Biogas Technology in Southeast Asia 9811988862, 9789811988868

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Green Energy and Technology

Pruk Aggarangsi Sirichai Koonaphapdeelert Saoharit Nitayavardhana James Moran

Biogas Technology in Southeast Asia

Green Energy and Technology

Climate change, environmental impact and the limited natural resources urge scientific research and novel technical solutions. The monograph series Green Energy and Technology serves as a publishing platform for scientific and technological approaches to “green”—i.e. environmentally friendly and sustainable—technologies. While a focus lies on energy and power supply, it also covers “green” solutions in industrial engineering and engineering design. Green Energy and Technology addresses researchers, advanced students, technical consultants as well as decision makers in industries and politics. Hence, the level of presentation spans from instructional to highly technical. **Indexed in Scopus**. **Indexed in Ei Compendex**.

Pruk Aggarangsi · Sirichai Koonaphapdeelert · Saoharit Nitayavardhana · James Moran

Biogas Technology in Southeast Asia

Pruk Aggarangsi Department of Mechanical Engineering Chiang Mai University Chiang Mai, Thailand

Sirichai Koonaphapdeelert Department of Environmental Engineering Chiang Mai University Chiang Mai, Thailand

Saoharit Nitayavardhana Department of Environmental Engineering Chiang Mai University Chiang Mai, Thailand

James Moran Department of Mechanical Engineering Chiang Mai University Chiang Mai, Thailand

ISSN 1865-3529 ISSN 1865-3537 (electronic) Green Energy and Technology ISBN 978-981-19-8886-8 ISBN 978-981-19-8887-5 (eBook) https://doi.org/10.1007/978-981-19-8887-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 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 Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

To the biogas community, let our collective vision for renewable energy, recycled waste and a cleaner environment keep progressing toward a carbon-free future

Preface

This is the first edition of our book, Biogas Technology in Southeast Asia. It is authored by Pruk Aggarangsi, Sirichai Koonaphapdeelert, Saoharit Nitayavardhana and James Moran. The authors have worked closely with the editorial team from Springer Nature to publish this book. There are a total of eight chapters covering biogas production, preprocessing, operation, cleaning and end use applications. Each chapter is a self contained unit. It is not necessary to read them all in sequence. Chapter 1 introduces biogas and makes a case for including it in a countries renewable energy mix. Chapter 2 introduces the production process. The biochemical reactions and pathways taken during biogas production are introduced. The terminology associated with biogas digesters is explained. Chapter 3 concerns itself with the wide variety of digesters currently available. Each digester type has unique attributes that make it suitable for a particular feedstock, climate and economic outlay. Particular attention is paid to those that are exclusively designed for use in Southeast Asia. Chapter 4 is a specialized chapter for the pretreatment of lignocellulosic feedstock (biomass) before it can be digested. Since most feedstock in Southeast Asia is wastewater, this pretreatment step is unnecessary. However if crops, grass, wood, etc. are being used to generate biogas, then it becomes necessary to pretreat. Chapter 5 is about the operation and control of the digester. Nowadays, automatic control is only sporadically used, but this is likely to change in the future as our understanding of the complex process broadens. Though each biogas plant is different, they all have some operational procedures in common that ensure a safe and stable digester. Chapter 6 is a post-treatment chapter. The output from the digester includes wastewater (cleaner but still too polluted to release back into the environment) and sludge in addition to biogas. Treating this wastewater so it can be released into the environment and drying the sludge for fertilizer are the processes discussed in this chapter. Chapter 7 introduces the many uses for biogas, from heat production to fuel cells. Several applications require the biogas to be cleaned and/or dried. The level of purity required is dependent on the destination. Chapter 8 concludes with a case study on the design and construction of a biogas digester in the South of Thailand. The growing importance to develop non-intermittent sources of renewable energies is the driving force behind extracting energy from biomass. Biogas is one such vii

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source and has its own advantages over other biomass energy products, such as bioethanol, because it can be produced from organic waste products. Southeast Asia produces plenty of organic waste but suffers from take-off options for the biogas. Farms and organic producers are located away from population centers, and unlike Europe, Southeast Asia is not networked with gas grids. Transporting biogas economically or using it close to production are challenges. Many biogas plants are constructed due to regulation requiring treatment of this wastewater. The traditional treatment is to place the wastewater in open ponds where the gases produced get released to the atmosphere. Methane has a global warming potential 25 times greater than carbon dioxide, and therefore, every effort should be made to capture and use it productively or at the very least flare it safely. The aim of this book is to provide a description of techniques, processes and necessary procedures to turn bioproducts into biogas. The microbial reactions that govern the process are discussed early, but the focus of this book is on the engineering required to select and construct biogas digesters. The pre- and post-treatments required for the feedstock and estimating the production rate from the feedstock supply and pollution level. The civil engineering involved in digester construction is a separate topic, not discussed in detail here but left to other dedicated books on this matter. Most of the subject matter comes from the design of biogas plants from agricultural farms across Southeast Asia, in particular Thailand which is where the authors are based. This book focuses on biogas produced from organic agricultural waste and food processing plants since these are the areas the authors have direct experience with. Biogas from landfill gas, sewage sludge and municipal waste are not discussed in this book mainly because Southeast Asia does not have many of these types of biogas plants. Biogas from these sources is typically more expensive as it contains more impurities that need to be removed prior to combustion. Several sections of the book deal with economic issues, such as plant construction and operating costs. Chapter 8 is a case study on a plant built in the South of Thailand. The costs are in Thai baht. Costs, especially labor cost, will of course be different elsewhere. Where possible the costs are presented in US dollars at a conversion rate of $1 = ฿33. Since this conversion rate fluctuates on a daily basis, prices presented in dollars should be used only as a guide. Technical data here is presented in SI units where possible. In some circumstances when the original data is presented in English units, then it is left as such. Units for production of biomethane are usually expressed as N m 3 / h,which stands for normal cubic meter per hour under standard conditions of 0 °C and 1 atm (101.325 kPa). Chiang Mai, Thailand

Pruk Aggarangsi Sirichai Koonaphapdeelert Saoharit Nitayavardhana James Moran

Acknowledgments

The assistance of many individuals who contributed material and suggested improvements is gratefully acknowledged. Thanks are also due to the companies and organizations who graciously provided data and gave permission for reproducing charts and figures. The number of such organizations is too large to permit individual recognition here; however, they are generally identified in the text as the sources of specific data. The authors wish to thank the researchers and staff at the Energy Research and Development Institute Nakornping of Chiang Mai University (ERDI-CMU) who have continuously been conducting research and development into renewable energies with a focus on biogas and its many applications. Special thanks also goes to Mr. Panutat Injaima and Mr. Warut Yuennan for their help with the graphic design. Among the many who have supported and financed this research over the years, special thanks goes to the Energy Conservation Fund (ENCON fund), the Thai Ministry of Energy. We would like to acknowledge our partners throughout the years include the Thai Biogas Trade Association, Agrikomp GmbH, Chiang Mai Fresh Milk Farm Co., and special thanks to Nam Hong Power Co. for allowing us to share the details of biogas design and construction as a case study for the book. Other significant contributor to this first edition is Chandra Sekaran our patient editor. Finally, we wish to express gratitude to our families for their support and patience during the preparation of this book. To all fellow researchers in the field of biogas, we wish to extend our deepest gratitude and thanks.

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About This Book

Biogas Technology in Southeast Asia is focused on the applications of biogas technology to waste and wastewater management problems commonly experienced in Southeast Asia. The book content is balanced between intensive biological process and the authors’ field experience of biogas digester designs and operation. The book begins with basic explanation on anaerobic digestion process of organic matter specifically on waste and wastewater produced from agricultural industries in the region, including swine farms, palm oil mills and tapioca starch factories. Specific biogas yields and digestion conditions are explained and verified using data provided by the Energy Research and Development Institute Nakornping of Chiang Mai University. This book also emphasizes digester selection and design based on feedstock input and other engineering conditions. Readers will be to estimate and visualize the project scale from the major components. Biogas operating conditions are discussed with crucial parameters suggested for monitoring. Lastly, biogas utilization in electricity and heat production are presented based on applicability in the region. This book should be suitable for general audiences up to project developers and plant operators. The authors aim the share existing experience and lessons learned, contributing to the biogas community.

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Contents

1 Introduction to Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Biogas Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Biogas History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Global Biogas Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Environmental Benefits of Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Biogas Production in Southeast Asia . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Cassava . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Palm Oil Mill Effluent (POME) . . . . . . . . . . . . . . . . . . . . . . . 1.5.4 Livestock Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.5 History of Biogas in Thailand . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 3 6 7 7 7 8 9 12 13 14

2 Anaerobic Digestion and Biogas Production . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Wastewater Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Solids in Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Biochemical Oxygen Demand (BOD) . . . . . . . . . . . . . . . . . . 2.2.3 Chemical Oxygen Demand (COD) . . . . . . . . . . . . . . . . . . . . 2.3 Anaerobic Digestion Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Acidogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Acetogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Methanogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Biogas Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Important Factors in Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Inoculum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3 Nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Toxic Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17 17 18 19 20 21 22 22 24 26 27 29 31 31 32 32 34

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2.5.5 Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.6 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.7 pH and Alkalinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 35 36 37

3 Biogas Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Pretreatment Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Biogas Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Suspended Growth Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Fixed Dome Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Plug Flow Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Covered Lagoon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Channel Digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5 Continuous Stirred Tank Reactor (CSTR) . . . . . . . . . . . . . . . 3.3.6 Anaerobic Sequence Batch Reactor (ASBR) . . . . . . . . . . . . 3.3.7 Anaerobic Baffled Reactor (ABR) . . . . . . . . . . . . . . . . . . . . . 3.3.8 Anaerobic Migrating Blanket Reactor (AMBR) . . . . . . . . . 3.3.9 Upflow Anaerobic Sludge Blanket (UASB) . . . . . . . . . . . . . 3.3.10 Expanded Granular Sludge Blanket (EGSB) . . . . . . . . . . . . 3.4 Attached Growth Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Anaerobic Filter (AF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Fluidized Bed Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Reactor Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Design Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Designing a Suspended Growth Reactor . . . . . . . . . . . . . . . . 3.5.3 Designing an Attached Growth Reactor . . . . . . . . . . . . . . . . 3.6 Palm Oil Biogas Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39 39 41 42 42 42 44 46 47 48 49 50 50 52 52 53 54 55 57 58 71 74 78

4 Lignocellulosic Feedstock Pretreatment for Biogas Production . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Crop Ensiling and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Feedstock Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Physical Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Mechanical Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Liquid Hot Water (LHW) Pretreatment or Hydrothermal Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3 Steam Explosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Ozonolysis (Ozone Reaction) . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 Wet Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.6 Microwave Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.7 Ultrasonic Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Chemical Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Acid Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Alkaline Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Oganosolv Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81 81 81 83 85 85 85 86 87 87 87 88 88 88 89 89

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4.6 Physicochemical Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1 Ammonia Fiber Explosion, AFEX . . . . . . . . . . . . . . . . . . . . . 4.6.2 CO2 Explosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Biological Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 Pretreatment Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

90 90 91 91 92 94

5 Biogas System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Plant Commissioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Mechanical and Electrical Completion (MEC) . . . . . . . . . . 5.2.2 Biological Start-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Biocommissioning Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Performance Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Early Treatment Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Digester Operation and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Operational Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

97 97 97 98 99 101 101 102 102 108 113

6 Processing Biogas Effluent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Biogas Effluent Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Stabilization Pond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Anaerobic Pond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Facultative Pond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Aerobic and Maturation Pond . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Constructed Wetland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Free Water Surface (FWS) . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Subsurface Flow (SF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Nitrogen and Phosphorus Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Sludge Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Sludge Drying Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Sludge Lagoon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Sludge Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 POME Biogas Post-treatment Case Study . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

115 115 116 117 118 118 121 121 122 123 124 125 125 126 126 127 132

7 Biogas Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Biogas Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Biogas Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Bioscrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Moisture Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Storage and Transportation of Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Biogas Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Biogas Piping System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

135 135 135 137 137 138 139 139 140

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7.5 Biogas Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Biogas Flaring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.2 Thermal Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Electric Power Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.1 Internal Combustion Engine . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.2 Gas Turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Upgrading to Biomethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.1 Transportation Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.2 Injection into a Gas Pipeline System . . . . . . . . . . . . . . . . . . . 7.7.3 Other Possible Biogas Applications . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

141 142 144 145 146 148 150 150 151 151 159

8 Designing a Biogas Plant—Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Plant Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Annual GHG Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Plant Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Plant Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

161 161 162 167 167 168 171

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

About the Authors

Dr. Pruk Aggarangsi is currently the Director of Energy Research and Development Institute Nakornping, Chiang Mai University and Assistant Professor in Mechanical Engineering Chiang Mai University, Thailand. He has a wide scope of expertise covering areas such as renewable energy, waste water treatment, numerical and finite element modeling, simulation and analysis, energy balance analysis, robotic system design and mechanical vibration analysis. For the past 15 years, he has participated and in charge of multiple biogas projects mostly supported by the Thai Ministry of Energy. Dr. Pruk has had crucial roles in engineering of many biogas constructions projects as well as conducting more than 40 biogas-related technical training workshops. He also plays an important role in driving Chiang Mai University’s Smart City-Clean Energy Project aim to initiate sustainable development for the communities around the world. Dr. Sirichai Koonaphapdeelert has a Ph.D. in chemical engineering from Imperial College London and has been teaching and working in the fields of environmental engineering and renewable energy for more than 10 years. During his time as the deputy director of Energy Research and Development Institute Nakornping of Chiang Mai University, he was involved in a number of projects related to biogas. Also, he initiated and helped driving a legal framework related to biogas in his country such as the drafted engineering standard for local gas grid and the regulatory property of upgraded biogas for vehicular uses. Dr. Saoharit Nitayavardhana is an Assistant Professor of the Environmental Engineering Department, Chiang Mai University and an advisor to R&D Department at Energy Research and Development Institute-Nakornping (ERDI), Chiang Mai University. Dr. Nitayavardhana received her Ph.D. in Molecular Biosciences and Bioengineering from University of Hawaii at Manoa, HI, USA. Her research interests focus mainly on biorefinery, biofuel/bioenergy waste utilization and waste-to-energy.

xvii

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About the Authors

Dr. James Moran received a Masters degree and Ph.D. degree in Mechanical Engineering from the Massachusetts Institute of Technology. He is currently an Associate Professor at the Department of Mechanical Engineering in Chiang Mai University. For the past 10 years, he has also been a consultant to the Energy Research and Development Institute (ERDI), Chiang Mai University. His research interests include biogas upgrading, biomethane grids, biomethane low pressure storage and meso-scale combustion. Along with his colleagues at ERDI, he has published many peer-reviewed papers on biogas and a book entitled, “Biomethane: Production and Applications”. All authors have published several pier-reviewed papers on biogas, and this book represents an agglomeration of this work.

