibited the type of microorganisms selected acting in the evaporation ponds, based on favorable temperature growth. An increase of the microbial communities like aerobic bacteria, yeasts and fungi were observed during the OMW management in open evaporation ponds investigations. In the same case, the decrease of monomeric phenolic compounds, sugars and proteins was also reported by Jarboui et al. (2010). This degradation exhibits the various microbial enzymatic activities held in the evaporation ponds. This biological activity is another process taking place in evaporation ponds, and closely related to the ponds environment. The microbial growth depends on the availability of easily biodegradable organic compounds such as total sugars, simple phenolic compounds, fats, total P and NH4 + , which are available in the complex OMW composition. In these natural reactors, microbial flora plays a crucial role since the effluent is the culture media selecting efficient microorganisms (bacteria) adapted to its composition. These acclimated flora progress and modify environmental conditions in the ponds, enhancing the activity of other more adapted microorganisms such as yeasts and moulds, which are rather acidophilic. During the microbiological and physicochemical characterization study of OMW stored in evaporation ponds, Jarboui et al. (2010) evidenced positive correlation between OMW organic matter (COD, BOD, TS, VS, polyphenols), minerals (K, Na, Mg and EC) and temperature. In addition, the same authors reported a negative correlation between the microbial flora (bacteria, yeast and fungi) and the nutrients compounds (total P, NH4 + , total sugar and monomeric phenolic compounds), explained by the crucial role of microbial flora in reducing both of these nutritive compounds and the OMW organic load consequently. The same results were mentioned by Amaral et al. (2008) when studying a continuous olive mill process in Northeastern Portugal, they noticed that very little anaerobic degradation occurs at cold temperatures (lower than 15 °C), since at these temperatures, bacterial growth and metabolism kinetics’ are reduced. Nevertheless, psychrophilic microorganisms may develop and relatively slight organic matter degradation may occur. The optimum temperature for mesophilic degradation is considered to be between 30 and 35 °C, at this temperature range, acidogenic and methanogenic bacteria become more active (Saqqar and Pescod 1995; Papadopoulos et al. 2003). On the other hand, the waste stabilization ponds and storage lagoons may offer a number of advantages in comparison to other biological processes for wastewater treatment, such as activated sludge and anaerobic digestion processes.

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Gas Emission and Atmospheric Impact

Olive mill waste contains a high quantity of phenols, potentially dangerous for the human health and environment. Moreover, the fermentation and digestion processes in the waste release an unknown quantity of polluting gases into the atmosphere (Perez et al. 1992). Since the evaporation of water is mainly caused by solar radiation, it can be deduced that the emission of phenols and sulphur dioxide was also highly influenced by the same microclimatic parameter. The volatilization of substances contained in OMW and their emissions into the atmosphere, such as phenols and sulphur dioxide, following the spreading of the wastewater on agricultural soil were evaluated by Rana et al. (2003). The decantation and the accumulation of OMW in ponds during several months could create an anaerobic zone in the bottom of the storage ponds (Fig. 7.3), that would enhance an anaerobic fermentation of the wastewaters. During this process, biogas, like methane and CO2 could be released in the atmosphere (Fig. 7.4). The biomethanisation is an anaerobic process where specific microorganisms eliminate the phenolic content, and the biogas production yield can be increased (Fiestas Ros de Ursinos and Borja-Padilla 1996; Niaounakis and Halvadakis 2004).

Fig. 7.4 Different processes held in evaporation ponds (Jarboui et al. 2010)

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Borja et al. (2006) reported a higher concentration of total volatile acids in the upper part of the OMW storage pond and they mentioned the increase of cumulative methane production after 21 days of the storing. This could be attributed to the initial stabilization of the simplest organic matter by anaerobic microorganisms.

7.4.1.4

Infiltration and Impact on Soil

In evaporation ponds, the OMW cumulative storage during several years may have a potential impact on soil characteristics; an infiltration through the soil’ layers would occur. Indeed, the OMW infiltration in an evaporation pond used for the storage over eight years was investigated by Jarboui et al. (2008). In the ponds, the granulometric characteristics, the physico-chemical and the biological parameters of the soil profile from the contaminated pond were compared to those of a control soil, located near the contaminated pond. The characteristics modifications’ of the contaminated soillayers were revealed, especially pH, electrical conductivity, COD and microflora content. These changes can be explained by the OMW constituents infiltration, which were noticed in the soil layers, especially phenolic compounds that have a negative impact on the shallow groundwater. This study confirmed the OMW behavior while stored in clay-sandy soil, and the possibility of effluent infiltration. Mahmoud et al. (2010) studied OMW application for 5 and 15 years; they showed a decrease in infiltration rate compared to the control after 5 years. However, the infiltration rate increased after 15 years of application. This fact was attributed to the crack formation after clay dispersion. Several ecological concerns arise including the possibility of groundwater contamination if the bottom of the lagoon is not properly lined against infiltration and leakage, and the emission of methane in the atmosphere due to the anaerobic fermentation of the waste that occurs in the lagoons. These lagoons should be located far enough from residences to avoid the insect and odor nuisances related to the effluent specifications storage (Rozzi and Malpei 1996; Azbar et al. 2004; Mohawesh et al. 2020). The application of high OMW amounts in a relatively short time and the presence of channels in the soil could induce leaching of ammonia, urea, and other organic nitrogen forms, which are generally retained by the adsorption complex of the soil and are found only in traces in the drainage water (Regni et al. 2017).

7.4.2 OMW Storage in Open Ponds: Valorisation Process in a Circular Economy Context Actually, the most common method used for OMW management is its storage in evaporation ponds (Kapellakis et al. 2015). Considering its several negative impacts,

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it becomes crucial to look for an eco-friendly management issue to the exacerbate problem of this particular effluent discarding (Yeneziani et al. 2017; Dutournié et al. 2019; Mohawesh et al. 2020).

7.4.2.1

Soap Manufacturing

The OMW is known for its relatively high concentration in oily residues. These constitute a raw material for oleic acid production, considering their high acidity about 3.39%, which is beyond the admissible value by the Codex Alimentarius (Elkacmi et al. 2016). Elkacmi et al. (2016) suggested a new approach to produce oleic acid from Moroccan OMW. The simple technique is based on the effluent inclusion with urea. The obtained results mentioned that the OMW oleic acid has nearly the same characteristics as the one resulting from raw olives. The separated oily phase from the OMW has very poor quality and nutritive value due to its strongly acidic aspect. Thus, it can be used to manufacture biodegradable high quality soap and glycerin. This new upgrading technique tends to purify, as much as possible, the food wastes to ensure a better environmental protection, contributing to the improvement of the olive sector profitability, through the recovery of a multiple valuable products. The evaporation ponds constitute a rich environment for the recovery of the oil extraction residues, which concentrate at the upper layer of the OMW storage open ponds. This oily layer is collected and transported in soap manufacturing for producing a biodegradable soap (Jarboui et al. 2010).

7.4.2.2

Soil Irrigation and Amendment

In the Mediterranean region, among the proposed solutions for OMW disposal, its direct spreading on agricultural lands was practiced. Some researchers have shown that the OMW controlled spreading on agricultural land may have a positive effect on the olive tree. Magdich et al. (2012) demonstrated that the application of three OMW levels (50, 100 and 200 m3 ha−1 year−1 ) over six successive years on olive tree in an orchard, increased the nutrient resources availability for plant growth as well as soil amendment. The same OMW doses applied by Abichou et al. (2009) on poor sandy soil showed an increase in aggregate stability and consequently an improvement in the soil structure. Similarly, Barbera et al. (2013) and Levy et al. (2017) reported an increase in the aggregate stability of an OMW-amended clay soil as well as sandy loam soil. Moreover, the controlled land application according to the available regulations is commonly permitted and considered as practical option for rural OMW management in olive oil–producing countries (Saadi et al. 2013; Steinmetz et al. 2015; Magdich et al. 2021). Mellouli et al. (2000) exhibited the OMW beneficial effects on sandy-soil aggregation, structure stability and hydrodynamic properties.

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Furthermore, while practicing sustainable controlled application of OMW into the land at five rates (20, 40, 60, 80, and 120 m3 ha−1 ) to enhance soil properties and evaluate the effect on barely production under rain fed conditions; Mohawesh et al. (2020) didn’t evidence any harmful effect of the effluent application on barley growth as well as soil properties. Indeed, at all of these rates used, soil organic matter and nutrient contents were increased, playing a fertilizing effect, and barley yield was improved. More recently, Dakhli et al. (2021) revealed an important improvement of soil fertility when spreading OMW amounts varying from 15 to 45 m3 ha−1 for three consecutive years. They noticed the soil pH and phosphorus content stability during the three years of investigation. However, the soil salinity was increased for the higher dose (45 m3 ha−1 ) treatment and electrical conductivity exceeded 6 dS m−1 in the arid area of the Southern Tunisia. In this perspective, the OMW agronomic valorisation by its spreading in olive field improved the soil chemical and microbial properties, these have beneficial effects on soil fertility and seems to stimulate soil enzymatic activities by increasing the organic matter, nutrients and microbial activity. Nevertheless, the addition of 200 m3 ha−1 could increase the salinity of the soil after long-term and frequent applications of OMW, which impacts negatively the soil chemical properties (Magdich et al. 2012, 2013, 2020). However, the OMW application effect has shown contrasted results due to the differences in soil texture and the period as well as the delay of application. At shortterm, OMW application increased soil aggregate-stability (Kavdir and Killi 2008; Barbera et al. 2013; Regni et al. 2017). This aggregate-stability may be degraded after repeated spreading of OMW for a long term as soil calcium is replaced by potassium, sodium, and magnesium from OMW (Mekki et al. 2006). Therefore, repeated application of OMW on clay soils is not recommended to avoid the soil mineral chelation. The beneficial effect of OMW controlled application on soil quality was confirmed. However, at long-term of OMW application, more studies are required to legislate this use (Delgado et al. 2022). Barbera et al. (2013) reported that OMW exerted two contrasting actions on soil microflora: it stimulated the microflora development by temporarily soil carbon enrichment and inhibited some microorganisms and phytopathogenic agents by phenolic compounds. According to Casacchia et al. (2012), soil amendment with OMW can be an interesting agricultural practice for supporting and stimulating soil microorganisms in sustainable olive orchards.

7.4.2.3

Dried and Residual OMW Integrated in Co-composting

The OMW evaporation in open-ponds is one of the most widely accepted management alternative because of the low investment required and the favorable climatic conditions in Mediterranean countries. This management process is used in Tunisia, Morocco, Spain, Greece and Cyprus (Alfano et al. 2009). In evaporation multiponds,

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the OMW management acts on the effluent physico-chemical and biological parameters. Furthermore, at the end of the storage, and in the last ponds the olive mill wastewater sludge (OMWS) is accumulate in the bottom of storage ponds and then rule out and stacked along the ponds. The OMWS utilization in agricultural fields is gaining popularity as a means of waste disposal. This organic material can enhance soil productivity because of its high organic matter and plant nutrients content. This solid waste could be valorized by its integration in composting process, leading to long-term soil fertility within sustainable agriculture (Chtourou et al. 2004; Hachicha et al. 2008a; Sellami et al. 2008). The composting process by the integration of solid waste resulting from the evaporation of OMW in open ponds could decrease the accumulation of toxic wastes that are rich in recalcitrant compounds, and their phytotoxic and antimicrobial properties, which limit their direct use for agronomic purpose. Composting has gained momentum over the past few decades as soil amendments because agricultural soils continue to lose their fertility and the soil amendment is needed to replenish organic material and nutrient content. Compost fulfils these needs at fordable prices, too. Additionally, compost can improve soil water capacity and cation exchange, increase microbial activity and reduce pesticide numbers (Khdair and Abu-Rumman 2020). The valorisation of OMW and the wine by-products is a promising strategy for the OMW sustainable management, allowing the transformation of environmental threats into valuable products. Indeed, the produced composts for soil amendment significantly improved the soil fertility and did not have any phytotoxic effect on radish growth (Majbar et al. 2018). In the same way, Martínez-Gallardo et al. (2019) proved that composting is a sustainable way to recycle OMW along with organic wastes, solving simultaneously their negative environmental impact. The composting process assisted with the earthworms enhanced phenolic compounds depletion and OMW ecotoxicity reduction more efficiently than composting alone, especially during the maturation stage (Sáez et al. 2021).

7.4.2.4

Incorporation in Brick Making Processing

The OMW incorporation in brick-making process represents a hopeful solution, which improves building materials quality and could reduce the heat required during ceramic production process, as well as its disposal impacts (Mekki et al. 2003; Ammar et al. 2004). These reductions would lower greenhouse gas emissions (GHG) and offer economic benefits. However, many factors should be considered, including the OMW transportation from the storage ponds to the factory that can affect both of the environmental and economic benefits. Indeed, among eco-friendly finding and economically viable solutions for OMW treatment and disposal, their valorization in building ceramics, represents a significant challenge in the major oil-producing countries (Mekki et al. 2003; Ammar et al. 2004; Zabaniotou et al. 2007). Indeed, the OMW incorporation in bricks can represent a promising way to valorize this effluent, to rid the environment of a highly

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polluting wastewater and to save huge and precious amounts of water when water shortage becomes a serious problem. This newly-prepared material has three positive impacts: it protects the environment, improves the building material properties, and allows water economy (Mekki et al. 2008). Actually, previous investigations held at factory scale showed that if introduced in brick raw materials mix, a substantial OMW volume is recycled, saving on the fresh water currently needed for the brick manufacturing process. Moreover, during the brick drying process, most of the OMW included water (around 98%) is released in a vapour form. Then, in the kiln, the bricks solids’ remaining would release additional heat (calorific value of 21–23 MJ kg−1 ), that may reduce the unrefined energy from fossil fuel currently needed during bricks firing (Mekki et al. 2008). According to Spiliotis et al. (2019), the replacement of fresh water (approx. 20% of the plastic mass) by OMW in ceramic brick manufacturing results in the production of lighter ceramic bricks of decreased thermal conductivity and reliable mechanical performance. Furthermore, these authors reported that the energy consumption measurements revealed that production energy save was as much as 30% during the firing. Consequently, this process is expected to lead to CO2 emission reduction and to contribute to the alleviation of the intense environmental problems associated with the safe disposal of this industrial effluent. In the same way, the overall Global Warming Potential (GWP) decreases up to 3.1% for OMW-based bricks with respect to conventional ones, as well as the Abiotic Depletion of fossil fuels which is reduced by 4.3%. On the other hand, no significant variation was observed for the toxicity impact category, that ranges from − 1.1 to 0.7%. In addition, the water consumption decreases for OMW-based brick production up to 7.8%. Finally, in terms of GWP, it has been found that to make the benefits persist, the oil mill should be located in a distance of not less than 150 km from the brick factory, indicating a more restrictive constraint than the economic one, corresponding to an optimal distance of 207 km (Silvestri et al. 2021).

7.4.2.5

Biochar Production

In the recent years, an increasing number of research studies have highlighted the benefit of using biochar in terms of mitigating global warming and as a strategy to manage soil health and productivity. Application of biochar to soils is currently gaining considerable interest globally due to its potential to improve soil nutrient retention capacity, water holding capacity, and also to sustainably store carbon, thereby reducing greenhouse gas (GHG) emissions (Lehmann and Joseph 2009; Verheijen et al. 2010; Dume et al. 2015). Biochar is a form of charcoal produced through the thermochemical process of biomass under low oxygen conditions known as pyrolysis. It is a carbon-rich, porous substance, produced by thermal decomposition of biomass under oxygen-limited conditions and at relatively low temperatures (< 700 °C) (Laird 2008; Laird et al. 2010). The definition adopted by the International Biochar Initiative (IBI) specifies the need for the application of this material to soil for both agricultural and environmental gains (Sohi 2012). Various types of

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biomass such as agricultural crop residues, forestry residues, wood waste, organic portion of municipal solid waste (MSW) and animal manures have been proposed as feedstock for biochar production. However, the suitability of each type of biomass as feedstock is dependent on the nature, chemical composition, environmental, as well as economic and logistical factors (Verheijen et al. 2010). Pyrolysis conditions for biochar production, together with feedstock characteristics largely control the physical and chemical properties (e.g. composition, particle and pore size distribution) of the resulting biochar, which determine the suitability for a given application. Challenges focusing on the OMW utilization are bivalent: on the one hand the necessity to treat OMW to minimize environmental hazards and on the other hand the need to capture reusable materials from the waste biomass. The utilization of this feedstock for energy production, carbon sequestration, or soil fertility improvement (Libra et al. 2011) is impeded by biomass-specific limitations, i.e. the high degree of heterogeneity in form, composition and water content, and OMW-specific limitations (phytotoxicity, biodegradation-resistant organics, seasonal nature of production, high regional scattering of olive mills). Thus, economically feasible conversion processes for more efficient utilization are required; this ought to be environmentally friendly and operationally decentralized. This strategy gains further momentum if associated with preventing potential ecotoxicological hazards. The OMW sludge is a solid waste, which could be valorized by its transformation into biochar, leading to long-term soil fertility within sustainable agriculture. Recently, Haddad et al. (2017) proposed the thermal conversion of OMW, after impregnation on dry biomass, by slow pyrolysis at 500 °C. They produced a biochar rich in phosphorus and nitrogen, with a low amount of volatile fraction and a high porosity media. In addition, these authors mentioned that the produced biochar was applied as bio fertilizer and showed an equalled impact on the growth of rye grass leaves, when compared to a commercial fertilizer. In addition, the impregnated biochars with OMW and cypress sawdust pre-treated with potassium chloride are exceptionally rich in potassium, phosphorus nitrogen and also sodium. Potassium bioavailability was significantly improved by biochar addition to soil. Furthermore, these biochars also contribute to various pathogens suppression and could be considered as attractive and promising organic fertilizer for acidic agricultural soils (Haddad et al. 2021).

7.4.2.6

Bioactive Molecules Recovery with Health Benefits

OMW is a by-product of olive oil production, rich in water-soluble bioactive compounds that could be separated by different technologies including industrial membrane technology (Caporaso et al. 2018).

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OMW has long been considered a waste and its disposal is expensive. Recently, many studies have pointed out its content richness in polyphenols and other biologically important molecules, shifting its perspective from waste to an economical and natural source of antioxidants (Caporaso et al. 2018; Delgado et al. 2022).

7.4.3 Main Recovered Biomolecules Promoting Health Qualifications Olive processing by-products are attractive source of bioactive and valuable compounds. Their use is consistent with the outcomes of numerous researches held, exhibiting their multiple benefits, promoting health as alternatives to synthetic molecules with major limitations that chemicals may present. In particular, synthetic molecules have relatively long life-cycles and come back to the environment causing many problems. Based on the cost-effectiveness, natural biomolecules are more active and effective. Considering the antioxidant properties of natural sources, these do not have imitated uses, especially as oil preservatives, without any negative health potential effect while compared to their synthetic analogues; they are also characterized by limited usages in many countries. Moreover, aware of negative environmental impacts of chemical synthetic compounds, and the recycling/reuse of biomolecules from bio wastes would need the consumers demand and enhance a circular economy approach implementation. It should be noticed that such natural compounds valorization would significantly reduce the negative environmental impact of olive processing by-products by preventing waste direct disposal. Such practice may contaminate both of the soil and water ecosystems been highly loaded by organic content with particularly phytotoxic effects related to high aromatic compounds rate. Consequently, their relatively high concentration in phenolic compounds and their derivatives, and long-chain residual fatty acids make their treatment and disposal difficult, notably in the countries where they are produced; these are mainly located in the Mediterranean region, where 97% of the olive is proceeded (Galanakis and Kotsiou 2017).

7.4.3.1

Bioactive Compounds Chemical Structure

Since 2017, many studies are interested in the extraction and identification of specific compounds found in the OMW. Therefore, different conventional as well as innovative technologies based on distillation, ultrafiltration, membrane separation technologies, liquid–liquid solvent extraction, microwave assisted extraction, ultrasound assisted extraction, high voltage electrical discharges, γ-irradiation extraction, enzymatic/acidic transesterification, etc. were used to recover OMW biomolecules

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(Yangui and Abderraba 2018; Gullón et al. 2020; Khounani et al. 2021). Then, the identification of the extracts was achieved by high performance chromatography and performing detectors (HPLC–DAD-MS etc.). Consequently, a panoply of bioactive and appreciated compounds were evidenced and their specific uses were developed based on their multiple benefits, especially their effect on promoting health, in replacement to synthetic chemical molecules characterized by their long live-cycle and environmental problems. Considering both of cost-effectiveness and their natural origin, the identified biomolecules are attractive with notable performances among which phenolic compounds identified were the most abundant with a wide range of 0.5–24 g l−1 (Delgado et al. 2022). Considering their biological activity, this seems to be highly correlated to the phenolic compound structure and its rate. The OMW isolated phenolic compounds are mainly derivatives of phenol substituted with different kinds of functional groups (Table 7.2), radicals R1 to R6 fixed on the aromatic ring. Besides benzoic and cinnamic acids as well as their derivatives, phenyl ethyl alcohols were found such as tyrosol, hydroxytyrosol, 3,4-dihydrooxyphenyl glycol, 4-methyl catechol, 4-methylcatechol, pHydroyphenylacetic acid, 2-(4-hydroxy-3-methoxy)phenyl ethanol and 2-(3,4dihydroxyphenyl)-1,2-ethandiol. A specific phenyl ethanoid glycoside or verbascoside, was also identified in OMW, as well as flavonoids (euteolin, quercitin, rutin, apigenin, lutolein, quercitin, rutin, apigenin, luteolin and β-glucoside. Secoiridoid commonly known as oleuropein and its derivatives are also present characterizing olive transformation by-products. Finally, a complex lipid: the squalene was also evidence in OMW (Chtourou et al. 2004; Galanakis et al. 2010; Galanakis and Kotsiou 2017; Delgado et al. 2022).

7.4.3.2

OMW-Derived Bioactive Molecules Application in Plant Growth and Protection

Bioactive molecules with growth promotion and antimicrobial effects, identified and characterized in OMW by-products, have stimulated many reports to employ these compounds as biostimulants, biopesticides, and as plant protectants for crop improvement (Sciubba et al. 2020). Palumbo et al. (2018) indicated that humic acids extracted from an amendment, obtained by combining OMW with a pre-treated organic material derived from solid urban waste, can be used as plant biostimulant (PB) in agriculture, thanks to their positive effects on biomass production, nutrition, and activity of enzymes implied in nitrogen metabolism and glycolysis. Other investigations have shown that PB formed as the two-stage process by-product of squeezing olive oil, can induce an increase in the protein content of maize grains up to 19% (Drobek et al. 2019). Therefore at low concentrations, OMW can efficiently generate positive metabolic and physiological responses in plants enhancing their growth and production. Phenols are important molecules and it has been established that at adequate concentrations, they can induce several positive effects in plants, even when they are exogenously

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Table 7.2 Main phenolic compounds of olive mill wastewater (Chtourou et al. 2004) R1 R6

R2

R5

R3 R4

Compound

R1

R2

R3

R4

R5

R6

Benzoic acid

COOH

H

H

H

H

H

p-Hydroxy benzoic acid

COOH

H

H

OH

H

H

3-Protocatechuic acid

COOH

H

OH

H

H

H

Gallic acid

COOH

H

OH

OH

OH

H

Caffeic acid

COOH

H

OH

OH

H

H

Vanillic acid

COOH

H

OCH3

OH

H

H

Veratric acid

COOH

H

OCH3

OCH3

H

H

Syringic acid

COOH

H

OCH3

OH

COH3

H

Cinnamic acid

CH=CH–COOH

H

H

H

H

H

p-Coumaric acid

CH=CH–COOH

H

H

OH

H

H

Ferulic acid

CH=CH–COOH

H

OCH3

OH

H

H

Synapic acid

CH=CH–COOH

H

OCH3

OH

OCH3

H

applied or present in PB formulations (Ertani et al. 2018). Conversely, at high concentrations as those normally recorded in OMW, phenols may be responsible for the inhibition of soil microbiome activity and the induction of several phytotoxic effects, including seed germination reduction, plant growth impairment and drops in productivity (Leopoldini et al. 2011). The OMW application in crop protection against pests takes advantage of their antifungal and antimicrobial properties without negative impact on plant growth (El-Abbassi et al. 2017). Several experimental evidences reported the fungicidal activity of OMW against dangerous phytopathogens. The inhibitory activity of OMW on in vitro mycelium growth of Fusarium oxysporum, Pythium spp., Sclerotinia sclerotiorum, Verticillium dahlia, and Botrytis cinerea was reported (Hachicha et al. 2009; Vagelas et al. 2009). Some researchers suggested the OMW incorporation into soil as an eco-friendly alternative to fumigants for crops protection against V. dahlia. Curative control activity was demonstrated through pot experiment monitored on tomato crops under field conditions (Yangui et al. 2011). These authors attributed to the OMW compounds’ a direct fungicide effect such hydroxytyrosol, or an induction of defence response mechanisms in the plant. Besides, Lykas et al. (2014) reported that in vitro,

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filtered OMW inhibited mycelium growth of Aspergillus, Botrytis, Fusarium and Penicillium spp. Moreover, OMW caused infections when it was sprayed on uninfected bulbs: malformations on 30% of the plant grown and reduction of the bulbs production by 50%. In this perspective, pre- or post-plant application of OMW and up to 400 ml pot−1 showed no phytotoxicity effect on tomato, but lower rates in post plant application increased crop shoot and root dry weights over parasite-free control. Total shoot number and dry weight of O. ramosa at 400 ml per pot were reduced by 70% and 74% for both parameters, respectively (Qasem 2019).

7.4.3.3

Biomolecules Health Benefits

OMW is a low-cost material rich in phenols, due to the hydrophilic nature of olive oil by-products and the phenols polarity, most of them are solubilized in the olive oil residues rather than in the oil (Plastina et al. 2019). Phenolic compounds in the olive industry’s by-products is attracting great interest as a potential source of such components’, with special attention oriented to the OMW, which is the primary potential source of these molecules. The major components in OMW include hydroxytyrosol, tyrosol, oleuropein, ligstroside and their secoiridoids derivatives, as well as a variety of hydroxycinnamic and hydroxybenzoic acids (Obied et al. 2005). Given the potential environmental impact, active molecule extraction from olive oil by-products should embrace methodologies that employ green technologies, considering their possible exploitation as food antioxidants or nutraceuticals (Posadino et al. 2021). In addition, phenolic compounds from olive oil by-products have shown so far great potential in improving sensory quality in milk and antioxidant capacity in fatty food matrices. Thus, the use of phenolic extracts from olive wastes could be a cost-effective solution to synthetic additives usually used in food industry, simultaneously contributing to the sustainability of olive oil industry and to a better byproducts management (Araújo et al. 2015). OMW extracts use in environments in which Fenton and Haber–Weiss reactions take place and in which the concomitant production of superoxide and nitric oxide would yield the powerful oxidant peroxinitrite. It is noteworthy that OMW extracts may add stability to products exposed to high superoxide levels (Visioli et al. 1999). OMW is added as such or as extracts, concentrated and stabilized and, in some cases, microencapsulated. Specifically, encapsulation protects them from degradation due to different factors reducing the amount of compounds required to be efficient and controlling their release into the food matrix (Mohammadi et al. 2016). De Leonardis et al. (2007) proposed the addition of lard with olive phenols as a “novel food”, showing that the natural antioxidants of OMW were highly effective in oxidative stabilization of lard. Indeed, the phenol extract significantly increased the lard oxidative stability, and the applied doses (100–200 ppm) were not cytotoxic when tested on mouse cell lines (embryonic fibroblasts). In addition, several studies have tested phenol extracts in dairy products to enhance antioxidant activity and better stabilize the products (Foti et al. 2021).

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In a similar tendency, OMW is rich source of bioactive substances with various biological properties that can be used as ingredients in the food industry for obtaining functional and nutraceutical foods as well as in the pharmaceutical industry. By using a spray drier system at low temperatures, an excellent extract rich in phenols with different biological properties was produced (Benincasa et al. 2022).

7.5 Conclusion and Future Prospects In this chapter, the multiple benefits of OMW storage in open ponds, where it undergoes a natural evaporation, were evidenced. However, besides the opportunities, some weaknesses are presented by using this wastewater treatment process. Consequently, a SWOT analysis was established to synthesise these pertinent results (Table 7.3). The OMW characteristics make difficult to apply a cost-effective treatment method. In Mediterranean countries, the common practice for its management has been the storage in evaporation ponds. However, with this management strategy, these effluents are transformed into a more polluting and recalcitrant solid sediment in which toxic compounds are concentrated. In addition, construction of evaporation ponds rarely meets engineering criteria for safe accommodation of liquid wastes, therefore, OMW often overflows or leaks and pollutes neighboring agricultural and underneath soils and groundwater. Nevertheless, OMW storage in open ponds was valorized in different sectors such as soap manufacturing, soil amendment, co-composting, biochar production and brick manufacturing, which reflected good Table 7.3 The main strengths, weakness, threats and opportunities related to the OMW storage in open ponds Strengths • Collection is regulated by a local law • Timely disposal of the effluent • Simple technology required • Low cost • Environmental advantages by collecting all the polluting effluent • Fat recuperation after floating and reuse in soap factory • OMW paste energetic valorisation after drying • OMW solid part co-composting and agronomic use (soil amendment) • “C credit” possible acquisition Treats • Environmental pollution load depending on meteorological conditions

Weakness • Effluent transportation cost • Specific area need for ponds construction with clay dominance • Fat collection and transport to soap factory • Transportation cost for fat scraping following the flotation phase Opportunities • Residual fats reuse • Effluent volume reduction with drying • Composted waste valorization as soil fertilizer • OMW dried solid energetic valorisation for furnace operating • Credits account

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and efficient results. In addition, OMW is rich source of bioactive substances especially, phenolic compounds considering their exploitation as biostimulants, biopesticides, food antioxidants or additives and pharmaceutical industry. Nevertheless, the OMW bioactive substances still need new biotechnological methods for better use in a circular economy of various sectors contributing to the sustainability of olive oil industry.