Nomenclature

Q˙ µ BOD C COD D d H RT kd Ks kt n n P R S S RT T T SS U LV V V SS X Y OLR

Wastewater volume flow rate, (m3 /day) Volatile solids specific growth rate, (day–1 ) Biochemical oxygen demand, (mg/L) Substrate concentration, (mg/L) Chemical oxygen demand, (mg/L) Dissolved oxygen, (mg/L) Water depth, (m) Hydraulic retention time, (day) Microbe decay rate, (day–1 ) Monod constant, (mg/L) BOD removal constant, (–) Number of moles (–) Porosity, (–) Absolute pressure, (atm) Universal gas constant, (0.082L atm/mol K) Nutrient concentration, (mg/L) Solids retention time, (day) Temperature, (K) Total suspended solids, (mg/L) Upflow liquid velocity, (m/day) Volume, (L) Volatile suspended solids, (mg/L) Average microbe concentration, (mg/L) Biogas yield, (gVSS/gCOD) Organic loading rate, (kg COD/m3 day)

xix

List of Figures

Fig. 1.1

Fig. 1.2 Fig. 1.3 Fig. 1.4 Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6 Fig. 2.7 Fig. 3.1 Fig. 3.2 Fig. 3.3 Fig. 3.4 Fig. 3.5 Fig. 3.6 Fig. 3.7 Fig. 3.8 Fig. 3.9

Energy potential of biogas or biomethane by feedstock source (With permission from International Energy Agency, [4] All rights reserved). Note Woody biomass feedstocks are available only for biomethane production . . . . . . . Minor case study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minor case study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RE power group, 120,000 Nm3 /day biogas plant . . . . . . . . . . . . . Aerobic and anaerobic degradation of organic material . . . . . . . . Sludge solids example for wastewater with 5% solids by weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open and closed reflux COD measurement (with permission from Ma [4]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic diagram of the anaerobic digestion process for biogas production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microorganisms involved in anaerobic methane production . . . . Biodegradation rates and retention times of various organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth rate of methanogens in different temperature ranges . . . Screening as a primary treatment method (Source In-house) . . . . Screen compactor as a primary treatment method (Source In-house) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Grit chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fixed dome biogas reactors (Source With permission from [3]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plug flow reactor (Source In-house) . . . . . . . . . . . . . . . . . . . . . . . . Covered lagoon reactor (Source In-house) . . . . . . . . . . . . . . . . . . Schematic of a modified covered lagoon (Source In-house) . . . . Channel digester (Source In-house) . . . . . . . . . . . . . . . . . . . . . . . . Channel digester for treating pig farm wastewater (Source In-house) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 10 11 14 18 20 22 23 24 33 36 40 40 40 43 43 44 45 46 47

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xxii

Fig. 3.10

Fig. 3.11 Fig. 3.12 Fig. 3.13 Fig. 3.14 Fig. 3.15 Fig. 3.16 Fig. 3.17 Fig. 3.18 Fig. 3.19 Fig. 3.20 Fig. 3.21 Fig. 3.22 Fig. 3.23 Fig. 3.24 Fig. 4.1 Fig. 4.2 Fig. 4.3 Fig. 4.4 Fig. 4.5 Fig. 4.6 Fig. 4.7 Fig. 4.8 Fig. 4.9 Fig. 4.10 Fig. 4.11 Fig. 5.1 Fig. 5.2 Fig. 6.1 Fig. 6.2 Fig. 6.3 Fig. 6.4

List of Figures

(a) Continuous Stirred Tank Reactor (Source Inhouse) (b) Anaerobic contact reaction tank (Source with permission from Tauseef etal. [15]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The four phases of an ASBR cycle . . . . . . . . . . . . . . . . . . . . . . . . Anaerobic baffled reactor (Source In-house) . . . . . . . . . . . . . . . . . Anaerobic migrating blanket reactor . . . . . . . . . . . . . . . . . . . . . . . Granular sludge inside a UASB and EGSB tank (with permission from [5]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UASB reaction tank (with permission from [7]) . . . . . . . . . . . . . . Expanded granular sludge blanket reaction tank . . . . . . . . . . . . . . Upflow anaerobic filter reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluidized bed reaction tank (Source In-house) . . . . . . . . . . . . . . . Continuous stirred tank reactor model . . . . . . . . . . . . . . . . . . . . . . Monod equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CMU-Hybrid digesters for treating palm oil mill effluent . . . . . . Drawing of a CMU-Hybrid reactor with internal hydraulic mixing (Plan View) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drawing of a CMU-Hybrid reactor (Profile and cross-sections) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biogas digesters for treating palm oil mill effluent (Source ERDI 2021) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant cell wall and chemical composition . . . . . . . . . . . . . . . . . . . Effect of pretreatment on biogas yield during anaerobic digestion of corncob (Source In house) . . . . . . . . . . . . . . . . . . . . . Ensiling of biomass for biogas production . . . . . . . . . . . . . . . . . . Effect of pretreatment on biomass cell wall structure . . . . . . . . . . Physical biomass structure change during biomass pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chopped Napier grass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of steam explosion on Napier grass at various temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of acid and alkaline pretreatment on biomass chemical compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic diagram for organosolv pretreatment . . . . . . . . . . . . . . White-rot fungus grown on lignocellulosic biomass (Shutterstock No: 1905632008) . . . . . . . . . . . . . . . . . . . . . . . . . . . Improvement of methane yield during anaerobic digestion by Trichoderma longibrachiatum (Source In-house) . . . . . . . . . . Plant commissioning test procedure . . . . . . . . . . . . . . . . . . . . . . . Factors affecting production and stability of biogas system . . . . . Stabilization ponds for biogas digester effluent . . . . . . . . . . . . . . A series of free water surface ponds for post-treatment . . . . . . . . Subsurface artificial wetland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A drying bed for treating sludge from a biogas reactor (Source In-house) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48 49 49 50 51 52 53 54 55 62 63 74 76 77 78 82 82 83 84 84 86 87 89 90 92 92 98 110 117 121 122 125

List of Figures

xxiii

Fig. 6.5 Fig. 6.6 Fig. 6.7

126 128

Fig. 6.8 Fig. 6.9 Fig. 7.1 Fig. 7.2 Fig. 7.3 Fig. 7.4 Fig. 7.5 Fig. 7.6 Fig. 7.7 Fig. 7.8 Fig. 7.9 Fig. 7.10 Fig. 7.11 Fig. 7.12 Fig. 7.13 Fig. 7.14 Fig. 7.15 Fig. 7.16 Fig. 8.1 Fig. 8.2 Fig. 8.3 Fig. 8.4 Fig. 8.5 Fig. 8.6

Sludge drying bed (Source In-house) . . . . . . . . . . . . . . . . . . . . . . . Post-treatment ponds for POME biogas effluent . . . . . . . . . . . . . . Cross-sectional profile of an anaerobic lagoon for POME biogas effluent treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-sectional profile of a facultative pond for POME biogas effluent treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-sectional profile of a maturation pond for POME biogas effluent treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications for biogas end use . . . . . . . . . . . . . . . . . . . . . . . . . . . Biofilter installation at a livestock farm (Stock Photo ID: 1844345287) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open flare gas system (Stock Photo ID: 2035273256) . . . . . . . . . Closed flare gas system (Shutterstock: ID: 1361985284) . . . . . . . Biogas burner (Stock Photo ID: 341516294) . . . . . . . . . . . . . . . . Dual biogas oil burner (Stock Photo ID: 2140597087) . . . . . . . . Biogas Akricomp BGA 095, 150 kWel CHP unit (with permission) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jenbacher type 6, Biogas CHP generator (with permission) . . . . Internal combustion engine (Stock Photo ID: 180736850) . . . . . Gas turbine operating principle . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas turbine (Stock Vector ID: 1259213491) . . . . . . . . . . . . . . . . . The distance traveled using different biofuels (with permission from [6]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel cell operating principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PEM fuel cell (Stock Vector ID: 1142997314) . . . . . . . . . . . . . . . Biofuel production from algae (with permission from [10]) . . . . Production of liquid fuels by the Fischer-Tropsch process (with permission from [12]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nam Hong palm oil mill (with permission) . . . . . . . . . . . . . . . . . . Inlet and outlet digester mass flows . . . . . . . . . . . . . . . . . . . . . . . . Nam Hong hybrid digester construction (Source in house) . . . . . Nam Hong power biogas system layout . . . . . . . . . . . . . . . . . . . . Biogas filter and flare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nam Hong palm oil biogas plant . . . . . . . . . . . . . . . . . . . . . . . . . .

129 130 131 136 139 143 143 144 145 146 147 148 149 149 151 152 153 155 158 162 163 164 166 166 170

List of Tables

Table 1.1 Table 1.2 Table 1.3

Table 1.4 Table 1.5 Table 1.6 Table 2.1 Table 2.2 Table 2.3 Table 2.4 Table 2.5 Table 2.6 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 3.5

Table 3.6 Table 3.7

Properties of biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equivalent heating value of 1 m3 of biogas at normal temperature and pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gross final energy consumption—world (Compiled from multiple reports from the British Petroleum Company [6]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biogas production (With permission from World Bioenergy Association [7]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . POME discharge standards in Malaysia, Indonesia and Thailand (With permission from Tan et al. [12]) . . . . . . . . . Design criteria for animal farms . . . . . . . . . . . . . . . . . . . . . . . . . Differences between BOD and COD . . . . . . . . . . . . . . . . . . . . . Microorganisms used for hydrolysis of complex organic molecules [1, 7] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microorganisms involved in acidogenesis [1, 7] . . . . . . . . . . . . Microorganisms involved in methanogenesis [1, 11] . . . . . . . . . Wastewater substrate characteristics . . . . . . . . . . . . . . . . . . . . . . Important trace elements in anaerobic digestion process . . . . . . Classification of various biogas production systems . . . . . . . . . Reactor technology comparison . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of biogas systems commonly used in Thailand [11] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The organic loading rate for the design of a microbial suspension treatment system . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kinetic constants used in the design of suspended microbial systems ([12], Wastewater Engineering: Treatment and Reuse, copyright McGraw Hill) . . . . . . . . . . . . . Recommended criteria for the design of channel digesters . . . . Operating conditions for a continuous stirred reaction tank for various waste types (with permission from [15]) . . . .

2 2

4 5 8 9 22 25 25 27 33 34 41 48 56 59

60 61 64

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Table 3.8 Table 3.9 Table 3.10 Table 3.11 Table 3.12 Table 3.13 Table 3.14 Table 4.1 Table 4.2 Table 4.3

Table 5.1 Table 5.2 Table 5.3 Table 5.4 Table 5.5 Table 5.6 Table 6.1 Table 6.2 Table 7.1 Table 7.2 Table 7.3 Table 7.4 Table 7.5 Table 7.6 Table 7.7 Table 8.1 Table 8.2 Table 8.3 Table 8.4

List of Tables

Operating conditions for an anaerobic contact reactor for various waste types (with permission from [15]) . . . . . . . . . Example design values for anaerobic contact reactor . . . . . . . . Operating values for UASB reaction tank for industrial wastewater [16, 17] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ASBR operating conditions for various waste types . . . . . . . . . Operating conditions of a anaerobic filter reaction tank for various waste types [12, 15] . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters for fluidized bed reactors for various wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters for downflow attached growth reactors . . . . . . . . . . Chemical composition of lignocellulosic raw material (With permission from [1]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages and disadvantages of lignocellulose pretreatment processes (with permission from [19]) . . . . . . . . . The impact of pretreatment processes on chemical composition and structure of waste (with permission from [19]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inoculum for anaerobic wastewater treatment systems . . . . . . . Microbes suitable for different substrates . . . . . . . . . . . . . . . . . . Suitable anaerobic conditions after commissioning . . . . . . . . . . Optimal OLRs for various feedstocks . . . . . . . . . . . . . . . . . . . . . Measurement and monitoring checklist for biogas systems . . . Troubleshooting common problems found in a biogas system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Post-treatment system design (Environmental Engineering Association of Thailand) . . . . . . . . . . . . . . . . . . . . Artificial wetland design criteria . . . . . . . . . . . . . . . . . . . . . . . . Approximate biogas yield and methane composition according to the type of raw material (Source in house) . . . . . . Comparison of H2 S reduction methods . . . . . . . . . . . . . . . . . . . Biogas storage arranged by pressure . . . . . . . . . . . . . . . . . . . . . . Biogas properties required for different technologies . . . . . . . . Comparison between open and closed gas flares . . . . . . . . . . . . Biogas Akricomp CHP unit specs . . . . . . . . . . . . . . . . . . . . . . . . Thailand CNG demand in transportation (ton/day) . . . . . . . . . . Parameters for digester sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters for CO2e calculation . . . . . . . . . . . . . . . . . . . . . . . . . Capital cost for digester construction and equipment . . . . . . . . Plant annual operation and maintenance costs . . . . . . . . . . . . .