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Chapter 8

Management of Olive Oil Mill Wastewater in Morocco Khattabi Rifi Safaa, Abdelkader Anouzla , Younes Abrouki, Hayat Loukili, Malika Kastali, and Salah Souabi

Abstract With over one million hectares of 998,000 ha, the olive tree is the most crucial permanent crop in Morocco. Production, which reached 1,559,000 tonnes for the 2017–2018 campaign, has averaged around 1.2 million tonnes over the past ten years, ranking the country 5th for olive producers after Spain, Greece, Italy, and Turkey. Olive growing is one of the most developed crops in Morocco. Concerning the development of this olive growing and the increased demands of the internal and external markets for olive oil, the olive industry has also experienced significant development in the country, thus causing a significant discharge of liquid residues loaded with toxic elements. The oil mill wastewater problem for Morocco is that the olive crushing units, whether traditional or modern, are concentrated in the most critical area in terms of water resources. The impacts of wastewater from olive oil mills on surface water, groundwater in particular, on drinking water production downstream of the city of Fes have been widely studied. The results showed that wastewater is heavily loaded with oils, trace metals, polyphenols, detergents, etc. The cost of producing drinking water from Oued Sebou becomes increasingly crucial during the olive growing period due to the discharge of olive oil mill wastewater directly discharged into Oued Sebou. To minimize the impacts, the Moroccan government has encouraged olive growers to use two-phase machines allowing the production K. R. Safaa · H. Loukili · M. Kastali The Water and Environmental Engineering Laboratory, Faculty of Sciences and Techniques of Mohammedia, Hassan II University, Mohammedia, Morocco e-mail: [email protected] H. Loukili e-mail: [email protected] A. Anouzla (B) Faculty of Science and Technology Mohammedia, University of Hassan II Casablanca, BP. 146, Mohammedia, Morocco e-mail: [email protected] Y. Abrouki Mohammed V University, Rabat, Morocco S. Souabi Faculty of Science and technology Mohammedia, University of Hassan II Casablanca, Casablanca, Morocco © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_8

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of oils without producing margins and avoiding the use of conventional systems (press), which produce effluents heavily loaded with oils, and grease compared to the discharges produced by two or three-phase machines. In this work, we studied the different methods of managing vegetable waters produced during the olive growing period in Morocco, the impacts on the environment, the treatment of rejects the valuation of olive oil mill wastewater as raw materials for the production of several products (activated carbon, cosmetic products, detergents such as the extraction of polyphenols, production of compost with a view to sustainable development). The olive oil mill wastewater produced is currently stored in large tanks to be evaporated naturally. However, this technique is too slow and does not allow a sound reduction in pollution. In addition, it is more advantageous during storage to scrape off the oils that rise at the interface at least twice to three times a week (two to three months). This technique has shown that natural flotation would considerably impact reducing oil pollution. In Morocco, different techniques are currently used to reduce the impacts of olive oil mill wastewater on the environment in particular: • Infiltration percolation on the ground. • Anaerobic treatment of olive oil mill wastewater. Keywords Olive mill wastewater · Coagulation · Flocculation · Valorization · Removal efficiency · Extraction process

8.1 Introduction The increase in olive oil production is not limited to the Mediterranean basin. It has also increased in other countries such as France, Serbia, Montenegro, Macedonia, Cyprus, Turkey, Israel, Jordan, the United States, Australia, and the Middle East (Achak et al. 2019a b). The olive tree is the main cultivated species in Morocco; it covers an area of (998,000 ha) (MAPM 2019). According to the Ministry of Agriculture and Maritime Fishing, the olive industry contributes to 5% of the agricultural gross domestic product (GDP). It actively contributes to the commercial balance, knowing that Morocco holds important international olives and olive oil production (MAPM 2019). The olive sector contributes to 16% of the country’s deficit in edible oils. It ensures an intense agricultural activity generating more than 15 million working days/year, equivalent to 60,000 permanent jobs (DEPF 2014). In Morocco, the total production of olives in the traditional mills of the province of AlHoceima is estimated at 435 T/year, of which 59% are concentrated in the commune of Tifarouine and 41% in the commune of Nekkor (El Yamani et al. 2020a). Due to the encouragement and support adopted by the State to improve this sector, olive growing has experienced a significant increase which has benefited in particular from the implementation of the “Green Morocco Plan” (Akesbi 2012). This increase was accompanied by a significant increase in the number of byproducts (solid olive waste, olive mill wastewater) (Dermeche et al. 2013). The olive

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pomace is easy to transport and store. It can be reprocessed to obtain a second olive oil using solvents or recovered as a primary material (Paz et al. 2020). The major problem generated by the olive sector is caused by the OMW produced by the traditional extraction methods and the continuous processes of decantation in three phases (Massadeh and Modallal 2008; Souilem et al. 2017). Reducing annually produced OMW is a real environmental challenge for traditional mills and threephase extraction systems. OMW is composed of (88–94%) water, (4–16%) organic compounds, and (0.4–2.5%) mineral salts (McElhatton and do Amaral Sobral 2011). Generally, they are characterized by a high organic load, low pH, a large number of suspended solids, and a high concentration of phenolic compounds and long-chain fatty acids (Amor et al. 2019). The high organic content of OMW (ranging from 45 to 220 g/L) makes it a very potent industrial liquid waste, reported to be 100– 150 times higher than domestic wastewater (Azbar et al. 2009; Sampaio et al. 2011). OMW is phytotoxic and antimicrobial due to its high phenolic compound load, which makes its uncontrolled release into ecosystems a strong source of pollution (S’habou et al. 2009). In Morocco, there is no specific legislation governing the management of OMW to the present. Almost all olive mills in Morocco, and the world, dispose of olive wastewater in evaporation basins (Jalo et al. 2018). The considerable storage period in these basins can be considered as a natural biological treatment method (Jarboui et al. 2009). This technique generates several problems such as foul odors, emission of greenhouse gases, infiltration into the soil, and proliferation of insects (Sampaio et al. 2011). In Morocco, some olive mills discharge the wastewater produced into the environment without any treatment, which generates pollution far from negligible (Souilem et al. 2017). This situation will become more worrying in the coming years due to the continuous increase in national and international demand for olives and olive oil. Preventive measures have been taken to better manage this situation, such as the prohibition of any discharge of OMW into the natural environment or into the public sewage system and the obligation to build adapted and specific storage basins for OMW. However, these actions do not completely solve the problem. Developing an appropriate approach to OMW management is a matter of great importance. It has been the subject of many studies that aim to decontaminate OMW by physicochemical processes such as coagulation-flocculation (Achak et al. 2008), nanofiltration (Coskun et al. 2010), electrochemical oxidation (Gotsi et al. 2005), and electrocoagulation (Flores et al. 2018) and on biological treatments using aerobic or anaerobic micro-organisms (Bernardi et al. 2017; Jaouad et al. 2020). Physical–chemical and biological treatments can also be combined to improve pollution reduction (Esteves et al. 2019; Khattabi Rifi et al. 2021). Some studies aimed to reduce the phytotoxicity of OMW to reuse them in agriculture or treat them in urban water treatment plants (Enaime et al. 2020). Other techniques have been much more focused on extracting phenols for pharmaceutical purposes (Alfano et al. 2018). However, most of the processes proposed for the treatment/recovery of OMW are either inefficient in reducing polyphenols and organic load or technically complex and require significant investments.

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8.2 Olive Sector in Morocco Morocco is the sixth-largest producer of olive oil globally after Spain, Italy, Greece, Turkey, and Tunisia (El Yamani et al. 2020a). The olive tree is the main cultivated species in Morocco; it covers an area of (998,000 ha) due to the agronomic and adaptive characteristics of this type of species (MAPM 2019). Generally, it is present on the whole national territory because of its adaptability to all the bioclimatic regions of Morocco, going from the humid mountainous zones to the arid climates of the Saharan region (Elabdouni et al. 2020). The varietal range of the olive tree is based on the national varieties Picholine marocaine, Haouzia, and Menara (Hadiddou et al. 2013). It is found mainly in the following regions: Fez Boulmane Taounate, Taza, Meknes Tafilalet, Marrakech Tensift Haouz, Beni Mellal Tadla Azilal, Oriental (Nador, Taourirt, Oujda, Guercif, Berkaneet Jerrada), Tangier Tetouan (Bouknana et al. 2021; Mouhtadi et al. 2014). The province of Taza, located in northern Morocco, is one of the essential olive-growing regions with about 78,800 ha, representing 36% of the province’s agricultural land and 9% of national olive trees. The olives produced by the province represent 7% of the national production, or 90,000 tons in a normal year (El Yamani et al. 2019). Morocco is one of the significant olive-producing countries with an annual production of 1.56 million tons, part of which is devoted to the production of olive oil (Ameziane et al. 2020). Implementing the “Green Morocco Plan” has led to a remarkable development of the olive sector in Morocco (Balaghi et al. 2010; Bouknana et al. 2021). The objectives of this plan are to have an area of 1,220,000 ha, a total olive production of 2,500,000 tons/year, olive oil production of 330,000 tons/year of which about 1/3 exported (El Mouhtadi et al. 2014). In Morocco, the olive sector plays a very important role on the socio-economic level because it actively contributes to the settlement of the population in rural areas by creating millions of working days (El Hazzat et al. 2015). On the socioeconomic level, several regions in Morocco are based primarily on agricultural products and their derivatives. The region Taza-Al Hoceima Taounate and particularly the provinces of Taza, Guercif, and Taounate (three provinces out of four in the region) are known for the cultivation of olive trees and the production of table olives and olive oil. The olive sector contributes to 5% of the agricultural GDP and represents 15% of the agro-food exports (Rouas et al. 2015). Morocco does not represent a fundamental law, especially in terms of respect for the environment of the olive industry. In addition and since 2003, following the example of food industries, the olive industry is required by Law No. 12.03 on Environmental Impact Assessment (EIA) to carry out such studies before implementing the related projects. However, it has been noted that the pre-project impact study (EIA) procedure is reduced to an administrative process before the start of the project and results in concrete provisions making it possible to preserve the environment, particularly the water resources. Therefore, a law for spreading OMW and pomace is being prepared.

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Several provisional laws and ministerial decrees have been promulgated at the international level, and strategic plans have been developed and implemented in some countries. One of them has foreseen the spreading of the OMW on the ground, as in Italy (Zema et al. 2019). Spain has almost completely changed the production techniques from the press and three-phase process (oil-water-pulp) to continuous production (oil-water-pulp) to two-phase systems to produce less waste (Lama-Muñoz et al. 2019).

8.3 The Olive Oil Extraction Process The different types of olive oil extraction at the national and regional levels can be grouped into two processes: The discontinuous process (pressing) in traditional mills and the continuous process (centrifugation) in modern units (De Bruno et al. 2021). The pressing system (batch process) is the oldest and most often used method in small mills (El Rhaouat et al. 2014). The olives are washed, crushed with stone rollers, and mixed with hot water in this process. The purpose of kneading is to promote the coalescence of tiny oil drops into more significant drops that can be more easily separated in a centrifugal field. On the other hand, to reduce the value of the viscosity of the olive paste to optimize the separation of the phases inside the decanter (oil/vegetable water/pomace) (Clodoveo 2013). The resulting pulp is spread on fiber disks stacked on top of each other and placed in the press to drain the oil. These discs were traditionally made of hemp or coir fibers. However, they are made of synthetic fibers for easy cleaning and maintenance today. The extraction press allows obtaining a liquid waste (OMW) and a solid fraction called olive pomace (Leone et al. 2017). Finally, vertical centrifugation or decantation separates the olive oil from the water. This process has many advantages, including low-cost equipment, technical simplicity, minimal mixing time, and low volume of OMW produced (40–60 L/100 kg of olives) (Achak et al. 2019a, b). However, this process remains a relatively obsolete technology because it has many disadvantages, such as the discontinuity of the process, the contamination of the oil, and the high pollution load of OMW (Clodoveo 2013; Leone et al. 2017). Generally, OMW from this type of process had a higher phenolic content than that obtained by other extraction processes (Aggoun et al. 2016). In the centrifugation process, a horizontal centrifuge allows continuous operation. The three-phase centrifugal extraction system uses a three-phase decanter that generates solid waste, olive oil, and wastewater (OMW) (Kiritsakis et al. 2017). Olive oil production is based on the differences in density of the components of the olive paste. Three-phase decanters require the addition of water to the system. The benefits of this process include increased production, reduced labor, a smaller footprint, better oil quality, better process control, and more straightforward automation (Akbarnia and Rashvand 2019). However, it also has disadvantages such as higher water and energy consumption levels, higher OMW output, and more expensive installation (Souilem et al. 2017).

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Olive fruit

Waching Discontinuou s process

Continuous process

Crushing + malaxation with hot water

Crushing + malaxation with hot water

Olive paste

Olive paste

Pressing

Horizontal centrifugation

Oil+OMW

Dry Pomace

After centrifigal decanting: oil+OMW

Dry pomace

Olive oil

Moist pomace

After oil rinsing with hot water: oil

3 phases 2 phases

Fig. 8.1 Main extraction processes of olive oil

The two-phase centrifugation process is the olive oil production process established over the last decades in the main producing countries. In this process, minimal amounts of water are added to dilute the olive paste (Ochando-Pulido et al. 2020). This method separates the olive paste into two phases: olive oil and humid pomace (Camacho et al. 2021a). The wet pomace generated is a semi-solid by-product (a combination of olive shells and OMW) called two-phase oil mill waste (TPOMW) (Fernández-Hernández et al. 2014). The main advantage of this process is that it minimizes the high cost of wastewater handling and disposal (Ochando-Pulido et al. 2020) (Fig. 8.1). Table 8.1 shows the distribution of oil mills according to the grinding process in Eastern Morocco. In eastern Morocco, the crushing of olives is governed by a weak infrastructure (Bouknana et al. 2021). The table shows that most crushing units are located in the provinces Taourirt-Guercif with 53%, followed by the provinces of Oujda and Nador with 21% and 17%, respectively, and the province of Berkane 9% (Bouknana et al. 2021).

8.4 Characterization of OMW OMW is a highly polluted waste product with a variable composition. This variability depends on the nature of the olives, their degree of ripening, the cultivation practices, and the process used for the extraction of olive oil (Ouabou et al. 2014; Elabdouni

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Table 8.1 Distribution of oil mills according to the grinding process in Eastern Morocco (Bouknana et al. 2021) Number of oil mills per prefectures

Number of oil mills per extraction process PP

Oujda-Jerrada Berkane Nador-Driouach Taourirt-Guercif Score Percent (%)

Percent (%)

PC3P

PC2P

89

28

10

109

34

14

9

42

21

7

21

17

76

12

6

53

326

82

34

100

74

19

8

100

PP press process, PC3P Process continuous 3 phases, PC2P Process continuous 2 phases

et al. 2020). The OMW from ben Karrich area (Tetouan Provence-North Morocco) obtained from a modern two-phase olive oil extraction unit shows acid pH, high suspended solids, total solids, fats, and polyphenols (Bouharat et al. 2018). Generally, OMW is a dark blackish-brown liquid effluent due mainly to phenolic compounds (Aytar et al. 2013; Genç et al. 2020). They have a bitter taste and a strong smell of olives when fresh. This bitterness is mainly due to the easily hydrolyzed glycoside oleuropein (Niaounakis 2011). When fermented, they have a foul smell (Khdair et al. 2019). OMW has high values for chemical oxygen demand (COD), biological oxygen demand (BOD), organic matter, suspended solids, lipid compounds, phenolic compounds, and minerals (potassium, phosphorus, and calcium) (Galanakis 2011). An acidic pH characterizes OMW. This is explained by auto-oxidation reactions and polymerization of phenolic compounds that are converted during storage (Rajib et al. 2016). The conductivity values vary between 3.4 and 45.5 ms/cm. The high conductivity is due to the salting practices for the preservation of the olives before the crushing, in addition to the natural richness of the olives in mineral salts (Hattab et al. 2020). This type of discharge’s chemical oxygen demand (COD) varies between 12 and 356.1 g/L, as shown in Table 8.2. The biochemical oxygen demand (BOD5) ranges from 15.82 mg and 93 g/L. The organic load is 200–400 times higher than domestic wastewater (Özdemir et al. 2010). Generally, the physical and chemical properties of OMW, more precisely its chemical oxygen demand (COD) and its phenolic content, depend on several factors such as the olive extraction process, the olive ripening, the cultivar as well as the climatic and agronomic conditions (Justino et al. 2010). OMW contains 83–94% water, 4–16% organic compounds (including 2– 15% phenolic compounds) and 0.4–2.5% mineral salts (Babi´c et al. 2019). Phenolic compounds in OMW range from 1.5 to 9.37 g/L. Various phenolic compounds have been isolated from OMW (Mwakalukwa et al. 2019). Zghari et al. (2017) showed that the Oued Oussefrou of the Dir El Ksiba (province of Beni Mellal-Morocco), is characterized by the presence of phenolic compounds, alcohols, and fatty acids that is due to the contamination by OMW because several oil mills are located nearby. Phenolic acids present in most OMWs include o- and p-coumaric, hydroxybenzoic,

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cinnamic, caffeic, ferulic, gallic, sinapic, chlorogenic, verbascoside, protocatechuic, syringic, vanillic, and elenolic acids (Alfarawati et al. 2013; Abdel-Shafy et al. 2015; Buchmann et al. 2015; Pinho et al. 2017; Palumbo et al. 2018; Vavouraki et al. 2020; Landete et al. 2021). The most abundant phenolic alcohols are tyrosol and hydroxytyrosol (Azzam and Hazaimeh 2021). Other phenolic compounds detected in OMW include oleuropein, demethyloleuropein, catechol, 4-methyl catechol, p-cresol, and resorcinol (Rharrabti and Yamani 2018; Ramires et al. 2020; Saad et al. 2020; Landete et al. 2021; Makris 2021). OMW also contains significant amounts of secoiridoid compounds (Hamimed et al. 2020). More recently, two other secoiridoids have been identified: hydroxytyrosol acyclodihydroelenolate (HT-ACDE) and comselogoside (Azzam and Hazaimeh 2021). Finally, the most important flavonoids isolated are apigenin, hesperidin, cyanidin flavone, anthocyanin, and quercetin (Vavouraki et al. 2020; Abbattista et al. 2021; Squillaci et al. 2021). Concerning the suspended solids, the OMW presents an average load that varies between 71 and 2.1 g/L. These suspended solids come from olive paste and stone (Galanakis et al. 2010a). Phosphorus represents values of 0.1 and 186 g/L. In OMW, phosphorus can be present in the form of salts and inorganics (orthophosphates, polyphosphates) and as organic compounds dissolved or attached to suspended solids (Achak et al. 2019a, b). High concentrations of nitrate and phosphorus favor an intensive multiplication of algae and fungi, which cause the phenomenon of eutrophication in the storage basins (Bouknana et al. 2021). A study showed a total absence of heavy metals like Cadmium and Zinc with some traces of Nickel (El Yamani et al. 2020b). The microbiological composition of OMW effluents is essentially related to the extraction process, the operating conditions adopted, the storage conditions and the physicochemical characteristics (Zaier et al. 2017). The majority of the flora in OMW are yeasts and fungi. These micro-organisms are able to develop more than bacteria in this type of effluent (Bleve et al. 2011; Sar et al. 2020). However, they appear to be best adapted to the high acidity and salinity of OMW, and are more resistant than bacteria to phenolic toxicity (Abrunhosa et al. 2013). Firmicutes (mainly represented by Clostridiales), Gammaproteobacteria and Chloroflexi are thus among the bacterial flora that predominate in OMW (Venieri et al. 2010). Halotolerant bacterial strains such as Bacillus sp. have also been detected in OMW (Ertu˘grul et al. 2007). OMW samples from the southern region (Sfax, Zarzis and Châal) are characterized by a more significant number of total aerobic mesophilic flora (Zaier et al. 2017). Thus, a microbiological study showed the presence of a C2A strain of Klebsiella sp. in olive oil mill waste (Elmansour et al. 2020). This type can tolerate and degrade tannic acid at very high concentrations (Pepi et al. 2013). Pseudomonas sp are also present in OMW (Venieri et al. 2010). These bacteria probably originate primarily from the soil. Indeed, Pseudomonas is a Gram-negative organism, which is more resistant to phenols than many other species (Chebbi et al. 2021). Fecal coliforms and Escherichia coli are present (Jail et al. 2010). These bacteria probably come from flying insects or workers’ hands during harvesting.

n.d.

NH4 + (mg/L)

Cl−

0.24

Ca2 + (g/L)

n.d.

n.d.

n.d. n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

1.5

0.01582

0.03516

13.9

11.5

4.6

(Kitane et al. 2020)

n.d.

n.d.

n.d.

n.d.

0.041

n.d.

6.33

n.d.

0.00132

8.7

n.d.

264.05

2.1

28.23

5.01

(El Ghadraoui et al. 2021)

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

12.5066

2.7

3.4

4.77

(Camacho et al. 2021b)

n.d.

n.d.

n.d.

n.d.

186

n.d.

n.d.

n.d.

n.d.

5.20

n.d.

73.60

n.d.

n.d.

4.98

(Di Mauro et al. 2017)

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

6.82

n.d.

n.d.

4.4

93

213.44

6.82

18.25

4.9

(Elabdouni et al. 2020)

BOD Biological oxygen demand, COD Chemical oxygen demand, EC Electrical conductivity, SS Suspended solids, NO3 Ammonium, Cl − Chloride, P Phosphorus, PO4 2− Phosphate ion, K + Potassium, Na+ Sodium ion, Ca2 + Calcium ions

2.45

2.06

(g/L)

K+

Na+ (g/L)

2.77

PO4 2− (g/L)

0.054

1972

n.d.

4.62

0.1

n.d.

n.d.

0.798

n.d.

39,095

P (g/L)

(g/L)

n.d.

5.43

NO2 − (g/L)

9.37

Polyphenols (g/L)

NO3 − (g/L)

n.d.

BOD (g/L)

101,033

13,355

71

356.11

SS (g/L)

5478

7353

4.72

45.5

pH

EC (mS/cm)

COD (g/L)

(Rajib et al. 2015)

(Belaqziz et al. 2016)

References

Table 8.2 Physico-chemical characteristics of OMW given by several authors

−

Nitrate, NO2

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

9.22

41.21

129.55

26.35

19.37

4.44

(Babi´c et al. 2019)

−

Nitrite, NH 4 +

n.d.

n.d.

n.d.

0.32

n.d.

n.d.

n.d.

n.d.

n.d.

5.8

55

105

n.d.

12.4

5.5

(Gargouri et al. 2013)

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8.5 Environmental Issues of OMW in Morocco OMW discharge is a significant problem in Al-Hoceima (Morocco) province. It contains a significant organic fraction that causes several types of pollution (Elabdouni et al. 2020). The lack of appropriate treatment methods leads the owners of the olive oil factories in Morocco to reject these waters in nature without any control of the toxic substances in the sewage networks. OMWs contain toxic substances: phytotoxins and antimicrobials (Asfi et al. 2012; Daâssi et al. 2014; Tafesh et al. 2011). They are often discharged into natural receptors waterways without any pre-treatment and strongly harm the quality of these surface waters (Rahmanian et al. 2014). Generally, the OMW is discharged directly into the Oued Oussefrou located in Dir El Ksiba, Beni Mellal region (Morocco), without treatment (Zghari et al. 2017). The discharge of OMW into rivers causes the appearance of black spots (Elabdouni et al. 2020). The Oued Oum Er Rbia (river) is one of the principal rivers of Morocco. It represents a source of water used for human consumption, crop irrigation, industrial and hydroelectric production in large part of Morocco. On-site in recent years, the number of pollution incidents generated by wastewater from olive mills has increased in the Oued Oum Er Rbia (El alami and Fattah 2020). The rejects during the campaigns of the oil mills make increase the DBO5 of the waters of a dramatic way to the point of making fall to zero the rate of dissolved oxygenated (Rajib et al. 2015). The self-purification capacity of the wadi is thus annihilated. This impact, combined with the effects of the salinity of the OMW, means that all aquatic life is totally inhibited (Larif et al. 2012). OMWs contain high concentrations of phosphors and tannins (Kralji´c Rokovi´c et al. 2014; Haddad et al. 2021). Large amounts of nutrients cause algae growth, increase the probability of eutrophication and lead to ecological imbalance in natural waters (Withers et al. 2014). The presence of tannins from the olive, causes a discoloration of the water (Hanafi et al. 2013). In addition, OMW contains high amounts of sugars and lipids (Galanakis et al. 2010b). The increase in dissolved oxygen consumption is due to the increase in the microbial population caused by the sugars (Wang et al. 2017). The lipids form an impermeable film, which prevents the penetration of light and oxygen (Mechnou et al. 2021). In the Haouz plain, near the city of Marrakech, the analysis of the water of fourteen wells, located in an agricultural area, close to four OMW storage basins showed that the physicochemical and biological quality of this groundwater is primarily influenced by the pollution of OMW (Boukhoubza et al. 2008). In eastern Morocco, in the absence of sound and sustainable management of OMW, the soil of the region is negatively impacted (Bouknana et al. 2021). Soil quality is reduced by OMW (Steinmetz et al. 2015). These wastes are at the origin of the increase of the salinity of the soils and the decrease of the pH, which could be at the origin of the change of the physicochemical characteristics (Kavvadias et al. 2010). Some of the toxic substances in OMW, such as phenols, can inhibit microbial activity and destroy soil microflora (Naija et al. 2014). Some of the toxic substances in OMW, such as phenols, can inhibit microbial activity and destroy soil microflora (Sar et al. 2020). Odors are a major nuisance to residents of the river.

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8.6 OMW Treatment Techniques Management and treatment of OMW have been considered the primary objective of most research studies to reduce and eliminate these environmental impacts. However, treatment is based on physical, chemical and biological processes. Some of these processes seem to be more efficient and their use depends mainly on socio-economic constraints. Evaporation is the most common practice followed in Morocco and in the Mediterranean countries for OMW management. It involves disposal and storage in evaporation ponds (Bouknana et al. 2021; Kitane et al. 2020). This technique accumulates the OMW in basins, which are then dried for several weeks or even several months depending on the climatic conditions (Rajhi et al. 2018). This method is widely used in rural communities and some industries worldwide for wastewater treatment (Jarboui et al. 2010b). The evaporation method can be considered a natural biological treatment technique due to the high evaporation rates that prevail during the summer, especially in Morocco (Jarboui et al. 2010b). The evaporation effect depends on seasonal variations and the direction (Jarboui et al. 2010a). However, this OMW disposal system is one of the cheapest, easiest to use and does not require skilled labor (Otles and Selek 2012). These evaporation ponds allow the liquid phase of this effluent to be reduced to a large extent. However, they do not contribute to the elimination of the sludge resulting from the evaporation (Martínez-Gallardo et al. 2021). This technique is technically and economically feasible only for small to moderate wastewater flows due to the long evaporation (Sáez et al. 2021). This removal method is prolonged and requires relatively large evaporation areas (Michael et al. 2014). Several problems related to OMW storage in ponds are the risk of infiltration of polluting constituents of these effluents and the contamination of soils and groundwater by toxic elements such as phenolic compounds, nitrates, phosphates, and mineral elements, etc. (Bouknana et al. 2021). The dark color of the OMW, the scum cover, and the top layer of oil prevent sunlight penetration. However, the recovery of the floating oil allows sunlight to penetrate, which increases the pond’s temperature and improves evaporation (Wu et al. 2020). These recovered oils can be used for the manufacture of soap (Kayombo et al. 2002). On the other side, forced evaporation requires an important technology, but allows the improvement of the yields and the reduction of the surface and volume of the treatment plants (Benyoucef et al. 2015). The principle is based on the physical process of forced evaporation by the diffusion of wastewater in a dry air stream. This treatment alternative has the advantage of generating dried sludge which can be disposed of or reused as fertilizer, combustion material (Hodaifa et al. 2020; Shaheen and Karim 2007). Incineration is an expensive, complex, and highly energy-intensive technique, which is even less appropriate when the OMW is primarily composed of water. It allows to evaporate first the aqueous phase of the OMW and then burn the sludge (Doula et al. 2017). Air pollution is a significant problem for incineration (Lam et al. 2010).

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Biological processes use microorganisms to degrade organic compounds in olive oil mill effluents. They are subdivided into aerobic and anaerobic processes. Several modern anaerobic technologies have demonstrated their ability to effectively treat OMW (Paraskeva and Diamadopoulos 2006; Aboutaleb et al. 2018). Biological treatments are among the treatment strategies for OMW proposed in Morocco (El Gnaoui et al. 2020; Tallou et al. 2020). Anaerobic processes have the advantage of reducing organic matter in municipal and industrial wastewater while producing energy (Skouteris et al. 2012). Sampaio et al. (2011) treated the OMW using a hybrid anaerobic up-flow digester. This reactor removed 81–82% of the COD with 3.7–3.8 m3 biogas production. The biomass developed in this reactor was not affected by raw OMW or organic shocks. Anaerobic digestion experiments of OMW without pretreatment were performed. Signs of inhibition in the biological process were observed due to a high concentration of inhibitors in OMW. Therefore, pre-treatment to reduce phenolic compounds is usually necessary (González-González and Cuadros 2015). Many authors have focused on a more promising alternative such as codigestion of OMW with other substrates such as municipal wastewater, cow dung, sewage sludge (Al bkoor Alrawashdeh 2019; Vavouraki et al. 2019; Tallou et al. 2020). El Gnaoui et al. (2020) studied the result of combining OMW with wastewater from the campus restaurant of Ibn Tofail University Morocco to improve the anaerobic digestion of OMW in terms of methane production and process stability. The best anaerobic digestion performance was observed for the (FW:OMW) mixture (80:20). The methane yield and biodegradation were higher. Thus, they found that this technique is an effective and appropriate method to solve the problem of oil mill effluents. Ruggeri et al. (2015) have developed an anaerobic digestion system, which allows the recovery of energy from olive waste, solving the problem of its disposal. However, a pre-treatment was carried out in front by adding CaCO. This process resulted in biogas production of 21.6 NL/L. Anaerobic treatment consists of a series of microbiological processes that convert organic compounds under anaerobic conditions into biogas that can be used for energy production (Afilal et al. 2013). Anaerobic digestion of OMW offers significant benefits such as low biological sludge production, production of renewable energy such as biogas, production of liquid fertilizer and reduction of greenhouse gas emissions (Bampalioutas et al. 2019; Tufaner 2020). A major disadvantage is the high toxicity of phenolic compounds (La Cara et al. 2012). Enaime et al. (2020) showed that anaerobically treated samples improved the germination of maize. On the contrary for tomato, a high dilution was necessary to reduce the inhibitory effect of OMW. Anaerobic processes are significantly influenced by a number of factors such as the type and variability of wastewater, the type of organic contaminants in the influent, its pH (Gao et al. 2010). Anaerobic processes can be more interesting than aerobic processes, as they can be used to treat compounds that are recalcitrant under aerobic conditions (Xiao and Roberts 2010). Many authors have combined anaerobic treatment with other processes (Amor et al. 2015; Vavouraki et al. 2021). Khattabi Rifi et al. (2021) showed that anaerobic treatment removed 23.9% nitrate, 29.1% turbidity, and 16.2% polyphenol. However, the combination with aeration treatment increased the pollution removal efficiencies of the OMW. Aerobic biological reactors have been studied

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to treat OMW (Günay and Çetin 2013; Michailides et al. 2011). These effluents are characterized by the presence of aerobic microorganisms (González-González and Cuadros 2015). These microorganisms use phenols as an energy source and allow their incorporation during the neoformation of humic substances (El Hajjouji et al. 2014). The treatment of OMW by the aerobic process seems to reduce the level of some phenolic compounds (Hussain et al. 2015). Aerobic bacteria largely degrade low molecular weight phenolic compounds but are relatively ineffective against the more complex polyphenols that are responsible for OMW pollution (Otles and Selek 2012). In addition, aerobic processes can only work efficiently if the OMW is diluted before treatment (Landeka Dragiˇcevi´c et al. 2010). Aerobic treatment was used as a pretreatment step to improve the anaerobic digestion of the OMW, whose goal is to reduce the concentration of phenolic compounds and decrease the total chemical oxygen demand (TCOD), due to the aerobic microorganisms (González-González and Cuadros 2015). Jalilnejad et al. (2011) studied the effect of aerobic pretreatment with Ralstonia eutropha. They found that a TP concentration of 250 mg/L was completely degraded within 24 h. This pretreatment reduced the COD, polyphenol concentration of undiluted OMW by 42% and 56%, respectively. In another study, aerobic treatment was studied during a period of 30 days on OMW obtained from an activated carbon column mixed with 15% lime, neutralized and diluted 15 times with distilled water. The results showed a significant percentage reduction of 79.78, 69.43 and 60.67%, respectively, for BOD5 , COD and polyphenols (Benamar et al. 2020). In a different study they found that aerobic pretreatment is able to improve the performance of the anaerobic digestion that follows it (Al bkoor Alrawashdeh and Al-Essa 2020). This aerobic biological treatment can also be done by a mixed culture of bacteria isolated from the olive pulp. Approximately 50% of the COD and phenols could be easily removed during a period of 24 h (Michailides et al. 2011). Aerobic bioreactors have a major drawback which is the excessive consumption of oxygen. Physical, chemical or physicochemical pretreatments of these wastes will be essential in order to reduce the high oxygen consumption, as well as the toxicity of the OMW towards the purifying microorganisms. Infiltration-percolation is a biological treatment by bacterial cultures fixed on a fine support used in Morocco to treat OMW (Achak et al. 2009). El Herradi et al. (2016) treated wastewater from oil mills in the city of Sefrou (Morocco) by infiltrationpercolation on two types of filters. The first one is based on soil and sand (F1) and the second is based on soil, sand, and fly ash (F2). The results show that the F1 and F2 filters allowed an increase in pH, decreased electrical conductivity, and significant removal of COD, BOD5, total polyphenols, TSS, nitrogenous matter, and phosphate. Membrane processes such as ultrafiltration and reverse osmosis are used in the treatment of OMW to remove pollutants from the water (Akdemir and Ozer 2009; Akdemir and Ay˘gan 2019). In wastewater treatment plants, adsorption processes are applied to remove dissolved pollutants (De Gisi et al. 2016). In the treatment of OMW, the main objective of adsorption is to biodegrade organic compounds with bactericidal or coloring effects (tannins–phenols, etc.) (Solomakou and Goula 2021). Adsorption is a mass transfer process that involves the accumulation of substances at