65 66 68 71 72 73 73 85 93

94 99 100 107 108 109 111 119 122 136 138 140 142 143 146 156 164 167 169 169

Chapter 1

Introduction to Biogas

1.1 Biogas Definition The resulting gas mixture produced when organic matter breaks down in the absence of oxygen is referred to as biogas. It can be produced from any organic matter with sewage, manure, industrial, municipal waste, agricultural waste and plant material being typical sources. Since organic matter is biologically recycled, biogas is considered a renewable energy source with a neutral or negative carbon footprint. The gas mixture in biogas contains primarily methane (CH4 ) and carbon dioxide (CO2 ). It may also contain small quantities of carbon monoxide (CO), hydrogen sulfide (H2 S), moisture and siloxanes. Biogas is combustible with the energy released suitable for heating applications. Biogas can be cleaned and upgraded to a fuel with properties identical to natural gas where it is then referred to as biomethane. In this book, biogas is taken to mean strictly the gaseous product of anaerobic digestion of organic material. The gas obtained from pyrolysis of biomass can also be used for power generation. But this gas is closer to a synthesis gas which is different to biogas (Tables 1.1 and 1.2). The composition of biogas depends on the type of feedstock and the production methods, such as: • Digesters: These are containers or tanks in which organic material in water is broken down by microorganisms. Contaminants in the biogas, such as hydrogen sulfide and moisture, are usually removed after from the digester. • Landfill gas recovery systems: The decomposition of municipal solid waste (MSW) under anaerobic conditions at landfill sites produces biogas. This can be captured and used although its gas quality is not as good as that produced from digesters. • Wastewater treatment plants: These plants produce sewage sludge which can be further digested to produce biogas. They tend to focus more on sludge volume reduction (waste treatment) than biogas production.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_1

1

2 Table 1.1 Properties of biogas Biogas properties Volume CH4 Volume CO2 Volume H2 S Lower heating value Flame speed Theoretical air/fuel rate Temperature of combustion in air Heat capacity (c p ) Density (Under normal temperature and pressure)

1 Introduction to Biogas

Biogas values 45–75% 30–40% 100–20,000 ppm 16–28 MJ/Nm3 25 cm/s 6.2 m3air /m3biogas

650 ◦ C 1.6 kJ/m3◦ C 1.15 kg/Nm3

Table 1.2 Equivalent heating value of 1 m3 of biogas at normal temperature and pressure Equivalent energy in 1 m3 of biogas LPG: 0.46 kg Gasoline: 0.67 L Diesel fuel: 0.60 L Fuel oil: 0.55 L Firewood: 1.50 kg

The methane content of biogas typically ranges from 45 to 75% by volume, with most of the remainder being CO2 . This variation means that the energy content of biogas can vary; the lower heating value (LHV) is between 16 and 28 MJ/m3 .

1.2 Biogas History Anaerobic digestion (AD) was first discovered back in seventeenth century as the relationship between flammable gas and organic waste decaying in a covered environment. Baptita Van Helmont first determined in the seventeenth century that flammable gases could evolve from decaying organic matter. Count Alessandro Volta concluded in 1776 that there was a direct correlation between the amount of decaying organic matter and the amount of flammable gas produced. In 1808, Sir Humphry Davy determined that methane was present in the gases produced during the AD of cattle manure. The first digestion plant was built at a leper colony in Bombay, India in 1859 [1]. Anaerobic digestion reached England in 1895 when biogas was recovered from a “carefully designed” sewage treatment facility and used to fuel street lamps in Exeter [2]. The development of microbiology as a science led to research by Buswell [3]

1.3 Global Biogas Trends

3

and others in the 1930s to identify anaerobic bacteria and the conditions that promote methane production. For digesters, farm-based facilities are the most common. Six to eight million family-sized, low-technology digesters are used to provide biogas for cooking and lighting fuels with varying degrees of success. In China and India, there is a trend toward using larger, more sophisticated systems with better process control that generate electricity. In Europe, AD facilities generally have had a good record in treating the spectrum of suitable farm, industrial and municipal wastes. The process was used quite extensively when energy supplies were reduced during and after World War II. Other factors influencing success have been local environmental regulations and other policies governing land use and waste disposal. As a result of these environmental pressures, many nations have implemented or are considering methods to reduce the environmental impact of waste disposal. More than 35 industries that use digesters have been identified, including processors of chemicals, fiber, food, meat, milk and pharmaceuticals. Many use AD as a pretreatment step that lowers sludge disposal costs, controls odors and reduces the costs of final treatment at a municipal wastewater treatment facility. From the perspective of the municipal facility, pretreatment expands treatment capacity. Although the first digester to use MSW as a feedstock operated in the USA from 1939–1974, it is receiving renewed interest. MSW processing facilities have made significant progress toward commercial use in recent years. MSW digestion poses many technical problems, including a large time needed to digest. High-solid digestion (HSD) systems were developed with the potential to improve the economic performance of MSW systems by reducing digester volume and the parasitic energy required for the AD process. These HSD designs can operate with total solids (TS) concentrations greater than 30%. Processes such as AD and composting offer the only biological route for recycling matter and nutrients from the organic fraction of waste. Composting technology is commercially available and in use, but its further application is limited mainly by environmental aspects and process economics. AD is a net energy-producing process, with around 1600–2800 MJ of heat per ton of organic waste input. The contribution of biogas to the energy economies of most industrialized nations is minimal—but is generally increasing slowly in line with increasing renewable energy use. The specifics of biogas production from wastewater are discussed in the proceeding chapters.

1.3 Global Biogas Trends Globally, renewable energy made up 14.4% of global energy consumption in 2019; see Table 1.3. This is every kind of renewable energy of which in 2019, biogas made up only 1.6%. The development of biogas has been uneven across the world, as it depends not only on the availability of feedstocks but also on policies that encourage its production and use. Almost two-thirds of biogas production in 2018 was used to generate electricity and heat (with an approximately equal split between electricity-

4

1 Introduction to Biogas

Table 1.3 Gross final energy consumption—world (Compiled from multiple reports from the British Petroleum Company [6]) Total (EJ)

Coal (EJ)

Oil (EJ)

Natural Gas (EJ)

Nuclear (EJ)

Renewable Renewables (EJ) (%)

1990

367

93

135

69

22

47

12.8

1995

386

92

141

75

25.5

51

13.2

2000

420

97

153

86

28

54

12.9

2005

481

125

167

99

30

59

12.2

2010

538

153

173

114

30

67

12.5

2015

571

161

181

123

28

76

13.4

2017

588

158

187

132

29

81

13.8

2018

598

161

188

137

30

83

13.8

2019

606

162

191

141

30

85

14.1

2020

557

151

174

138

24

70

12.5

only facilities and co-generation facilities). Around 30% was consumed in buildings, mainly in the residential sector for cooking and heating, with the remainder upgraded to biomethane and blended into the gas networks or used as a transport fuel. A report on the global potential of biogas published by the World Biogas Association (WBA) in July 2019, noted that just 2% of global wastes and feedstocks suitable for the production of biogas are currently being used for that purpose. The report estimates that the biogas recovery from these readily available feedstocks, with existing technologies deployable immediately, would be capable of meeting up to 6–9% of current global primary energy needs or 36–54 EJ. A report from the IEA (International Energy Agency [4]) estimated global biogas and biomethane potential to be 24 EJ, divided between the continents as shown in Fig. 1.1. The uncertainty in these figures is exemplified by Kummamuru [5]who estimated the global potential biogas energy from agricultural residues within a range of 17–128 EJ. Either way, considering biogas produced 1.43 EJ in 2019, the potential global growth of the biogas industry is therefore extraordinary. Biogas production in 2019 was 1.43 EJ, which was an increase of about 5.1% over 2018. Almost 54% of the production occurred in the EU. Global biogas production increased almost 5 times during 2000–2019 from 0.28 EJ to 1.43 EJ (Table 1.4). The increase in production was the highest in Europe; however, despite this progress, the overall contribution of biogas to the global energy scenario remains negligible (0.2%). In Europe, due to the low cost of available raw materials and regulatory support resulting in a strong market for biogas, the annual production of biogas was 13.5 million tons from 1.5 billion tons of agricultural biomass. There are more than 15,000 biogas plants in Europe. Germany is a pioneer with an installed capacity of approximately 25% of the global supply, due to strong development of agricultural biogas plants. Energy crops were the primary choice of feedstock that underpinned

1.3 Global Biogas Trends

5

Fig. 1.1 Energy potential of biogas or biomethane by feedstock source (With permission from International Energy Agency, [4] All rights reserved). Note Woody biomass feedstocks are available only for biomethane production Table 1.4 Biogas production (With permission from World Bioenergy Association [7]) World World Africa Americas Asia (EJ) Europe Oceania (EJ) (Billion (EJ) (EJ) (EJ) (EJ) m3 ) 2000 2005 2010 2011 2012 2013 2015 2017 2018 2019

0.28 0.50 0.94 1.09 1.21 1.28 1.29 1.33 1.36 1.43

12.4 22.0 37.1 42.5 48.3 53.2 56.0 57.7 59.3 62.3

0.00 0.00 0.00 0.00 0.00 0.00 0.002 0.002 0.002 0.002

0.13 0.17 0.24 0.25 0.28 0.28 0.21 0.19 0.19 0.19

0.05 0.15 0.33 0.38 0.39 0.40 0.40 0.41 0.44 0.5

0.10 0.17 0.36 0.44 0.52 0.57 0.68 0.73 0.73 0.73

0.01 0.01 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.03

the growth of Germany’s biogas industry, but policy has recently shifted more toward the use of crop residues, sequential crops, livestock waste and the capture of methane from landfill sites. A major effort in Europe to support industrial production of fuels from biomass by adjusting tax exemptions and supporting biogas research and development programs. The European Union aims to set environmental sustainability criteria for green gas fuels. In the USA, the pathway for biogas has been through landfill

6

1 Introduction to Biogas

gas collection, which today accounts for nearly 90% of its biogas production. There is also growing interest in biogas production from agricultural waste, since domestic livestock are responsible for almost one-third of methane emissions in the USA.

1.4 Environmental Benefits of Biogas A dominant driver for biogas production is the environment, particularly the need to control greenhouse gas emissions. The most common greenhouse gas is carbon dioxide, but methane is a far more potent greenhouse gas in equivalent amounts. The Intergovernmental Panel on Climate Change is constantly re-evaluating the relative potential of various gases. In recent years methane has been considered to have around twenty eight times the global warming potential of carbon dioxide. For a smallscale biogas plant a methodology for assessing emissions is given by the UNFCCC, United Nations Framework Convention on Climate Change [8]. As a result, the need to control methane emissions as well as displace carbon dioxide produced by the burning of fossil fuels will drive biogas development. This is likely to persist throughout this decade and increasingly favor biogas production as the environment moves up the political agenda. The World Biogas Association found that AD has the potential to reduce GHG emissions by 3290–4360 Mt CO2 eq., which is equivalent to 10–13% of the current greenhouse gas emissions, Jain et al. [9]. In addition to issues of greenhouse gas emissions, there are a number of environmental benefits of biogas schemes. AD facilitates a reduction of pollution through effective waste management. Agricultural slurries have a high biological oxygen demand (BOD), which can pollute waterways and ground water. There is also odor nuisances for people living close to farms, due to digestion in uncontrolled open lagoons. Implementing a biogas system can reduce this odor significantly and simultaneously reduce the wastewater BOD. Biogas can also be produced from energy crops. Bachmaier et al. [10] analyzed ten agricultural biogas plants and carried out a detailed balance of greenhouse gas emissions (GHG). They included all emissions including those from plant construction and operation, crop/waste transportation and residue post-treatment. Compared to a reference system based on fossil fuels, electricity produced from biogas plants avoids GHG emissions by 573 − 910 gCO2−eq /kWhel . The range of these results highlight that estimates of GHG emissions or savings can be only made on the basis of individual digesters. These GHG savings are likely underestimates for GHG savings in Southeast Asia. Being in Europe, the digesters analyzed required heat. Several also used energy crops which have a lower net GHG saving due to fertilizer use. The base case was electricity produced from power plants fired with natural gas (30%) and coal (70%) which on average had a carbon footprint of 825 gCO2−eq /kWhel . The actual GHG emissions from the biogas plants ranged from −85 to 251 gCO2−eq /kWhel with the lowest −85 gCO2−eq /kWhel coming from a chicken manure plant. A similar range of GHG mitigation from biogas production was modeled by Aurich et al. [11] who reported that electricity from biogas was 22–75% less than the GHG emis-

1.5 Biogas Production in Southeast Asia

7

sions caused by the energy mix in Germany at the time. In their scenarios analyzed GHG emissions calculated from biogas were between 100 and 400 gCO2−eq /kWhel .

1.5 Biogas Production in Southeast Asia Having explored biogas production globally, in this section the focus will be on production in Southeast Asia. Asia produces the second highest quantity of biogas after Europe but has the potential to produce the most. Biogas development was helped through remuneration via the Clean Development Mechanism (CDM), particularly between 2007 and 2011. The tropical climate and heavy rainfall promote lush vegetation ideally suited to biogas production. The main substrates found in Southeast Asia are discussed next. The substrate itself is the main factor in determining the biogas yield although other factors such as temperature, pH, C/N ratio, organic loading rate and hydraulic retention time also influence the biogas yield.

1.5.1 Substrates The top three substrates used for biogas production in Southeast Asia are cassava, palm oil and livestock waste. Livestock waste is composed of mostly swine and chicken manure. There are some molasses and ethanol biogas plants, but they are far fewer in quantity.

1.5.2 Cassava Cassava originated in South America. Tapioca is dried cassava in powder form. Nigeria has the world’s largest cassava production, followed by Brazil and Thailand. However, Thailand has a higher yield per area out of the three and is the world’s largest cassava exporter. In 2016 it produced 33 million tons. It is grown in all regions of Thailand except the South. Cassava industries, in their production process, use large quantities of water generating high volumes of wastewater. In Thailand, more than 90% of the cassava-starch factories also produce biogas from the wastewater. The wastewater contains high levels of suspended organic solids (SS). Biological treatment systems are capable of operating at a high organic loading rates (OLRs) and can process wastewater with a high biochemical oxygen demand (BOD). A 3–4 m deep, anaerobic pond with a small surface area is suitable. Without sunlight penetrating, photosynthesis cannot occur and the pond condition becomes anaerobic. An Anaerobic Fixed Film Reactor (AFFR) was developed to treat wastewater and produce biogas from cassava starch plants. This is discussed in more detail in Sect. 3.4.1.

8

1 Introduction to Biogas

Table 1.5 POME discharge standards in Malaysia, Indonesia and Thailand (With permission from Tan et al. [12]) Parameters Limits of discharge Indonesia Malaysia Thailand BODa (mg/L) CODa (mg/L) Total solids (mg/L) Suspended solids (mg/L) Oil and grease (mg/L) TKN (Total Kjeldahl N) (mg/L) pH Temperature (◦ C) a These

100 350 – 250

100 – – 400

20 120 3000 50

25 50

50 200

5 200

6–9 –

5–9 45

5–9 40

parameters will be explained in Sect. 2.2

1.5.3 Palm Oil Mill Effluent (POME) Southeast Asia countries are the largest producers of palm oil in the world. The leaders are Malaysia and Indonesia followed in third place by Thailand. Indonesia has over 850 palm oil mills; however, as of 2019, less than a tenth of palm oil mills in Indonesia are equipped with biogas capture. For every palm oil mill equipped with biogas capture, approximately 40,000–50,000 tCO2 e can be abated annually. The increasing global demand for the palm oil results in more palm oil mill effluent (POME) which needs to be treated. Fresh POME is hot (temperature 60–80 ◦ C), acidic (pH of 3.3–4.6), thick, brownish liquid with high fat, oil and grease content. Its discharge requirements are shown in 1.5. This is a slightly controversial waste as several environmental groups and NGO’s find POME indirect land use cost high, and there are long-running deforestation and sustainability concerns. The European Union’s Renewable Energy Directive II states that palm oil should be phased out of the biofuels mix by 2030. Open pond systems are the most commonly used at the majority of palm oil mills for effluent treatment, due to their low maintenance costs, system reliability and simple design. Any gases produced are not captured but escape into the atmosphere. These systems have long hydraulic retention times (>100 days) which require large pond areas. Nowadays, an anaerobic system for biogas production is the preferred treatment method and is far more beneficial for the environment. Some modifications from standard anaerobic plants have been developed for POME wastewater including, attached growth anaerobic reactors, anaerobic fluidized bed reactors, anaerobic filters, anaerobic sludge blanket processes, anaerobic baffled bioreactors and integrated anaerobic treatment processes. These reactors and others are discussed in Chap. 3.

1.5 Biogas Production in Southeast Asia

9

Table 1.6 Design criteria for animal farms Index Conversion factor Biogas production

0.85 1.0 0.85 0.45

Fertilizer production

Unit m3 /day. LU. Pig m3 /day. LU. Hen m3 /day. LU. Cow kg/day. LU.

1.5.4 Livestock Waste Another popular source for biogas production is livestock waste. Historically the roots of biogas systems in Thailand stem from agricultural waste, especially wastewater and waste from pig farms. Support for these plants began in 1990 from the Ministry of Energy. Thailand has the potential to produce over one billion m3 of biogas per annum from its agricultural industry alone. Utilization was only 36% of this potential Aggarangsi et al. [13] in 2013. Using a biogas plant to manage pig farm waste can help reduce expenses, especially the farm’s electricity bills. It also helps reduce the problem of odor in the community. In order to estimate the potential biogas production rate per day from this waste the following method can be used. In Thailand a conversion factor is used to convert a livestock unit (LU) into a biogas production rate, Kashyap [14]. One livestock unit is an average of 500 kg of animal weight. This is about the weight of an average cow, hens which weigh around 2.5 kg each and pigs vary but a fattening pig weighs around 60 kg (Table 1.6). Example 1.1 A farm has 50 cattle, 100 breeder pigs and 850 hens. What size of biogas system is suitable for this farm? Estimate its biogas production potential and its fertilizer potential. Solution The first step is to calculate the livestock unit for each animal. The cattle are 50 LU, the breeder pigs have (60 kg ∗ 100)/500 = 12 LU, and the hens have (850 ∗ 2.5 kg)/500 = 4.25LU. Animal Cow Pig Hen Total

LU 50 12 4.25

Biogas potential = 0.85 ∗ 50 = 42.5 m3 /day = 0.85 ∗ 12 = 10.2 m3 /day = 1.0 ∗ 4.25 = 4.25 m3 /day = 56.95 m3 /day

So the total biogas potential, assuming all waste is used, for this farm is 56.95 m3 /day. This can be used to design and size the biogas system. The fertilizer production rate per day will be: (50 + 12 + 4.25) ∗ 0.45 = 29.8 kg/day Municipal Solid Waste Municipal solid waste (MSW) consists of solid waste generated within a municipality by households, industries and a commerce. Its composition

10

1 Introduction to Biogas

Fig. 1.2 Minor case study 1

and quantity are variable with organic matter constituting about 25–75% of the total MSW. The MSW production rate typically varies between 1.1 and 2.2 kg/person/day based on a country’s income. Two major conversion pathways exist for MSW and they are the biochemical conversion process (anaerobic digestion) and thermochemical conversion (incineration). The thermochemical conversion mechanism is rarely used because of the low calorific value and high moisture content of MSW. Anaerobic digestion of MSW has attracted more attention because of the possibility of separating the organic biodegradable fraction from the total MSW. Additionally, the generation of renewable energy, reduction of land-filling and mitigation of pollution are other advantages (Fig. 1.2). Minor Case Study 1 In 2013, anaerobic digestion technology in Thailand was increasingly gaining popularity for its benefits to both the environment and energy. Swine farms in Thailand needed to make the decision whether to invest into the then novel technology. Khana Hybrid Co., Ltd. owns multiple swine farms across central Thailand including a farm located in Chanchongsao province, 100 km east of Bangkok. It is a medium size farm, possessing 24 swine houses holding 20,000 nursery piglets and 25,000 fattening pigs on average throughout the year. The facility was expected to produce as much as 810 m3 of wastewater from animal waste with a COD concentration up to 18,000 mg/L. A channel digester, with a capacity of 3750 m3 , was constructed which produces 3060 m3 of biogas per day. It gets combusted in a generator and produces 6120 kW-hr per day. The estimated annual greenhouse gas savings is 9300 ton CO2 per year.

1.5 Biogas Production in Southeast Asia

11

Fig. 1.3 Minor case study 2

Industrial waste Industrial wastes are byproducts, residues and wastes that result from various industrial activities. They include waste from the pulp and paper industry, food industry, petrochemical refinery waste, textile industry and liquid biofuel production waste. Besides waste from the food industry, other industrial wastes have not been widely used as a substrate in anaerobic digestion due to their recalcitrant chemical properties and low biodegradability of about 30–50%. In the pulp and paper industry, a large quantity of wastewater with a high organic load is produced during the paper manufacturing process. Anaerobic treatment of this wastewater has an added benefit of lower treatment cost because of the possibility of utilizing the biogas produced for energy generation. Due to low solid content (TS < 1%) of this wastewater, they are co-digested with other substrates or pretreated to improve their biodegradability. The textile industry is another industry that generates a significant quantity of wastewater through the production process of washing, dyeing and finishing. Some studies have reported on the anaerobic treatment of textile wastewater for the production of biogas, but a full-scale commercial AD has yet to be built for this substrate (Fig. 1.3). Minor Case Study 2 KCF Green Energy Co., Ltd. from Nakorn Pathom province in Thailand, ran a business of raising more than 2 million laying hens, producing about 100 tons of chicken manure per day. They spotted an opportunity to manage chicken manure disposal, while generating electricity and selling bio-fertilizer. This waste is combined with 200 m3 of ethanol wastewater daily. Ethanol wastewater has a large COD of 200,000 mg/L. A dual CMU hybrid digester, each with a capacity of 9000 m3 , and nine CSTR reactors each with a capacity of 3000 m3 were con-

12

1 Introduction to Biogas

structed. The total biogas produced each day is 40,000 m3 . It gets combusted in a generator and produces 88,000 kW-hr per day. The estimated annual greenhouse gas savings is 100,000 ton CO2 . Other waste sources Less common sources of substrates for biogas production include, wheat straw, corn stover and sugarcane. Wheat straw is a typical example of an agricultural waste which is a suitable substrate for the production of biogas, although its lignocellulose content slows down the degradation process. Corn stover is a potential substrate for biogas production that is leftover from the maize harvest. Sugarcane bagasse is another agricultural waste that can serve as a substrate for co-digestion purposes due to its energy potential. It is a byproduct of the sugar milling industry.

1.5.5 History of Biogas in Thailand This book focuses on the development and use of biogas in Southeast Asia. The ASEAN countries which includes Brunei, Cambodia, Malaysia, Myanmar, Indonesia, Laos, the Philippines, Vietnam, Singapore and Thailand have high potential for producing biomass energy, due to the predominance of agriculture in these economies. The tropical climate is suited for biogas. Thailand is the regional leader in biogas technology and production. Some notable milestones are listed below: • 1950: Biogas technology was introduced into Thailand by the Ministry of Agriculture. The purpose was to produce biogas for domestic cooking • 1960s: Thailand began to utilize biogas from anaerobic waste treatment systems. • 1972: Chiang Mai University began to cooperate with the Department of Agriculture to develop biogas technology. • 1988: The Ministry of Agriculture established a “Thai-German Biogas Project” with the support of the German Technical Cooperation agency (GTZ) of the Federal Republic of Germany to promote the use of biogas technology in Thailand. • September 1988 - December 1994 the Thai-German co-operative supported the “Biogas in Animal Farms” program from the Ministry of Agriculture. This program built small biogas plants at household or small-scale farms and was implemented in the northern and eastern parts of Thailand. • 1990, Rolf Kloss from the National Institute for Agricultural Research in Braunschweig, Germany, designed a parallel biogas system known as the Channel Digester with the UASB for pig farms. The first such swine farm facility built in Thailand was by Chiang Mai University in the Mae Hia district. • 1992: Mr. Wolfgang Tentscher, a German expert from the Asian Institute of Technology (AIT), designed a hybrid biogas system for pig farms. It was built on the Kamphaeng Saen Campus of Kasetsart University.

1.6 Case Study

13

• 2007: The Institute for Energy Research and Development (ERDI) was formed by combining two research institutes together. It operates under the authority of Chiang Mai University. Some of their systems developed include the CMU-CD system suitable for animal farms, the CMU-CD biogas system for livestock waste water management, CMU-Modified Cover Lagoon for waste water treatment from industrial plants and CMU-CSTR biogas plant for the production of biogas from energy crops.