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the interface of two phases, such as the liquid–liquid, gas–liquid, gas–solid or liquid– solid interface (De Gisi et al. 2016). Adsorbents widely used for OMW treatment include mainly activated carbons (Aliakbarian et al. 2015), ion exchange materials (Víctor-Ortega et al. 2016), zeolites (Aly et al. 2014). Adsorption results from specific interactions between an adsorbent and a compound to be adsorbed (the adsorbate) (Desbrières and Guibal 2018). The adsorption process is influenced by the dose of adsorbent, the pH of the solution, the contact time and the temperature (Elayadi et al. 2021). Dehmani et al. (2020) studied the adsorption efficiency of OMW on clay. The results showed that adsorption on clay increased the pH of the sample to 6.14 and removed 42% of the COD and 57.4% of the phenolic compounds. OMW treated by adsorption on clay showed a significant effect on the germination of Lepidium sativum seeds and the growth of Vicia fabaplants. The treatment by this type of process is more respectful of the environment because of the use of a very abundant product in Morocco and which is purely natural. Achak et al. (2014) have developed a wheat bran-based adsorption process for the removal of phenolic compounds from OMW. This treatment allowed to adsorb 45–67% of phenolic compounds at a dosage of 10–50 g/L of wheat bran. The main advantages of this type of adsorption are the speed of adsorption, the cost and the availability of the bio adsorbent. Vavouraki et al. (2021) performed adsorption experiments in the presence of different adsorbent resins (XAD 4, XAD 16, FPX 66). FPX 66 showed the best efficiency compared to other resins with a 75% reduction of phenolic compounds. Adsorption technology has gained more comprehensive application for treating different types of wastewater due to its low cost, simplicity, versatility and robustness (Lakherwal 2014; De Gisi et al. 2016; Burakov et al. 2018). Several works have used activated carbon as an adsorbent for the treatment of OMW (Ziati et al. 2017; Eder et al. 2021). The use of activated carbon has disadvantages, such as the impossibility of its reuse. Chitosan has been used as a coagulant for the treatment of OMW, but it also represents strong adsorption characteristics of the pollutants (Rizzo et al. 2008; Saheed et al. 2020). Filtration is a process used to remove large particles, suspended solids, oil and grease (Zirehpour et al. 2012). However, membrane fouling is one of the common problems associated with wastewater purification and OMW in particular (Bottino et al. 2020; Hube et al. 2020). A pre-treatment step is necessary to reduce membrane fouling and increase filtration efficiency. The correct selection of membrane type and operating conditions affect the performance and efficiency of the filtration process (Zirehpour et al. 2015). Enaime et al. (2019) developed a combined process, including the filtration of raw oil mill wastewater (OMW) from an extraction system in Morocco on two successive olive stone (OS) filters followed by coagulation-flocculation. The results showed that this filter provided better removal of total suspended solids (TSS), fatty matter (FM) from water, total phenolic compounds and COD with percentages of 82.5, 73.8%, 11.3 and 23.2%, respectively. This process represents advantages due to using a natural filter widely available in the Mediterranean countries, particularly in Morocco. Mounia Achak et al. (2019a, b) studied the efficiency of a Moroccan sand filter filled with 50 cm of sand and 10 cm of gravel at the top and bottom of the filter. This process was performed for OMW diluted with domestic wastewater. Indeed, the sand filter was very efficient for the removal of pollution from the OMW with

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average efficiencies of 60.4%, 74.4%, 80.61%, 97.85%, 96.45%, 79.75% and 76.75% for NTK, NH4 + , NO3 − , PO4 3− , total COD PT and dissolved COD respectively. Ultrafiltration is an advanced separation technology used in various industrial sectors (Lafi et al. 2018; Li et al. 2018). It was established initially as a wastewater treatment technique (Ren et al. 2021). In particular, the treatment of OMW effluents (El-Abbassi et al. 2011). This technique is recognized as a separation process of clarification and disinfection (Al Aani et al. 2020). Yahiaoui et al. (2011) studied the effect of combining ultrafiltration with the electrocoagulation process on removing chemical oxygen demand (COD) from OMW. Ultrafiltration has achieved a COD removal efficiency of approximately 96%. Ultrafiltration (UF) membranes have come a long way as a safe, clean, economic and powerful separation tool for a wide range of constituents and contaminants in water and wastewater (Al Aani et al. 2020). This treatment technique is a simple treatment method due to the low energy consumption, the reduced number of control methods, the absence or less importance is given to process chemicals operating temperature and treatment quality (Buonomenna and Bae 2015). This process allows many industries to become more environmentally friendly by facilitating waste recycling and resource recovery (Bajracharya et al. 2016). Reverse osmosis (RO) is a process that uses spiral wound semi-permeable membranes to separate and remove dissolved solids, organics, pyrogens, heavy metals and chemicals, submicron colloidal material, color, nitrate and bacteria from water (Garud et al. 2011). OMW treatment tests were carried out using this technique, and the results obtained showed that the maximum COD removal efficiency was 96.3 and 96.2% for the XLE and BW30 reverse osmosis membranes (Coskun et al. 2010). The OMWs were centrifuged in the first experiment and then passed through reverse osmosis. At the same time, in the second experiment, ultrafiltration was placed between the centrifuge and reverse osmosis. The reverse osmosis experiments were conducted at 10, 15, 20 and 25 bar. Chemical oxygen demand (COD) removal efficiencies at 25 bar were 97.5% (Coskun et al. 2013). Reverse osmosis represents several advantages such as high efficiency of the membranes in the selective rejection of minerals, high water permeability, reduced production costs, environmental protection (Madaeni and Samieirad 2010). But this type of process seems to be very expensive because of the requirement of higher pressures and the high cost of the membranes (Mungray et al. 2012). Electrocoagulation (EC) is an electrochemical technique used in Morocco that generates coagulant species in situ from the electro dissolution of sacrificial anodes, usually iron or aluminum, destabilizes the pollutants in suspension, dissolved or emulsified using an electric current (Elkacmi et al. 2017a; Garcia-Segura et al. 2017). It can potentially remove various types of pollutants, including organic and inorganic contaminants for different types of wastewater (Shahedi et al. 2020). EC includes the production of cationic metal species inside the reactor by accelerated corrosion of the consumable metal anode induced by the electrical energy exerted on the plates (Ghernaout et al. 2019). Electrocoagulation combines different mechanisms which can be electrochemical (dissolution of metals and reduction of water, electrooxidation or electroreduction of pollutants etc.), chemical (acid/base equilibrium with

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Fig. 8.2 Interactions occurring within an electrocoagulation reactor (Hakizimana et al. 2017)

modification of the pH, precipitation of hydroxides, the oxidation–reduction reaction in the mass etc.) and physical (physical adsorption, coagulation, flotation etc.). They can be sequential and/or parallel. They are all summarized in Fig. 8.2, which highlights the complexity and interaction between the mechanisms of the EC process (Hakizimana et al. 2017). Several factors affect the electrocoagulation process, such as pH, electrode materials, current density, supporting electrolytes (Garcia-Segura et al. 2017). A great deal of work has been done on treating OMW by electrocoagulation (Hanafi et al. 2010; Salameh et al. 2015; Ghahrchi et al. 2021). Elkacmi et al. (2020) studied the possibility of OMW detoxifying in an external loop airlift reactor by electrocoagulation powered by a solar photovoltaic system. The results show an abatement rate of about 79.24% for COD, 94.82% for polyphenols and 97.87% for dark color. This process offers a low operating cost (0.2 USD/m3 ) compared to other treatment processes due to a renewable and sustainable energy source. Electrocoagulation has advantages because of the low use of chemicals, low sludge production, and the simplicity of the equipment. However, some disadvantages are that the sacrificial anodes must be replaced when exhausted, the oxide film can passivate the electrodes, and the aqueous medium must be conductive (Barrera-Díaz et al. 2011). The coagulation-flocculation technology has been widely used in various disciplines of water treatment internationally and in Morocco (Baptista et al. 2015; Teh et al. 2016; Dotto et al. 2019; Hasna et al. 2020). Coagulation-flocculation aims to grow colloidal particles by destabilizing suspended matter and then forming flakes by absorption and aggregation. These will be decanted and filtered (Choumane et al. 2017). Colloidal particles in the solution are negatively charged. They tend to repel each other and therefore remain in suspension. We say that there is the stabilization of particles in the solution. The principle of coagulation is the destabilization of particles in suspension, using chemical reagents called coagulants by neutralizing their negative charges. This method requires significant agitation (Bratby 2016). Flocculation

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is the phenomenon of forming larger flakes (agglomeration of discharged colloids in a three-dimensional network). Flocculants or flocculation additives are used to do this. Unlike the coagulation stage, flocculation requires slow agitation (Oladoja et al. 2017; Osborn 2015). Flocculants or flocculation aids are, in their great majority, polymers of very high molecular weight. They can be mineral, natural organic, or synthetic organic (Lapointe and Barbeau 2020). Coagulation-flocculation depends on pH, coagulant/flocculant dosage, speed, mixing time, temperature, retention time, etc. (Kim 2016; Rodrigues et al. 2013). The addition of a coagulant often changes the pH of the water. This variation must be taken into account to stay within the optimal range for coagulant precipitation. The optimal pH range is the range within which coagulation takes place sufficiently quickly (Ghernaout and Boucherit 2015; Ratnaweera and Fettig 2015). The reagent dose is a parameter to be considered during the treatment process. Coagulant, which is usually strongly acidic, tends to lower the pH of the water. Add acid or a base (Sher et al. 2013). Even though these techniques increase the efficiency of flocculation coagulation, they represent several disadvantages such as the need for pH adjustment before or after the treatment, the sensitivity to temperature changes, the need for higher dosages, the sensitivity to the characteristics and composition of the OMW, as well as the excessive production of sludge.

8.7 Valorization of OMW In Morocco, olive by-products have been mainly valorized as bioenergy until now (Donner and Radi´c 2021). OMW is rich in mineral and organic nutrients. This criterion has led researchers to develop many processes for the valorization and exploitation of OMW at the laboratory and pilot scale. The OMW can be valorized and used to produce some valuable components (Akretche et al. 2019; Paz et al. 2021). The application of the anaerobic digestion process to the OMW has allowed the transformation of organic substances into biogas (65–70% methane and carbon dioxide) through biochemical reactions (Olatunji et al. 2021; Vassalle et al. 2020). Methane energy can be used in thermal form or converted into electrical energy (Negro et al. 2017). El Gnaoui et al. (2020) investigated the combination of OMW with wastewater from the Ibn Tofail University campus restaurant to improve the anaerobic digestion of OMW in terms of methane production and process stability. The results show that co-digestion of OMW with FW Food Waste is an effective and suitable method to solve the problem of oil mill effluents. The best anaerobic digestion performance was observed for the (FW:OMW) mixture (80:20). The methane yield and biodegradation were higher with values of 302.16 ± 03.04 mLSTP CH4 gVS−1 and 86 ± 04.7% respectively. Compost is obtained mainly by aerobic-anaerobic degradation of the organic substance of the solid residues, which has for essential goal to fix the fertilizing elements on a carbonated substrate to restore them to the ground according to the needs of the plants (Pandit et al. 2019; Naserian et al. 2021). Zorpas and Costa (2010) studied the co-composting of olive oil solid residues (OSR) and OMW treated with

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Fenton. Results indicated that OMW is detoxified at the end of the Fenton process, and COD is reduced by up to 70%. The final co-composted material of OOSR with treated olive mill wastewater (TOMW) has optimal characteristics and is suitable for agricultural purposes. Sáez et al. (2021) compared the effect of composting and the combination of composting followed by vermicomposting to eliminate the potential toxicity of OMW-derived sediments stored long-term in evaporation ponds. The results showed that in situ composting and composting combined with vermicomposting were effective in addressing the environmental problems associated with the toxicity of OMW. The OMW contains a significant quantity of oil. These can be used to manufacture soap and glycerine (Elkacmi et al. 2017b). Elkacmi et al. (2016) showed that the fractional crystallization technique successfully separated oleic acid from OMW to be used as a commercial product for soap and glycerin production. OMWs are of economic interest because of their high load of bioactive phenolic constituents, notably polyphenols, whose beneficial effects on health have been examined (Quiñones et al. 2013; Khdair and Abu-Rumman 2020). More than 50 different phenolic compounds have been identified in OMW, including hydroxytyrosol (HT), tyrosol, and other minor phenolic compounds, such as caffeic and ferulic acids, and secoiridoid compounds, which among other things, are associated with antioxidant, anti-inflammatory and antimicrobial properties (Carrara et al. 2021). Akretche et al. (2019) evaluated the potential of OMWs for the synthesis of antioxidant phenols that can be recovered for industrial applications as food additives. They adopted two green and environmentally friendly procedures: free radical-induced grafting using H2 O2 /ascorbic acid and complexation grafting using CaCL2 . The results show a content of 2 g/L (gallic acid equivalent). The antioxidant activities were estimated to be 61 ± 0.4% and 84 ± 0.8% after 96 h for the CaCl2 grafting and free radical grafting, respectively.

8.8 Conclusion The increase of the olive sector in the world over the last decade, combined with an increase in the use of large quantities of water, has generated a large amount of OMW. However, OMW is produced in a limited period and in large quantities in Morocco. Their chemical and physical characteristics vary depending on the cultivars, the harvest period, the type of olives, and the extraction technology used. Different methods to reduce the pollution of OMW have been studied. This review focuses on the relevant state of the art on recent treatment processes, management and recovery of OMW developed in Morocco and internationally, including biological treatments, coagulation-flocculation, and infiltration percolation. Each treatment process has its advantages and disadvantages. The choice of the most appropriate treatment is based on several factors: simplicity, safety, energy efficiency, waste sludge production,

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and operating cost. The disadvantages of biological methods, such as the need to dilute the effluent several times, which increases the cost of treatment, should be considered. In conclusion, further research on the technical–economic evaluation for future industrialization is needed.

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Zema DA, Lucas-Borja ME, Andiloro S, Tamburino V, Zimbone SM (2019) Short-term effects of olive mill wastewater application on the hydrological and physico-chemical properties of a loamy soil. Agric Water Manag 221:312–321 Zghari B, Doumenq P, Romane A, Boukir A (2017) GC-MS, FTIR and 1 H, 13 C NMR structural analysis and identification of phenolic compounds in olive mill wastewater extracted from Oued Oussefrou effluent (Beni Mellal-Morocco). J Mater Environ Sci 8:4496–4509 Ziati M, Khemmari F, Cherifi O, Didouche FY (2017) Removal of polyphenols from olive mill wastewater by adsorption on activated carbon prepared from peach stones. Rev Roum Chim 62:865–874 Zirehpour A, Jahanshahi M, Rahimpour A (2012) Unique membrane process integration for olive oil mill wastewater purification. Sep Purif Technol 96:124–131 Zirehpour A, Rahimpour A, Jahanshahi M (2015) The filtration performance and efficiency of olive mill wastewater treatment by integrated membrane process. Desalin Water Treat 53:1254–1262 Zorpas AA, Costa CN (2010) Combination of Fenton oxidation and composting for the treatment of the olive solid residue and the olive mile wastewater from the olive oil industry in Cyprus. Bioresour Technol 101:7984–7987

Chapter 9

Changes in Olive Mill Waste Water Management in Turkey Renan Tunalıoglu

Abstract Olive is a fruit that cannot be consumed raw. Therefore, it has to be processed and one of these technologies is the olive oil processing technology. Different technologies are used when processing olive oil into oil. Whatever technology is used (classical or modern: 2 phase-3 phase etc.), Olive Mill Waste Water (OMWW) is revealed. According to some people, OMWW is waste, and according to others, it is not waste. The reason for this is that when OMWW is released directly to nature (soil, stream, sea, etc.), it pollutes the environment due to the organic components it contains. OMWW appears in all technologies/systems used in olive oil production. While health authorities in the world describe olive oil as a very valuable reward given to human beings, environmental authorities claim that nature is polluted due to OMWW that occurs while processing olives into oil. This dilemma is evaluated from the same point of view in Turkey, which is known as the homeland of olives, as in the world. Because Turkey is an important olive oil and table olive producer, ranking fourth among the world olive producers which has 8% of the total amount of olives and 9% of olive oil production in the world. In Turkey, approximately 180 thousand tons of olive oil is produced each year. For this reason, in this section, the problems with OMWW in Turkey and the solution possibilities of these problems are investigated. As a result of this research, it has been observed that there is a very serious change in Turkey. In recent years, it has been understood that olive oil factories prefer ecological processing systems and pollute the environment less. Because, on the one hand, the enterprises that could not make the necessary investments for waste management before are supported by the state, on the other hand, plants used only as fuel (Olive Pomace + OMWW) are now used as fertilizer, feed for cattle and cosmetics. Keywords OMWW · Olive oil · Dilemma · Changes · Turkey

R. Tunalıoglu (B) Department of Agricultural Economics, Faculty of Agriculture, Aydın Adnan Menderes University, South Campus, Aydın, Turkey e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_9

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9.1 Introduction Olive is not consumed directly so it needs to be processed into table olives and olive oil. The process through which olive oil is produced results in by-products that may be harmful to the environment. One of such by-products is pomace olive oil which can be re-processed as oil and used as raw material in food, industrial and energy sectors, while another by-product, namely Olive Mill Waste Water (OMWW), is not yet considered to be efficiently reusable (Tunalıo˘glu and Sefero˘glu 2012). There are still nearly 1200 olive oil factories in Turkey. Approximately 2% of these factories are Classical System, 38% are three-phase Modern System, and 60% are two-phase Modern System. Olive Mill Waste Water (OMWW) mostly leaves the factories operating with classical systems and modern three-phase systems, so it is seen as a problem with these factories (Figs. 9.1 and 9.2). However, olive oil factories in Turkey were operating in the classical system with very few 2-phase systems and many 3-phase systems for many years (until ten years ago). After the olive oil was obtained, the olive pomace was sent to the pomace factories, the OMWW was kept in the pits next to the olive oil factories, and after the pits were filled, it was discharged to the nearest river, lake, pond, sea and soil. For this reason, OMWW was a serious problem for olive oil factories and some Ministries (environment-agriculture-industry) in Turkey for many years. Existing olive oil enterprises were penalized when they discharged OMWW to the environment, starting from the 2005/2006 production season, within the framework of the Environment Law. These penalties are (a) administrative fine starting from approximately 13,000 EURO in accordance with the relevant paragraphs b,f,ı,i, of Article 20 and 23 of the Environmental Law No. 2872 due to the high organic matter contained in the OMWW of the factories, and (b) Turkish. It was considered as a crime against

2 Phases 3 Phases Clasiccal

Fig. 9.1 The olive oil systems in Turkey (Murat Hocao˘glu 2015)

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OLIVE (On year-off year-average 1200 thousand tons/years)

TABLE OLIVE (300 thousand tons/year)

OLIVE for OIL

)

Modern (Continiu System) Olive Oil Plant

(900 thousand tons/year )

3 PHASE SYSTEM (Water+Centrifuge) Olive for oil is processed total 38% by 3 phase system in Turkiye (342 thousand tons )

Olive Oil (20%) (68 thousand tons)

Olive Pomace(40%) (137 thousand tons) (7 thousand tons. row pomace oil)

OMWW (40%) (137 thousand tons)

2 PHASE SYSTEM (Centrifuge) Olive for oil is processed total 60% by 2 phase system in Turkiye (540 thousand tons) Olive Pomace +OMWW (80%) (416 thousand tons olive pomace) (16 thousand tons row pomace oil) (208 thousand tons OMWW)

Olive Oil (20%) (108 thousand tons)

Classical Sysem Olive Oil Plant

HDYROLIC SYSTEM (Water + Centrifuge) Olive for oil is processed total 2% by 2 phase system in Turkiye (18 thousand tons)

Olive Oil (20%) (4 thousand tons)

Olive Pomace (32%) (5.7 thousand tons) (0.3 thousand tons row pomace oil)

OMWW (48%) (8 thousand tons)

Total OMWW (353 thousand tons)

Total Olive Oil (180 thousand tons)

Total Olive Pomace (559 thousand tons) Total Olive Row Pomace Oil (23.3 thousand tons)

in Practice -Sewage system -Rivers -Lakes -Stream -To the ground Suggestions Evaporation Pool/lagoon Purified Transition to 2 phase system

Fig. 9.2 Olive mill waste water processing and using in Turkey (by prepared by author that case of study which calculated with last 4 years in 2020, adopted by Tunalıo˘glu and Arma˘gan 2008)

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the environment within the framework of paragraph 1 of Article 181 and paragraph 1 of Article 182 of the Penal Code, and was given a prison sentence (from 6 months to 2 years) (Yıldırım 2014). In the past years, while the legal regulations were being implemented, many scientific studies with technical, economic and social dimensions were carried out in Universities and Research Institutes on the treatment and evaluation of OMWW (Tunalıo˘glu 2010). The direction of the research was primarily to evaluate the OMWW by treating it (establishing a common treatment plant or an individual lagoon system) or to switch all factories in the country to a two-phase system. But then it was decided that OMWW is difficult and expensive to purify due to the oil, COD and BOI values it contains, and therefore there is no single, ideal and economical treatment system. Because, giving it to central systems or treatment plants in the form of waste water has only created a temporary solution and its negative effects on the environment have continued. Despite everything, it was determined that the treated black water, even if it was treated, did not comply with the current environmental regulation of Turkey (Tunalıo˘glu et al. 2012). On the other hand, OMWW cake, which is kept in the pits and whose water evaporates, started to be used as an additive to the rations in animal feeds, for fertilizer and weed control. While agricultural soils in Turkey are poor in terms of organic matter and high fertilizer prices increase the use of OMWW as fertilizer, it has been found to have positive effects on weed control and especially on soil nematode prevention (Tunalıo˘glu and Arma˘gan 2008). Other alternative solutions were then proposed. These recommendations are described below.

9.2 Options for Solution Although it is known that OMWW causes environmental pollution in Turkey, official authorities claim that it is an organic source of water. For this reason, several different solution options have been reviewed, especially since the last ten years. While determining these solution options, the most important issue is to take into account the technical and social costs. The solutions proposed by the government in Turkey are as follows. 1. Establishment of integrated facilities: (long term) It is the most ideal option. In this system, olive oil plant, pomace factory, refining plant and even table olive processing plant should be together. However, since establishing such facilities requires long-term and heavy investments, a serious organization is required. This organization will only be possible with the increase of cooperatives in the sector. In this proposal, besides financial resources, social costs should also be discussed. Establishing an integrated facility for olive cultivation in Turkey, which is still dominated by the private sector rather than cooperatives, is only a long-term proposition and time is needed.

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2. Conversion of three-phase olive oil enterprises to two-phase system: (long term) Olive variety etc. in two-phase system. Since relatively less water needs to be added to the process compared to other characteristics, the resulting OMWW is disposed of together with pomace. Since the waste water generation is limited to the washing water, the pomace that comes out has a humidity of 50–60%. In this respect, the two-phase system is the most valid option for the solution in terms of environmental protection. In order for this option to be implemented, the costs of conversion to two phases of the machines in the existing three-phase olive oil enterprises in Turkey should be taken into account. In addition, since the OMWW comes out together with the pomace in the two-phase system, some arrangements should be made, such as an area where the relatively more watery pomace can be stored, and the drying facilities of pomace factories should be increased. If a twophase system is preferred, the biggest problem is transportation. After all; the cost of converting from three phases to two phases, the establishment and renewal of an additional drying facility of pomace factories, the transportation cost of water pomace, the storage cost, and the pomace income of olive oil factories in case of switching to a two-phase system will decrease by approximately 50%. Serious financial resources are required for this solution option. In order to support this proposal, a transition program of at least 5 years should be established with the support of the state to olive oil factories and investment support should be provided in the transformation into enterprises in this process (TBMM 2006, 2008). 3. Lagoons: If olive oil enterprises continue to operate in three phases, the solution is for each enterprise to use a lagoon. The lagoons should be constructed in accordance with the dimensions requested by the Ministry of the Republic of Turkey (Republic of Turkey Ministry of Environment, Urbanization and Climate Change). It is necessary to be careful that these pools, which seem to be the most suitable solution in a short time, do not cause wrong investments and waste (Tunalıo˘glu and Bekta¸s 2012) 4. Treatment systems: In case the olive oil enterprises continue to operate in three phases, the OMWW must be purified so that it does not pollute the environment. In this option, the cost of construction and operation of the treatment plant and the cost of collecting and transporting the OMWW will form the investment basis of this option. In this management approach for the treatment of black water, it is recommended to use central treatment plants instead of waiting for individual treatment solutions from each plant. It would be a wise proposal to collect the wastewater of olive oil enterprises, most of which were established on a small scale and in remote areas, and bring it to the central treatment facilities to be established in certain regions and purify it. The establishment of such facilities around Organized Industrial Zones (OIZ) or municipal waste water treatment facilities is important in terms of technical infrastructure. Thus, the waste water that has passed a pre-treatment in these facilities is conveyed to the city waste water treatment plant.

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If the options determined in detail by the above state are summarized as a priority; 1. Transition and conversion to the two-phase system 2. Continuing the three-phase system a. Continuing three-phase production + making the lagoons suitable for the criteria (enlargement etc.) b. Continuing three-phase production + on-site treatment + discharge c. Continuing three-phase production + central treatment + discharge d. Continuing three-phase production + gasification of pomace in the central plant and evaporation of waste heat and black water. In fact, the solution proposals of both the state and the private sector (businesses) did not differ much from each other. The priority order in the evaluation of the operators regarding the OMWW is, 1. Continuing the three-phase system a. Continuing to use the three-phase system and discharging directly to the receiving environment, b. Continuing to use the three-phase system and using it as fertilizer, irrigation water and weed control kept in the lagoons (after keeping it in the lagoons), c. Continuing to use the three-phase system and giving it to the sewer, creating an evaporation system, making treatment, 2. Transition and transformation to the two-phase system–to switch to the twophase system with the support of the state-to obtain olive pulp and animal feed from olive pulp.

9.3 Conclusion and Comments In Turkey, while the state has claimed for many years that OMWW is polluting the environment, operators stated that they do not believe this in a study measuring environmental awareness (Tunalıo˘glu and Yıldırım 2014). However, Turkey’s agricultural and olive growing policies have changed since 2000, farmers were provided with certified saplings and garden plant support, and as a result, the number of olive trees and olive oil production increased. With this increase, it has started to bring the territorial water problem to Turkey’s agenda again and again, and the environment has become more polluted. On the other hand, olive oil factories had two reasons to continue the three-phase system. The first was the concern of being deprived of the oily pomace income from the three-phase system, and the second was not having the capital to convert to the two-phase system. Question: Is OMWW no longer The Weakest Link of Olive Oil Production in Turkey? Answer: Not now!

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Because in the following period, Turkey’s most intensive olive production is in the Aegean, Marmara, Mediterranean and Southeastern Anatolia regions (Fig. 9.1). Countless meetings, symposiums, congresses and workshops were held for the solution of the OMWW problem, projects made with the funding of Universities and Research Institutes and Ministries were completed, and an olive research commission was established twice in the Turkish Grand National Assembly. In summary, all official institutions and organizations and the private sector have made a serious effort to solve the problem. Fund resources of the state created through different ministries (Republic of Turkey Ministry of Environment, Urbanization and Climate Change, Republic of Turkey Ministry of Agricultural and Forestry and Republic of Turkey Ministry of Industry and Technology), GEKA (South Aegean Development Agency), KOSGEB (Small and Medium Enterprises Development Organization of Turkey), with ARDSI [Agriculture and Rural Development Support Institution (EU-IPARD)] provided support to operators for establishing a two-phase system or converting from a threephase system to a two-phase sytem. An important point is that 78% of the machines used in olive oil production facilities The existence of these domestic companies in Turkey and the fact that 88% of the existing factories working with the 3-phase system are suitable for conversion into two phases accelerated the solution of the OMWW problem. In the last ten years, the state has supported the operators by creating different funding sources in the transition from three phases to two phases, taking into account all the research results. Acknowledgements This paper is based on two ongoing projects 1—between the Department of Agricultural Economics at Aydın Adnan Menderes University in Turkey and School of Management at the University of Southampton in the UK. The financial support from the Scientific and Technological Research Council of Turkey (TUB˙ITAK: 2219/2010) and 2—Republic of Turkey Ministry of Environment, Urbanisation and Climate Change, “Olive Sector Waste Management” Project (TUBITAK MAM: 2014–2015).

References ˙ Murat Hocao˘glu S (2015) TÜBITAK—The Scientific and Technological Research Council of Turkey MAM Zeytin Sektörü Atıklarının Yönetimi Projesi 5148602 (ÇTÜE.15.223) Nihai Rapor T.C. Çevre-Sehircilik ¸ ve ˙Iklim De˘gi¸siklikleri Bakanlı˘ga Çevre Yönetimi Genel Müdürlü˘gü, Temmuz 2015 Gebze, Kocaeli-Turkey TBMM (The Grand National Assembly of Turkey) (2006) 22. Dönem T.B.M.M” (14 Subat—14 ¸ Haziran 2006) Türkiye Zeytin ve Zeytinya˘gı ˙Ile Di˘ger Bitkisel Ya˘gların Üretimindeki Sorunların Ara¸stırılarak Alınması Gereken Önlemlerin Belirlenmesi Amacıyla Kurulan Meclis Ara¸stırması Komisyon Raporu (10/41, 170, 177, 263, 295) Ankara, Türkiye (in Turkish) TBMM (The Grand National Assembly of Turkey) (2008) 23. Dönem T.B.M.M. (11.03.2008– 11.07.2008) Zeytin ve Zeytinya˘gı ile Di˘ger Bitkisel Ya˘gların Üretiminde ve Ticaretinde Ya¸sanan Sorunların Ara¸stırılarak Alınması Gereken Önlemlerin Belirlenmesi Amacıyla Kurulan 10/27, 34, 37, 40, 102 Esas Numaralı Meclis Ara¸stırması Komisyon Raporu, Ankara, Turkey (in Turkish)

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Tunalıo˘glu R (2010) Environmental impacts and solutions olive vegetable water investigation of possibilities in Turkey: Aydın Province. TUBITAK 2010/1 BIDEB. Project Report. Ankara, Türkiye (in English) Tunalıo˘glu R, Arma˘gan G (2008) Aydın ˙Ilindeki Zeytinya˘gı ˙I¸sletmelerinde Elde Edilen Yan Ürünlerin Tarım-Sanayi ve Çevre ˙Ili¸skileri Boyutunda De˘gerlendirilmesi Türkiye VIII. Tarım Ekonomisi Kongresi Bildiri Kitabı. Cilt 2. Bursa, Turkey (in Turkish) Tunalıo˘glu R, Bekta¸s T (2012) The problem of olive mill wastewater in Turkey and some solution alternatives. Agriculturae Conspectus Scientificus 77(1):57–60 Tunalıo˘glu R, Sefero˘glu S (2012) The evaluation of olive mill waste water in Turkey: is it really a waste or no? In: Symposium on olive oil mill wastes and environmental protection, proceedings, 16–18 Oct 2012, Chania, Crete, Greece, p 32 Tunalıo˘glu R, Sefero˘glu S Arma˘gan G (2012) The weakest ring of olive oil production in Turkey: olive oil mill waste water (Proceedings of the book 2008, p 308). In: Proceedings of the sixth international symposium on olive growing, 9–13 Sept 2008, Evora, Portugal. Acta Horticulture, vol 949. ISHS, pp 587–593 Tunalıo˘glu R, Yıldırım R (2014) The different points of view of olive mill waste water in Turkey: a result of new ecological paradigm. In: International conference for olive tree and olive products. Proceeding book, 2–6 Nov 2014, OLIVEBIOTEQ 2014, Amman-Jordan, pp 400–403 Yıldırım R (2014) Aydın ilinde Karasu Sorunu ve Zeytinya˘gı ˙I¸sletmelerinin Çözüme Yönelik Tercihlerinin De˘gerlendirilmesi. Adnan Menderes Üniversitesi, Fen Bilimleri Enstitüsü, Master Theses (in Turkish) (Supervisor: Tunalio˘glu R)

Chapter 10

Future Trends in Olive Industry Waste Management: A Literature Review Aysen Muezzinoglu

Abstract Olive oil is one of the major features of the Mediterranean diet signifying healthy nutritional practices of both ancient and modern times. It is an exceptional food and contains several components beneficial for wellbeing and well-being. Olive oil, as well as the olive tree leaves are rich in oleuropein. During maturation of the fruit and the leaves, oleuropein is broken down to hydroxytyrosol and elenolic acid that are valuable components. These are antioxidants that strengthen the immune system. Oil mills use different technologies and consume a lot of water and energy. Depending on the technology, along with the main product line, side-streams such as cakes, pomace, kernel, and different types of olive mill wastewaters are generated. Wastes are also generated from subsequent treatment facilities for pomace oil and soap production. Captured oils are reclaimed and refined if possible. Only part of the beneficial chemicals such as polyphenols in the olive fruit is retained in the olive oil phase and a substantial part goes into the waste streams. More than fifty different valuable phenolics and other organic compounds have been identified in olive mill wastewaters. Pomace contains another good portion of polyphenols depending on the extraction technology. These chemicals need to be processed or removed from wastewaters by pretreatment for easier environmental management as the wastes are not acceptable to the receiving environmental media, and moreover they inhibit waste treatment. Integrated with this treatment, biotechnological conversions, or recovery of pure ingredients from the wastes are advisable to produce valuable raw materials for food, energy, agriculture, and pharmaceutical sectors. With the rising of 3R (reduce, reuse, recycle) trends, conservation of water and energy has become an issue in the olive industry. This strategy extends from preservation of these resources to enrichment of the beneficial ingredients in the product oil and reclamation of chemicals from olive by-products and wastes. Another option is adding wastes into adjusted cultures as substrates for biomass production and use the products in food, energy, and fertilizer industries. Innovative, and revised conversion technologies are needed for treatment, reuse, and reclamation of these chemicals from by-product and waste A. Muezzinoglu (B) Retired Professor, Department of Environmental Engineering, Dokuz Eylul University, Tinaztepe Campus, Buca, Izmir, Turkey e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Souabi and A. Anouzla (eds.), Wastewater from Olive Oil Production, Springer Water, https://doi.org/10.1007/978-3-031-23449-1_10

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streams. There is a high number of scientific publications to address these more challenging goals. In this article new trends for stand-alone or integrated methods of waste treatment/pretreatment in management schemes for olive agro-industry with waste minimization, bioconversion and recovery/purification of beneficial chemicals are evaluated. Among the options, biotechnology of producing fuel, food, animal feed, pharmaceuticals, and fertilizers is especially important. These are being discussed for development of a profile leading to future research interests in view of the big number of published scientific research related to olive agro-industry. Keywords Olive products · Olive waste treatment · Olive industry material side-streams · Biotechnology of olive wastes · Agricultural uses of olive wastes · Treatment of olive oil mill wastes · Treatment of table olive wastes · Waste minimization in olive industry · Biorefinery of olive industry wastes

10.1 Introduction Olive tree has been the symbol of wealth for millennia in the Mediterranean areas. It alludes to a healthy life and correct nutrition in our day and is popular even at parts of the world that naturally cannot grow olive trees because of their climate. Olive oil is a unique feature of the Mediterranean diet and culture. Olive tree is a species restricted to the exclusive climate and one of the major cultivated crops of the Mediterranean regions. Its life span is long, so that sometimes it is named the ‘immortal tree”. There are examples claimed to be thousands of years old in the area. Fruits (drupes) ripen, turn from green to purple black at the end of dry and warm summer period. They are harvested and processed in autumn/winter which is the rainy season. This period between harvest and oil production takes about five months and is called a “campaign”. Fruit harvest and therefore the amount of production fluctuate biannually. This phenomenon of alternate growth may be very severe between “on” years and “off” years. Products such as the olive oil, various types of green or black table olives, olive tree leaf teas and extracts, balms and medicines, traditional olive soaps, balms, skin care products and even less usual wood kitchenware and furniture parts constitute the beneficial uses olive trees. Oleuropein, that is the main phenolic matter is rich in the olive products. That substance is in fact the taxonomic marker of the plant. During maturation of the fruit, oleuropein is broken down into three parts: hydroxytyrosol, elenolic acid, and glucose. These are all valuable compounds for health. The scientific evidence supports the role of oleuropein as a potential agent in diabetes, hypertension, cholesterol and inflammation control, and has excellent antioxidant properties that strengthen the immune system. Health care experts recommend Mediterranean diet against cardiovascular problems, weight control, skin care practices and for maintaining overall health including the cognitive strength. The medicinal benefits and the homeopathic healthy-life style trends all over the world created an increased demand to olive and olive products.