1.6 Case Study Agro-industries are very important for the economy in the poorest region of Thailand, Issan. The RE Power Group introduced a project to capture biogas from a starch factory in the city of Korat, using a 195,000 m2 hybrid covered lagoon reactor (HCLR); see Fig. 1.4. The installed biogas plant removes organic waste materials from the factory wastewater with an efficiency above 85%. It produces 120,000 Nm3 of biogas daily. Of this, 65% is used to generate electricity through 6 MW gen sets and sold to the national grid, 30% is used for heat in the starch factory and 5% is purified and compressed for sale as compressed biogas (CBG). The CBG is sold in the transport sector, replacing fossil fuel-based compressed natural gas (CNG). The total project construction cost was $8-million. The project has had a significant positive environmental impact on people living in the vicinity of the starch factory. Using biogas for CBG production was a pioneering decision in Thailand. The biogas purified and compressed to CBG has a 85% methane content and is suitable for legal use in transportation. Up to 60 trucks or 500 cars per day can be served by the CBG filling station. The CBG is priced about 10% less than CNG. The availability of CBG therefore has a direct economic impact on the region. The project had the following objectives: 1. Efficient wastewater treatment (>85%) in a dual-feedstock HCLR 2. Economic utilization of the produced methane for electricity production, starch drying and CBG generation 3. Provision of clean effluent for use in irrigation and as liquid fertilizer 4. Building the first industrial-scale CBG plant and a gas station to sell CBG to the public as car fuel Project Innovations • Introduced the use of HCLR and CBG production to sell a clean fuel to the public. • The first reactor to digest both wastewater and waste starch pulp simultaneously. • The project provides an alternative use for biogas to conventional electricity generation.

14

1 Introduction to Biogas

Fig. 1.4 RE power group, 120,000 Nm3 /day biogas plant

Financial benefits Operating cash flow is the best measure for sustainability in biogas projects. This project’s operating cash flow was positive from the first month after project completion—with an operating margin of over 45%. The technical solution is economically attractive and applicable in starch factories and other agroindustries across Thailand. The starch factory saves up to 35% in its starch drying costs by replacing fossil fuel oil with biogas supplied by the project. The starch factory benefits to the tune of $1 million per year through savings in its cost for boiler fuel and electricity, while switching to a carbon negative fuel that greatly reduces their carbon footprint. This provides the starch factory with a competitive edge over its competitors. Environmental Impact The project aimed to reduce greenhouse gas emissions by 350 t-CO2 eq per day. This goal was already reached in October 2018 and the size of the emissions reduction continues to grow in line with an increasing biogas yield. Water sources in the area are protected from the factory’s waste. 8000 m3 of wastewater per day is now treated with an 85% or above efficiency.

References 1. Meynell P (1976) Methane: planning a digester. Sochen Books, Dorset, Clarington 2. Eckenfelder W (1957) Biological treatment of sewage and industrial wastes, vol 2. Reinbold Publishing, New York 3. Buswell W, Hatfield AM (1936) Anaerobic fermentations. Technical Report 32, Urbana, IL. https://www.isws.illinois.edu/pubdoc/B/ISWSB-32.pdf

References

15

4. International Energy Agency (2020) Outlook for biogas and biomethane: Prospects for organic growth. Technical report, World Energy Outlook Special Report, Paris. https://www.iea.org/ reports/outlook-for-biogas-and-biomethane-prospects-for-organic-growth 5. Kummamuru B (2016) WBA global bioenergy statistics. Tech Report, World BioEnergy Association 6. British Petroleum Company (1981) Bp statistical review of world energy. Techreport, British Petroleum Co., London 7. World Bioenergy Association (2021) Global bioenergy statistics 2021. Technical report, WBA 8. United Nations Framework Convention on Climate Change (2019) Methane recovery in wastewater treatment. online https://cdm.unfccc.int/methodologies/DB/ K7FDTJ4FL3432I1UKRNKLDCUFAMBX7.AMS-III.H 9. Jain S, Newman D, Nzihou A, Dekker H, Feuvre PL, Richter H, Gobe F, Morton C, Thompson R (2019) Global potential of biogas. Technical Report 10. Bachmaier J, Effenberger M, Gronauer A (2010) Greenhouse gas balance and resource demand of biogas plants in agriculture. Eng Life Sci 10(6):560–569 11. Meyer-Aurich A, Schattauer A, Hellebrand HJ, Klauss H, PlÃchl M, Berg W (2012) Impact of uncertainties on greenhouse gas mitigation potential of biogas production from agricultural resources. Renew Energy 37(1): 277–284. ISSN: 0960-1481. https://doi.org/10.1016/j.renene. 2011.06.030. https://www.sciencedirect.com/science/article/pii/S0960148111003235 12. Tan KA, Wan Maznah WO, Morad N, Lalung J, Ismail N, Talebi A, Oyekanmi AA (2021) Advances in pome treatment methods: potentials of phycoremediation, with a focus on south east asia. Int J Environ Sci Technol ISSN 1735-2630. https://doi.org/10.1007/s13762-02103436-6 13. Aggarangsi P, Tippayawong N, Moran J, Rerkkriangkrai P (2013) Overview of livestock biogas technology development and implementation in thailand. Energy Sustain Dev 17(4):371–377. ISSN 0973-0826. https://www.sciencedirect.com/science/article/pii/S0973082613000239 14. Kashyap P (2017) Pollution control and policy measures for piggery wastewater management in Thailand. In: WEPA Group Workshop on Piggery Wastewater Management in Asia, Chiang Mai Feb

Chapter 2

Anaerobic Digestion and Biogas Production

2.1 Introduction The decomposition of organic matter under anaerobic conditions (defined as the absence of oxygen) occurs commonly in nature, such as in animal guts, swamp/mash land or sediments in lakes/ponds. Organic decomposition in the presence of oxygen is known as aerobic digestion. The major difference with aerobic digestion is that methane gas is not produced as a by-product, see Fig. 2.1. In anaerobic conditions, the biochemical pathway uses carbon dioxide (CO2 ) as a final external electron acceptor to produce energy for cell respiration. Although complete aerobic degradation of organic substances occurs much faster than that of anaerobic degradation, the final products are carbon dioxide and water, which have little benefit for energy applications. Anaerobic digestion of organic waste is becoming more popular worldwide since it addresses major global issues including energy security, waste management and environmental concerns. Organic materials, including industrial wastewater, animal manure, sewage sludge, municipal solid waste, crop residue and biomass, can be degraded in an anaerobic digester through a series of metabolic reactions. In an AD, the environmental conditions are closely controlled to maximize biogas production. The biogas produced from waste/residue digestion in AD is composed of methane (50–75% by volume) and carbon dioxide (25%–40% by volume), with small amounts of nitrogen (< 5% by volume), hydrogen (< 1% by volume), oxygen (< 1% by volume) and hydrogen sulfide (50–5000 ppm) [1]. Biogas can be used for energy applications ranging from stationary power to transportation. In the case of transportation, the biogas needs purification (biogas applications are discussed in Chap. 7). The digestate leftover from the AD process is nutrient rich as it contains biomass of bacterial cells which are great for soil amendment, as discussed in Chap. 6.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_2

17

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2 Anaerobic Digestion and Biogas Production

Heat loss

Influent (100 kg COD)

Aerobic

+

Effluent (10-12 kg COD)

Aeration

Sludge (30-60 kg) Biogas 40-45 m3 (~70% CH4 )

Influent (100 kg COD)

Anaerobic

Effluent (10-12 kg COD)

Sludge (5 kg) Fig. 2.1 Aerobic and anaerobic degradation of organic material

2.2 Wastewater Analysis Large quantities of organic waste are found in wastewater. Wastewater is untreated water or water that contains any substance or unwanted waste that causes water quality degradation. It cannot be reused until it has been treated. Wastewater refers to all effluent from household, agriculture, commercial establishments, hotels, hospitals, industries and so on. The pollutants in the wastewater have different characteristics depending on the source, such as color, odor, oil, fat, detergent, soap, insecticide, organic and inorganic matters. The energy is locked inside the biodegradable organic matter. This energy is both sustainable and environmentally friendly. A modern wastewater treatment system has the ability to treat wastewater and produce energy at the same time. There are many types of contaminants in wastewater such as:

2.2 Wastewater Analysis

19

1. Organic substances such as carbohydrates, proteins or fats. These can be degraded by microbes that use oxygen which cause the dissolved oxygen levels in the water to decrease and cause putrid conditions. The amount of organic material is usually measured by the biochemical oxygen demand (BOD). This parameter is introduced in Sect. 2.2.2. 2. Inorganic substances, such as minerals and heavy metals, including mercury, chromium and copper can be harmful to living things. 3. Oils and grease which float and hinder photosynthesis, disrupting oxygen distribution in water. 4. Nutrients such as nitrogen and phosphorus. When high in quantity, they cause algae bloom, (eutrophication) making a significant change in pH and dissolved oxygen. Wastewater sources can be divided into 3 types: 1. Domestic wastewater comes from routine activities of people such as consumption, washing and cooking. It comes from residences, housing villages, condominiums, hotels, markets, schools, hospitals, etc. 2. Industrial wastewater comes from industries such as food, power plants and chemical industries. There are many types of industrial wastewater according to each different industry. Each sector produces its own wastewater characteristics. 3. Agricultural wastewater comes from farms and includes manure, field residue and process residue. It is often loaded with organic substances. This type of wastewater generally contains suspended solids which can lead to sludge formation.

2.2.1 Solids in Wastewater Figure 2.2 shows a typical composition of wastewater with 5% total solids. The solids are in turn categorized into volatile and non-volatile components. Volatile solids are typically referred to as organic matter in the solid fraction. The amount of solids in wastewater can be obtained experimentally by the following process. An initial mass of wastewater is placed on a dish and weighed. The dish is heated to 103–105 ◦ C to evaporate all of the water. The remaining solids are weighed. The solids percentage is calculated from Eq. (2.1): %Total Solids =

Weight of dry solids × 100 Weight of original sample

(2.1)

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2 Anaerobic Digestion and Biogas Production

Fig. 2.2 Sludge solids example for wastewater with 5% solids by weight

The percentage of volatiles in the solid matter is determined by heating the solids to 550 ◦ C for 15 mins. This releases the volatiles, Spellman [2]. The mass before and after is measured. The percentage of volatile is calculated from Eq. (2.2): % Volatiles =

Weight of volatiles × 100 Weight of dry solids

(2.2)

2.2.2 Biochemical Oxygen Demand (BOD) Biochemical oxygen demand (BOD (mg/L)) is the amount of dissolved oxygen needed by aerobic biological organisms to break down the organic material in water. It is a direct measure of oxygen requirements and an indirect measure of the biodegradable organic matter. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter, during 5 days of incubation at 20 ◦ C. It is often used as a robust surrogate for the degree of organic pollution in water. Microorganisms

Organic matter + O2 −−−−−−−−→ CO2 + H2 O + New Cells + Stable Products The test for wastewater BOD is based on the premise that all the biodegradable organic material contained in the sample can be oxidized to CO2 and H2 O. As the microbial activity depends on temperature, this is kept in a tight range between 20 and 27 ◦ C . For example, a sample of wastewater is taken, xww mL and diluted with clean water up to xdw mL. The dissolved oxygen content is measured initially and after 5 days. The BOD is calculated from Eq. (2.3). BOD, (mg/L) = where

(D1 − D2 ) P

(2.3)

2.2 Wastewater Analysis

21

D1 Dissolved oxygen in the sample immediately after preparation, (mg/L) D2 Dissolved oxygen in the sample after the 5 day incubation period at 20 ◦ C, (mg/L) . P Fraction of the initial sample to the diluted sample, P = xxww dw

2.2.3 Chemical Oxygen Demand (COD) Chemical oxygen demand (COD (mg/L)) indicates the amount of oxygen that is needed for the oxidation of all organic substances in water. Chemical oxygen demand is an important water quality parameter because, similar to BOD, it provides an assessment on the environmental effects from discharged wastewater. The greater the amount of oxidizable organic material in the sample, the higher the COD level. This causes a reduction in dissolved oxygen (DO) levels. A reduction in DO is detrimental to aquatic life. The COD test is often used as an alternate to BOD due to its shorter testing time. Many governments regulate the maximum chemical oxygen demand allowed in wastewater before returning it to the environment. There are two methods for COD measurement, open and closed reflux. The basis for both tests is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. The amount of oxygen required can be thus calculated, American Public Health Association [3]. The method involves using a strong oxidizing chemical, potassium dichromate (K2 Cr2 O7 ), to oxidize the organic matter in solution to carbon dioxide and water. After adding the potassium dichromate the sample is then digested for approximately 2 h at 150 ◦ C. Once complete, the amount of excess potassium dichromate is measured. To do this, ferrous ammonium sulfate is added until all of the excess potassium dichromate has been reduced to Cr3+ , see Fig. 2.3. The amount of Cr3+ is used as an indirect measure of the organic content in the water sample. 2h

+ 3+ Organic matter + Cr2 O2− → CO2 + H2 O + NH+ 7 +H − 4 + Cr

Chemical oxygen demand (COD) will always be higher than or equal to the biochemical oxygen demand (BOD) since it is the total measurement of all organics in the water that can be oxidized chemically. Biochemical oxygen demand (BOD) relies on bacteria to oxidize readily available organic matter during a five-day incubation period. COD uses powerful chemicals to oxidize organic matter. Generally, COD is preferred to BOD for process control measurements because results are more reproducible and are available in just four hours rather than five days. The main differences are summarized in Table 2.1. The ratio between these parameters (COD/BOD) is called the biodegradability of the sample. The lower the ratio, the higher the biodegradability. Typical values for raw wastewater would range from 1.5 to 2.3, Pirsaheba et al. [5] and for biologically treated effluent would be from 4 to 8, Kewu and Wenqi [6].