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Although olive is an indigenous plant of Mediterranean climate areas, it is recently being grown in plantations established in several other regions of the world having similar climatic conditions. According to the International Olive Council 2022 annual report (https://www.internationaloliveoil.org), provisional production in 2021/22 crop year points to 3,197,500 t olive oil, and 3,134,000 t table olives. Almost all table olives and more than 95% of olive oil is produced in the Mediterranean region. It may be calculated that the total water use will amount to more than 30 million cubic meters. That means that about this much wastewater will have to be discharged into the catchment areas of the Mediterranean basin; causing eutrophication in the bays of the coastal seas and contaminating the surface and underground water resources as well as the soils. The recent rise of demand in olive products created more pressure on the olive growing regions of the world, and especially in the Mediterranean countries. The substantial stress over the limited land-resources for alternative uses is another big problem in growth areas. In many rural regions olive trees and olive groves are already under the dangers brought by the urbanization and economic development. So, even though the demand of olive production is economically pressing, extending the olive growth areas is difficult. Traditionally, olive oil production was from simple olive presses that produce extra virgin oils. This term covers the oils obtained only by mechanical means with no excessive heat, no chemical processing and necessarily having very low acid value (usually less than 0.8% by weight of free oleic acid). This traditional method was known for centuries and gives the most valuable olive oil. For higher capacities, however, continuously working horizontal centrifuges are in use for a few decades to separate the oil from solids in water mixture. In the older three-phase method (3-P) there are three main outgoing streams: olive oil, pomace (olive solids) and the wastewater. A comparatively new method works with less water inputs and produce only two outgoing phases of oil and wastes (2-P). The combined form of wastewater and pomace is quite different from the wastewaters of 3-P systems. Olive fruit produces only about one fifth its amount of oil. In 3-P systems after adding somewhat more than this amount of fresh water during the production, the yield of oil on the basis of olive plus added water amount is only 1/10 or less. Keeping this overall yield at mind, minimum 90% or more of the summed olive and water flows out of the mill in the form of highly polluting waste streams. Water and energy are two main inputs in olive oil production once the harvested olives reach the mills. Volume of the water the olive oil mills process largely depend on the technology and machinery used. These waste streams are wastewaters and solid wastes (3-P) or alternatively one watery solid waste stream (2P). Fader et al. (2020) mentioned that water resources in the Mediterranean are scarce and often mismatching with the human and ecological needs. Water scarcity is a vicious problem with the average share below 1000 L of water per capita. The total renewable freshwater resources of the countries in the Mediterranean basin are estimated to vary between 1212 and 1452 km3 yr−1 of which 59.6 km3 yr−1 is used by industry and 193 km3 yr−1 . Agricultural use constitutes the use of 64–69% of all renewable water resources in the area. So, water use in olive agro-industry is a

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sizable portion of the industrial use depicted above. Also considering the pollution problems and their results, the cost of olive products on the water environment is high in the Mediterranean region. On the other hand, relatively stronger impacts of global warming are being forecasted in the Mediterranean exerting significantly increased risks and vulnerabilities in the freshwater resources. Droughts, wild-fires, and shortages in domestic uses will be more frequent issues. Agricultural expansion in the Mediterranean region will be limited by the lower levels of productivity and water resources. It is known that a bigger part of the olive produce goes into commercial olive oil production. Scientific literature is rich for treatment possibilities of olive mill wastewaters and solid wastes with low treatability. Moreover, there are several scientific reports published for methods of managing table olive wastes, as well as other olive related wastes like tree pruning materials, and biomass consisting of leaves, branches from harvesting as well as preliminary washings of olive fruits during storage, pits, etc. Olive oil production is highly energy consuming, too. Energy use figures by different methods of olive oil mills are given by Azbar et al. (2004). When energy, water and land resource depletions considered together, it is realized that olive products make a high impact on the regional ecosystem balances. In addition, when high amounts of hard-to-treat liquid, solid and gaseous wastes, as well as the associated odor problems are noted, it can be said that olive products are environmentally expensive. Olive oil mills may generate significant odor complaints if the mills are located by the residential areas. Anaerobic treatment units and biogas production plants may cause odor problems, especially in relation to sludge handling. An additional big source of odor is evaporation ponds or wastewater holding wells if used instead of the wastewater treatment plants. Emitting vapors, gases and steam have pungent smells due to volatile components with low boiling points. Odor is a serious problem in the surroundings of these mills, ponds and even engineered evaporators. Some ingredients of olive mill wastewaters, like fatty acids, have a characteristic irritating odor that may be recognized from long distances. When evaporation ponds are used for waste collection; the inherent odor problems might be accompanied by fly and other pest nuisances, too. Uncovered storage of extracted olive pomace cakes is another cause of seepage and odor nuisances. Closed drainage systems must be provided for olive pomace storage areas to avoid seepage. There may be air quality complaints around dried pomace burning facilities. The composition of the flue gases from solid waste combustion systems is well known. Research related to odors resulting from the solid waste storage and drying facilities, olive fruit storage areas at oil mills, wastewater holding tanks, and evaporation and distillation units could not be found in the scientific literature. Keeping in mind the Mediterranean diet, Lacirignola et al. (2014) calculated the combined food losses and waste footprints in the Mediterranean countries. That study is not dedicated to the olive products but included olive and olive oil where they are produced and consumed most. These authors suggested that current consumption patterns imply high ecological, carbon, and water footprints of consumption and

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unfavorable national virtual-water balances. From the perspective of olive agroindustry it is important to minimize the water, energy, and resource depletion when food becomes refuse. Difficult and expensive treatment of the wastes is the main reasons why there are so many scientific studies published around the world by reputable researchers and journals. Although, the future trend of olive agro-industry waste management is delineated by this very high number of research reports, for practical reasons unfortunately only part of them could be mentioned, discussed, and the expected future developments could be covered in this chapter. Although the Mediterranean diet has been linked to health benefits and has considerable nutritional importance, to qualify this diet as sustainable, biodiversity and ecosystems should be protected. Olive industry must be managed to have lower environmental impacts and optimize natural resource depletion. On the other hand, Mediterranean region already faces severe environmental problems such as land use challenges, land degradation, water scarcity, environment pollution, biodiversity loss, and climate change.

10.2 Materials and Methods This literature review intends to predict the environmental, economic, and practical future of the waste management in olive agro-industry and discuss what might be done to avoid losses. Relatively new environmental challenges and ideas of the last two decades are used and predictions for the not so-far-future of the olive agroindustry has been attempted. The method used in its preparation was to go through the published reviews, research articles, book chapters and technical reports of the last two decades. The number of scientists working with different aspects of the waste management problems and the number of research investments in the olive related areas is astonishing. That reflects the severity and importance of the problems. Also, the energy, food, and water shortages, as well as the social, environmental and employment problems are very important in regional economies of olive producing areas. Besides, it is obvious that otherwise valuable materials are being wasted and are polluting the environment. In this review not only the olive oil production wastes, but also the table oil process wastes are briefly mentioned. These wastewaters are even more difficult to treat due to salinity contents and contain the same valuable compounds in olive mill wastes that can be valorized if technology and investments were existed. When the scientific study results published during the last two decades are evaluated, following points could be noted: 1. Scientists working in many different branches of science are deeply interested in the management of olive wastes 2. Pathways were recommended leading to problem solving by way of,

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(a) valorizing the high amounts of wastes (b) minimizing the water usage in novel production techniques (c) using the wastes as raw materials for bio-energy, bio-fuels, and bio-power to promote the carbon capture and sequestration for mitigation of climate crisis, as part of both circular economies of olives as well as out-of-cycle solutions to the olive agro-industry management deficiencies (d) pretreating the wastes for toxic and harmful components to increase their treatability (e) valorizing the wastes as algal, bacterial, or fungal substrates to obtain several high value bio-products such as food and nutrient additives, animal feed additives, pharmaceuticals, chemicals, polymers, etc. (f) studying whatever wastes are left from the valorizing processes, and benefit from them by way of reclamation, further reuse, or discharge with less harm into the environment. 3. More recently the focus of the scientific interest has shifted towards integrated management schemes to include (a) to (f) above. A novel concept brought by among these are related with the olive biorefinery concepts. 4. Achieving circular bio-economies involving added-value material production and using life-cycle analyses (LCA) to help the decision-making process aiming at preserving the climate, human health, food security, ecosystem, and resource sustainability goals. By utilizing the waste minimization opportunities using recycling, reclaiming or reuse (3R) alternatives, many problems could be solved in different areas of agriculture, industrial processes, and everyday life. The same should be made possible for olive agro-industry. However, care should be exercised in jumping into 3R advocacies. For example, in continuous olive oil mills, the (2-P) process is less water consuming compared to the more traditional (3-P) techniques. However, switching from 3-P into 2-P may not necessarily diminish the wastewater management problems as in 2-P systems the quality of the (wastewater + pomace) waste stream is no better for treatment. In any case, one must also be careful about the raw material and product quality of implications of the 3R in the mainstream production line before such programs are underway. Olive biorefinery containing economic, environmental, and social design elements aiming at the high value-added products seems to be the next generation olive agroindustry waste management strategy. This concept will soon bring by the economic incentives to the olive industry growth which is already an employment-intensive regional sector. On the other hand, water resources management would require less use of water in this industry or seek for the water reuse possibilities. This might be possible at least to replace the irrigation water demand, provided the toxic, oily and excessively polluted wastewaters from the industry can be controlled.

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All these strategic inputs are based on a good number of scientific research reports published in the domain of integrated management of the olive wastes. Recently, more technologically advanced, and more environmentally friendly ways of obtaining bioenergy and bioproducts from olive wastes have been under development.

10.3 Results Data used in this chapter exists in books, book chapters and review and research articles. They are mostly published in the period beginning with the millennium. For example, the book edited by Galanakis (2017) presented relevant ideas throughout the twelve chapters written by groups of prominent scientists. There are also several other books, special journal issues on olive mill waste management, research articles and a high number of review articles that requires attention. Some of them are cited for their main points in this section. For the sake of conserving time and space, details of the main-stream production methods and processes are skipped. Only the existing difficulties, proposals for solving the olive waste management problems, novel ideas of treatment, possibilities of valorization and schemes for biorefineries are being mentioned very briefly. Olive farming and olive products keep generating an increasingly large amounts and a wide range of low-cost by-products (wastes). In the oil mills, part of the olive tree leaves, raw olive pomace, and olive stones are already available in the same facility along with the main olive oil production line. Olive pomace is a raw material for oil extraction facilities which separate the oils retained in the solid wastes by chemical extraction methods and send it to oil refineries. These are reclamation units that are feasible only if the transportation cost of the influent material is not too high. Following categories of production from the main groups of olive agro-industry wastes of woody nature (prunings/leaves/twigs/branches/olive pit stones), wastewaters and solid wastes (pomace, solvent extracted solid wastes, etc.) can be summarized for this strategy: Renewable energy and power . Biohydrogen . Biomethane . Bioethanol and biodiesel. Biochemicals production . . . . .

Enzymes Polysaccharides Biosurfactants Bioactive compounds Agricultural material (compost, fertilizer, irrigation water, antimicrobial products)

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. Fungal biomass, edible mushrooms . Food and feed products . Biochars for various uses. Water reuse . Irrigational reuse . Recycle to the mill . Recreational reservoirs or recharges into the aquifers. Presently, there are only a few examples of commercialized beneficial uses of the compounds in the waste streams. Yet, there are many examples of conceptual novel processes and biorefinery schemes to be integrated with the mills, so that olive biomasses can be converted into marketable products. The co-products obtained in these add-on processes and the biorefinery schemes include antioxidants, mannitol, xylitol, furfural, paper, and biochar along with biogas, biohydrogen, bioethanol, power, and heat. Selection of these depends on the chemical composition, volume of the olive waste, as well as the market features and regulatory aspects. Also, further engineering, techno-economic and environmental impact assessment studies are necessary to create the best biorefinery settings for an energy- and nutrient-sufficient and zero-waste operation in the future. According to Vlyssides et al. (2004) mass balance of inputs and outputs of a 3-P olive oil mill are given per 1000 kg of olive introduced: For elements of C, N, P, K, Ca in product and by-products, 186.14 kg org. C 1.39 kg org. N 0.48 kg P 3.13 kg K 0.46 kg Ca

and masses of outgoing flows for the same input, 1670 kg wastewater 359 kg emissions 212 kg olive oil 195 kg olive stones 1.11 kg ash 29.1 kg phenolics 83.5 g antioxidants 11.84 kg seed oil

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This mass balance included the water inflow nearly 1.5 times the amount of olives into the mill. High waste production and discharge needs necessitate sustainable handling of these materials. These authors calculated that out of every 100 kg olive fruit input plus the 4 kg attached leaves coming with them, there could be a 53.7 kg irrigation water return to the olive groves if a good treatment could be possible. This is in addition to the production of various marketable products. According to this research, the marketable products included 0.05 kg crude antioxidants, 1.14 kg biogas and 22.71 kg total oils for direct use and to be refined. In their proposed mode of operation of the 3-P mill, only a destoning step before malaxation was added to the system. Di Giacomo and Romano (2022) discussed the present and future of the olive agroindustry in their recent review article. They think that olive cultivation and production technology have evolved to almost a final point for economic feasibility, for a fixed quality of the oil product. They also think that the problems have now spread to non-Mediterranean areas, as more regions and countries are growing olive trees in similar climate regions all over the world. As for the oil production technologies, they foresee more replacements of 2-P with older 3-P techniques, which they think can be done easily. But they also predicted that in the future the continuous 2-P and 3-P mills will co-exist, also together with small traditional presses in many countries. They think that the exploitation of dried pomace by solvent extraction will be an obsolete practice. Obviously, if 2-P techniques will be applied more, dried pomace extraction will be more and more difficult for technological and waste transport reasons. Then of course other novel technologies should come into picture for solid waste management and pomace oil extraction deficits. Ahmed et al. (2019) as well as many other scientists working in this field of research foresaw the future trends in the olive mill wastewater management in the domain of transforming its valuable constituents into value-added products. This strategy should replace the traditional wastewater treatment facilities that can only be established with high investments and usually having little efficiency at high cost. That is because the presence of high organic loads, pH and toxic compounds inhibit microbiological growth during treatment. To put these wastes to a more balanced form for treatment many studies were carried out. For this, either the wastes are diluted, or pretreatment steps are added into schemes of biological treatment to make these challenging wastes more treatable. But instead of that, there is an entirely different strategy to use: cleaner production methods could be devised, where using olive mill wastewaters as raw materials. These wastewaters will then be novel resources in production of many feed and food additives, medicines, nutraceuticals, soil additives, fertilizers, plastics as well as biofuels. In the following sections technology and suitability to novel technological improvements versus the expected developments are briefly discussed in view of published research papers.

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10.3.1 Olive Mill Wastewaters and Treatment Olive mill wastewater which is the main liquid waste stream of the olive oil industry was studied by many scientists. It has always been a hot topic in many olive producing countries. Several olive mill wastewater disposal techniques have been applied at olive mills for centuries, extending from the spreading on land or simplest lagoons or wells, to more complex and higher technologies of today. Olive mill wastewaters carry excessive amounts of organic matter acting as pollutants as soon as they are out of the olive oil extraction plant. These are toxic or hard-to-treat substances resisting to biological degradation therefore inhibiting the treatment of wastewaters. That is also why organic loads are seldomly expressed in units of biological oxygen demand (BOD) or (BOD5 ) which opposes the usual practice in wastewater engineering. Instead, Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) are used as more appropriate parameters for assessing the organic loads. Also, the total phenolic (TP) component concentration is a good measure to see the inhibitory potential. Wastewater streams from olive oil mills have considerable different characteristics highly depending on the production technology used, together with other factors related to the olive product. This variability in characteristics along with the different national or regional requirements of treatment efficiencies for disposal of wastewaters necessitate thorough evaluations of the wastewaters before selecting the treatment strategies. There are several books and review articles in the literature about the characteristics in relation to growth, soil, climatic conditions, harvest year and time, time after harvest, variety as well as the main production methods. Thus, wastewater characterization and treatment options have always been a most popular subject for several decades for the scientists trying to find solutions to the environmental impacts in production areas. They have put gigantic efforts making use of the scientific and technological expertise they had. But at the end an efficient, easy, and still a low-cost and widely acceptable treatment could not be established. Olive mill wastewaters are high in volume, highly loaded with solids and oily organic matter, contain highly toxic organic compounds such as phenolics, and are discharged seasonally during a few months in late autumn–winter period. In short, they are difficult wastes. It is nearly impossible to treat these wastewaters at an acceptable efficiency and reasonable cost in a single pass treatment unit. On the other hand, olive agro-industry has a growing trend and this is of crucial importance for agricultural societies and it provides employment for poorer regions. Also, there is an increasing demand for more olive products all over the world as olive is not exclusively grown or processed in Mediterranean countries anymore. In more traditional regions one of the mostly applied practices is direct discharge onto the soil. Those advocating this application are underlining the benefits of bioremediation and irrigation occurring simultaneously. But olive mill wastewater discharge to fields causes strong odor nuisance, pest management problems, risks of soil contamination and growth inhibition for crops and plants unless maximum

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acceptable dosages are exceeded. Unavoidable leakage into the aquifers or surface water bodies is a likelihood, too. They might cause severe impacts both on land and aquatic flora and fauna, in other words the impact of land-spreading of wastewaters might probably be the deterioration of the agricultural areas in the medium to long run. Some countries prohibited direct discharges of untreated olive mill wastewaters, and some try to keep this application under control. Because of the characteristics of these wastewaters, they are not accepted into the municipal treatment plants. By now, a good number of methods from simple wastewater treatment like lagoon systems to the most sophisticated systems are under discussion. Also are the problems associated with the pollution caused by olive mill wastes. Thus, increased social, environmental pollution concerns versus the global demand to olive products, as well as climate change and water shortages in growth areas forced the use of highly sophisticated multi-stage treatment options be tried for an acceptable solution. In this part of this review, it is not intended that a full coverage of the key elements of wastewater treatment be given, although the efforts by many scientists for many years should not be overlooked in the field of wastewater treatment of olive mill wastewaters. Indeed, in many books, book chapters, reviews and research articles, many treatment processes have been proposed. Those who are interested may read them to begin with. However, more recent treatment processes mostly ending in valorized products will be mentioned here in a classification mostly used in the wastewater treatment educational practices. To begin with it must be said that even this classification is difficult to make as it inherently combines a variety of different treatment concepts, such as biological/physical chemical/algal/fungal, etc. These combined applications may vary from simple wastewater treatment to most advanced methods.

10.3.1.1

Biological Treatment of Olive Mill Wastewaters

Engineered systems to meet wastewater discharge legislation are needed in any industry sector. Usually for cost reasons they better be biochemical (biological) systems or their combinations than physical–chemical or advanced systems. In the olive industry however, difficulties are encountered in biological treatment due to toxicity and heavy loads of the wastewaters. Therefore, preliminary, and sometimes even second preliminary steps must be added to the treatment schemes to adjust the properties of the oil mill wastewaters. This makes them multi-stage treatment plants of usually of combined nature. Most widely applied treatment methods for industrial wastes including the industrial wastewaters rely on biological methods. This is true for the municipal wastewaters treating mixtures of domestic and industrial wastewaters, and valid for discrete industrial plants including the olive mills. However, due to several unique substances in olive mill wastes, inhibition is a big problem to directly use biological treatment. Among these inhibitors phenolic compounds constitute one of the most important group of compounds. Olive industry waste discharges are of high strength, i.e., they are highly loaded with organics and contain organic compounds like long chain

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fatty acid molecules that are difficult for biochemical decomposition. These wastewaters have a seasonal discharge pattern which necessitates huge balancing structures considering the high volumetric input into the treatment plant. Also, olive oil mills have a wide territorial scattering, again another problem to locate centralized treatment plants. These difficulties in treatment urges small-scale oil mill owners to use low-cost disposal methods with no precautions to stop their serious environmental impacts. But the new trends in olive mill wastewater treatment is the recovery of addedvalue compounds from olive mill wastewaters while adjusting the influent properties. This is a necessity to lower the cost of treatment, recycle and reclaim the valuable materials and reuse the waters in land irrigation if possible. There are two main types of biological treatment of wastewaters: aerobic and anaerobic. They have their own pros and cons for treating olive mill wastewaters. Aerobic treatment is the basic first step in treatment, but anaerobic treatment is better for high organic loads, and it produces gas products at the end that can be used as sources of bioenergy. Also, as an extreme example against normal mixed cultured biological treatment, the units may be run by using pure cultures. Inoculating acclimatized pure cultures has been proposed for use in treatment of hard-to-treat wastewaters like olive mill wastewaters for maximum efficiency. Several pure cultures of bacteria, fungi and algae have been tested and reported. For example, Amaral et al. (2012) used a wild strain of Candida oleophila isolated from olive mill wastewater for its detoxification. Incubation with the C. oleophila isolate was found to be able to remove 50% of the organic load, and 83% of total polyphenol content, from undiluted and non-supplemented olive mill wastewater samples. The impacts of this isolate on detoxification were evaluated by using germination tests, bioluminescence assays, and studies on mitochondrial bioenergetics. Authors decided that Candida oleophila isolate could be used for future application in biological treatments of olive mill wastewaters. To decrease the toxicity and lower the concentrations of difficult-to-treat substances, olive mill wastewaters have been diluted in many cases until it is safe to treat them biologically. If the dilution can be done with other wastewater streams, it may become less costly for dilution water requirements and in some cases the induced nutrient deficiency in the diluted olive mill wastewater can be resolved. Farabegoli et al. (2012) have found that a more suitable treatment plant unit is sequential batch reactor (SBR) fed with previously sieved and diluted raw olive mill wastewater. Four dilution ratios were tested (wastewater/tap water, v/v): 1:25, 1:32, 1:16 and 1:10. Results showed that there was a complete removal of the biodegradable organic content at all the investigated influent loading efficiencies of around 90 and 60% for COD and TP, respectively. The authors also tested adding preor post-treatment operations using membrane technologies such as ultrafiltration, nanofiltration and reverse osmosis. Also, a combined treatment of different waste streams (such as with domestic sewage from the work place) along with the olive mill wastewater might be planned. This would allow adjustment of the influent load

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and characteristics to reach a highly efficient biological process in the SBR plant. Total phenol concentration in the treated effluent did not achieve the limit required for reuse. For full-scale application, they recommended mixing the olive mill wastewater with other wastewaters to provide nutrients along with dilution. Olive oil mills with batch (small-scale traditional presses) and continuous operation of 3-P and 2-P are the main systems are used in processing olives to obtain virgin olive oils. Wastewaters from these three technologies are quite different because of the difference of points of introduction of water, as well as the equipment in use. 3-P systems use more water given in more than one point and therefore has higher wastewater volumes, and they also have a separate stream of solids consisted of the extracted olives. In the newer 2-P olive mills effluent is made up of a mixture of the wastewater and the solid wastes. Therefore, the characteristics and volumes of the waste streams are different in these two types of mills. More detailed descriptions can be found in the literature such as Azbar et al. (2004). Borja et al. (2006) described the treatment technology for 2-P olive oil mill waste. Their report pointed out the main features and characteristics of 2-P wastewaters and pomace as compared to the more conventional olive mill wastewater and olive cake derived from the 3-P process. The advantages and disadvantages of the 2-P production process were also discussed. They recommended biological treatment methods like aerobic processes in completely mixed activated sludge reactors with high COD removal efficiencies. Their report also included the anaerobic digestion of wastewaters from the washings of olives and oil and their combination using fluidized-bed and hybrid reactors. Wastewater treatment methods including chemical oxidation with several chemicals, as well as physical–chemical treatments such as by using different coagulants and flocculants were introduced. Awad et al. (2004) introduced the basic terminology used in this sector, methods of oil production, equipment used, wastewater volumes and properties, and methods as well as success rates of treatment methods. They indicated that the quantities and contents of the olive mill wastes are known to vary with geographical and climatic conditions, tree age, olive type, extraction technology used, use of pesticides and fertilizers, harvest time, and stage of maturity. The wastes are characterized by dark colors of intensive violet to dark brown/black; strong odors; high degree of organic pollution. They have COD values around 220 g/L, in some cases reaching up to an extremely high value of 400 g/L with very high COD/BOD5 ratios. Typically, it is acidic with pH values between 3 and 5.9; high acidity, has a high content of polyphenols of up to 80 g/L; essentially at high contents of solid materials (total solids up to 102.5 g/L) and oil (up to 30 g/L). Most of the phenolic compounds such as hydroxytyrosol, tyrosol, caffeic acid, rutin, luteolin, and flavonoids in olives and olive oil are insoluble in oil. Thus, they remain in wastewater. The selective recovery of these phenolic compounds in a preprocessing step ends in both the reduction of the intrinsic wastewater environmental toxicity and the production of high added value molecules. Ahmed et al. (2019) summarized the olive oil mill technology and related water, energy use as well as volumes of oil mill wastewaters with respect to these technologies in a review article. Azbar et al. (2004) reviewed input/output relations for olive

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raw material, water, and energy usage per 1 ton of olive fruits producing about 200 kg olive in all technologies. What can be noted from this data showed that the presses that work batch-wise with less water usage, therefore less wastewater is produced. Yet, it must be remembered that it is difficult to standardize water and energy use in presses. Roughly out of the 1ton input olives, 200 kg oil and 400 kg solid wastes of 25% moisture and 6% oil contents are produced from presses. In the continuous processes that are run with horizontal centrifuge systems, when more traditional 3-P methods are used, out of 1ton olives 200 kg oil together with 500–600 kg solid wastes of 50% water and 4% oil are produced. However, in different 3-P units around 1000– 1200 kg of wastewater is discharged with a water content of 94% and unseparated oil of 1%. However, the 2-P plants that are comparatively new are with less use of water. Liquid wastes from these units are discharged as mixed with the solids. This watery waste stream contains more than 60% water and 3% oil. All oil mills are also highly energy consuming and work with 40–117 kWh power per ton of olives to cover all units. More traditional treatment methods of aerobic and anaerobic treatment, as well as their combinations are also considered. Fungal treatment is another biological option. Coagulation/flocculation, adsorption, filtration (biofiltration, membrane filtration, etc.), electrolysis as well as wet air oxidation and ozonation are recently investigated as advanced treatment methods. Unconventional methods such as high pressure and temperature, ultrasound, electrocoagulation, photochemical, ozone, hydrogen peroxide oxidation techniques were recently investigated (Evci et al. 2019). However, these methods are expensive, and not so simple for easy application in the field. They also may produce waste streams, for example like a difficult-to-handle sludge, that must either undergo further treatments or be disposed of with care (Foti et al. 2021). This section scanned some of the publications in view of the aerobic, anaerobic, combined processes, physical chemical including the advanced-oxidation methods and the new trends in recovering added-value compounds during pre- and posttreatment steps. Aerobic Treatment Aerobic biological treatment is always the first method that comes to mind by the wastewater engineers. However, due to the treatment difficulties of olive mill wastewater, modifications of aerobic treatment have long been proposed, such as using pure microorganisms to decompose phenolic compounds in pretreatment. For this purpose, scientists tested several bacteria and fungi such as Geotrichum candidum (Asses et al. 2009), Pleurotus ostreatus (Olivieri et al. 2012), Phanerochaete chrysosporium, Aspergillus niger, Aspergillus terreus and Geotrichum candidum (Garcia Garcia et al. 2000). Aissam et al. (2007) studied yeasts and fungi in reducing the phenolics and COD of olive mill wastewaters. Other strategies to lower the toxicity of the olive mill wastewaters before the aerobic biological treatment is to dilute the inflow. One such research was carried out by Chiavola et al. (2014). These authors sieved and prediluted influent wastewater (1:10–1:25) and treated it in a sequencing batch reactor (SBR). They found good removal efficiencies of COD (90%) and total phenolics (60%). They also added pre-

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and post-steps of membrane technologies such as ultrafiltration, nanofiltration and reverse osmosis into the treatment scheme. But they argue that phenolics removal did not achieve the limit required for reuse. Yet, they concluded that in full-scale application, mixing the olive mill wastewaters with other liquid streams to lower its toxicity by way of dilution and provide missing nutrients into the influent can be considered as an option. Hafidi et al. (2005) tried aerobic treatment for olive mill wastewaters after adjusting its low pH using lime. They have inoculated the wastewaters with soil micro-flora and by bakers’ yeast (Saccharomyces cerevisiae) both in natural acidic pH and after neutralizing its acidity. Neutralization of pH of the wastewater before aerobic digestion enhanced the microbial activity, but the means of neutralization has a great influence on organic structures of the molecules for microbial degradation and on types of compounds incorporated. Benamar et al. (2020) studied two successive treatment steps for treatment of olive mill wastewaters, the first one is physical–chemical pretreatment by infiltrationpercolation and the second one is biological treatment by soil microorganisms under aerobic conditions. They reported that dilutions of 15× were used. Then, they examined different columns filled with filtration sand, granular activated carbon, and lime. The second step of aerobic biological treatment with soil microorganisms was applied to effluents from the column filled with this mixed material. The total removal efficiency of the wastewaters after these steps were 87.86, 87.39 and 81.59% for COD, BOD5 , and polyphenols, respectively. However, the high dilution rates and longer than usual aerobic treatment times in the order of days seem to be the drawbacks of this method. Anaerobic Treatment Anaerobic treatment is a low cost and low energy demand process depending on the characteristics of the influent wastewaters. Usually, anaerobic treatment of wastewaters with high organic carbon values is beneficial. Yet, there are pros and cons of this method in treating the olive mill wastewaters. If it works well, the wastes are treated with high efficiency and high yields in biogas production may be achieved. That is why many wastewater engineers and scientists have studied anaerobic treatment as a method of high efficiency and yields of bioenergy pre- and/or post-treatment units of the olive oil wastes. Anaerobic technology is usually coupled with biogas and other gaseous fuel production when applied to olive mill wastes. Beyond biomethane/biogas production, biohydrogen can be produced from olive mill wastes under anaerobic conditions. Such processes are briefly mentioned below. Fungal Treatment Díaz et al. (2021) investigated the ability of Phanerochaete chrysosporium to treat the olive mill wastewater. Experiments were carried out at 26 °C at the optimal pH range of 4–6 without the addition of glucose for 10 days. Parameters followed in treatment were COD, BOD5 , biodegradability index, reducing sugars, total phenolic compounds, and color. The results obtained in this study revealed that the use of

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Phanerochaete chrysosporium gave COD and color removal efficiencies of about 60%, and 32% of total phenolic compounds in treatment of 2-P mill wastewaters. Cibelli et al. (2017) investigated fungal treatment of olive mill wastewater samples following thermal and high-pressure homogenization. Then the samples were taken into reactors for the growth of twelve different species of soil-borne and/or pathogenic fungi. Fungi growth measurement results of high-pressure homogenization samples indicated increased bioactivity, in 3.0–4.8 days. Their analyses pointed to the success ratings of following fungal groups: first group of Colletotrichum gloeosporioides, Alternaria alternata, Sclerotium rolfsii, and Rosellinia necatrix was strongly inhibited by the olive mill wastewater and the second group of Aspergillus ochraceus and Phaeoacremonium parasiticum stimulated the fungal growth. It was shown that the high-pressure homogenization improved the treatment. Caffaz et al. (2007) proposed a novel fungal biological process to treat olive mill wastewaters. They investigated the growth of a biomass rich in fungi in a batch reactor filled with the wastewater and its capacity to degrade the organic and phenolic load. Under aerobic conditions olive mill wastewater degradation for COD and TP removal efficiencies of 86 and 70% were reached, respectively. They reported that fungi biomass was able to completely decompose pure phenolic compounds. Algal Treatment Many scientists have indicated that treatment of olive mill wastewaters with cultures of algae converts them into feedstocks for algal products. These can be utilized for several beneficial materials, for example algal lipids can be extracted and can be used for biodiesel production. Algal biofuels constitute another large area of scientific research and education. However, it is not possible to investigate all possibilities, methods and hurdles connected to algal biomass growth and bioconversion of the ingredients into beneficial end products. This review must be dedicated to olive agroindustry waste management. But a very brief overview just at the interface between olive products and algal studies exists below.

10.3.1.2

Advanced Treatment Processes

Although almost all olive mill wastewater treatment options involve a series of unit processes for success, special and even unique ideas of high technology applications are worth to mention here. There are several recent studies examining different sets of combined treatment of olive mill wastewaters. Among them are photo-Fenton process in combination with fungi treatment, coagulation/flocculation, nano-catalytic conversion of olive mill wastewater constituents, solar-driven advanced oxidation process combined with coagulation/flocculation, de-phenolization, etc. This is an area open to much further inspiring research and scientific advancement. Among the physical–chemical treatment alternatives used as advanced oxidation processes (AOPs) those resulting in a reduction of toxicity aim at reaching higher biodegradability (Amor et al. 2019). These authors carried out studies with AOPs or treatments combined with physical–chemical or biological treatments.

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Canizares et al. (2007) evaluated and compared the technical and economic feasibilities of three treatment applications on a real industrial wastewater that was studied: (a) conductive diamond electrooxidation (CDEO), (b) ozonation and (c) Fenton oxidation. The wastewater they tested contained a COD of nearly 3000 mg L−1 . CDEO method achieved complete mineralization of the waste. Likewise, both ozonation and Fenton oxidation were able to treat the wastes, but their efficiency and rate of mineralization were very different. They concluded that only the conductive-diamond electrooxidation method could achieve complete organic matter mineralization of the pollutants in the wastewater. However, the efficiencies were found to depend on the concentration of the specific pollutants, whereas oxidation with ozone (at pH 12) or by Fenton’s reagent was found to depend on the nature of the pollutants. Papastefanakis et al. (2010) studied the treatment of olive mill wastewaters using electrochemical oxidation by applying cyclic voltammetry and bulk electrolysis with Ti/RuO2 and Ti/IrO2 anodes. They found that the oxidation at 28 Ah L−1 and 50 mA cm−2 leads to quite high color and phenols removal (86 and 84%, respectively). Removal of toxicity, ending in COD and total organic carbon reductions of 52 and 38% were found, respectively. The authors concluded that these electrodes indicated good treatment potential for olive mill wastewaters but warned against the formation of a complex composition of the treated water which might compromise the activity and stability of the used anodes. Chatzisymeon et al. (2008a, b) studied the photocatalytic treatment of a 3-P olive mill wastewater with TiO2 in a batch laboratory-scale photoreactor. They provided UV-A irradiation by a 400 W and high-pressure mercury lamp and Degussa P25 TiO2 was used as the catalyst. They found that the COD removal was affected by the contact time and the influent COD, all other variables being of no significance. The energy consumption per unit mass of pollutant removed was found lower for higher influent CODs. They suggested that TiO2 photocatalysis can be a promising process for OMW treatment. OMW was almost completely detoxified at low influent COD, although toxicity was only slightly reduced at very high organic loadings. Ochando-Pulido et al. (2013) studied at laboratory scale the photocatalytic degradation of the wastewater brought from a 2-P olive mill. In this investigation, the photocatalytic degradation process was used as an alternative for the treatment. A novel photocatalyst having ferromagnetic properties was used in this work which gave good results compared to other commercial catalysts. Removal rates up to 58.3% COD, 27.5% total phenols and 25.0% total suspended solids were reached. Also, they reported that as a pretreatment pH-temperature flocculation was performed, which increased the overall COD removal efficiency up to 91%. According to the results obtained in this investigation, the photocatalytic degradation process was suggested as an alternative with high possibilities in the treatment of olive mill wastewaters. Michael et al. (2014) investigated the treatment of 3-P olive mill wastewaters by means of a solar-driven advanced Fenton oxidation process combined with coagulation/flocculation pretreatment. They reached a high COD removal of 87% and elimination of the polyphenolic fraction at the wastewater pH. They suggested that the proposed process could accomplish a reduction of the olive mill wastewater toxicity.

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Wet Air Oxidation Catalytic wet air oxidation (CWAO) was studied by Pham Minh et al. (2007) for use in combination with anaerobic digestion. They used laboratory-prepared platinumand ruthenium-supported titanium or zirconium catalyst and 50% diluted olive mill wastewater. They reported a high efficiency of total organic carbon (TOC) removal, up to 97%, and a nearly complete removal of the phenolic content using CWAO at 190 °C and 70-bar total air pressure. A decrease in the phytotoxicity of the treated effluent was also shown with Vibrio fischerii test. Methane production yield was enhanced in the subsequent anaerobic treatment. Combination of hydrogen peroxide catalytic wet oxidation and microbial technologies for the treatment of olive mill wastewater was used by Azabou et al. (2010). They used a test reactor which was at the semi-batch mode at atmospheric pressure and aluminum-iron-pillared inter layer clay ((Al–Fe)PILC) were tested in two different catalytic processes: [(Al–Fe)PILC/H(2)O(2)/ultraviolet radiation)] at 25 °C and [(Al–Fe)PILC/H(2)O(2)] at 50 °C. The second option operating at 50 °C reduced the COD, color and total phenolics considerably. Toxicity of the effluent was tested using the Vibrio fischerii luminescence inhibition test by 70%. Feasibility of using Wet Hydrogen Peroxide Catalytic Oxidation process as a pretreatment to anaerobic treatment was also tested. Results showed that the pretreatment for more than 2 h ended in higher methane production. Fenton Process Fenton reaction is a widely advised solution for the treatment of olive mill wastewaters. The quality of the treated water from this process is suitable for irrigation. Alrousan (2021) studied separate and combined applications of H2 O2 , O3 , and UVA irradiation on olive mill wastewater in a glass tube photoreactor over a wide range of dosages and dilution ratios. The treatment efficiency was evaluated for reduction of dissolved organic carbon (DOC) and the change in biodegradable organics content expressed by BOD5 . The highest DOC reduction in this study was around 40% by UVA/peroxonation, while the highest enhancement in BOD5 (209%) and biodegradability (254%) was achieved by dark peroxonation. The author also noted that a wide range of doses resulted in the same degree of change in DOC reduction and BOD5 (and biodegradability) enhancement. This lab-based study demonstrated the potential of the studied systems to significantly reduce the organic fraction of real wastewater applications and increase the biodegradability. Martínez-Nieto et al. (2011) tested Fenton chemical oxidation process using ferric chloride or potassium permanganate as catalysts for the activation of H2 O2 on an industrial scale. By using potassium permanganate in the system, the final water was transparent with a slight yellow tinge, but odorless and with a low total phenol content. They decided that treated water could be used for irrigation or discharged directly into the municipal wastewater. Their results showed that a Fenton-like reaction was with good efficiency and was relatively cheap.

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There are several more research studies testing Fenton process in different settings to treat olive mill wastewaters. These can be followed in detail in recent literature. A review study by Ochando-Pulido et al. (2016a) is a good source of information in this respect. In another published paper Agabo-García et al. (2021) the authors studied the heterogeneous catalysts that can be an efficient and economical option for olive mill wastewater treatment as an advanced oxidation process. In this work, heterogeneous photo-Fenton reaction (artificial ultraviolet light/H2 O2 /HFeO2 ). HFeO2 were tested at 0.04; 0.3; 0.8; 5.0; 10.0; 20.0; 30.0, and 50.0 g/L. At pH = 3.0, temperature = 20 °C, agitation rate = 700 rpm with 50 g/L of HFeO2 the results showed high removal rates for COD of 62.8% and total phenolic compounds (TPCs) of 88.9%. These results were also compared with advanced oxidation systems, such as UV, H2 O2 , and UV/H2 O2 . They added that in addition, the catalyst was reused three times. Among many other recent research published that are using Fenton process treatment, Baycan et al. (2020) can be mentioned. These authors used real olive oil mill wastewater for treating with the advanced oxidation process (photo-Fenton) method in a batch reactor. Initial oxidant and catalyst concentrations and pH of water were measured, and reductions of total organic carbon (TOC) and color were calculated using the central composite experimental design method. The samples were analyzed at different times during the experiments to determine water quality parameters. Increasing oxidant concentrations did not give a noticeable rise in the removal percentages. On the contrary, the removal efficiencies for both color and TOC were decreased by increasing catalyst concentration. Optimum color and TOC removal efficiencies were achieved as 95% and 65%, respectively. Ozonation Ozone treatment has been known as a very effective method in environmental engineering for decomposing organics in wastewater and waste gas streams for a long time. However, until recently, due to its cost and difficulty of transportation of ozone to the site of use, it has been disadvantageous. Recently, however, advances in ozone manufacture equipment technology and reduced cost of in situ production capabilities, a new era of treatment technologies with ozone has been possible. Ianni et al. (2018) studied the effects of ozone treatment on the phenolic fraction of the olive mill wastewater from a 3-P system. Ozonation was applied for 1, 3, and 8 h, using an ozone generator. Total polyphenols, antioxidant activity, and the amount of tyrosol and hydroxytyrosol were determined on the treated samples. The germination test of radish seeds was used to measure the biotoxicity of the treated wastewaters. This test showed that, at 50% dilution the biotoxicity decreased, but also it was found that a significant decrease of antioxidant activity of the wastewaters occurred. Hydroxytyrosol was significantly lowered, depending on the duration of treatment, while tyrosol was less affected.

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Bettazzi et al. (2006) applied ozone and Fenton’s reagents on olive mill wastewaters and on phenolic synthetic solutions and obtained polyphenol removal efficiencies up to 95%. They stated that the pretreatment of the olive mill wastewater by means of these oxidation processes increased their biological treatability. Electrochemical Treatment Unconventional methods like the electrochemical treatments have been tried for the treatment of olive mill wastewaters. This involves electro-coagulation, electroFenton, electrochemical oxidation with polyaluminum chloride (PAC), conductive diamond electrochemical oxidation (CDEO), electro-oxidation with in situ generated active chlorine, as well as by means of cyclic voltammetry and bulk electrolysis using Ti/RuO2 or Ti/IrO2 anodes. Tezcan et al. (2007) used electrochemical oxidation with PAC in the presence of H2 O2 on fresh and containing very strong organics-loaded olive mill wastewater of COD 45 g L−1 . Results showed that the Fe electrode was more effective than the Al electrode in electrochemical oxidation by up to 62–86% COD removal efficiency and 100% turbidity removal. Oil and grease removal was possible at 20–75 mA cm−2 current densities. These researchers also investigated the electrochemical oxidation of olive mill wastewaters from a 3-P mill, using Ti/RuO2 anode. Papastefanakis et al. (2010) studied also tried electrochemical treatment of olive oil mill wastewaters using Ti/RuO2 anodes. Membrane Technologies Use of membrane processes using state-of-the-art membranes have been studied among scientists working with olive mill wastewater treatment. Akdemir and Ozer (2009) designed an ultrafiltration process for olive oil mill wastewater treatment. Filtration experiments were performed with raw wastewater and no pretreatment was applied. Effects of major operating variables such as the transmembrane pressure, feed flow rate and operation time were investigated. A Box–Wilson statistical experiment design method was used by considering the transmembrane pressure (1– 3 bar), recycling flow rate (100–200 L/h) and operation time (30–120 min) variables while COD removal and permeate flux variation were the objective functions. COD removal efficiency and permeate flux variations were determined by this statistical approach. The optimum flow rate and pressure selected were 100 L/h and 1 bar with 89.5% COD removal efficiency. Stoller et al. (2016) studied several methods of membrane technologies to treat ultrafiltration, nanofiltration, bio-filtration and reverse osmosis experimentally. The authors added biofiltration as a novel step forward to make the final effluent quality to conform with the regulatory requirements, They used a system with small residence times and is capable of treating the reverse osmosis permeate to the target values. Another recent research paper on this topic is by Cifuentes-Cabezas et al. (2021). These authors worked on a real olive oil washing wastewater using four different organic ultrafiltration membranes and evaluated the recovery of the phenolic compounds in the wastewaters. Selection of two permanently hydrophilic polyethersulfone (PESH) membranes with MWCO of 4 and 50 kDa, respectively, one

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polyethersulfone (PES) membrane with a MWCO of 5 kDa and a regenerated cellulose acetate (RCA) membrane with a MWCO of 10 kDa. They used four UF membranes (UH004, UP005, RC70PP and UH050) and compared the olive mill wastewater pretreated with ultrafilter with the aim of obtaining a permeate enriched in phenolic compounds. Transmembrane pressures and crossflow velocities were varying from 1 to 3 bar and from 1.5 to 3.4 m s−1 , respectively. Low rejection of phenolic compounds was observed in all cases, while the rejection of COD varied between 19.5 and 62.9% depending on the membrane and operating conditions tested. Decolorization Treatment options that are good for the removal of phenolics and organic matter usually decrease the intensity of darkness of the olive mill wastewaters. Many of the studies aiming at these removals also mention the color removal as a side effect of treatment. However, a publication by Jalo et al. (2018) made an investigation for accomplishing decolorization. Real mill wastewater was brought into the laboratory and tested after 20× dilution using electrocoagulation or the photo-Fenton process. It was found that an electro-coagulation treatment of two hours, at 22 V DC with aluminum plates, gave nearly clear and colorless water (93% decolorization). A phenolic content reduction by 92.4% was obtained at the same time. Also the photo-Fenton reaction was applied with the use of H2 O2 /Fe(II), O2 /Fe(II), and H2 O2 /O2 /Fe(II) at a wavelength of 254 nm. With this technique, under the best operating conditions they reported 78% decolorization. Evaporation/Drying This is the most extensively applied method of wastewater elimination for many centuries in the open areas. However, in the sense of an engineered treatment technology, evaporation ponds or drying units for thermal treatment are being anticipated. But, due to the high energy costs, and other environmental restrictions this method can only be applied under special circumstances. Taralas and Kontominas (2005) treated the olive oil wastewaters by thermochemical methods in a laboratory-scale reactor at total pressure conditions slightly above atmospheric pressure (103–104 kPa) and high temperatures (700–855 °C). Wastewater was thickened by forming a mixture of olive husks (kernels) and was used as fuel to obtain the heat required. Concentration by evaporation/drying following the preliminary mixing operation of the solid waste effluent by washing (leaching) and fractionation was applied. Gas emissions from the pyrolysis were analyzed for CO, CO2 , O2 , CH4 , and C2+ gases. Along with the amounts of benzene and toluene, tar/liquid phase, and char total yields are given. In thermal treatment experiments in air, the major gas emission was CO2 . The effect of the moisture content and the particle size class of the biofuel samples on the variation of the devolatilization rate, relative to devolatilization time, was determined at a given furnace temperature regime.

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Coagulation/Flocculation Coagulation-flocculation is a widely used method in wastewater engineering, usually with industrial wastewaters. It can either be a pretreatment before another main treatment unit or can be the main treatment process depending on the characteristics of the wastewater. It can also be used as a pretreatment method for olive mill wastewaters. Recently, in addition to the usual flocculant applications, several alternative chemicals such as biopolymers like chitosan (Rizzo et al. 2008). Throughout this review, several citations involving the use of coagulation/flocculation method as a pretreatment in front of other biological or chemical treatments are notable. Eskikaya et al. (2017) investigated the effect of different coagulant doses on reduction of chemical oxygen demand (COD), suspended solid (SS) and color removal performance on olive mill wastewater using acid cracking and coagulation-flocculation. Acid cracking was applied at pH 1.5 and reaction time of 30 min using hydrochloric acid (HCl). Coagulation experiments were carried out at three different concentrations of FeSO4 (1000–5000 mg/L) in a jar test by adjusting the pH to an optimum. The experiments demonstrated that the 3000 mg Fe2+ /L was found as the optimum coagulant dose for the coagulation giving 79% COD removal. A color removal rate of 48.8% was obtained using optimum coagulant dose corresponding to 1500 mg/L.

10.3.1.3

Combined Treatment

To partially treat the olive mill wastewater ending in detoxification for the main biological treatment units or pretreat the wastewater for any other further treatment, combining processes are quite logical. Such combinations are advised for olive mill wastes and might as well be the first steps of conceptual olive biorefineries. For full treatment options on site before final discharge to receiving medium, olive mill wastewater can be sent to either of the two subsequently proposed complete biological treatment systems. Anaerobic–aerobic treatment or the other way around. Anaerobic processes are especially suited for the treatment of high-load wastewaters like olive mill wastewaters with a COD concentration sometimes in the order of thousands of mg L−1 . The climatic conditions in the olive-growing and olive oil producing countries are suitable for anaerobic processes. Combining anaerobic and aerobic processes eliminates the disadvantages of individual applications. For example, the first step includes the advantages of the anaerobic process concerning degradation efficiency, energy self-sufficiency, and minimal excess sludge production. Awad et al. (2004) gives good schemes for these options which may prove to be practical as well as environmentally and economically viable. However, given the characteristics of the olive mill wastewaters and its inhibitory properties to bacterial growth, treatment schemes usually must start with physical and chemical correction steps of these characteristics. So, no single unit can complete

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the treatment to receive a legally acceptable treated water effluent for discharge. This is also true for the valorizing systems working together with treatment units, such as anaerobic/biogas systems. So combined treatment can be counted a must for olive mill wastewater treatment. Main combined treatment schemes are discussed widely in olive mill wastewater treatment practice. Most of the treatment schemes discussed in this review is combined or hybrid systems, too. Sarika et al. (2005) reported removal of wastewater solids in the treatment of olive mill wastewaters using coagulation/flocculation. They tested three cationic and one anionic polyelectrolytes. They found minimum dosage requirements to initiate the separation being 2.5–3 g/L. Lime and ferric chloride were also tested as reference coagulants and found to be quite effective in terms of TSS removal although the degree of COD reduction was generally lower than that with polyelectrolytes. To decrease the high required doses of lime treatment and long processing time necessities, polyelectrolytes can also be helpful, too. A preliminary cost analysis showed that lime treatment for complete solids removal was generally less costly than that with polyelectrolytes. An interesting process proposed by Sarris et al. (2019) combines olive mill wastewaters with a stream of crude glycerol. Glycerol is the main by-product of alcoholic beverages and biodiesel transesterification reaction and is a surplus material in the world, creating a serious environmental challenge. These authors used crude glycerol to dilute the olive mill wastewater and tested the ability of Yarrowia lipolytica strain ACA-DC 5029 to grow on this nitrogen-limited substrate. They worked with submerged shake-flask cultures, using the crude glycerol and olive mill wastewater blends with high initial glycerol concentration to produce biomass, cellular lipids, citric acid and polyols. The research showed satisfactory growth in blends; citric acid production was not affected by olive mill wastewater addition. Vuppala et al. (2021) in their research carried out a multi-response optimization by grey relational analysis, on the data of coagulation and flocculation treatment of olive mill wastewaters to select the optimum coagulant dosage. The coagulation/flocculation process was carried out by adding aluminum sulfate (alum) to the waste stream at different doses, from 100 to 2000 mg/L. This method was applied for selecting the best operating conditions for lowering a combination of multi-responses such as chemical oxygen demand (COD), total organic carbon (TOC), total phenols and turbidity. From the analysis, a dosage of 600 mg/L alum found to be the optimum. A multiplicity of different flocculants has been used for successful treatment of olive mill wastewaters. Likewise wet H2 O2 catalytic oxidation; advanced oxidation by UV and/or O3 was investigated in combination with aerobic/anaerobic biochemical treatment schemes. Higher treatment efficiencies were achieved by the sequential coagulation and Fenton system. Recent scientific literature is very rich in such applications of advanced wastewater treatment research. Stoller et al. (2016) studied the possibility to treat olive mill wastewater by coagulation/flocculation, membrane technology and bio-filtration. Membrane processes are successful in olive mill wastewater treatment; however, membrane fouling issues must be tackled with. To stop the fouling, these authors used boundary flux method.

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Yet, the membranes may not be sufficient to reach the desired level of wastewater treatment. Thus, at the end of the treatment scheme a bio-filter was equipped. This step made it possible to treat the wastewater to a quality suitable to recharge them into superficial aquifers. The adopted system is compact, have small residence times and is capable to treat the wastewaters adequately. In some cases, residues from other industrial or treatment activities can be introduced into the olive mill wastewater treatment. For example, Fragoso and Duarte (2012) used the sludge produced at drinking water treatment plants. This suggested that this sludge has a similarity to bentonite with respect to the presence of aluminum silicate and is at a similar particle size and high pH. Results showed that it was possible to reduce 40–50% of the COD, 45–50% of the TP, a maximum of about 70% total suspended solids, 45% for total solids and total volatile solids in the influent wastewater by using this sludge.

10.3.2 Olive Solid Wastes, Utilization and Treatment 10.3.2.1

Woody Wastes: Tree Prunings, Twigs and Leaves

Pruning of the trees, along with several other agricultural practices may level off the harvest fluctuations, as well as giving the trees suitable shape to ease the fruit growth. To ease the harvesting practice and shape the trees for better yields, mechanical or manual pruning techniques are used (Dias et al. 2020). The main woody wastes of the olive oil industry are olive tree pruning debris, fruit pit stones and leaves. A sizable part of these come from the olive agricultural practices. They can be directly burned using combustion equipment at varying levels of technology or gasified, pyrolyzed, or used in the production of biofuels such as bioethanol and biodiesel. Pruning biomass from olive groves usually comes in bigger masses every other year. Garcia Garcia et al. (2020) reported that one hectare of olive plantation generates more than 5 t of these by-products, annually. They estimated this material contains 25% of leaves, 50% thin branches and 25% woods by weight. This composition may change according to the grove management, tree age, pruning practices, soil, climate, and location properties of the area. Because of the woody structure of these wastes some researchers have investigated the production of pulp and paper. According to Diaz et al. (2005) pulping was carried out in 25% of active alkali at temperature of 175 °C in 90 min. Also, paper sheet making was demonstrated to be possible. Micro- and nano-celluloses have been receiving increasing attention during the last few years, too. Their many advantageous properties promote their use in a wide range of applications, such as paper or polymer reinforcements, packaging, construction material, membranes, bioplastics, bioengineering applications, optics, and electronics. Micro- and nano-cellulose production is possible from olive tree pruning using physical pretreatment (PFI refining) before the micro-fibrillation stage (Ibarra

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et al. 2021). These authors found good micro-cellulose yield and the quality obtained in their research was cellulose in films with good properties for use. Fillat et al. (2018) tested olive tree pruning residue and produced cellulose nanofibers. They used bleached pruning pulp which was treated by TEMPO-mediated oxidation and subsequent defibrillation in a microfluidizer. The properties of cellulose nano-fiber from pruning debris made this nanomaterial suitable for several applications, also makes it a good candidate for packaging having improved functional properties to polymeric matrices (Fillat et al. 2018). Requejo et al. (2012) investigated the woody solid wastes due to their large potential as source of energy and for the recovery of chemicals. These authors proposed establishing a biorefinery for the recovery of valuable energy and chemicals. The biorefinery was considered to begin with the separation step of olive pruning residues into two parts: main part (stems > 1 cm diameter), and residual part (stems < 1 cm diameter, and leaves). They found that dry pruning material in the larger size group has cellulose (29%), hemicellulose (21.4%), lignin (27.7%) and extractives (13.2%). This size group is the in sizes above 1 cm. The smaller size group of less than 1 cm contain more leaves and tiny stems and has more extractives (16.5%). The main part was sent to hydrothermal treatment and from this operation they separated two streams: a liquid fraction (HL) that is rich in products of hemicellulose decomposition, and a solids (HS) fraction that is rich in cellulose and lignin. Up to 42% of the polysaccharides were recovered in the HL as valuable compounds. However, solids fraction (HS) was sent to heated pulping reactors in ethanol–water, resulting in a liquid fraction (HPL) rich in lignin, and other solid (HPS) rich in cellulose. HPS can be used for the bioethanol production in saccharification and fermentation process, giving a bioethanol conversion yield of 90.6% of the theoretical value. After the pulping, HPS obtained was converted to paper, but this product was with lower strength than those of paper obtained from the main line of prunings, directly. For the heating values pruning residuals provided 18.70 MkJ/t, and 1094–2234 °C flame temperature, which is a cost lower than fossil fuels. Olive tree pruning biomass is a good raw material for bioethanol production due to high amounts of potentially fermentable carbohydrates. Steam explosion was used by Ballesteros et al. (2011) to remove the effect of extractives and to avoid the condensation reactions during extraction. Thus, the enzymatic hydrolysis could be made more effective. They reached to 20% more total sugars efficiency through the steam explosion. They thought that this operation could be used as a pretreatment. Romero-Garcia et al. (2022) in their comprehensive research on olive tree prunings, concluded that valorizing this biomass is a good part of an olive biorefinery. They applied liquid hot water (160–230 °C) and steam explosion (180–240 °C) on these agricultural wastes. A water extract rich in antioxidants (mostly oleoeuropein) and sugars can be obtained from phosphoric acid treatment. Both applications proved to be good as pretreatment and they obtained sugar recoveries of (92%) from the hot water extraction at 180 °C and 80.4% from the steam explosion at 220 °C. Thus ethanol production was found to be better in the case of steam explosion pretreatment, resulting in 72% yield compared to 63% in liquid hot water treatment, at different temperatures of 220 °C versus 200 °C, respectively.

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Structural composition with respect to water soluble components of the olive prunings, olive leaves, olive stones, extracted olive pomace were determined by Domenech et al. (2021). Highly water-soluble contents varying between these different types of by-products were found to be 27.8% of the pruning biomass, 35% of the leaves, 63% of the stones, and 42% of the extracted pomace. When cellulose, hemicellulose and lignin amounts were determined, it was found that they varied in ranges of 10–20%, 10–25%, and 23–36%, respectively. Sugar contents in water solutions was varying between 20 and 41% which is quite high in amount. This confirms the possibility of obtaining biofuels and other valuable by-products from these materials. Garcia-Martin et al. (2011) carried out research for obtaining xylitol production from prunings using sulfuric acid hydrolysis followed by fermentation with Candida tropicalis. Olive leaves may represent about 10% of olive harvest weight. These leaves have a strong potential for antioxidants and are used to obtain their extracts. This antioxidant potential is mainly due to oleuropein and hydroxytyrosol as major compounds, but other phenolics and their potential synergistic effects add to this potential. These extracts are commercially available but, more studies on the encapsulation of olive leaf extracts are being searched. For example, Markhali et al. (2020) studied the olive tree leaves as a source of active substances of value. A nano-encapsulation technology using olive leaf extract (OLE) and whey protein concentrate (WPC) was mentioned by Soleimanifar et al. (2020). Nanoencapsulation is an emergent growing research field in several disciplines such as food, pharmaceutical, and cosmetics industries. In their work, nanoparticles containing olive leaf extract (OLE) were prepared from whey protein concentrate (WPC) by electrospraying.

10.3.2.2

Olive Stones

Olive stone is a lignocellulosic material. Obtaining biofuels and other bioproducts from this by-product was discussed by Padille-Rascon et al. (2020). Main process is the sequential fractionation of the lignocellulosic components based on acid pretreatment and steam explosion followed by enzymatic hydrolysis. Xylose recovery of 83% of total sugars in olive stones was obtained. Olive stones constitute about 10% of the weight of olives and are mainly composed of cellulose (20.9%), hemicellulose (26%), xylose (26.6%), galactose (1.4%), arabinose (1.3%), and lignin (35.6%) (Padille-Rascon et al. 2020). Funez-Nunez et al. (2020) studied the olive stones as a raw material for furfural production in two stages (a) by autohydrolysis of hemicellulosics to recover pentoses (mainly xylose), (b) subsequent dehydration of the pentoses into furfural. Autohydrolysis led to hemicellulosic liquors with varying xylose concentrations. They found maximum yields of 23% and efficiency of 96% under different conditions in this experimental study. Brozzoli et al. (2010) Studied fermentation of the stoned olive pomace as a raw material for livestock. Pleurotus ostreatus and Pleurotus pulmonarius led to significant increases in the limited crude protein, ranging from 7 to 29%. Also, a marked decrease in phenols was noted by about 50–90% after 6 weeks. Both species, however,

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led to moderate delignification associated with significant consumption of hemicelluloses. So organic matter digestibility and net energy of olive stone mixtures were not improved after the fungal colonization. Padille-Rascon et al. (2020) evaluated the potential of sugar production from olive stones. Tests consisted of a sequential fractionation process of the lignocellulosic components of olive stones by acid pretreatment and steam explosion followed by enzymatic hydrolysis. In the first acid pretreatment step at 128 °C, 10.5 g acid/100 g olive stones having 33% solids, the yield of xylose recovery found as 71%. In the steam explosion method, and at adequate steam explosion conditions of 195 °C for 5 min overall sugar production yield of 83% of the total sugar content was obtained. Olive stone biomass pretreated by NaOH and by reactive extrusion technology was discussed in Domenech et al. (2020). They obtained maximum carbohydrate conversion rates of 57.7% xylan. 31.57 g of total sugars was obtained per 100 g of raw olive stone biomass at NaOH to biomass ratios of 15% and at 125 °C.

10.3.2.3

Pomace and Deoiled Solid Wastes

Solid wastes from presses and 3-P olive oil mills are often reclaimed by reprocessing in central plants. Solids are transported, dried, and residual oil is reclaimed by hexane extraction. Spent hexane is recovered by condensation. Traditionally, this extracted oil is a good starting material for the soap-stock manufacturing industry. Further refining and blending with high-quality “virgin” oils results in edible oils, provided its composition is suitable. However, the oil obtained from solid wastes by hexane extraction might contain incomplete combustion products and polyaromatic hydrocarbons due to direct contact with combustion gases during drying. Therefore, a careful examination of their presence and concentrations is important before deciding if the reclaimed oil could be consumed as edible oil. To assure good quality, and for elimination of such residuals, filtering the formed gases through activated carbon columns is practiced. A total switch over from 3-P to 2-P oil production would probably force the pomace oil recovery sector to go out of business. For solid wastes from 2-P production systems, composting is the recommended method of beneficiation, and cocomposting of this wet material together with other agricultural wastes seems to be a reasonable approach. Borja et al. (2006) described the characteristics of the solid residue from 2-P mills as a waste with high organic matter concentration. They underlined that due to its high polluting load this waste cannot be easily handled by traditional treatment concepts that try to solve the problems of conventional 3-P mill olive cakes. They investigated the anaerobic digestion of this waste at different strengths, found the kinetic parameters of anaerobic decomposition and carried out mass balances. They described the behavior in the successive phases of anaerobic process, i.e., the hydrolysis, acidogenic and methanogenic steps in one and two-stage anaerobic treatment schemes.