22

2 Anaerobic Digestion and Biogas Production

Fig. 2.3 Open and closed reflux COD measurement (with permission from Ma [4]) Table 2.1 Differences between BOD and COD BOD COD Only measures the amount of oxygen consumed by microbes for organic material degradation 5 days required for BOD BOD is not suited for industrial wastes, rich in cyanide’s, heavy metal ions or toxic compounds

Measures all organic materials that are oxidized by a chemical oxidizing agent 4 h to complete Industrial waste or any toxic-containing wastewater can be measured by COD

2.3 Anaerobic Digestion Process Anaerobic digestion uses diverse microbial communities to break down biodegradable material and produce biogas (Fig. 2.4). It consists of four processes carried out in series. These four major steps are as follows: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Briefly, large organic molecules are hydrolyzed into small molecules which are consequently converted into acids and eventually biogas is produced as a product, see Fig. 2.5.

2.3.1 Hydrolysis Hydrolysis is the first step in biogas production. It converts complex biochemical molecules such as carbohydrates, proteins and lipids into simpler molecules such as oligomeric and monomeric sugars, amino acids and long-chain fatty acids, see

2.3 Anaerobic Digestion Process

23

Complex Organic Compounds

Proteins

Hydrolysis (H)

Lipids

Carbohydrates

H

H

H

Simple Biomolecules Amino acids, Sugars

Acidogenesis (ACD)

Fatty acids

AC D

AC D ACT Syntrophic Acetogenesis

ACD

Intermediate Products (Propionate, Butyrate, Lactate, Ethanol) ACT Syntrophic Acetogenesis

Acetogenesis (ACT)

Acetate Methanogenesis (M)

M Acidiclastic Mathanogenesis

ACT Homoacetogenesis

Methane, Carbon dioxide

Hydrogen, Carbon dioxide M Hydrogenotrophic Mathanogenesis

Fig. 2.4 Schematic diagram of the anaerobic digestion process for biogas production

Fig. 2.4. The degradation of organic matter occurs through the action of hydrolytic microorganisms where the anaerobic bacteria produce extracellular enzymes to decompose the complex organic compounds, see Table 2.2. These hydrolytic bacteria can be facultative or obligate anaerobic microorganisms. Therefore, they actively digest organic materials with or without oxygen. The hydrolysis reaction rate varies according to the type, complexity and size of the biodegradable material. Hydrolysis of feedstocks with high lipid or high solids contents can take several days. Consequently, hydrolysis is considered the rate-limiting step in biogas production. In general, for feedstocks with complex structures such as lignocellulose, a pretreatment process is often required to improve the hydrolysis rate and overall digestibility. Pre-

24

2 Anaerobic Digestion and Biogas Production

Fig. 2.5 Microorganisms involved in anaerobic methane production

treatment is where the organic material is broken down prior to entering the digester. This process is discussed more in Chap. 4. As a biological process, this step depends largely on environmental conditions such as organic matter content, pH, temperature, surface area for enzyme degradation process and inhibitory compounds. As it only breaks down complex material to smaller molecules, total organic content after hydrolysis remains unchanged.

2.3.2 Acidogenesis Acidogenesis occurs after the hydrolysis reaction. It makes intermediates for further digestion. This fermentation step converts simple biochemical molecules into short-chain fatty acids, commonly known as volatile fatty acids (VFAs), alcohol and hydrogen (H2 ). Fermentative bacteria, known also as acidogenic bacteria or acidogens (Table 2.3), implement this step. During acidogenesis, acetic (C2), propionic

2.3 Anaerobic Digestion Process

25

Table 2.2 Microorganisms used for hydrolysis of complex organic molecules [1, 7] Complex organic Resulting simple Extracellular enzymes Type of hydrolytic compounds biochemical molecules microorganisms Carbohydrates (i.e., starch, cellulose, hemicellulose and pectin)

Proteins

Lipids

Soluble sugars

Amylase, cellulase, cellobiase, hemicellulase, xylase, xylanase, pectinase

Acetovibrio celluliticus, Bacillus, Bacteriodes, Clostridium, Lactobacillus, Staphylococcus Peptides and amino Protease and peptidase Bacteriodes, acids Butyrivibrio, Clostridium, Fusobacterium, Peptococcus, Proteus vulgoris, Selenomonas, Streptococcus Long chain fatty acids Lipase Clostridium, and glycerol Eubacterium limosum, Mycobacterium, Streptococcus

Table 2.3 Microorganisms involved in acidogenesis [1, 7] Biochemical molecules Intermediates Sugars

Fatty acids and alcohol

Amino acids

Fatty acids, acetate and ammonia

Type of acidogenesis bacteria Clostridium, Eubacterium limosum, Streptococcus Bacillus, Desulfobacter, Desulforomonas, Desulfovibrio, Escherichia, Lactobacillus, Pseudomonas, Sarcina, Selenomonas, Staphylococcus, Streptococcus, Veillonella

(C3) and butyric (C4) acids are the main products with small amounts of caproic (C6), valeric (C5), lactic (C3) and succinic (C4) acids produced. Acidogenesis occurs rapidly, and it is the fastest reaction in biogas production. This can cause accumulation of VFAs leading to lower pH in the system and can lead to process failure. Similarly to bacteria responsible for hydrolysis, acidogens (Bacteroides, Clostridium, Butyribacterium, Propionibacterium, Pseudomonas and Ruminococcus) can be facultative or obligate anaerobic microorganisms, which means the reaction can occur with or without oxygen.

26

2 Anaerobic Digestion and Biogas Production

2.3.3 Acetogenesis Acetogenesis is the process of making acetate (acetic acid) from the intermediates (VFAs and alcohol) produced in acidogenesis. This step is critical as acetate is responsible for 60–70% of the total methane produced [8]. It is produced from two major mechanisms that occur in parallel in AD: 1. Syntrophic acetogenesis, where VFAs are oxidized into smaller molecules (acetate) with H2 production (Eqs. 2.4–2.10). 2. Homoacetogenesis, where acetate is synthesized by H2 and CO2 (Eq. 2.11). All reactions are non-spontaneous reactions (positive Gibb’s free energy changes or endergonic reactions). To make these reactions happen, low concentrations of H2 are required and the hydrogen must be utilized by the H2 -consuming bacteria (such as hydrogenotrophic methanogens, homoacetogens and/or sulfate reducers (if sulfate is present in the feedstock)). Low H2 concentrations, < 100 ppm, are required for H2 producing acetogenic bacterial growth and activity. There is a syntrophic relationship between the both groups of bacteria. Therefore, H2 -producing acetogenic bacteria are commonly known as syntrophic acetogenesis bacteria or syntrophic acetogens. Although the reaction time of acetogenesis is short, this anaerobic bacteria grows slowly with a long lag phase required for environmental adaptation. Syntrophomonas wolfei and Syntrophobacteri wolinii are the syntrophic acetogens often found in the anaerobic digester [9]. Moreover, in the absence of sulfate, Desulfovibrio spp. could metabolize lactate or ethanol for acetate generation [10]. Propionate: + CH3 CH2 COO− + 3H2 O → CH3 COO− + HCO− 3 + H + 3H2 

G o = +76.1 kJ/mol

(2.4)

Butyrate: CH3 (CH2 )2 COO− + 2H2 O → 2CH3 COO− + H+ + 2H2 

G o = +48.1 kJ/mol

(2.5)

Valerate: CH3 (CH2 )3 COO− + 2H2 O → CH3 COO− + CH3 CH2 COO− + H+ + 2H2 

G o = +48.1 kJ/mol

(2.6)

Caproate: CH3 (CH2 )4 COO− + 4H2 O → 3CH3 COO− + 2H+ + 4H2  G o = +96.2 kJ/mol

(2.7)

2.3 Anaerobic Digestion Process Table 2.4 Microorganisms involved in methanogenesis [1, 11] Group of microorganisms Reactions Hydrogenotrophic methanogens

H2 + CO2 to methane

Acetotrophic methanogens

Acetate to methane + CO2

27

Methanogens Methanobacterium, Methanobrevibacter, Methanoplanus, Methanospirillum Methanosaeta, Methanosarcina

Glycerol: + C3 H8 O3 + 2H2 O → CH3 COO− + HCO− 3 + 2H + 3H2 

G o = +76.1 kJ/mol

(2.8)

Lactate: + CH3 CHOCOO− + 2H2 O → CH3 COO− + HCO− 3 + H + 2H2 

G o = +4.2 kJ/mol

(2.9)

Ethanol: CH3 CH2 OH + H2 O → CH3 COO− + H+ + 2H2  G o = +9.6 kJ/mol

(2.10)

Another group of microbes are called homoacetogenic bacteria or homoacetogens, which include Acetobacterium, Acetoanaerobium, Acetogenium, Butyribacterium, Clostridium, Eubacterium and Pelobacter [11]. The autotrophic homoacetogens utilize CO2 as a carbon source and H2 to produce acetate (Eq. 2.11). 2HCO3− + 4H2 + H+ → CH3 COO− + 4H2 O  G o = +104.6 kJ/mol

(2.11)

2.3.4 Methanogenesis Methanogenesis is where the acetic acid, hydrogen and carbon dioxide are used by methanogens to create methane under strictly anaerobic conditions. Archaea, obligate anaerobic microorganisms, are extremely sensitive to oxygen and other environmental conditions (pH, temperature and inhibitors). These are the key microbes for biogas production, see Table 2.4. Moreover, they can use only a very specific substrate. Con-

28

2 Anaerobic Digestion and Biogas Production

sequently, for most wastewater, methanogenesis is often the rate-limiting step. An exception is when complex organic material (such as biomass) is used, the ratelimiting step is hydrolysis. All methanogenesis reactions are thermodynamically favorable (exergonic reactions) which means that they are spontaneous reactions (Eqs. 2.12–2.19). Therefore, acetogenesis occurs when coupled with methanogene sis (the resulting final value of G o is less than 0). Acetate: 

o CH3 COO− + H2 O → CH4 + HCO− 3 G = −30.9 kJ/mol

(2.12)

Carbon dioxide: 

+ o 4H2 + HCO− 3 + H → CH4 + 3H2 O G = −135.5 kJ/mol



4H2 + CO2 → CH4 + 2H2 O G o = −131.0 kJ/mol

(2.13)

(2.14)

Carbon monoxide: 

4CO + 2H2 O → CH4 + 3CO2 G o = −210.8 kJ/mol

(2.15)

Formate: 

o 4HCOO− + H2 O + H+ → CH4 + 3HCO− 3 G = −130.4 kJ/mol

(2.16)

Methanol: 

+ o 4CH3 OH → 3CH4 + HCO− 3 + H + H2 O G = −314.3 kJ/mol



CH3 OH + H2 → CH4 + H2 O G o = −113.0 kJ/mol

(2.17)

(2.18)

Ethanol: 

2CH3 CH2 OH + CO2 → CH4 + 2CH3 COOH G o = −116.3 kJ/mol

(2.19)

There are two major pathways and thus group of methanogens for methane production. Acetotrophic methanogens Acetotrophic or aceticlastic methanogens directly convert acetate to methane and carbon dioxide via several biochemical reactions. Acetate is the key element for efficient AD as more than 70% of methane produced during AD comes from this reaction by the two essential genera which are Methanosarcina and Methanosaeta [12]. Methanosarcina (spherical-shaped microorganisms) can use several substrates such as methanol, methylamines and H2 /CO2 while Methanosaeta

2.4 Biogas Yield

29

(rod-shaped microorganisms) can grow only on acetate. Typically in the AD with low substrate concentration as found in CSTR, Methanosaeta would be predominant due to their lower values of biokinetic parameters, such as half-velocity constant, yield coefficient, specific growth rate and specific substrate utilization rate, when compared to that of Methanosarcina. At high substrate concentrations, Methanosarcina would be dominant [13]. Hydrogenotrophic methanogens The other thirty percent of methane production in AD comes from the action of hydrogenotrophic methanogens. This type of methanogens reduce CO2 to methane while utilizing the H2 produced in acetogenesis. Hydrogenotrophs play a critical role in stable and efficient AD, as low H2 concentrations are required during acetogenesis (previously mentioned in Sect. 2.3.3). Some of hydrogenotrophic methanogens can use formate, carbon monoxide, and alcohols to reduce CO2 for methane production [8].

2.4 Biogas Yield Biogas yield varies depending on chemical composition of the organic material. Theoretical methane yield can be calculated from the chemical formula of the substrate using Eqs. (2.20) and (2.21) [14, 15].      o h o h o h c c H2 O → + − CO2 + − + CH4 Cc Hh Oo + c − − 4 2 2 8 4 2 8 4 (2.20) 

    o 3n s h o 3n s h c + H2 O → + − − − CO2 Cc Hh Oo Nn Ss + c − − + 4 2 8 2 2 8 4 8 4   h o 3n s c − + + + CH4 + nNH3 + sH2 S + (2.21) 2 8 4 8 4 Moreover, typically, theoretical methane yields can be calculated based on the organic content. This is obtained from the COD (chemical oxygen demand) of the substrate. Note that COD represents the amount of O2 consumed during digestion of organic matter. Theoretical biogas yields of organic compounds including carbohydrates, proteins and lipids are 750, 800 and 1390 mL/gVS with methane content of 50, 60 and 72%, respectively. Example 2.1 What is the theoretical methane yield when one kilogram of COD is anaerobically digested at STP (0 ◦ C, 1 atm). Step 1. Calculation of COD (O2 ) equivalent to CH4 . The balanced reaction for the oxidation of methane is given by: CH4 + 2O2 → CO2 + 2H2 O

30

2 Anaerobic Digestion and Biogas Production

So a mole of methane (16 g) is oxidized by 2 mol of oxygen (64 g). Or, put another way, one gram of methane is equivalent to 4 g of COD or 0.25 g of methane per g COD. Step 2. Conversion of CH4 mass to volume at standard temperature and pressure condition (STP). = 0.0156 moles of methane. Given, 0.25 g of methane, this is equivalent to 0.25 16 To convert this mass of CH4 to a volume (at STP), the ideal gas law can be used: V =

0.0156(0.082)273.15 n RT = = 0.35 L P 1

where V n R¯ T P

volume of gas (L) number of mole of gas (mol) universal gas constant (0.082 L atm mol−1 K−1 ) temperature (K) absolute pressure (atm).