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Other treatment methods and reuse of the pomace were also mentioned, such as composting, production of alcohols, mannitol, and other added-value compounds such as monosaccharides, oligosaccharides, arabinose, and glucose. Furfural and activated carbon production from pomace were also included in this report. Fernandez-Rodriguez et al. (2014) investigated the co-digestion of 2-P solid wastes with the algal co-substrate of Dunaliella salina biomass for methane production. Wastes alone have a high C/N ratio which avoids high methane yields. In blending algal biomass with wastes at 25–50% mixtures, digestion was improved. They reported a C/N ratio of 26.1 at 25% mixture of microalgae and the methane yield was found 330 mL CH4 /g VSadded . D’Annibale et al. (2006) investigated the valorization of olive-mill wastewater as a growth medium for the microbial production of extra-cellular lipase. Strains of Geotrichum candidum, Rhizopus arrhizus, Rhizopus oryzae, Aspergillus oryzae, Aspergillus niger, Candida cylindracea and Penicillium citrinum were screened. All strains were able to grow on the undiluted olive mill wastewater, producing extracellular lipase activity. C. cylindracea NRRL Y-17506 showed the highest lipase activity on all the samples of olive mill wastewater used. Its lipase production with olive mill wastewater was markedly good. Growths were affected by the type of nitrogen source and was induced by the addition of olive oil. The highest activity was obtained with supplement of NH4 Cl and olive oil of 3.0 g L−1 .

10.3.2.4

Direct Combustion of Solid Wastes, Cakes and Pomace for Bioenergy

Direct firing of olive mill solid wastes had been practiced historically, as the material contains high heating value especially after drying. After oil extraction the solids have more convenient properties for burning, compared to other biomass. Many olive oil mills use dried and de-oiled residual solids as an alternative fuel source due to the high heating value of these wastes. Recovered heat can be used to help with the malaxation process in cold winter days. In some mills the olive stones are separated from the vegetable parts and either burned directly or valorized in manufacturing activated carbon. De-oiled olive solid wastes are partly burned in solid waste extraction plants to dry incoming olive solid wastes with high moisture content before extraction, and the excess is sold to other industries as an energy source. In Turkey, officials have recently permitted the use of olive oil mill solid wastes as “fuel” under certain conditions requiring changes in design of the boiler furnaces. But, incomplete combustion resulting in high gaseous and particulate-phase unburned hydrocarbons and carbon monoxide emissions is a significant problem in such combustion systems. Emission studies revealed that if conventional combustion systems are used to burn these solid wastes, they do not comply with national air quality regulations. It is also suggested that only dried and de-oiled olive solid wastes be burned (Azbar et al. 2004). This

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material has low sulfur content varying between 0.12 and 0.26%. That is why it has been extensively studied by energy and fuel researchers for use as a direct or co-firing fuel in suitable combustion equipment. These studies cover firing solid wastes alone and their co-combustion trials in laboratory, pilot scale and full-scale furnaces. Azbar et al. (2004) also gave an extended report of the combustion studies. According to this report lower heating value (LHV) of the dried olive solid wastes is in the range of 3922–4445 kcal kg−1 . In order to study their behavior during combustion fuel, reduction in mass with increasing temperatures was followed by preparing temperature–gravimetric analyses charts (TGA). After drying at 100 °C, the material loses its volatile fractions, and at 250–260 °C, its original mass is reduced by 70–80%. To burn the wastes efficiently, a special boiler design with secondary air injection and high residence time in the primary combustion zone is required. Otherwise, significant levels of highly fluctuating concentrations of carbon monoxide and unburned hydrocarbons are emitted from conventional furnaces designed for other fuels. A series of full-scale combustion tests for the combustion of de-oiled olive cakes were carried out in the Institute for Process Engineering and Power Plant Technology Department of Stuttgart University, Germany. Several combustion parameters were examined in different types of boilers. These were full-scale combustion performance tests in grate-fired and rotary-type furnaces. It was noted during the tests that the size distribution of the olive oil mill solid wastes was important for combustion. Normally, the percentage of fines with less than 3 mm size is above 50%. When firing in gratetype units, small particles fall down and burn with less available oxygen under the grate to create much CO and unburned hydrocarbons (Zuberbuehler et al. 2000). Combustion characteristics of olive pomace were studied by several researchers. In her Ph.D. thesis, Dumanoglu (2003) proximate analyses of pomace was made. It was shown that in raw and n-hexane extracted pomace, moisture levels are variable depending on process and external storage conditions. After drying, oil in the pomace is extracted by hexane which leaves 10% moisture, less than 1% oil and around 90% solids in the solid wastes. This residue contains lignin and cellulosic material in fiber form. Volatile matter is quite high, and it vaporizes at low temperatures to increase incomplete combustion products. Thus, to burn solid wastes from olive mills specially designed furnaces are necessary and the air/fuel ratio is a very important combustion parameter. In this work combustion tests were carried out in a 70 kW capacity pilot furnace under laboratory conditions to monitor the emissions. Dumanoglu (2003) concluded that to have low CO and other incomplete combustion product emissions, automated and continuous feeding of the fuel, secondary air injection and controlled air/fuel ratios are important parameters. Direct firing of the olive mill pomace, as well as the leaves, branches, olive stones and even pelletized dry pomace has been common practice for a very long time. However, to comply with the newly emerging air pollution criteria, combustion and power generation technologies were further developed and reported by many researchers. Amirante et al. (2016) discussed a high technology Combined Cooling,

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Heating and Power (CCHP) plant of 281 kW capacity in Italy. These authors recorded that this plant fired with olive tree pruning residues have advantages both in economic terms as well as for reduction in CO2 emissions. Another research report that can be cited on olive pomace combustion plant performance is Colantoni et al. (2019). A 7.5 kW nominal electric power Rankine cycle (ORC) system was powered by firing olive pomace at an olive farm situated in the central part of Italy. Olive pomace has been shown as a suitable fuel for this application. It was reported that this material answered the energy consumption of the olive farm and was even shown to supply power for other users. Overall power conversion efficiency was found to vary between 12.7 and 19.4%, depending on the organic pomace flow and the working pressure in the steam generator. More design studies in higher combustion technology units of fluidized-beds are also investigated. One such research was published by Atimtay and Varol (2009) and contains data from a pilot scale fluidized-bed combustion system. Combustion performances and emission characteristics of olive cake and coal mixtures were investigated. Several co-combustion tests of coal with olive cake were conducted with mixing ratios of 25, 50, and 75% of olive cake by weight. Operational parameters (excess air ratio, secondary air injection) were varied and the variation in pollutant concentrations were measured, and combustion efficiencies were calculated. For the set-up used in this study the highest combustion efficiency was found as 99.8% at an excess air ratio of 1.7, secondary air flow rate of 40 L min−1 for the combustion of 25 wt% olive cake mixtures. These authors have earlier publications on the combustion of olive pomace with or without co-firing with other fuels in fluidized bed systems like Varol and Atimtay (2007), Atimtay and Varol (2009) and Akpulat et al. (2010). Dependency of larger combustion units to the fuel stocks, as well as the shipment costs of the wastes, mixture preparation techniques and practical difficulties encountered in storage are problematic. These include odor and polluting gas emissions, too. Recently, however, more technologically advanced combustion methods and environmentally benign bioenergy forms are being obtained from olive wastes. These methods are under development. One such method is “oxy-fuel” combustion which is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. A recent study of modern technologies with co-firing of lignocellulosic biomass main components (cellulose, hemicellulose, and lignin), lignite, and their mixtures by Xiao et al. (2022) under air and oxy-fuel conditions. These authors reported research results under the oxy-fuel condition with the same oxygen mole fraction as air, data for co-combustion of the biomass-lignite. Cellulose and hemicellulose were shown to have a positive synergistic effect on the release of lignite volatiles, but lignin showed a negative synergy. In other words cellulose and hemicellulose improved the co-combustion performance, while lignin inhibited. Although the authors did not test the olive mill solid wastes, it is probable that de-oiled and dried pomace could be a good candidate for this biomass, too.

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High Technology Biofuels from Olive Solid Wastes

Biogas (methane) production from 2-P olive mill solid wastes after a phenol recovery step following a steam explosion application under high pressure was investigated by Serrano et al. (2019). These researchers evaluated semi-continuous mesophilic anaerobic digestion of dephenolized steam-exploited olive mill solid wastes during a long operational period (275 days), at varied organic loading rates. Recovery of the phenols, also ending in improvement of the influent wastes, helped subsequent bio-methanization of the olive wastes to produce energy. This work evaluated a semi-continuous mesophilic anaerobic digestor to work with steam-exploded olive solid wastes. The process was stable when organic loading rate (VS) was fixed at 1 gVS/(L d) giving a specific methane production rate of 163 ± 28 mL CH4 /(gVS d). They indicated that regardless of the organic loading rate, the concentration of phenolic compounds was always lower than the inhibition limits. Therefore, they concluded that steam-exploited olive mill solid wastes could be a suitable substrate for anaerobic digestion at a suitable rate. Serrano et al. (2019) have been publishing a high number of publications with valuable data on the olive oil mill solid wastes.

10.3.2.6

Biological Treatment of Olive Solid Wastes

Biological treatment of olive solid wastes occurs in two big groups: aerobic (composting) and anaerobic (fermentation). Fermentation, anaerobic with mixed fermentation and fungal treatment methods are logical to investigate. However, standalone, and unprocessed solid wastes is not suitable for anaerobic treatment because of its low water content.

10.3.2.7

Composting of Olive Solid Wastes

Composting method for solid waste treatment is preferable to other methods. Since operational and personnel costs are rather low, and the output is well-known by the farmers as a soil additive, composting is an acceptable method for many plant owners. Yet, the application depends on the cost of the compost which determines the sales potential of the final product. Solid waste from olive agro-industry is a good starting material for compost. In this field of research many studies have been carried out. For example, full-scale co-composting with municipal sewage sludge, green cutting waste straw and olive mill wastewater was described by Hassen et al. (2021). Akratos et al. (2017) presented a report on the design and operation of cocomposting using olive mill wastes. They presented results of two case studies of olive mill wastes at pilot-scale and full-scales. Experimental results in the pilot and

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full-scale units revealed that composts obtained from olive mill wastes have excellent physical–chemical characteristics with no phytotoxic effects. However, they suggested that genotoxic and cytotoxic effects should be evaluated before using these composts for agricultural purposes. Tuzel et al. (2020) reported three different kinds of composts obtained from 2P, 3-P olive mill solid wastes and olive oil wastewater sludge after co-composting with dairy manure, poultry manure, and straw. They were separately enriched by rock phosphate and potassium salt and mixed with peat in ratios of 0, 25, 50, 75, and 100% by volume. These researchers grew tomato from seeds and showed that increasing the compost ratios in the growing medium and the enrichment of the growing medium increased organic matter content, electrical conductivity, and macro and micro-nutrient concentrations in the vegetables. They concluded that the composts obtained from 2-P and 3-P olive mill solid wastes and olive mill wastewater sludge can be used without any need of enrichment. A mixing ratio of 25% was found suitable in most of the measured vegetable properties. Chowdury et al. (2013) reviewed recent research on composting wastes of olive mill wastes. The authors submitted the design and operating factors of composting process and the operational factors effecting final compost properties.

10.3.2.8

Other Methods of Treatment of Solid Wastes

Drying, evaporation, thermal treatment methods are in this group. Drying/evaporation followed by condensation of the water vapor was discussed briefly for reuse of olive mill wastewaters with possibility of irrigation. However, this might be an energy consuming process which must be advised with care. In solid waste treatment, however, drying is a better option. Discussions of technical nature may be found in the related field of science and technology for different types of dryers such as fixed, fluidized or moving bed units. Combustion, pyrolysis and gasification, chemical extraction, can be counted in this group, too. These methods end in energy production, and some were already discussed above.

10.3.3 Beneficial Uses of Olive Agro-industry Wastes It is obvious that the need to decrease the dependence on conventional fossil fuels and prefer alternative bioenergy resources is a top global priority. Green energies could effectively contribute of reduction of greenhouse gas emissions and to mitigation of climate change with serious unwanted effects. It is of importance that olive mill wastewater is a promising raw material for bioenergy and biofuel production. This is thanks to its high content of sugars, volatile acids, poly-alcohols, and fats.

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Its low nitrogen content makes it a suitable substrate for biohydrogen with photofermentative process because high NH4+ concentrations inhibit nitrogenase synthesis and activity (Dermeche et al. 2013).

10.3.3.1

Biogas from Olive Wastes

In biofuels research related to olive mill wastes, anaerobic digestion is a good application as their organic loads are readily transformable into biogas. Biogas is a good replacement of natural gas and can be easily used for power production, heating, and even for motor vehicles and jet fuel. Anaerobic digestion ending in methane formation proceeds in four major steps. It starts by hydrolysis, which refers to cleavage of chemical bonds added by the H–O bonds of water. It continues with acidogenesis, or fermentation stage followed by acetogenesis as the third step. Methanogenesis is the final stage for methane production. At this final stage acetic acid, ethanol, H2 and CO2 formed in earlier stages are used to produce methane (CH4 ) along with the ammonia (NH3 ) from nitrogen containing compounds and hydrogen sulfide (H2 S) from sulfur containing compounds. Digestate which is a mixture of the liquid and sludge solids remains after the gaseous products are removed to be cleaned and used as biogas. Digestate is rich in nutrients and can be used in many beneficial ways following various post-processes. Anaerobic digestion technology not only allows the treatment of wastewaters but also produces biogas. It is known that for an efficient anaerobic bioconversion process, the wastewater should have a balanced carbon/nitrogen/phosphorus (C/N/P) ratio and a pH in the range of 6.5–7.5. Reactions are exothermic and carried out in specially designed anaerobic digesters. Although olive mill wastewaters have unsuitable C/N/P ratio for anaerobic digestion, when they are mixed with nutrient-rich streams, process performance is significantly enhanced. Intermixing with other wastewater streams not only has the potential to provide the balancing of nutrients and correct the pH, but also improves the biological process as it may decrease the inhibitory effects of phenolic compounds and lipids present in the olive mill wastes by way of dilution. Several studies assess the use of pretreatments for removal of hard-to-treat compounds before anaerobic digestion step. Advanced oxidation processes, coagulation-flocculation or Fenton oxidation are few examples to these combined treatments involving anaerobic treatment. After removing the phenolic components, for example by extraction, olive mill wastewater transforms into a good substrate for fermentative transformations. However, anaerobic degradation of olive mill wastewaters faces some difficulties due to the high content of hardly degradable cellulosic materials and toxic substance, such as phenols, long-chain fatty acids, ethanol, tannins, etc. (Gunay and Karadag 2015). As mentioned earlier, phenolic compounds have a strong toxicity on microorganisms and hinder the potential of biological degradation of the wastes by anaerobic bacteria. Combined physical, chemical, and biological methods have been applied prior to anaerobic digestion to eliminate toxic compounds and enhance methane productivity.

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Tamborrino et al. (2021) studied a full-scale anaerobic treatment plant that uses olive mill by-products. This is a two-stage plant at the size of 100 kWe power. They tested two biomass types: olive pulp and pitted pomace, as well as a biomass consisting of 10% crushed cereal. In both cycles, the retention time was 40 days. The production of biogas was between 51 and 52 m3 /h, with insignificant fluctuations. The specific production values of biogas indicated that a volume of biogas greater than 1 m3 /kg was produced in both tests. The biogas output had a methane concentration of about 60% and the specific production of methane based on the total volatile solids (TVS) was in the order of 0.70 m3 CH4 /kgTVS. The ratio between volatile organic acids and alkalinity was always lower than 1 and tended to decrease in the second stage digester. Regarding the co-substrates studied for olive mill waste co-digestion, manure is investigated as it contributes to the nutrient balance, has a high pH and high buffering capacity. Azbar et al. (2008a) used laying hen litter and cheese whey for co-digestion of the olive mill wastewaters. Co-digestion has proven as a good alternative for treatment of olive mill wastewaters. Other pretreatment steps that can be added to the anaerobic treatment scheme are numerous. In another study, Azbar et al. (2008b) suggested the pretreatment of using Fenton’s process which resulted in an almost 3.5-fold enhancement in the biodegradability of the olive mill wastewater. Pretreatment in the form of UV-H2 O2 application also significantly increased the biogas production (1.75-fold higher gas production). However, the use of H2 O2 alone had negative effect on the biogas production. Gunay and Karadag (2015) reviewed recent developments in anaerobic digestion of olive mill wastewaters and tried co-digestion with several different waste streams (slaughterhouse wastewater, whey, manures, wastewater treatment plant sludge and microalgae waste). They have investigated the process performance in different cosubstrates, and also tested different possible pretreatment technologies. The effects of organic loading, hydraulic retention time, and temperature on suspended, biofilm, and granular reactors are discussed. They reported that co-digestion enhanced the methane productivity by balancing nutrient and alkalinity levels. Anaerobic digestion experiments of olive mill wastewater without pretreatment were carried out and signs of inhibition in the biological process were observed after a time equal to 1.5 times the Hydraulic Residence Time (HRT) due to the presence of high inhibitor concentrations in the wastewater (Gonzales and Cuadros 2014). Based on the findings of these authors, olive mill wastewaters subjected to an aerobic pretreatment to reduce the concentration of phenolic compounds and decrease the total Chemical Oxygen Demand (TCOD), yielded a reduction of 78% and 90% of the initial polyphenol concentration and 18% and 21% total COD reduction when the substrate was aerated for 5 and 7 days respectively. The best methane yield was found to be 0.39 m3 CH4 /kg COD removed at aeration time of 5 days and was 2.4 times higher than that for untreated olive mill wastewaters.

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Tsigkou, K. and colleagues have been studying the anaerobic reactor design, kinetics of the anaerobic digestion, inoculum, that maximizes the biogas production in UPBR reactors and published a series of recent articles in this field of research. Only three of the most recent ones this group of researchers have published are mentioned here. Tsigkou et al. (2019) studied the optimal conditions such as the temperature and solids content for the anaerobic digestion of the olive mill wastewater and worked to maximize the methane yield. Inoculum from a wastewater treatment plant and two lab-scale bioreactors treating the wastewaters from a mesophilic Up-flow Anaerobic Sludge Blanket (UASB) reactor and thermophilic Up-flow Packed Bed Reactor (UPBR) were tested. Treatability of the wastewaters from the olive mill and methane yield under mesophilic and thermophilic conditions were determined through Biochemical Methane Potential (BMP) assays. Tsigkou and Kornaros (2022) studied removal of high organic loads in the olive mill wastewaters by converting them into biogas. They used an upflow packed bed reactor (UPBR) with mesophilic inoculum colonization acclimatized from thermophilic. An anaerobic inoculum was acclimatized under thermophilic conditions and subsequently added into the reactor. Its performance was examined for twicecentrifuged olive mill wastewater of 40 g COD/L, and tests were carried out for several runs of gradually decreased hydraulic retention time, reaching steady-state conditions until the minimum of 4.2 days. Optimum performance was achieved at 5.6 days. The gas release of 9.51 ± 0.37 L CH4 /L-Feed (methane gas under normal conditions). COD removal of the system was higher than 75% for all the tested scenarios. Phenols removal value (33–67%) depended on the tested run (HRT, feedstock). The authors considered this technique quite promising for development the high-rate anaerobic treatment plants for olive mill wastewaters, without the need of co-digestion. Tsigkou et al. (2022) studied the olive mill wastewater from a 3-P mill, by feeding it into a thermophilic high-rate upflow packed bed (UPBR) anaerobic digester for converting the organic load into biogas. An active anaerobic inoculum originating from a mesophilic reactor was acclimatized to thermophilic conditions and used. Reactor performance was tested towards the treatment efficiency for thermophilic operating conditions at the minimum hydraulic retention time of 4.2 d. Work was focused on characterizing the microbial community and its variation during the operating runs, via 16S rRNA amplicon sequencing. Identification of new microbial species and taxonomic groups were carried out and the microbial dynamics of the high-rate thermophilic anaerobic process was examined. Anaerobic treatment is a successful method when the characteristics of olive mill wastewaters are suitable. The advantages of this type of treatment are low nutrient requirement, low sludge production, waste stabilization and biogas (methane) production. However, it is difficult to apply this treatment method alone to olive mill wastes when the toxic effects of polyphenolic compounds, and hard-to-treat polymeric substances are excessive. Yet, if preceded by pretreatment for removal of toxic components in the influents, it usually gives good results.

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Anaerobic digestion has high COD removal efficiency, low biological sludge production and low energy consumption, and therefore is a suitable method at high organic loads. A basic study of the anaerobic treatment of olive mill wastewaters using thermophilic and mesophilic microorganisms was carried out by Borja et al. (1995) to determine kinetic parameters of the digestion process. Following these leading information and data, many anaerobic digestion research reports have been published for the olive mill effluent treatment. For example, Blika et al. (2009) demonstrated that heat treatment supplemented with further pretreatment with a white-rot fungus for removal of the phenolics content, caused a stable operation at a Hydraulic Retention Time (HRT) of 30 d. Also, dilution of the raw wastewater, without any solids removal, lead to a stable operation at an HRT of 30 d and ended in higher production of biogas. Erguder et al. (2000) studied the anaerobic biodegradability of olive mill liquid and solid wastes. Their data indicated that anaerobic treatment had high efficiencies (85.4–93.4%) and ended in the production of 57.1 ± 1.5 m3 of methane gas for each m3 olive mill waste waters. They also mentioned that anaerobic treatment of the olive mill residual solids (OMRS) when they are treated alone was poor. But when mixed with olive mill wastewaters efficiency of their treatment increased. Anaerobic cultures needed an adaptation period of 15–25 days for treatment of the solid wastes with and without olive mill wastewater addition. Anaerobic treatment options of olive mill waste with co-digestion or in a mixture with digested or otherwise treated biomass is also available in the literature. For example, Martinez-Garcia et al. (2009) tried piggery effluents fermented with Candida tropicalis for co-digestion of olive mill wastes. Azaizeh and Jadoun (2010) used a co-digestion strategy for treatment of a mixture of olive mill wastewater and swine manure. Adapted mesophilic microorganisms were used in up-flow anaerobic sludge blanket (UASB) continuous reactors. During 170 days of operation, this system showed high biodegradability. A COD removal rate of 85–95% and biogas production rate of 0.55 L g−1 -COD were obtained for a mixture of solid waste with one third of its volume of wastewater. This mixture ended at an organic load of 28,000 mg L−1 COD and after treatment it was reduced to 1500–3500 mg L−1 . These results suggested that co-digestion technology using UASB reactors is a highly reliable and promising technology for olive mill wastewater treatment and biogas production. Rubio et al. (2019) carried out batch experiments and investigated the mesophilic anaerobic co-digestion of 2-P olive-mill waste and cattle manure in mixtures at different ratios. For all the ratios used mixing had improved total methane production than that of both biomasses alone. These improvements were within 264–319% of stand-alone olive mill waste methane yield. However, their results showed that the mixing ratio is a very important variable. For best results to overcome process inhibition, the olive mill waste/manure ratio of 60/40% or 75/25% were recommended by these authors. Thus, they concluded that anaerobic digestion of wastes is an attractive method for solid waste treatment, too. Their results suggested that the mixture of these agro-industrial by-products could be effective to enhance biogas production and organic matter removal from 2-P olive mills.

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One of the studied forms of pretreatments is the use of ultrasound for biomass decomposition. For example, Oz and Uzun (2015) investigated the applicability of low-frequency ultrasound technology to olive mill wastewaters as a pretreatment before anaerobic digestion. Their results showed that the application of 20 kHz, 0.4 W/mL for 10 min on diluted wastewater increased the soluble chemical oxygen demand to total chemical oxygen demand (SCOD)/TCOD) ratio from 0.59 to 0.79. This led to 20% enhancement in methane production in trials using pretreated and diluted olive mill wastewater. Alagoz et al. (2015) investigated the effect of ultrasonic and microwave pretreatment on biogas production from the anaerobic co-digestion of olive pomace and wastewater sludges. It was found that co-digestion of wastewater sludge with olive pomace which yielded around 0.21 L CH4 /g VS added, whereas the maximum methane yields from the mono-digestion of olive pomace and unpretreated wastewater sludges were 0.18 and 0.16 L CH4 /g VSadded . Likewise, co-digestion increased methane production by 17–31%, compared to individual digestions of these substrates. Methane production from olive mill wastewaters might also begin with an anaerobic or aerobic pretreatment to lower the toxicity factors, followed by a two-phase anaerobic digestion process. In this case, fungi were shown to be successful microorganisms for pretreatment. During the first phase of the two-phase anaerobic digestion, macromolecules such as carbohydrates, proteins, and lipids are transformed into simple organic compounds (sugars, volatile fatty acids, and amino acids) and intermediates such as volatile organic acids (mainly acetic, propionic, and butyric), alcohols (mainly ethanol), ketones (acetone), CO2 , and hydrogen by hydrolytic processes and acidogenic fermentative bacteria. In the second phase, through the interactions between methanogenic and acetogenic microorganisms, all these metabolites are converted into CH4 and CO2 .

10.3.3.2

Biohydrogen from Olive Wastes

Another interesting approach for bioconversion of olive mill wastes into energy is its use for hydrogen production through anaerobic fermentation. Although biohydrogen is one of the most promising renewable biofuels that can be obtained from olive oil mill wastes, however, problems might arise due to the dark color of the wastewater. Light hindrance due to the dark color of the wastes may inhibit photosynthetic bacteria and high dilutions of the wastes are recommended to overcome this. Battista et al. (2016) used a mixture of olive mill wastewater and olive pomace to produce hydrogen and bioethanol anaerobic fermentation using Saccharomyces cerevisiae. They also tested different pretreatments (ultrasound, alkaline hydrolysis, and calcium carbonate addition), concluding that ultrasound application and alkaline pretreatment lead to the hydrolysis of the lignin and cellulose ending in soluble organic matter of sugars and enhanced methane production.

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In this field of research there are several research groups in the world, such as the Eroglu et al., and colleagues with dedicated studies of biohydrogen production for a long time. For example, Eroglu et al. (2008) studied Rhodobacter sphaeroides for the photo-fermentation of olive mill wastes under anaerobic conditions, achieving a production yield of 16 L of biohydrogen per liter. In this research by using photosynthetic bacterium Rhodobacter sphaeroides strain O.U.001 in photoreactors, obtained biohydrogen. They increased production significantly by using extremely diluted and iron-supplemented cultures of olive mill wastewater to a level of 125 mL biohydrogen. In another study Eroglu et al. (2004) investigated bio-hydrogen production from diluted olive mill wastewaters (1–20% (v/v)) in column photobioreactors. When olive mill wastewaters were the sole substrate, and when 2% dilution was used, they obtained hydrogen producing potential of 19.9 m3 m−3 at high organic loads of 9.71 kg m−3 and the carbon-to-nitrogen (C/N) molar ratio of (73.8 M M−1 ). Eroglu et al. (2006) in another research studied wastewaters for bio-hydrogen production under anaerobic photo-fermentative conditions using R. sphaeroides O.U.001. Eroglu et al. (2009) brought several olive oil mill effluents from western Anatolia (Turkey) and used them at dilutions of 4% (v/v). They found a linear relationship between C/N molar ratios and bio-hydrogen production capacities.) studied with the same group of authors the olive mill wastewater treatment using R. sphaeroides which ends in bio-hydrogen production, too. Eroglu et al. (2008) studied bio-hydrogen production linked to olive mill wastewater photodegradation by R. sphaeroides O.U.001 strain using diluted cultures of this wastewater (2% v/v) supplemented with iron and molybdenum. These two metals were used to modify the role of nitrogenase enzyme complex that catalyzes photosynthetic processes. The diluted and ironsupplemented olive mill wastewater-based cultures showed a significantly increased production (125 mL bio-hydrogen obtained in the presence of molybdenum versus the 62 mL bio-hydrogen production if none added). Cerqueira et al. (2021) investigated the biohydrogen production and carried out thermodynamic and energy efficiency analyses. They carried out a thermodynamic study by comparing the performances of different reactor configurations, traditional, sorption-enhanced, chemical looping, and chemical-looping sorption-enhanced for the steam reforming of olive mill wastewaters. They found that sorption-enhanced reforming tests showed higher biohydrogen yield for than the traditional and chemical looping reformers, with a hydrogen purity above 99%. By combining sorptionenhanced with chemical looping reforming, a H2 purity above 99% and a yield higher than that for chemical looping by itself were observed. Algal biohydrogen production research is another axis of research studies known for a long time. Based on this idea olive agroindustry wastes have been the subject of biohydrogen production through algal biomass. In one such study is by Faraloni et al. (2011) used Chlamydomonas reinhardtii biomass was used for bio-hydrogen production. These scientists and their co-workers published several research articles on using the olive mill wastes for biohydrogen production.

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Another option for biohydrogen production from olive mill wastes can be producing a suitable algal biomass first and after harvest converting them into biohydrogen such as covered in the review by Behera et al. (2015). This option is always viable except the economics involved must be carefully examined.

10.3.3.3

Biodiesel from Olive Wastes

Olive oils of inedible quality such as reclaimed oil from pomace, fusty oils that are unmarketable and refused for edible use, spent oils, used cooking oils etc., can be transesterified to obtain biodiesel. Although this method is suitable for valorization of low quality and inedible oils, for reasons of global food safety and ethical considerations it might involve challenges. Because if these oils could be refined to edible food products, an ethical problem arises for transforming them into fuels. Therefore, the feasibility and ethics of producing biodiesel fuels from olive oils should be assessed together. A dependable way of producing biodiesel from olive mill wastes can be through algae production first and utilizing the oil extracted from algal biomass to be transformed into biodiesel. Although the coupled aim is treatment of olive wastes, algal biomass lipids obtained have the potential to be converted into biodiesel through transesterification process. Oil extracted from the algae is mixed with alcohol and an acid or a base to produce the fatty acid methyl esters (FAMEs) that makes up the biodiesel. This utilizes the knowledge of algae growth (micro and macro as necessary) from olive mill wastes after fixing the carbon-to-nutrient ratios. Many good research papers are published in this field, covering a wide range of algal oil production techniques. But this requires another whole literature review and therefore is not included in this chapter. Following research studies are given as examples to the transesterification method that can be used to produce biodiesel from olive waste product oils into fuels. Jeguirim et al. (2020) studied pyrolysis products of olive pomace impregnated with olive mill wastewater in a pilot-scale pyrolyzer. According to these authors, pyrolysis of olive pomace impregnated with olive mill wastewater could be considered as a promising issue for bio-oil production. At the end they proposed a novel strategy for the olive mill wastewater conversion into irrigation water, green biofuels and biofertilizers. Results showed a bio-oil production yield of 36 wt%. Full characterizations of the end products of bio-oil, bio-chars and recyclable water were carried out. The bio-oil properties showed that viscosity and flash point values could reach the European standards. However, the lower heating value (26 MJ/kg) and the acidic character limit its direct use. They indicated that these properties were attributable to the presence of phenolic molecules and long chain organic acids. Therefore, they concluded that this bio-oil requires an upgrading step for better qualities for wider use. They found that the solid residue after the pyrolysis is a bio-char that could be used as a bio-fertilizer as it is rich in nutrients such as K and P. These authors stated that they obtained a treated water stream good for irrigation and bio-oil as a green fuel, as well as a bio-char that can be evaluated in several uses. They stated that the

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olive mill wastewater impregnation on olive pomace and pyrolysis of the mixture could be considered as a promising issue for bio-oil production. However, this bio-oil requires an upgrading step for a better valorization. In the experimental research by Sanchez and Vasudevan (2006) biodiesel was produced using catalytic transesterification of triglycerides (triolein) present in olive oil with methanol and Novozym® 435 as a catalyst. Stepwise methanolysis with a 3:1 methanol to triolein molar ratio gave the best results. The final conversion rate and yield of biodiesel were unaffected by initial enzyme concentrations above 500 U/mL olive oil. The optimum reaction temperature was 60 °C. Experiments were also carried out with used cooking oil; the conversion rate had no significant difference. The efficacy of Novozym435 was determined by reusing the enzyme; although the enzyme’s relative activity decreased with reuse, it still retained 95% of its activity after five batches and more than 70% after as many as eight batches. In another study by Coggon et al. (2009) biodiesel was produced by transesterification of triglycerides (triolein) present in olive oil with methanol and Novozym® 435 as an enzymatic catalyst. Methyl acetate was found to be an effective solvent and acyl acceptor. In this study the effects of different solvents and three different acyl acceptors on the transesterification of triolein (as a model compound) was investigated. The yield of biodiesel (methyl or ethyl ester) production was determined with respect to time and different mechanical stirring rates. The results indicated that the highest yield is obtained in a solvent-free system with mechanical stirring. The effects of the parameters of molar ratio of methanol to triolein, mode of methanol addition, enzyme activity and reaction temperature on conversion yield were shown. Obtained biodiesel after a reaction time of 24 h were found to be unaffected by changes in these parameters over the range studied. Results of a pilot study was presented by Serafini et al. (2016) at a conference on the production of biodiesel from 2-P olive oil mill wastewaters using zinc stearate as a catalyst. According to the records, they specified that the catalyst was able to catalyze simultaneously the triglyceride transesterification and the fatty acid esterification reactions with high activity and selectivity under moderate operating conditions. Serafini and Tonetto (2019) studied the synthesis of fatty acid methyl esters (FAMEs) using crude olive pomace oil as raw material and zinc stearate as catalyst. Pomace oil is a non-edible by-product of olive oil production. Responses to transesterification were followed by triglyceride and free fatty acid (FFA) conversion and FAME yield at 30 min of reaction time. Under optimum conditions of 140 °C, catalyst loading of 3 wt%, initial molar ratio of the reactants 30; 98 and 67% of triglyceride and FFA conversions were achieved, with a FAME yield of 84%. Authors indicated that pomace oil can be used as a raw material for biodiesel production. Khounani et al. (2021) discussed an experimental study they carried out also involving biodiesel production from pomace oils. This study will be further mentioned for other dimensions it has.