So, one gram of COD can be anaerobically digested into 0.35 L CH4 . One kg of g L 1 m3 × 1000 kg × 1000 ). Therefore, COD can produce: 0.35 m3 of CH4 (= 0.35 g COD L complete anaerobic digestion of 1 kg COD produces 0.35 m3 CH4 at STP. Note: Biogas normally contains ∼ 60% methane. So, one kg of COD can produce = 0.58 m3 of biogas. ∼ 0.35 0.6 The above example is based on the assumption that oxygen demand is only used for methane production. It is impossible that all COD is converted to methane, as some organic content is used for bacterial growth. The effluent also still contains some organic matter. Calculations based on COD removal are upper estimates for methane production. In situations where sulfate is present, some COD will be consumed for sulfate reduction. Also organic digestion is dependent on its biodegradability. Thus, a conversion factor from COD to methane is necessary to accurately estimate the actual methane potential from organic waste/wastewater. The biochemical methane potential (BMP) is normally used for a preliminary assessment of the biogas production potential of a given substrate. The substrate is tested in a laboratory environment under optimal conditions and not limited by digestion time. This test can last for several weeks to months depending on the given substrate. BMP is expressed as: BMP =

Methane Production (or biogas production) (mL) COD Removed (g)

Example 2.2 Given the following parameters we are interested in knowing how much biogas is produced per day at this plant: • Wastewater intake per day: 500 m3 /day • Chemical Oxidation Demand (COD): 10,000 mg/L

2.5 Important Factors in Anaerobic Digestion

31

• Biochemical Methane Potential: 150 mL/g COD (at 35 ◦ C and 1 atm) = 0.15 m3 / kg COD • COD removal efficiency of biogas system: 80% • Methane content in biogas: 60%. Step 1. Calculate of the amount of COD per day: = 10,000

1 L m3 mg COD × = 5000 kg COD/day × 500 3 L 1000 m day

Step 2. Calculate of the amount of methane produced per day: = 5000

kg CH4 kg COD × 0.8 × 0.15 = 600 m3 CH4 /day day kg COD

Step 3. Calculate of the amount of biogas produced per day: Given a methane content in the biogas of 60%. 600 m3 CH4 /day ×

1 = 1000 m3 biogas/day 0.6

2.5 Important Factors in Anaerobic Digestion Complete anaerobic digestion of organic material for biogas production relies on several biochemical steps and several groups of microorganisms (Sect. 2.3). Therefore, control is needed over factors affecting microbial growth and activity, such as temperature, nutrients, pH, toxicity and inhibitors. Moreover, methanogens, the most vulnerable microbes, have extremely low growth rates and are very sensitive to operating conditions. Consequently, close monitoring to maintain the desired operating conditions is necessary for stable and efficient biogas production. Some important factors for successful anaerobic production of biogas are discussed below.

2.5.1 Inoculum The anaerobic biological degradation of organic substances is complex. The organic matter that enters the system is transformed by several groups of microbes working in series. Because organic matters degrades naturally under anaerobic conditions, the AD can start naturally itself. Wastewater carries natural microorganisms which are capable of anaerobically converting the waste into biogas. However, these small amounts of naturally occurring microbes would take a considerable length of time to start the system. The high organic content in AD systems would result in an unacceptably long start-up period or unstable process. However, this method might

32

2 Anaerobic Digestion and Biogas Production

be applicable for animal manure. Normal AD requires the addition of inoculum starter or active seed sludge to provide sufficient degrading microbes. The seed sludge should occupy at least 10% of the reactor volume. A wide rage of inoculum starter, such as animal manure (cattle, chicken, pig, etc.) or sludge from a wastewater treatment unit, can be used for start-up. Steady-state operation happens when the inoculum starter has reached a concentration that the microbes have adapted to the digester. The initial organic loading rate (OLR) typical starts at low levels and increases until normal operation conditions are reached. A short start-up time is desirable to save costs and loss of organic material. Acclimated seed culture should be used to inoculate the AD system. These cultures reduce the start-up time and improve system performance. For example, cattle manure is a suitable inoculum for energy crops or high fiber substrates. The digester start-up can take several days to several months depending on amount and type of seed inoculum. In typical mesophilic AD, the startup time ranges from 1–3 months; whereas, it can take 3–6 months for thermophilic AD due to high degradation rate of organic materials [1]. It is also critical to maintain the correct inoculum-to-feed ratio. Large volumes of inoculum are costly. Low inoculum-to-feed ratios may lead to acidogens dominating over methanogens creating volatile acid accumulation, lower pH and eventually process instability. During start-up it is necessary to control the pH and alkalinity in the system.

2.5.2 Substrates AD is typically used in Thailand for waste treatment of animal farms (pig, chicken and cattle), tapioca starch effluent, palm oil and ethanol plants. These substrates have high biodegradability, see Table 2.5. However, biomass, a complex organic compound structure consisting of lignin, cellulose and hemicellulose, is not easily biodegradable. This material requires pretreatment step to break down the complex structure and enable enzyme accessibility for digestion. Biomass pretreatment techniques are discussed in Chap. 4. The qualitative relationship between biodegradation rate and retention time required for digestion is presented in Fig. 2.6.

2.5.3 Nutrients Nutrients are necessary for all biological processes. All cells contain macronutrients (such as carbon, nitrogen and phosphorus) and micronutrients (such as sulfur, potassium, calcium and magnesium) with small amounts of trace elements (such as iron, nickel, cobalt, cadmium, zinc, manganese, molybdenum and copper) for maintaining microbial growth.

2.5 Important Factors in Anaerobic Digestion

33

Table 2.5 Wastewater substrate characteristics Substrate COD (mg/L) Animal waste

2000–10,000

Tapioca starch wastewater

8000–30,000

Palm oil mill effluent

20,000–60,000

Ethanol distillery effluent

> 100,000

Fig. 2.6 Biodegradation rates and retention times of various organic compounds

Comments Very balanced, high alkalinity and contains necessary inoculum Rich in carbohydrates, low pH, lack of nutrients Contains lipids, low pH, high temperature, balanced in nutrients Low pH, lack of nutrients

Biodegradation rate

Sugar (mono/disaccharides)

Carbohydrate Protein Hemicellulose Lignin Wax Retention time

The carbon to nitrogen ratio (C/N ratio) of the organic material is another important parameter. The C/N ratio of organic substances suitable for biogas production should be about 20–30, with 25 being the optimal. If the carbon–nitrogen ratio is too high, the biological reaction is limited due to nitrogen depletion. On the other hand, a low C/N ratio leads to ammonia accumulation in the system which could be toxic for microorganisms. Codigestion of waste is normally done to balance the C/N ratio of the digester. For example, tapioca starch wastewater which has a high C/N ratio is mixed frequently with animal manure which has a low C/N ratio. Other than the C/N ratio of the feedstock, the COD:N:P ratio is another parameter for anaerobic treatment. For a highly loaded anaerobic digestion system (0.8–1.2 kg COD/kg VSS/day), the minimal COD:N:P ratio of 350:7:1 is required, while the recommended ratio for a lightly loaded (less than 0.5 kg COD/kg VSS/day) is 1000:7:1 [16]. Macronutrients and micronutrients are often found in the feedstock, but over the long run trace elements should be added to stabilize and improve performance. As an aside, animal manure is rich in nutrients and trace elements and thus there is no risk of trace element depletion. Table 2.6 summarizes trace elements necessary for AD process.

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2 Anaerobic Digestion and Biogas Production

Table 2.6 Important trace elements in anaerobic digestion process Processes

Enzyme/Cofactor

Trace elements

Reactions

References

Acetogenesis

Formate dehydrogenase

Fe, Se, W

HCOOH ↔ CO2 + 2e− + 2H+

Ljungdhal [17]

Acetogenesis

Carbon monoxide dehydrogenase

Fe, Ni, Zn

CO + H2 O ↔ CO2 + 2e− + 2H+

Ljungdhal [17]

Acetogenesis and methanogenesis

Hydrogenase

Fe and/or Ni

H2 ↔ 2e− + 2H+

Shima et al. [18]

Methanogenesis

Carbonic anhydrase

Zn

CO2 + H2 O ↔ + HCO− 3 + 2H

Glass and Orphan [19]

Methanogenesis

Formylmethanofuran dehydrogenase

Fe, Mo/W

CO2 + MF → CHO− MF + H2 O

Hochheimer et al. [20]

Methanogenesis

Methyltransferase/ factor III

Co

MeOH + CoM → CH− 3 CoM + H2 O

Ferry [21]

Methanogenesis

Methyl-CoM reductase/F4 30

Ni

CH− 3 CoM + [H] → CH4 + CoM

Ferry [21]

2.5.4 Toxic Substances Toxic or inhibitory compounds can come from the feedstock itself or generate as intermediaries during the digestion process. The accumulation of these substances can cause instability, low biogas yield and eventual process failure. Toxic compounds, such as high concentrations of heavy metals, disinfectants, antibiotics, herbicides, pesticides and solvents, also inhibit digestion. Although metals as trace elements are necessary for microbial metabolism, a high concentration is toxic. Ranking trace element toxicity to acid forming bacteria would be as follows: Cu > Zn > Cr > Cd > Ni > Pb, whereas the trace element toxicity to methanogens is: Cd > Cu > Cr > Zn > Pb > Ni [22, 23]. Typical feedstocks for AD contain small amount of these toxic compounds. Antibiotics and disinfectants could be an issue for AD treatment of animal waste. Nitrogen in the form of ammonia (NH3 ) is another concern in digestion. Feedstocks with high N content or proteins produce free ammonia via the ammonification reaction. Ammonia nitrogen in aqueous solution can be either, ionized ammonia (NH+ 4 ) and unionized ammonia (NH3 ), depending on pH and temperature. The higher the pH and temperature, the higher amount of NH3 . The unionized ammonia (NH3 ) is more toxic compared to NH+ 4 . At concentrations of 80 and 1500 mg/L of NH3 and NH+ , respectively, inhibition of microbial activity in biogas production begins 4 [24]. It is difficult to separately measure the ionized and unionized compounds so the total ammonia is normally what is measured. As a rule of thumb, the total ammonia nitrogen (NH3 − N) concentration should be limited to less than 3500 mg/L at a pH of 7.0. This inhibition could self-mitigate by microbial acclimatization. Hydrogen sulfide (H2 S) is another gaseous product produced in anaerobic digestion of sulfur-rich material or wastewater with high concentrations of sulfate. Large quantity of H2 S gas is corrosive; thus this biogas needs cleaning and upgrading,

2.5 Important Factors in Anaerobic Digestion

35

involving the removal of H2 S. Small quantities of H2 S can dissolve in aqueous solutions and form equilibrium with bisulfide (HS− ) and sulfide (S2− ) depending on pH. At acidic pH (< 7), the majority chemical species is H2 S (aq.). This can be toxic as only 200 mg/L can inhibit microorganisms especially methanogens [25]. In anaerobic treatment of sulfate-rich wastewater, sulfur toxicity is an issue. Therefore, it is recommended that the amount of sulfate in waste material should be less than 500 mg/L.

2.5.5 Oxygen Facultative anaerobic microorganisms, hydrolysis and acid forming bacteria, can tolerate some free oxygen in the system, but strictly anaerobic methanogens cannot. Free oxygen can enter the reactor from the feeding pump or excessive effluent recirculation. Methanogens are very sensitive to oxygen, and thus the O2 levels should be monitored. A direct oxygen probe can measure dissolved oxygen (DO) but is limited as there are other oxidizing agents which harm methanogens. A more general parameter is the oxidation-reduction potential (ORP) which measures the net values of oxidation-reduction reactions in an aqueous environment. Inside a typical digester the ORP should be kept below −200 mV. However, ORP should be between −330 to −300 mV [24, 26] for optimized methanogenesis. Other electron acceptors such as sulfate, nitrate and nitrite should be minimized for efficient methane production.

2.5.6 Temperature Temperature affects growth rate, nutritional requirements, cell permeability, enzyme activity and metabolism in biological processes. The key mechanism affected by temperature is the structure and the composition of cytoplasmic membranes which determines the rate of substrate utilization. Normally microbes can naturally adapt to temperature changes. However, methanogens, the most vulnerable group of microbe, are unable to do so. Methanogens can be classified to psychrophiles (< 15 ◦ C), mesophiles (∼ 20–50 ◦ C) and thermophiles (∼ 50–65 ◦ C) (see Fig. 2.7). The optimal temperatures for biogas production are 5–15 ◦ C, 35–45 ◦ C and 55–65 ◦ C for psychrophilic, mesophilic and thermophilic methanogens, respectively [8]. In general, thermophilic digestion allows higher loadings with reduced hydraulic retention times, higher conversion efficiencies and pathogen disinfection. Mesophilic digestion requires less process heat and is more stable as there are more mesophilic bacteria species than thermophilics. In Southeast Asia, winters are short and mild, so typical AD operates without temperature control in the mesophilic condition. Small temperature drops during winter lower biogas yields but operators find this tradeoff financially acceptable.