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Bioethanol from Olive Wastes

In the scientific literature there are many good research reports on obtaining bioethanol from olive wastes. As a preliminary step before biological treatment of olive mill wastes pretreatment with specific fungi, such as Aspergillus niger, Aspergillus terreus, and Pleurotus sajor-caju plays a key role to increase the final production of the reference bioenergy compound. The effects of dilution with water, heat treatment and treatment with H2 O2 were investigated, too (Foti et al. 2021). Further biological treatment was carried out with Saccharomyces cerevisiae by research groups of Sarris et al. (2014) as well as Nikolaou and Kourkoutas (2017). This report confirmed the role of S. cerevisiae to produce bioethanol under optimal fermentation conditions using the 50% dilution of the olive mill wastewater with water. Battista et al. (2016) used a mixture of olive mill wastewater and olive pomace to produce hydrogen and bioethanol with Saccharomyces cerevisiae with anaerobic fermentation using different pretreatments such as ultrasound, alkaline hydrolysis, and calcium carbonate addition. They found that the ultrasound and alkaline application pretreatments lead to the hydrolysis of the lignin and cellulose ending in an increase in soluble organic matter in the form of sugars. Calcium carbonate addition helped removing polyphenols that are not wanted in the fermentation process.

10.3.3.5

Algal Bioenergy and Biofuels

Like the algal treatment processes briefly mentioned in Sect. 10.3.1.1, algae biomass for bioenergy and biofuel production are covered in many research studies in relation to olive agro-industry wastes. The algae biomass can be transformed into various types of renewable biofuels including bioethanol, biodiesel, biogas, biohydrogen, and further processing for bio-oil and biofuels production through pyrolysis, liquefaction, and gasification. Several techniques such as centrifugation, flocculation, floatation, sedimentation, or filtration are used for harvesting and concentrating the algal biomass. This field of research shows many issues to be considered and many hurdles to be resolved, too. However, only directly affiliated research at the intersection with olive wastes are briefly introduced below. In a review Behera et al. (2015) studied the algae as the source of third generation biofuels. They give a view of the selection, growth, harvesting, drying, oil extraction, conversion methods that can be used. In fact, algal biomass has widely known for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In this review, authors discussed the recent findings and developments in algal biomass for improved biofuel production. Akubude et al. (2019) prepared a review article on production of biodiesel from microalgae via nano-catalyzed transesterification process. The review covered microalgae production, economic bioconversion of microalgae biomass into materials like fuels, food supplements, extracts and the merits of CO2 capture in an

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olive/algae biorefinery. Among the biofuels were options of producing biomethane, biohydrogen, bioethanol and other by-products. Application of nano-catalysis in biodiesel production was reviewed, showing comparative issues involved in different classes of catalysts such as homogenous/heterogeneous or enzymatic catalysts. Catalysts, especially the nano catalysts present a novel approach in the biodiesel production from microalgae. Utilization of this technology for biofuel production can be exploited for the commercial biodiesel production in industry. Kowthaman et al. (2022) reviewed the advantages of microalgae biomass oil extraction and biodiesel production, as well as the methods of algae isolation, growth, types of cultivation, microalgae harvesting, types of oil extraction as well as their effects on biodiesel production methods, biodiesel characterization and related work. Discussing of using the open algal pond systems versus the photo-bioreactors were discussed. Selection methods of the microalgae species with respect to lipids used for biodiesel production were presented and compared with conventional feedstocks. These authors also discussed process factors of the in-situ transesterification of algal oils for biodiesel production such as molar ratio, stirring rate, moisture, reaction time, catalyst type and temperature. The characteristics of the algal biodiesel are presented. Product properties such as flash point, cetane number, density, kinematic viscosity, pour and cloud point and calorific value were studied and compared with the results of conventional diesel oil. The properties of microalgae biodiesel meet ASTM standards. This study reports that oil from microalgae can be a suitable alternative compared to edible oils due to ease of growth, separation, and high lipid contents. Hodaifa et al. (2008) studied the rinse water from a 2-P olive oil mill for producing microalgae Scenedesmus obliquus (CCAP 276/3A). The characterization of the wastewater indicates that it is deficient in nutrient of nitrogen for algae growth. The highest value of maximum specific growth rate, 0.044 h−1 was registered in the culture with 5% raw waste. However, biomass productivity proved greater in the culture with 100% raw waste. It was found that the composition of the lipid fraction of the biomass depended on the percentage of wastewater used as the nutrient medium, reaching the highest percentages of monounsaturated, polyunsaturated, and essential fatty acids in the culture with 100% olive mill waste. Hodaifa et al. (2020) also studied the combination process of physical–chemical treatment and algal culture of Scenedesmus ubliquus for treating olive mill wastewater. They proposed a primary treatment with flocculation using Flocudex CS-51 as a first step, followed by microfiltration through 0.2 μm pore size membrane and microalgae growth as the main treatment stage. Study was made for wastewater dilutions of 5, 10, 25, 50, 75 and 100% in ultrapure water as the culture media. Experiments were performed on a laboratory scale in stirred batch tank reactors. High overall removal rates achieved after primary treatment for chemical oxygen demand (92.6%), total phenolic compounds (98.9%), total organic carbon (75.9%), total nitrogen (63.5%) and inorganic carbon (55.3%). Final harvested biomass rich in energetic compounds were harvested with highest values of carbohydrates (72.5%) in culture with 5% OMW and lipids (44.9%) in 100% OMW culture.

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These authors have shown that among the tests they made, cultures without dilution and at aeration rate 0.5 min−1 , agitation at a of speed 3.33 Hz, continuous illumination, and temperature of 25 °C gave highest biomass growth at (0.21 g/L). Lipids (44.9%) that were obtained under these conditions was suitable for biodiesel production. On the other hand, treatment for pollutants removal by using a culture medium with 50% of which resulted in the removal percentages of TOC (67.4%), IC 95.8%, and TN 98.2%. For phenolic compounds removal, the highest removal rates (54.4 and 59.1%) were determined in the culture media with 5% and 10% of OMW, respectively. These researchers reminded however, that for a real process, temperature and illumination would be the variables to be under natural conditions, which means that solar light and ambient temperature should be used. DiCaprio et al. (2015) studied simultaneous algal biomass production and biotreatment of olive mill wastewaters using Scenedesmus sp. Unsterilized wastewater at 9% dilution by volume with tap water added by nitrates to make up for the unsuitable C/N ratios for algal growth was applied. Heterotrophic growth was attainable due to the dark color of the olive mill wastewater even at such dilutions. Yet, they reported growth rates comparable (even bigger) were attainable as the rates in autotrophic cultures. They also reported that the contamination was not a risk and the addition of inorganic nitrogen prevented reduction in growth rates. Mostafa et al. (2019) investigated growing different bluegreen microalgae Nostoc muscorum, Anaebaena oryzare, in a BG11medium and Spurilina platensis in a Zarrouk medium and used them for culturing in undiluted and 1:2 diluted olive oil mill effluent mediums to see the difference. They observed biomass growth and the decrease the phenolics content. They found that bluegreen algae reduced the initial phenols by 50–60% within 15 days and 80% in 30 days. In case the wastewaters were diluted by 50% the growth was much better. They concluded that microalgae could utilize the phenol as carbon source for growth. In the research work by Malvis et al. (2019) studied the 2-P olive oil mill wastewater in an integral process with preliminary stage of one of the three alternative physical–chemical methods of flocculation, photolysis and microfiltration followed by microalgae growth as a second stage. COD removals were 57.5%, 88.8% and 20.5% for flocculation, photolysis, and microfiltration, respectively. The removal efficiencies of organic load in the primary treatment were 96.2% for COD, 80.3% for total organic carbon (TOC) and 96.6% for total phenolic compounds (TPCs). In secondary treatment, different experiments using the microalgae Chlorella pyrenoidosa were performed in laboratory-scale stirred batch tank reactors. The olive mill wastewater concentrations in each culture medium were: 5, 10, 25, 50, 75 and 100% (v/v). The common experimental conditions: pH = 7, temperature = 25 °C, agitation speed = 200 rpm, aeration rate = 0.5 (v/v) and illumination intensity = 359 μE m−2 s−1 were used. The maximum specific growth rate obtained was 0.07 h−1 and volumetric biomass production was 1.25 mg/L h that were achieved in the culture with 50% (v/v) mixture of olive mill wastewater. The final biomass produced had a high percentage of carbohydrates, with contents ranging from 30.3 to 89.2% and the highest lipid

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content (34.2%) was determined in the culture with 25% of OMW (v/v). The final treated water is suitable for use in irrigation, discharge to receiving waters or for being reused in the same process. El Shimi and Mostafa (2016) studied the role of cyanobacteria in wastewater phyco-remediation. Their research studied growth of three cyanobacteria strains (Nostoc muscorum, Anabaena oryzae and Spirulina platensis), either individually or in a mixture, on undiluted and low olive mill wastewater dilutions of 50, 75%. Best results of all growth parameters and degradation of phenolic compounds were obtained by mixed culture and at 50% dilution. Authors also indicated the produced algal biomass that can be directly used as a bio-fertilizer for olive trees. The cultivated algal species were suggested to be a promising feedstock for biodiesel, food, and animal feed production. These examples of micro algal research studies related to olive wastes can be increased in number.

10.3.4 Biotechnological Pathways and Conversions into High-Added Value Products Ingenuity of the scientists and producers in the field of advising new techniques especially in the biotechnology of olive wastes is almost limitless. That is how so many novel processes are proposed for alleviating the problem of sustainable olive waste management. Here is only just a few of such ideas. Goula and Lazarides (2015) presented an integrated approach for complete recovery of olive mill wastes also including the recycling of water to return into the mill. Specially designed fermentation, spray-drying and encapsulation technologies are addressed producing several valuable bio-products, some of them to be included in food formulation and pharmaceuticals. Biotechnology and microbial products from olive agro-industry became popular scientific areas of microbiology and engineering disciplines related to the environment. In the field of research related to olive waste management, many comprehensive review papers were recently published. For example, Foti et al. (2021) described that out of the 794 records of scientific study between 1996 and 2020, 298 records were found in the field of biotechnology and microbiology in Science Direct using the keyword of “olive mill wastewater”. This is one of the indicators of the big interest in using olive wastes as raw material in producing marketable biotechnology products. For example, phenolic compounds like hydroxytyrosol, tyrosol, verbascoside, acids (such as caffeic, gallic, vanillic, and syringic) and polymeric substances in the olive mill wastes are value-added products that can be obtained by several high technology approaches. Solvent extraction, enzymatic and chromatic methods, membrane processes, microfiltration, ultrafiltration, nanofiltration and reverse osmosis are examples of these methods. Foti et al. (2021) described the compounds with antioxidant, anti-inflammatory, immuno-modulatory, analgesic,

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antimicrobial, antihypertensive, anticancer, anti-hyperglycemic activities in the olive mill wastes. Phenols extracted from olive oil mill outgoing streams are valuable for pharmaceutical or cosmetic applications and attempts have been made to describe microorganisms and metabolic activity involved in their treatment and production. In a research study by Goldfarb et al. (2017) solid wastes (pomace) of a 3-P olive oil plant widely used over the world were presented as a first step. Supercritical CO2 extraction applied to biochars to obtain the polyphenols and polyunsaturated fatty acids. Both pure supercritical CO2 , and supercritical CO2 followed by ethanol co-solvent alternatives were tested. Remaining exhaust biomass was transformed by thermochemical conversion into heat/steam by way of combustion or by conversion into bio-chars after pyrolysis. In the pyrolysis pathway products were converted into bio-oils like in a petroleum refinery together with syngas. Activated bio-chars are valuable materials for catalytic chemical operations and for water treatment. As was discussed earlier, olive mill wastewater can also be used for production of algal biomass, which is another positive approach for sustainability. Algal biomass accumulates lipids and carbohydrates and therefore can be used to produce biofuels, food, feed, and several bioactive molecules. There have been just few references in this review to briefly mention such opportunities compared to the actual number of research available.

10.3.4.1

Enzymes and Sugars

Different types of sugars are among the major components of olive-derived biomass. For their beneficial uses however, they must be converted into some targeted compounds. Fuel production is a logical aim, but as an initial step pretreatment is necessary to open or break down the lignocellulosic matrix and facilitate access to the hydrolyzing enzymes for obtaining glucose which is the main structural compound for further processing. Like glucose, xylose, and other monomeric sugars can be the starting points for further valuable products. For obtaining simple sugars from olive mill wastes treatment with alkalis, acids, oxidants, liquid hot water, and methods like steam explosion, biological, ultrasound etc. chemical, physical, physical/chemical methods can be applied. In some methods, catalytic treatments and enzymatic hydrolysis are used to enhance delignification efficiencies. For the solid wastes like prunings, leaves, twigs, etc., size reduction of the wastes increases the efficiency. Nadour et al. (2015) mentioned the compounds of exopolysaccharides in the olive oil wastewater. These include glucose being the main monosaccharide, followed by galactose, arabinose, galacturonic acid, xanthan, and glucuronic acid, depending also on the type of microorganisms of fermentation. These can be used in many areas. The epoxypolysaccharides are produced through a fermentation process depending on the type of microorganism. There are several more research reports for epoxypolysaccharide manufacture processes published by using different microorganisms.

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Kalogerakis et al. (2013) reported extracting value-added products from olive mill wastewaters such as hydroxytyrosol, tyrosol, and total phenols using several solvents. Good antioxidant recovery yields with the least environmental impacts were searched. Ethyl acetate, diethyl ether, and mixture of chloroform and isopropyl alcohol were tested as solvents. After running LCA analyses, ethyl acetate was found to be with the least environmental impacts at highest yield of production. Unit operations of sedimentation and membrane filtration are recommended to develop processes to separate phenolic material into value-added products of hydroxytyrosol, tyrosol, caffeic acid, oleuropein and luteolin. Synthetic resins and polymers can also be utilized as sorbents and results. It was shown that by combining different pretreatments with sorbent options, it is possible to selectively adsorb specific biophenols. Tundis et al. (2020) described using membrane technologies for phenol recovery from the olive mill wastewaters for use in pharmaceutical, nutraceutical, and cosmeceutical applications. Membrane technology applied was a combination of microfiltration (MF), nanofiltration (NF), and reverse osmosis (RO) in a sequential setting. RO application gave the highest content of hydroxytyrosol, tyrosol, oleuropein, verbascoside, vanillic acid, and luteolin. Of particular significance, hydroxytyrosol gain of 1522.2 mg/L was reached with this application, that is about five times higher than the MF. Laccases are known to help decompose olive mill wastes. Some research studies involved using waste/wastewaters as a growth medium. White-rot fungi have been reported for efficiently decomposing the phenolic compounds because their extracellular ligninolytic enzymes are able to catalyze lignin-like structures and promote oxidation of difficult-to-decompose compounds (Ntougias et al. 2015). Mann et al. (2015) studied the production of laccases from white-rot fungi grown in olive mill wastewaters. This white-rot basidiomycete, previously found to reduce phenols and phytotoxicity of olive mill wastewaters, created substantial laccase activity during cultivation. Their results indicated that the olive mill wastewater could be a source of laccase mediators since the efficiency of removal of phenols increased when 1% olive mill wastewater was added. The authors suggested that olive mill wastewaters would be worth to keep trying for some laccase-mediated bioremediation applications.

10.3.4.2

Foods and Food Additives from Olive Wastes

Bolek (2020) showed that olive stones could be used as a healthy functional food ingredient thanks to their rich antioxidant properties, nutrients, and dietary fibers. In this study, wheat flour was substituted by olive stone powder at levels of 0, 5, 10 and 15% and used in biscuit dough. Wheat flour replacement by olive stone powder increased antioxidant activity, fat, and fiber content of biscuit samples (p < 0.05). According to results of sensory analysis, wheat flour could be substituted up to 15% by olive stone powder to prepare biscuits without causing unacceptable products in terms of sensory properties.

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Agricultural olive leaf wastes and eggshell and eggshell membrane wastes from food industry were used together to produce valuable functional products by Bayraktar et al. (2021). The adsorptive loading capacity of the eggshell membrane for bioactive compounds obtained from olive leaf extract were tested for preparing functional biomaterials. Using the foam separation method, both separation of the eggshell membrane and adsorption of bioactive compounds to the eggshell membrane were achieved simultaneously. It was shown that olive leave extract was successfully adsorbed by the eggshell membrane. Cytotoxicity and antimicrobial studies showed that prepared olive leaf extract-loaded membranes were functional materials with bioactive properties. In conclusion, ESM was determined as a promising protein in the production of functional antioxidative and antimicrobial food or dietary supplement after the adsorption of bioactive olive leaf polyphenols. Tree pruning debris including twigs and branches, as well as the leaves are currently used for animal feed. However, because of their polyphenolic composition, olive leaves have large potential in numerous other more valuable applications. In a review Espeso et al. (2021) analyzed the chemical composition of olive leaves and discussed methods of processing them, including thermochemical, biochemical, drying, extraction and condensation methods. They also examined the current applications of the olive leaves in sectors relating to cattle feed, fertilizers, novel materials, energy generation, and food and pharmaceutical products. Their aim in this review was to provide a source of knowledge for producers, policy makers, innovators, and industry in shaping environmentally sustainable decisions for how olive prunings and leaf wastes can be utilized and optimized. Lafka et al. (2011) carried out conventional liquid solvent extraction and phenolics as valuable ingredients. They found that the ethanol extract exhibited the highest antiradical activity, but no correlation was found between anti-radical activity and phenol content. The ethanol extract appeared to be a stronger antioxidant, and contained ascorbyl palmitate and vitamin E. The supercritical extract exerted good antioxidant capacity although its phenolic yield was not quite high. Various phenolic acids and flavonoids were also identified. HPLC analysis of the extracts showed that the predominant phenolic compound was hydroxytyrosol. Caporaso et al. (2018) studied olive mill wastewater and gave an overview of the techniques of separating valuable ingredients, especially the ones for use in food production. They have given effective methods to recover phenolics from the wastewater and some examples on how to utilize them in functional foods. They suggested that added into vegetable oils, they retard the lipid oxidation and improve the oxidative status of the product, whilst several challenges need to be faced. Obtained phenolic extracts were also used in food emulsions, milk products or other model systems, showing promising results and little or no negative impact on the sensory characteristics or other properties of these food products. Their possible use as antimicrobial agents is a promising approach for the application in meat products. Olive mill solid wastes after a few days in storage may create seepage water flows, odor and fly nuisances, and develop increased acidity in the residual oil product to make the quality unsuitable for edible oil. Research indicates that to eliminate such problems, olive solids must be extracted when they are fresh. In a biotechnological

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model, residual olive cakes after oil extraction can be utilized in furfural production or can be used as animal food additive and fertilizer. Studies have shown that olive cake may be used for composting and is a good raw material in biotechnology. Bio-oils, bio-char and biodiesel production research is underway for the pomace and pomace derived materials. 2-P process wastes cannot be easily transported and dried, as there is need to special equipment for their transport, storage, as well as driers to bring down the moisture content to suit the novel process requirements. Single-cell protein (SCP) which is a protein source derived from cells of microorganisms such as yeast, fungi, algae, and bacteria, can be grown on various carbon sources. There are many publications discussing the role of agricultural wastes in SCP production. For production the dried cells of microorganisms or the whole organism grown on the carbon source is harvested and consumed. SCP is a protein source for human food supplements and animal feeds. Rages and Haider (2021) studied single cell protein production using C. lipolytica from oil cake wastes of olive agroindustry. Alkaline hydrolysis of the waste was used to break down the cellulosic substances and isolate simple sugars at different temperatures and times. The hydrolyzed supernatant parts of olive fruit wastes were utilized as a carbon source, and as a basal medium for growth of the yeast Candida lipolytica. Single cell protein can be a remedy for human food and animal feed. Flamminii et al. (2020) evaluated the recoverable matter in olive mill wastes. In the top of the list are the phenolic compounds of increasing attention worldwide as they show a wide spectrum of beneficial including many antimicrobial activities and holding and emulsifying capacity. Phenolic extracts and their by-products are suggested for use in several food products to improve the oxidative stability of fatty matrices and in general the enhancement of the nutritional profile. Several research studies are published and are underway. According to these authors, several critical issues may arise and need to be addressed in some food matrices, though. Among them are the impairment of the physical properties and impacts on the sensory acceptability such as bitterness and pungent taste. However, criticisms arose especially when by-products were used with minimal purification pretreatments, as in the case of pomace flour. In some cases, encapsulation technologies have been proposed to overcome the main critical aspects of olive mill wastes and their recovered compounds; however, up to now, their application in food matrices has not been enough. In food and food additive use many research articles are published. As examples, a group by Galanakis, C. M., and co-workers have many proposed valorization methods to obtain dietary fibers (Galanakis et al. 2010a), and producing pectin and phenol containing solutions for beverages using nano- and ultrafiltration (Galanakis et al. 2010b) among other novel uses. Domenech et al. (2021) made full characterization of olive derived wastes for use in production of value-added compounds. Among the compounds detected, xylooligosaccharides, mannitol, 3,4-dihydroxyphenylglycol, and hydroxytyrosol were noted as potential enhancers of the valorization of said by-products. The extraction of these compounds is expected to be more favorable for OTP, OL, and EOP, given their high extractives content, and is compatible with other utilization strategies such as the bioconversion of the lignocellulosic fraction into biofuels and bioproducts.

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Koutrotsios et al. (2019) used the olive-mill wastes, and olive pruning residues as substrates for the cultivation of Ganoderma lucidum and Pleurotus ostreatus. They found that several substrates based on olive by-products had a positive impact on P. ostreatus mushroom production, whereas only one performed adequately for G. lucidum. Addition of phenols obtained from olive mill wastewaters to increase the beneficial effects of foods and to extend their shelf-lives is at the stage of becoming commercialized. In the USA adding olive pulp extracts up to 3000 mg/kg have been approved by in certain types of foodstuffs at the level of GRAS (Generally Recognized as Safe) status (GRN No. 459). Application of olive mill waste extracts for food supplement is a broad field of recent research. There are many proposed and developed methods for the healthiest method of purification for the beneficial polyphenols and their fractional separation.

10.3.4.3

Pharmaceuticals and Nutritional Additives

Olive mill wastewaters are rich sources of bioactive substances that can be used as ingredients in the pharmaceutical industry due to their antioxidants and antimicrobial components. In a review by Zbakh and El Abbassi (2012) studied the potential application of olive mill wastes for the preparation of functional beverages and the impact of beverage formulation factors on bioavailability of phenolics in their composition. They concluded that owing to the numerous reported biological activities of these wastes, and its phenolic content are associated with health promoting activities. Therefore, phenol extracts when used in beverage preparations may have a significant impact on the human health through the reduction in incidence of cardiovascular and chronic degenerative diseases. El Shafie et al. (2022) found that cobalt oxide (Co3 O4 ) nanoparticles supported on olive stone biochar was used as an efficient sorbent for rifampicin and tigecycline from wastewater. These pharmaceutical products could be adsorbed on biochar material obtained from olive stone had maximum adsorption capacities (qmax ) for rifampicin and tigecycline were 61.10 and 25.94 mg/g, respectively. By using a spray drier system working at low temperatures, extracts rich in phenols with different biological properties were produced in a research by Benincasa et al. (2022). Qualitative and quantitative analyses were carried out by high-pressure liquid chromatography–tandem mass spectrometry (HPLC–MS/MS). Following chemicals were detected in mg/kg dry powder weight: apigenin (9.55), caffeic acid (2.89), catechol (6.12), p-coumaric acid (5.01), diosmetin (3.58), hydroxytyrosol (1.481), hydroxytyrosyl oleate (564), luteolin (62.38), luteolin-7-O-glucoside (88.55), luteolin-4-O-glucoside (11.48), oleuropein (103), rutin (48.52), tyrosol (2043), vanillin (27.70), and verbascoside (700). They showed that the use of dehumidified air as a drying medium, with the addition of maltodextrin, was an effective way to produce a phenol-rich powder to be included in food formulations as well as in pharmaceutical preparations having different biological properties.

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D’Antuono et al. (2014) reported evaluations of Italian and Greek olive mill wastewaters using membrane technologies in successive filtrations and the products of microfiltrates, ultrafiltrates and nan filtrates were analysed for polyphenols. The ultrafiltrate fraction indicated good purification and polyphenol enrichment. Speciation of the compounds in the UF fractions were made to find secoiridoids and their derivatives, phenyl alcohols, phenolic acid and derivatives, and flavonoids. However, differences were observed among the cultivars regarding the presence of elenolic acid derivatives, hydroxytyrosol glucoside, and β-hydroxyverbascoside diastereoisomers. These showed that olive mill wastewaters can be regarded as valuable raw material for the isolation of several bioactive compounds able to be used in food, cosmetic and pharmaceutical industry. Several nutraceuticals of olive waste origin aiming to reduce the oxidative stress are currently available on the market. Olive polyphenols are renowned as the main compounds responsible for the health effects of the Mediterranean diet in prevention of several chronic diseases and diet-associated diseases. Gorzynik-Debicka et al. (2018) submitted helpful data on the olive products and olive polyphenols for their health benefits and potential medicinal uses. Health problems such as obesity, metabolic syndrome, type 2 diabetes (T2D), cardiovascular diseases, hypertension, and some cancers are included in these problems. The main healthy phenolic compounds present in olive mill wastewaters are derived from oleuropein hydrolysis, as hydroxytyrosol, tyrosol and elenolic acid. Although there are many other ingredients helpful for the health of the body.

10.3.4.4

Animal Feed from Olive Wastes

There are very promising research studies for the use of additives generated from olive mill wastes in animal feed and fodder formulations. Nutritional compounds in the olive agro-industry wastes is an economically reasonable factor in preparing livestock feed at less cost. There are several examples of this strategy starting with lamb, pig, chicken feed formulations. For example, Leskovec et al. (2019) mentioned that olive polyphenols exerting antioxidative effects in humans, does so in animals, too. They tried olive leaf extract known to be rich in phenols in pig feed having a linseed oil-enriched diet, which is a known cause of postprandial oxidative stress. This extract exerted some antioxidative effects in piglets with no negative effects, even when included in higher concentrations than recommended for humans. Therefore, they suggested that olive leaf extract might be used as an effective feed additive for beneficial impacts on intestinal health and meat quality, among others. Tzamaloukas et al. (2021) discussed the utilization of olive industry by-products as part of the ruminant diet to minimize the costs related to animal feeding and facilitate the olive waste management. They indicated that olive wastes can be safely included up to 15–20% on dry basis without negative effects on digestion. They suggested that nutritional properties of milk and meat are improved through dietary

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supplementation, without negative effects on growth performance and productivity. It was found that these additives increase the level of monounsaturated fatty acids and decreases that of saturated fatty acids in the milk and meat of ruminants with are both beneficial effects for consumer health. Makri et al. (2020) have shown that the introduction of olive mill wastewaters into the silage of lambs improved the antioxidant profile in the vital tissues of lambs, such as the heart, brain, and muscle.

10.3.4.5

Sustainable Packaging Materials

In a recent review by Khwaldia et al. (2022) discussed the beneficial uses of olive byproducts for food packaging at a laboratory scale. Bioactive compounds that can be obtained from olive wastes were used in packaging formulations, to improve the functional and biological properties of films and coatings as well as extending the shelf-life of food. These authors thought that use of bioactive compounds from olive waste materials in coatings such as in the commercial thermoplastic films can be feasible for application at an industrial scale. However, the issue needs more attention, and some hurdles need to be overcome. Also, they thought that legislation for packaging practice must be backed up by new governmental policies and financial support to promote the adoption of this new technology.

10.3.5 Agricultural Uses of Olive Wastes 10.3.5.1

Bioremediation

In a comprehensive review article Sciubba et al. (2020) discussed the antimicrobial activity and protective properties of olive wastes against plant pathogens with the bioactive molecules including phenols and polysaccharides. These authors reviewed the recent advances made in the identification, isolation, and characterization of olive-mill-wastewater-derived bioactive molecules that enables an influence on the important plant processes such as plant growth and resistance to pathogens. In another recent review Foti et al. (2021) mentioned that olive mill wastewaters are a growth substrate for Azotobacter vinelandii and the resultant effluents could be applied onto soil as fertilizer. Role of phenolic compounds in integrated pest management programs, has been mentioned by several studies, too. Recently, the microorganisms decomposing these compounds and convert them to residues have been studied for lower toxicity, COD, and phenolic content effluents from the olive wastes.

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Biopesticides from Olive Mill Wastes

Plant diseases are largely addressed using synthetic pesticides that have a negative impact on environmental pollution, are harmful to health, and can also generate pestresistance. Bio-pesticides have been recently described as the best candidates for the control of phytopathogens for a sustainable agriculture. Sciubba et al. (2020) suggested that the huge problem which is represented by pests such as fungal and bacterial pathogens causing significant crop losses and produce potentially harmful end products can be confronted with olive mill wastes. Di Ilio and Cristofaro (2020) studied the effect of polyphenolic extracts of olive mill wastewater to determine their possible use against the Mediterranean fruit fly. They suggested that the polyphenolic fractions derived from the olive mill wastewater can be used as a strong natural chemosterilant against the Mediterranean fruit fly.

10.3.5.3

Reuse for Irrigation Water

It might be underlined that the biggest help of olive mill wastewaters on the agriculture would be reclamation of water for irrigation purpose. Koufi et al. (2006) studied the reclamation of olive mill wastewaters for agricultural purposes by means of electroFenton method followed by anaerobic digestion. They suggested up to 65.8% of the total phenolic compounds can be removed by electro-Fenton, to lower the phenol induced toxicity. Electrocoagulation of the anaerobically digested effluent provided complete detoxification. Under the conditions of water shortages which is a very crucial issue in the olive growing regions, the huge volumes of wastewater generated by the olive industry attracts attention. However, using them as they are, is unthinkable for sustainable farming in the croplands as well as the produces. Possible reuse in irrigation needs serious research dealing with the adjustment of properties by treatment and choosing appropriate methods of irrigation. In a study by Dutournie et al. (2019), recovery of water from olive mill wastewater was proposed. The method included the impregnation of olive mill wastewater on a lignocellulosic biomass (such as oak sawdust, wood chips), followed by controlled drying of the impregnated biomass in a convective dryer. Water recovery was made by condensation of the vapors from the drying operation. Finally, the residual of impregnated biomass would be reused. The convective drier which was used for evaporation was at air velocity of 1 m s−1 and air temperature of 50 °C. The experimental results showed that the water recovery yield reached 95%. The condensate waters have low electrical conductivity and salinities but turn out to have low pH values and slightly high COD. However, they could be easily made more suitable for reuse in agriculture after additional low-cost treatment. In recent studies, challenges of using olive mill wastewaters for irrigation of olive groves was covered. In one of the research reports, problems originating from the salinity, sodicity, metal and trace element content, nutrients, organics, and pathogens are discussed (Salcedo et al. 2020). Their concentrations and safe dosages in irrigation use were indicated. These authors indicated that depending on the varieties, plant

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tolerances to salinity varies therefore they could respond differently to irrigation by wastewaters. In olive groves product oil quality may improve or be compromised. Although irrigation by treated olive mill wastewaters is a very remarkable topic for reuse purposes, more research is needed in this area like other valorization methods of the olive agro-industry wastes.

10.3.6 Table Olive Processes and Wastes 10.3.6.1

Table Olive Production Processes

Olives for table olive production are separated from leaves and twigs and classified with respect to fruit size. It is known that there are entirely different operations in preparing table olives of different assortments. Cappeletti et al. (2011) summarized the production processes for different types of table olives varying from green to black table olives. Each has different polluting steps and water uses. There are several different production processes for table olives depending of the kind of product required, variety of olives, natural composition, degree of ripeness, country of origin, local or regional customs, etc. But the table olives produced in this way have shorter processing times than other methods and are in high demand around the world (Parinos et al. 2007). The most important processing methods regarding economic importance have following three main steps involved: De-bittering or Lyeing In this step olive bitterness is removed using several chemicals such as NaOH, NaCl, lactic acid, etc., and high amounts of clean fresh water is needed. Successive rinses produce a high amount of wastewater. NaOH at 1–2% w/v concentration was used for lyeing for 8–12–15 h. The concentration of the NaOH used depends on the variety of olives, the degree of ripeness of the drupe, temperature, and characteristics of the water to be used. In this first step oleuropein is hydrolyzed to elenolic acid, glucoside and hydroxytyrosol. Rinsing This step uses large quantities of fresh water to separate the sodium hydroxide from the flesh of the olives at varying durations. The most used method for washing is to rinse for 18–25 h with an initial short rinse of 1–2 h and two more rinses of 8–12 h each. There are other options for longer or shorter duration rinses depending on the olives to be produced. Fermentation in Brine After the rinsing step, the olives are submerged in 9–10% w/v NaCl solution. The addition of used brines or “mother brines,” ensures the onset of a safe lactic fermentation.

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Finally, after the olives are washed with fresh water and after the olives of unacceptable quality are disposed off, they are packed using 3–5% brine solution and pasteurized at 90 °C for 1 h, according to traditional methods.

10.3.6.2

Wastewaters from Table Oil Processing and Their Treatment

During the three main steps of production multiple washes are required, and this leads to generation of 2–6 L of wastewater per each kg of olive used (Rincon-Llorente et al. 2018). Along with the wastewaters from these three successive stages, there are additional wastewaters from a table olive processing plant which are produced from the washing of the plant, cleaning of the vessels or containers, etc. Also, different kinds of wastewaters may be coming from specific table oil types such as Spanishstyle green olives, Californian-style, etc. All wastewaters from table olive processing cause serious environmental problems because of their high volumes and chemicals in composition. For example lye wastewaters and the subsequent washings have high pH and contain high alkalinity, and brine wastewaters are acidic and saline. All have high amounts of organics, too. Fermented green olive polyphenols in the wastewaters coming from fermentation brines is rich but no reducing sugars are left as they are consumed in the fermentation step. Table olive production wastes are a different group of wastes from the olive mill wastewater problems. In this regard they constitute an overlooked problem. The table olive industry and the more difficult problem of their wastewaters is concentrated mainly in the Mediterranean area. This is where most of the table olives is produced. These wastewaters are even more problematic not only because of their characteristics of high organic matter, high phenolic content but also due to high salinity and conductivity due to added salts and alkalis. At present, these wastewaters are no different than the olive oil wastes as there is no generally suitable treatment with acceptable results. Like Energy production from olive by-products (wastes) by direct firing alone or co-firing alternatives, has been one of the first approaches in valorization. In this respect, journals in the fields of sustainable energy and fuels have hosted a good number of scientific reports including excellent reviews of wide coverage. The olive mill wastewaters, presently the most common management method is storage in large evaporation ponds, an environmentally unacceptable solution to the problem. Different studies have been carried out on table olive wastewater treatment, but the reality is that at the industrial level, none has been successfully applied. A recent review (Rincón-Llorente et al. 2018) analyzed the present state of the table olive wastewater treatment and some promising technologies for the future. Some table olive industries use systems to remove suspended solids and active carbon absorption and ultrafiltration through membranes to discolor table olive processing wastewater. These wastewaters, just like the olive mill wastes deserve valorization to produce high added-value compounds, too. Huertas-Alonso et al. (2021) proposed a valorization scheme which can be divided into two main lines:

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the extraction of phenolic compounds and the transformation of the sugary fraction into selected compounds. Chemical conversions were carried out to obtain valuable components, such as levulinic acid and 5-hydromethylfurfural. The products were then analyzed using NMR identification of the antioxidant phenolic fraction and microwave single-phase reaction of the sugar fraction. According to the results, the highest concentration of phenolic compounds does not correspond to the sample directly obtained from NaOH treatment (S1), indicating that water washing steps (S2–S5) are fundamental to recover phenolic substances. Moreover, glucose was presented in the sugary fraction that can be transformed into levulinic acid by a single-phase reaction under microwave irradiation. Scientific research on the table olive waste management options included some articles such as the following. Papadaki and Mantzouridou (2016) reviewed the promising technologies for biological treatment of table olive processing wastewaters at laboratory to pilot-scale systems. These included methods of anaerobic digestion, lactic acid fermentation and fungal fermentation and advances in the manufacturing of value-added products (e.g. biogas, platform chemicals, natural antioxidants). Biological treatment of table olive processing wastewaters using different microorganisms, the required bioreactor design modifications, and operational conditions were discussed. Prospects of valorizing table olive wastewaters were presented in this article. Chatzisymeon et al. (2008a, b) investigated the photocatalytic treatment of effluents from black table olive processing over TiO2 suspensions. They studied the effects of various operating parameters such as initial organic load, type of catalyst, wastewater concentrations and possibilities of reuse, and addition of hydrogen peroxide. Depending on the conditions employed, nearly complete decoloration (> 90%) could be achieved, while mineralization never exceeded 50%. Shake-flask tests with non-acclimated activated sludge showed that both the original and photocatalyzed effluents were degradable aerobically. Their biodegradation rates were three times greater. On the other hand, photocatalytic oxidation of the original effluent was faster by at least two orders of magnitude compared to biological oxidation at comparable levels of degradation. Ozdemir and Keskinel (2018) studied the current methods of treatment of table oil industry wastewaters. Knowing that high pH, salt and phenol content of the wastewaters make it difficult to treat they suggested eco-friendly production methods to be developed from an environmental perspective. The extraction and recovery of added-value products from table olive manufacturing would reduce the operating costs of the advanced treatment processes required and would make them more applicable. It might also be possible that these valorization methods would result in an improvement in the efficiency of biological processes avoiding inhibitory processes. For example, Kiai et al. (2018) investigated a real wastewater from table olive processing plant and recovered phenols using cloud point extraction. This technique was used to preconcentrate the natural phenolic compounds from table olive processing wastewaters using nonionic surfactants. The optimum conditions for recovery of phenolic compounds were studied with respect to different experimental

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parameters, surfactant concentration, pH of the solution, equilibrium temperature and incubation time. The recovery of phenolic compounds with one step CPE under at optimized conditions were 62–68%. Extraction and recovery of added-value products can be considered as a viable solution to difficulties of treatment. For example, phenols that are strong antioxidants can be extracted from wastewaters. Kiai et al. (2014) studied membrane distillation process for treatment of table oil wastewaters. They used PTFE membranes with different pore sizes at different temperatures and achieved a very high separation of phenolics after 4 h. They report phenolics concentration factor up to 2.2.

10.3.7 Waste Minimization Practices and Biorefineries for the Olive Agro-industry The olive biorefinery includes processes for multiple products from a multiplicity of raw materials such as olive leaves, prunings, olive mill wastewaters, solid wastes, etc. Almost all of the fields of scientific research results on these materials can be included in an olive biorefinery.

10.3.7.1

Linear Versus Circular Economy

In a strategy of the circular olive agro-industry format which can be described as “grow-produce-consume-valorize the outputs-regrow”, the cost of the treatment and to the environment will be minimized. Olive biorefinery is the novel conceptual strategy developed in this regard. Completion and performance of the cycle can be followed by life-cycle-analyses (LCAs). This also minimizes the misuse of resources, and protect the resources of water, energy, and carbon footprints. Waste minimization possibilities are crucial in olive agro-industry. Some researchers investigated these possibilities with Life Cycle Analyses (LCA) approaches in mind. The research article by Vlyssides et al. (2004) is one of the frontiers of integrated olive oil production proposals with “from-cradle-to-cradle” strategy to replace the traditional “from-cradle-to-grave”. Yet, within the last 18 years there has not been seen any full-scale plant. There is no single global management strategy that can be adopted that is feasible in different socio-economic contexts and production scales. On the other hand, the most recent management approach is to propose and realize olive mill waste valorisation which is receiving more attention as European Commission is promoting the transition towards a circular economy (Ochando-Pulido et al. 2016b).

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Kourmentza et al. (2017) studied and demonstrated the potential of olive mill wastewater to be converted into sustainable resources of biofuels and bio-based products. They reviewed the most significant advances concerning a variety of promising valorization scenarios. Domenico et al. (2021) published a comprehensive review of the existing literature for their compliance with CE perspectives. They mapped the possible conversion and valorization pathways (theoretical and applied) in a circular olive oil supply chain. These pathways were meant as good ways of handling the bioresources and worked out in their capacity to lead to: (i) minimization of wastes; (ii) optimization of the use of the resources, (iii) enhancing the efficiency of the whole agri-food system, (iv) create new value chains, (v) enable regenerative and restorative ecologies. They concluded that the solutions are visible in the existing literature, with means and ways of the methods proposed for recovery and utilization of the waste streams in the olive oil supply chain. They thought that market aspects need more treatise as it generally disregarded it. Only a marginal attention has been paid to the innovative CE-based business models, to coordination challenges of the whole industry or to regulatory issues. They also noted the shortage of real case-studies which may hinder the application of circular management solutions. Only the subsectors of recovering high value-added bioactive compounds from the olive wastes, obtaining polymeric biomaterials production from pruning residues, and biofertilizers or biostimulants and biopesticides production were found to have progressed in this aspect. They underlined that the energy valorization routes must be given more attention for optimization, with more care to the conversion of by-products (such as digestates) tests to be carried out as beneficial for the ecosystems.

10.3.8 Beneficial Uses of Olive Biomass Streams in a Conceptual Olive Biorefinery As a general characteristic of the olive mill wastewaters, it must be said that they have high solids, and rich in salts and organics such as polyphenols, organic acids, polysaccharides, oils, proteins. These make their physical–chemical or biochemical treatment very difficult, slow, and therefore costly. Depending on the oil production technology these wastes may originate from 5 to 6 unit operations in the mill and have different properties. The specific properties of each stream are firmly established thanks to many scientific research papers and books. As their compositions are nicely established (of course in ranges for each parameter as they fluctuate for many reasons) and as the valorization techniques got clear, very comprehensive leading review papers have been and are being published. Waste minimization studies aim at water economy and wastewater reuse. Although 2-P mills might be considered in this category due to less water use, it creates a substantial quantity of a thick and more loaded liquid phase, which is not necessarily more easily treatable than the separate wastewater and solid phases of the

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3-P mills. So, it is not problem-free (Azbar et al. 2004). Mixed pomace and olive mill wastewaters of the 2-P technology might endanger the existing solid waste deoiling investments operating like reclamation units in essence. Likewise, valorization processes might require less intermixed waste streams to be more feasible than the 2-P mill wastes. An olive biorefinery might be considered like a petroleum refinery when the biomass (olive wastes) raw material is to be considered like crude oil and a wide variety of bio-products such as (bio-)energy, (bio-)fuels, (bio-)chemicals are obtained. Biomass in this case replaces fossil resources and this replacement is healthier for the future of the earth for several reasons. Firstly, biomasses are seasonally growing resources instead of a diminishing underground oil resource. It also replaces the carbon cycle adding net emissions of CO2 into the atmosphere by an annual carbon cycle renewing the CO2 . So, all the biofuels and bioenergy forms are much sustainable compared to fossil resources. This sustainability also covers the solutions for climate change problems and extreme weather management efforts. Energy production from olive by-products (wastes) by direct firing alone or cofiring alternatives, has been one of the first approaches in valorization. In this respect, journals in the fields of sustainable energy and fuels have hosted a good number of scientific reports including excellent reviews of wide coverage. A multi-feedstock biorefinery concept was discussed by Romero-Garcia et al. (2022) among several others. The idea of multiple feedstock and multiple product olive bio-refinery which can be established as proposed in Negro et al. (2017) can operate both on watery and relatively dry wastes from the olive agro-industry. The marketable bio-products will help the economies of the industry and the region. It will also change the waste discharge patterns and help solving the environmental difficulties. These authors recommend the optimization and integration planning with scale-up studies to be commenced as necessary. There are many different types of feedstocks in a multi-feedstock biorefinery such as the olive biorefinery, responses to mixed use of raw materials by way of co-transformations seem to be a healthy approach. These authors propose smaller scale, therefore less capital investment units to compete with centralized large biorefinery units. This seems a good approach for olive agro-industry regions that may integrate biorefinery concepts with growth and production of the olive by-products. Olive production and by-products were briefly mentioned in this article as scientific and technical literature is already very rich in these topics. Tree prunings, leaves, stones, solid wastes like pomace and stones and wastewaters including fruit washings, different wastewaters coming from the horizontal centrifuge in 3-P systems and solid matter rich wastewaters from 2-P systems and the oil wash waters from virgin oil tanks, all wastes are rich biomass resources. Their composition and amounts may vary drastically from one year to another, and according to variety of olives, location, time of harvest, delay time before milling, degree of ripening, etc. Using waste bioprocesses in an olive biorefinery to produce useful products and metabolites drives this agro-industry towards a sustainable circular bioeconomy. Bioenergy and biofuels from olive wastes might potentially replace the fossil fuels

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and help launch an ecologically friendly carbon cycle. These might as well be beneficial to the pathways of the carbon cycle outside of the olive agro-industry and GHG mitigation and better carbon management will be addressed. Cardoza et al. (2021) studied the selection of best areas for building biorefineries by appraising the locations of the olive-derived waste flows are generated. In this work, the biomass flows from olive crop fields, mills, and olive pomace-extracting industries where these wastes are generated were determined and their amounts were indicated using geographic information system (GIS) tools in Andulisia (Spain). Then, the environmental sensitivities were evaluated on the GIS system to determine the reception capacity of the study area. Information layers corresponding to the availability of the four different types of biomass/wastes, and layers corresponding to the environmental fragility of the study area were overlapped and they resulted in an overall map. Thus, identification of the best areas for the biorefineries could be made. Olive biorefineries are good examples of circular bio-economies which are gaining increasing attraction. These concepts bring about obtaining multiple added-value products while valorizing the olive biowastes. A few examples of biorefinery schemes based on olive biomasses are described and environmentally tested in the literature, i.e. Khounani et al. (2021). In their evaluation the Life Cycle Assessment (LCA) has been performed to assess the environmental impacts associated with the valorization of olive biomasses through several processes. It is important to know and lower the environmental footprint of the olive biomass when these biorefineries are established and processes applied. The selection of the processes, products and location of the biomass processing centers also seems crucial. More work is needed to establish the environmental impact of biorefineries based on olive biomass through LCA for political and economic decision making in a bioeconomy framework and for a better comparison of all these valorization processes. Contreras et al. (2020) wrote a review article and studied the usual practices for disposal of different olive biomasses versus the advantages of the wide range of products that can be produced from them. They summarized the most relevant practices of the products and the processes for their manufacture. They mentioned that for the present time the accumulation of process information for olive mill waste valorizations is satisfactory. In this research authors also studied the integrated valorization pathways in a single facility called a biorefinery, with due technoeconomic and environmental evaluations. In this they included the bio-conversions into bioenergy, biofuels, and a wide range of bioproducts. Examples of biorefinery schemes were given in their article, however, they added that the scale-up and industrial applications are still missing. They warned that although these wastes have a significant potential to being valorized, it must be kept in mind that their production is usually scattered geographically, wastes are generated from relatively small sources, and are present only for a short period of time during the year. Therefore, care should be exercised in calculating the economic turnover. For re-evaluation practices a good background scientific research and technical capacity is necessary. They also discussed the environmental consequences to be expected from this organization.

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In a special issue edited by Castro (2020), several contributing articles are published together to cover introductory ideas in a biorefinery concept. These ideas are applicable to conventional linear production schemes and circular bioeconomy models of producing olive and olive products. Beyond being treatment and disposal alternatives for the wastes, they aim at recovery of valuable materials. A comprehensive review prepared by Romero-Garcia et al. (2014) compiled the research studies on the olive industry by-products, starting from olive tree prunings, olive stones, olive pomace and wastewaters obtained from olive oil production. It included a wide range of compounds that can be produced in a biorefinery based on olive tree biomass. For example, bioethanol has been considered at length, as well as research on other value-added products. Recent technological advances, future perspectives, and challenges in each stage of the process have been attained. Negro et al. (2017) categorized all wastes from olive agro-industry into two main groups with respect to the water contents: (a) low to medium moisture level wastes (prunings, leaves, fruit stones, pomace), (b) wastewaters of different solids contents. Low-medium moisture content wastes, heat, steam, and power can be obtained by direct firing, or after gasification following pyrolysis or charring. Alternatively, through biochemical/chemical conversions using proper techniques, lignin, cellulose, and hemicellulose contents are transformed into ethanol, oligosaccharides, resins, composites, polymers, activated carbon, aromatics, etc. And whatever material is left from these conversions can be burned to obtain heat and power. On the other hand, wastewater group needs treatment to comply with wastewater discharge limits. They are not acceptable for direct land spreading or irrigation and with open storage in wells. Inclusion into the municipal wastewater network or treatment is not allowed, too. This necessitates treatment, which is difficult, low efficiency and expensive. But if certain steps can be added to the treatment schemes as pre- or post-processes that will valorize the chemicals that are hard to decompose in the main treatment units, both the difficulties and costs may get lower. For example, anaerobic digestion can be coupled with biogas units, bioethanol and biohydrogen production units can be integrated. Also, extraction of valuable chemicals after pre-processing is possible and after proper purification they can be sold as food and feed, pharmaceuticals, plastics, fertilizer, or soil conditioner. These reduce the overall costs of treatment. A high number of possibilities exist for evaluation of many different types of biomass flow in the olive agriculture and olive products industry. These extend from conversion to energy, gas, fuels, to numerous bioproducts. One must keep in mind that only one fifth of olive fruits by weight can be converted into oil and the other four fifths are wasted. The bioprocesses using hard-to-treat compounds integrated with wastewater treatment will replenish water resources by way of reuse or recycle the water such as use in irrigation. At the same time better treated wastes will not pollute the environment. Bioplastics or biopolymers to replace conventional petrochemical plastics can help to minimise plastic pollution with adverse impacts on the environment. Many authors including Leong et al. (2021) have concluded that a waste biorefinery–circular bioeconomy strategy could ensure an energy, food, and environment security.

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From leaves, stone pits and pruning debris, to wastewaters and pomace, olive industry wastes are rich biomass streams. Although these wastes have a significant potential to being valorized, their production place may be geographically scattered and sometimes too small for efficient economic uses. They are present only for a short period of time during the year, and for a steady flow for use they must be stored for a long time. For re-evaluation of these wastes a good background scientific research and technical capacity is necessary. This has certainly elevated in scientific and technical capabilities during the last few decades. Thus, the usual practices of direct disposal of these different biomasses which do not take advantage of the wide range of products that can be obtained from them, is currently changing (Contreras et al. 2020).

10.3.8.1

Biorefinery of the Pruning Biomass

Susmonaz et al. (2019) studied the technical and economic feasibility of an integrated biorefinery based on olive tree pruning materials using process simulation software to produce ethanol (109 L per ton of biomass), xylitol (27 kg per ton of biomass), antioxidants (47 kg per ton of biomass) and electricity. Required investment for the proposed refinery was calculated as 32.1 M Euros with a payback period of 5–6 years at current prices of the three compounds. These authors recommended establishing a small-scale prunings-based lignocellulosic biorefinery for further feasibility studies. Requejo et al. (2012) introduced an olive pruning biorefinery using various processes. Size segregated material with a stem size of 1 cm plus the leaves are separated and treated in a different way. The mainstream of olive prunings are processed hydrothermally to obtain a liquid fraction containing hemicellulose degradation molecules. After saccharification and fermentation, this liquid fraction was converted into bioethanol at a yield of 90.6%. Solid fraction however, was recommended for combustion as it was found to have 18.7 MJ/kg energy content.

10.3.8.2

Biorefineries of Olive Solid Wastes and Wastewaters

Martínez-Patino et al. (2019) used exhausted olive pomace as a source of natural antioxidant compounds, which can be extracted as a first step in a biorefinery strategy for valorization of this agro-industrial residue. This work investigated the impact of ultrasound-assisted extraction. Under the optimal conditions of 43.2% ethanol concentration, 70% ultrasound amplitude, and 15 min extraction time, an extract with high phenolic and flavonoid content, as well as high antioxidant activity. Valorisation of the oil agro-industry wastes is a matter of interest from both an environmental and a social point of view. An olive biorefinery involves a multiproduct process from different raw materials: olive leaves, exhausted olive pomace, olive stones and olive tree pruning residues. Biorefinery processes associated with

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these wastes would allow their valorisation to produce bioenergy and high valueadded renewable products. Therefore, location of the biorefinery must be in areas where many oil mills and olive groves with associated industries exist. Yet, there are studies for economic benefits of more widespread use of 2-P plants like Hocaoglu et al. (2018). They suggested its widespread use because it uses 60% less water and therefore less wastewater is produced. This study showed that any technical switch should be weighed with cost terms and such strategic changes should be given in terms of LCA evaluations. But quality considerations in main product line and different possibilities of valorization of the waste streams for obtaining other marketable products should also be given more consideration should a decision for a whole country be given. A most recent publication by Rapa and Ciano (2022) is a review covering the olive supply chain for farming, extraction, packaging, and waste treatment stages of this agro-industry. For sustainable management, authors evaluated the scientific literature for the last 10 years to overview the environmental impact assessment by means of Life Cycle Assessment (LCA) methodology. The aim is to underline the main environmental impact contributions in the olive supply chain. A recently published review by Rapa and Ciano (2022) included a very high number of scientific publications. The authors noted that most of these originated from the olive producing countries. Environmental impacts of the use of pesticides, fertilizers, water, and fuel for machinery put an emphasis on the environmental impacts to the farming stage which included the cultivation and harvesting operations. Organic systems indicated lower environmental impacts in almost all the categories studied. The location of production, both for the specific climatic conditions and for production regulations was found to be an important factor. The most significant influence on the results, however, was the quality of the final product. High quality standards in the extraction process, as well as in the packaging or farming stages, have strong influences on the impacts on the environment. Another strong impact group was the waste treatment. It was clear that a “best” waste disposal method could not yet have been reached. Authors agree that this was a real problem. However, they think that new approaches such as valorization of wastes seem to be promising and deserve a sustainability evaluation employing LCA. Another interesting aspect is the CO2 balance including the absorption of CO2 by the trees. This issue required, they think, more detailed analyses in the LCAs. A few examples of biorefinery schemes based on olive biomasses are described in literature, i.e. Khounani et al. (2021). In view of the circular bioeconomy prospective olive biorefineries are gaining more attraction. These authors investigated different by-products and residues generated in olive agro-industry and indicated many of them are not used and discarded as wastes. They designed and experimentally applied innovative platforms based on circular economy against the produce-pollute system and acting towards the zero-discharge principles. The main goal was establishing the sustainability in olive supply chain system they have proposed two different scenarios of biorefineries. First scenario was comprised of olive cultivationoil extraction-deoiled pomace residue as animal feed (after edible oil extraction). The second scenario was olive cultivation-oil extraction-pomace oil extraction for

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biodiesel production. In the second scenario in addition to the main production line, crude glycerol from transesterification of pomace oil for biodiesel production was used to produce two phosphate salts and an oxygenated fuel additive (triacetin). Also, the leaves were converted into antioxidants for biodiesel production in this scenario. These two scenarios were tested and compared for environmental impacts for the following environmental damage estimates: (a) climate change impacts, (b) human health, (c) primary energy consumption IMPACT2000+ method. The authors concluded that an olive biorefinery is the most promising solution as a good example of circular bio-economy to alleviate the environmental challenges. Also, the calculated environmental risks of producing all the products involved in this project in conventional methods are much higher than in a biorefinery. Intercomparison of the two scenarios differ for damage categories but ultimately both are at comparable levels. Finally, they have recommended using pilot scale tests to be implemented for evaluating the economic features of developed olive agro-biorefineries. Darvishi (2012) indicated the biotechnology in use for microbial systems to synthesize or purify high value metabolites of the carbohydrates as a carbon source in fermentation. The authors described olive mill wastewaters as a potential substrate for producing lipases, organic acids, microbial biopolymers, lipids, single cell oil or proteins, biosurfactants. Solid waste (pomace) is another rich raw material source for many industrial chemicals such as enzymes, organic acids, biopolymers, biosurfactants, food and cosmetics, pharmaceuticals, energy sources (including biofuels such as biodiesel, bioethanol, biohydrogen, biogas) as well as agricultural additives (biofertilizers, biomass, compost, animal feed). He put forth that as more research is carried out in the scientific community and industry, olive mill wastes might become a low-cost raw material of value-added end products.

10.4 Conclusions In this chapter a high number of scientific research reports have been reviewed. Mostly the knowledge and data from research carried out during the last two decades have been evaluated. But unfortunately, many good research works could not have been included for practical reasons. It is acknowledged that the number of emerging new valorization and treatment techniques is increasing almost logarithmically during a couple of decades. Seemingly, there will be continuing newer proposals and new ideas in the coming years, and some of them will hopefully be realized, soon. So, whatever solutions to management problems due to olive wastes/by-products are going to be used, must already be in front of us today. It is hoped that the lucky new techniques that will be used in full-scale olive agro-industry waste management to be implemented, have already been covered and discussed in this review.

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Supply chain of olive products from agriculture to food on the table is one of the most waste producing activities in the world. Tremendous volumes of olive agroindustry waste discharges that are highly polluting and barely treatable are being discharged. This industry also exploits the already scarce water and land resources of the regions where olive agro-industry is located. As a rule of thumb, only about one fifth of the olive drupes coming from the groves can be converted into oil. That is why in this agro-industry sector large quantities of biomass rich in wasted organics are produced. Besides, in the production lines very high volumes of water is used which make the problem even more critical. Under the pressure of increasing world-wide demand for olive and olive products, and for resource sustainability, some precautions will have to be immediately taken. Currently, the usual practices for disposal of the olive mill biomass are far from being perfect. It is a necessity that the inherent advantages of the valorization techniques proposed must cover a wide range of valuable materials and energy that can be produced from them. For a long time, roughly up to the beginning of this century, search for proper treatment at reasonable cost has been regarded as the key problem-solver in olive industry. Usually, finding the best treatment technique was the key factor in scientific research. However, it is accepted now that it is difficult and therefore expensive. So, side economic benefits are necessary for covering the excessive costs involved in olive mill waste management. The required incentives might be provided by way of integrating valorization of wastes into the traditional waste treatment. Many of the older methods of treatment for solid and watery phases of olive agroindustry wastes are insufficient. These effluent streams, however, can be broken down into marketable products for added economies. This has become more noticeable as the usual practices of waste management have failed to meet the evolving environmental restrictions emerged in the long run. So, there is a growing need to change the paradigm in the olive waste management strategies. A more circular vision to reintroduce the wasted materials into the production cycle, rather than the “producetreat(/or not)-dispose(/pollute)” strategy is needed. Expensive management practices on precious land, food, and water resources with high water, carbon, and energy footprints must be terminated for a sustainable life. Transition to a circular olive bioeconomy which aims at “closing the loop” of product lifecycle through more recycling and reuse will be beneficial for both the environment and the economy. It seems that a stepwise strategy as exemplified above must be the new research trend. The applicability and the problem-solving-capacity of the valorization and the treatment techniques will be the determining step in choosing between the proposed methods. Applicability covers economy, feasibility, and financial aspects along with the social, environmental, and administrative dimensions. However, the most important issue is the extent of the massive problem the specific proposed method will solve. For example, if the wastewaters are to be diluted with freshwater resources at a high ratio, the problem-solving capacity of the proposed method will be limited. In this case it will only depend on the significance and cost of the products obtained, i.e., how critically they are needed, what will be the economic feasibility and ecological expenses, etc. A solution could seem to be very attractive; however, the market size

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of the valorized material could be very small. Then this method bears no significance to the giant problem of management in the olive agro-industry. From the point of view of these perspectives, it can be said that the most crucial valorizations in the order of significance are, a. irrigation water production, with inclusive nutrients if possible b. energy production (technologies better than low-tech direct-firing) such as in the forms of biofuels for transportation, biogas and other gaseous energy carriers for use in industry and households c. soil improvement materials and farming supplies including physical, chemical, and biological additives d. nutritional ingredients to replace flour or other additives for food and feed to be used widely e. pharmaceuticals (these need not necessarily be in large amounts as they are highly expensive products, so they might not answer a substantial part of the discharge problems, but they are significantly helpful for health and well-being) f. bioactive molecules for biodegradable packing materials (again these need not to be in amounts as a big problem-solver, but very important for the environment). Another big jump needed at this point, is the scale-up from research to real life applications. Much of the research until now are either totally conceptual or depend only on laboratory-scale studies or test-tube experiments. However, economic feasibilities can only be trusted after having test data from larger size units. The positive results of the lab-scale experiments must be repeated at pilot-scale and even at larger full-scale. This is no easy task, considering the financial hurdles these projects would face. Results of the research must be convincing to the governments and financers. Governments and international associations would need to be attracted and convinced to activate programs based on these new ideas. Size of full-scale production units is another big problem in olive waste problem management. Traditionally, the sizes of olive oil mills may be so small in a region that expensive treatment models needed in these units could have never been realistic. So large storage capacities must be provided for the wastes so that the production facilities to be established can be continuously fed. There is also the problem that the olive wastes are generated during the campaign periods of a few months in fall/winter. Alternatively, the valorization facilities might operate on multiple influents and raw materials so that when olive wastes are not available for a long period in a year, they do not become idle. It seems that the most reasonable approach is to regard oil mill wastewater treatment/valorization as a regional problem. That requires collection and treatment to take place for a group of olive oil mills in the same geographic area. This strategy would lead to an economy of scale, allowing the adoption of more expensive technology normally unaffordable by individual mills. To comply with environmental regulations, optimizing resource recovery from wastes would be convincing. As a historical remark, Muezzinoglu and Uslu (1986) submitted a regional project for collecting the olive mill wastewaters from more than 60 small olive oil producers in and around a major producer town at the Aegean Sea coast of Turkey. The project

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aimed at diverting the courses of individual wastewater flows away from the coast, and behind the hills to a barren land with good soil permeation properties. Project received strong incentives from the municipal authorities. An engineered system consisting of a pressurized collection network, and several sequential ponds to uphold the wastewaters were the main elements of the project. The ground would be strengthened with impermeable membranes to avoid infiltration into the aquifers. These ponds would act as evaporation ponds in essence, and odor and pest problems were addressed. Besides, they were considered as the real beginning of further projects including valorizations. Obviously, wastewaters must be collected during the campaign period and would need to be processed around the year. However, the collection network and pond system have never been realized although the mill owners were required to contribute only minimally to its finance. Instead, they preferred to continue facing the stringent individual wastewater obligations which is much more costly for them. At such local scales the problem solving is very slow or even impossible if it needs to develop centralized solutions. Especially the finance is the biggest downside of such centralized projects. Up to this moment, no strategy is available that can be adopted in a regional or global scale. More than thirty-five years after the incidence mentioned above, today one can anticipate that the way the administrative handling of the olive mill wastewater management is the most important element in solving paradigms. There are many valorization method proposals and biorefinery schemes to integrate into oil mill and olive product manufacturing workplaces to make maximum use of the chemical constituents of olive biomasses. The co-products obtained in the biorefineries include antioxidants, mannitol, xylitol, furfural, paper, and biochar along with biogas, biohydrogen, biodiesel, bioethanol, heat, steam, and power. In addition, there are other valorization methods, which have been commented in a high number of very successful research and review articles. Many other beneficial uses can be explored in future studies and new biorefinery schemes will continue being proposed, too. Further assessments are necessary to find the best biorefinery scheme to contribute to sustainable and zero-waste operation integrated into the olive agro-industry of the future. Local environmental, economic, ecological, and social implications of these proposals will have to be evaluated to help the political decision-making. Yet, it should be underlined that much more work is required for political, regulatory and economic decision-making in a bioeconomy framework. Standard LCA procedures can be used for inter-comparisons among doable valorization options. The idea behind the maximum use of valuable materials instead of discharging them as environmental pollutants is a very up-to-date and healthy way of thinking for our sustainable future. There aren’t many production sectors like olive agro-industry so widely and strongly effective regionally and with so much scientific efforts spent in this direction. Although the industrial scale application of the biorefinery facilities is still missing (Khounani et al. 2021), there is a high expectation that soon there will be some investments realized as integrated olive biorefineries in olive producing countries.

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It is strongly hoped that with so much intellectual and scientific intelligence accumulated during the last few decades and with such a high amount of valuable data in hand to start with, olive agro-industry will be the first large-scale application of a bio-refinery concept.

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