36 100

Thermophiles Growth rate methanogens (%)

Fig. 2.7 Growth rate of methanogens in different temperature ranges

2 Anaerobic Digestion and Biogas Production

80 60 Mesophiles 40 Psychrophiles 20 0 0

20

40

60

Temperature (°C)

2.5.7 pH and Alkalinity The pH value is a critical factor. There are different pH ranges for different groups of microbes that work together during anaerobic digestion. The ranges are as follows: 5.5–6.5 for hydrolysis bacteria and acidogens and 7.8–8.2 for acetogens and methanogens. Maintaining the system pH level between 6.8 and 7.4 is crucial in order to maximize activities of all microbial groups. The methanogenic bacteria are the most susceptible microbe to pH change. In systems with large quantities of nitrogen, a high level of free ammonia (NH3 ) forms at a pH above 8.0 resulting in a significant drop in methanogen activity. Low pH also adversely affects methanogenic activity. A drop in pH can come from VFA accumulation (as discussed in Sect. 2.3.3), which also drastically reduces methane yield. This could happen with high organic loading rates (OLR) or if readily digestible organic material is used the VFA utilization rate cannot compete with the VFA production rate. Reducing the OLR would help lower the VFA production rate and allow time for VFA consumption. When both VFA production and consumption rates are balanced the pH restores itself to neutral. Then methanogens start actively working again. Alkalinity (ALK) is the chemical parameter that indicates the system buffering capacity. When VFA are produced, the pH remains unchanged in the system with sufficient alkalinity (Eq. 2.22). Alkaline chemicals, such as sodium bicarbonate and sodium carbonate, are commonly used to adjust the pH to neutral. Typical alkalinity values are 1000–5000 mg/L as CaCO3 with the optimal ranging between 2000 and 3000 mg/L as CaCO3 [27]. The ratio of VFA/ALK is critical for operational stability and should be kept below 0.4. − H2 CO− 3 + HAc ↔ H2 O + CO2 + Ac

(2.22)

Other reactor design parameters, such as organic loading rate, retention time and mixing, often govern the biogas yield and thus will be discussed in Chap. 5.

References

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References 1. Li Y, Khanal SK (2017) Bioenergy: principles and applications. Wiley, New York 2. Spellman FR (2008) Handbook of water and wastewater treatment plant operations, 2nd edn. CRC Press, Boca Raton 3. American Public Health Association (1975) Standard methods for the examination of water and wastewater. Technical report 4. Ma J (2017) Determination of chemical oxygen demand in aqueous samples with nonelectrochemical methods. Trends Environ Anal Chem 14:37–43. https://doi.org/10.1016/j.teac. 2017.05.002. https://www.sciencedirect.com/science/article/pii/S221415881730034X. ISSN 2214-1588 5. Pirsaheba M, Ghayebzadeh M, Moradia M, Gharagozloua F, Sharafia K (2015) Ratio variations of soluble to total organic matters at different units of a fullscale wastewater integrated stabilization pond. J Chem Pharma Res 5(7):1326–1332 6. Kewu P, Gong W (2008) Biodegradability enhancement of municipal landfill leachate. Water Sci Eng 1(4):89–98 7. Nayono SE (2010) Anaerobic digestion of organic solid waste for energy production. KIT Scientific Publishing 8. Khanal SK (2008) Anaerobic biotechnology for bioenergy production: principles and applications. Wiley, New York 9. Ariesyady HD, Ito T, Okabe S (2007) Functional bacterial and archaeal community structures of major trophic groups in a full-scale anaerobic sludge digester. Water Res 41:1554–1568 10. Borja R, Rincon B (2017) Biogas production. In: Reference module in life sciences. Elsevier, Amsterdam. https://doi.org/10.1016/B978-0-12-809633-8.09105-6. https://www. sciencedirect.com/science/article/pii/B9780128096338091056. ISBN 978-0-12-809633-8 11. Archer DB, Kirsop BH (1990) The microbiology and control of anaerobic digestion. In: Anaerobic digestion: a waste treatment technology, pp 43–89 12. Mata-Alvarez J (2003) Fundamentals of the anaerobic digestion process. In: Mata-Alvarez J (ed) Biomethanization of organic fraction of municipal solid waste. IWA Publishing, Cornwall, pp 1–20 13. Conklin A, David Stensel H, Ferguson J (2006) Growth kinetics and competition between methanosarcina and methanosaeta in mesophilic anaerobic digestion. Water Environ Res Res Publ Water Environ Fed 78:486–96 14. Boyle WC (1977) Energy recovery from sanitary landfills—a review. In: Microbial energy conversion. Pergamon Press, Oxford 15. Bushwell AM, Mueller HF (1952) Mechanisms of methane fermentation. Ind Eng Chem 44:550 16. Hansen KH, Angelidaki I, Ahring BK (1998) Anaerobic digestion of swine manure: inhibition by ammonia. Water Res 32(1):5–12. https://doi.org/10.1016/S0043-1354(97)00201-7. https:// www.sciencedirect.com/science/article/pii/S0043135497002017. ISSN 0043-1354 17. Ljungdhal LG (1986) The autotrophic pathway of acetate synthesis in acetogenic bacteria. Annu Rev Microbiol 40(1):415–450. https://doi.org/10.1146/annurev.mi.40.100186.002215. PMID: 3096193 18. Shima S, Warkentin E, Thauer R, Ermler U (2002) Structure and function of enzymes involved in the methanogenic pathway utilizing carbon dioxide and molecular hydrogen. J Biosci Bioeng 93:519–530. https://doi.org/10.1016/S1389-1723(02)80232-8 19. Glass JB, Orphan VJ (2012) Trace metal requirements for microbial enzymes involved in the production and consumption of methane and nitrous oxide. Front Microbiol 3:61 20. Hochheimer A, Hedderich R, Thauer RK (1998) The formylmethanofuran dehydrogenase isoenzymes in methanobacterium wolfei and methanobacterium thermoautotrophicum: induction of the molybdenum isoenzyme by molybdate and constitutive synthesis of the tungsten isoenzyme. Arch Microbiol 170:389–393 21. Ferry JG (2010) The chemical biology of methanogenesis. Planet Space Sci 58(14):1775–1783. https://doi.org/10.1016/j.pss.2010.08.014. https://www.sciencedirect.com/science/article/pii/ S0032063310002527. ISSN 0032-0633

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22. Lin C-Y (1992) Effect of heavy metals on volatile fatty acid degradation in anaerobic digestion. Water Res 26(2):177–183. https://doi.org/10.1016/0043-1354(92)90217-R. https://www. sciencedirect.com/science/article/pii/004313549290217R. ISSN 0043-1354 23. Lin C-Y (1993) Effect of heavy metals on acidogenesis in anaerobic digestion. Water Res 27(1):147–152. https://doi.org/10.1016/0043-1354(93)90205-V. https://www.sciencedirect. com/science/article/pii/004313549390205V. ISSN 0043-1354 24. Deublein D, Steinhauser A (2008) Energy supply in the future—scenarios. In: Biogas from waste and renewable resources, chap 2. Wiley-VCH Verlag GmbH ‘I&’ Co, Weinheim, pp 7–26 25. McCarty PL (1964) Toxic materials and their control. In: Anaerobic waste treatment fundamentals. Public works 95, pp 91–94 26. Gerardi MH (2006) Nitrogen, phosphorus and sulfur bacteria. In: Wastewater bacteria. Wiley, New Jersey, USA 27. MetCalf and Eddy (2013) Wastewater engineering: treatment and resource recovery, 5th edn. McGraw-Hill Inc., New York

Chapter 3

Biogas Reactors

3.1 Pretreatment Units Wastewater has different characteristics according to its origins. Some are digested easily and produce a large specific volume of biogas. However, solids and/or chemicals may be present which are potentially harmful to the microbial activities and unfavorable for digestion. The wastewater may also be at high temperatures, causing damage to microbes. Industrial wastewater usually contains more impurities and contaminants than municipal wastewater. Therefore, such wastewater must be pretreated before entering the reactor. There are several methods to do the task; • Bar screen: Bar screens are used for the separation of solid wastes. The screens are often made from metal gratings. There are two types: coarse and fine screens, see Fig. 3.1. An automatic screen may be installed, instead of manually removing the solid wastes. A screen compactor is also used as shown in Fig. 3.2. • Pre-sedimentation basin: Wastewater containing high solid contents might be retained in an open basin with sufficient time to allow some solid sedimentation. The retention time can be hours to days depending on the solid concentration. Also, the wastewater temperature decreases as heat has time to dissipate. Natural or mechanical-assisted aeration can be employed to accelerate the cool down rate and to remove unwanted volatile chemicals from the wastewater. • Comminutor: A comminutor crushes large-size particles, e.g., chunks of feces, vegetables, meat, etc., into small pieces. It is indispensable for wastewater containing large organic solids, as they block pipes easily. • Grit Chamber : A grit chamber is usually located before a pump inlet to protect it from physical damage. The main function is to allow gravels and sands to settle as shown in Fig. 3.3. • Grease Trap: For wastewater containing high levels of fat and oil, they can be removed by a grease trap which allows fat and oil to float, where they can be scooped out.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_3

39

40

Fig. 3.1 Screening as a primary treatment method (Source In-house)

Fig. 3.2 Screen compactor as a primary treatment method (Source In-house)

Fig. 3.3 Grit chamber

3 Biogas Reactors

3.2 Biogas Reactors

41

There are other types of wastewater contamination. Industrial wastewater with acids, alkaline, heavy metals or toxic chemicals can deteriorate digester microbial activity potentially leading to total process failure. Sometimes toxic compounds are formed in the reactor, for example, excessive volatile fatty acids are present in wastewater with low alkalinity and ammonia in wastewater with high organic nitrogen. These will be addressed in Chap. 5.

3.2 Biogas Reactors There are numerous reactor types for biogas production. They are classified in several ways, such as the characteristics and concentration of the waste, the microbes used and operational temperature as shown in Table 3.1. Biogas reactors are mostly classified according to the types of microbes (mesophilic or thermophilic) in addition to their operational setting, whether they are mixed, suspended or attached to any medium. Another common way to distinguish reactors is to classify them into single or multi-stage reactors. In a single-stage fermentation process (Single-stage reactor), all biological reactions occur together simultaneously. On the other hand, multi-stage reactors separate the acidogenic process and the methanogenic process from each other. Multi-stage reactors are more stable for biogas production [1]. They can handle waste with high volatile organic acids [2] and can treat organic waste with high nitrogen content, such as chicken manure, cow manure, biological sediment and food waste. However, multi-stage systems are more expensive to construct and more complicated to operate and maintain. Substrates with low solids content can be pumped. This is know as operating in “wet condition”. A dry fermentation reactor is used when the solid concentration is so high that pumps are inoperable. Screw conveyors can be used to move substrates with solid concentrations above 20%. Reactors can be operated batch-wise or continuously depending on the feed availability.

Table 3.1 Classification of various biogas production systems Condition of tank microbes Suspended growth/attached growth Degradation process Operating temperature Waste concentration The amount and type of treated waste Waste feed

Single-phase/two-phase Mesophilic/thermophilic Wet system/dry system Single substrate/cosubstrates Batch/continuous

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3.3 Suspended Growth Reactors A variety of suspended growth reactors have been developed and used throughout Southeast Asia. The reactors are either made from reinforced contrete or fabricated metals. Some are just earth excavated lagoons. The suspended growth reactors are relatively simple to construct and operate since they do not have any media inside. However, their biodegration efficiency is lower than sludge blanket and attached growth types of reactors.

3.3.1 Fixed Dome Reactor Early biogas reactor designs were fixed dome reactors, which are a low rate anaerobic reactor. It is suitable for small-scale wastewater treatment. In Thailand, fixed dome reactors used to be popular for treating wastes from small pig farms (less than 500 pigs). The reactor is usually built with concrete and half-buried underground. The structure of this fixed dome reactor is divided into 3 main parts, as can be seen in Fig. 3.4. 1. A Mixing Chamber or A Mixing Pit: It is used for mixing the organic substrates and water before feeding into the digester chamber. 2. A Digester Chamber: This is where the organic substrates ferment. The chamber needs to be strong and leak-proof. The biogas accumulates at the top of the chamber, pressurizing the chamber and moving the digester effluent into the overflow tank. The height of effluent in the overflow tank governs the biogas pressure. 3. An Overflow Tank and a Sand Bed Filter: The overflow tank is a reservoir for the digester effluent. It is connected to a sand bed filter where the excess sludge can be dried by sunlight. The dry sludge can be used as a fertilizer for soil improvement. This kind of biogas reactor is operated by a self-balance mechanism. When the biogas pressure increases, the digester liquid is driven into the overflow tank. When the biogas is extracted the slurry in the overflow tank flows back. Due to its simple design, this type of reactor can last for more than 10 years. Concrete fixed dome reactors used to be common in Southeast Asia during the 90s but were replaced with pre-fabricated fiber composite designs.

3.3.2 Plug Flow Reactors A plug flow reactor is a long narrow tank made of reinforced concrete, steel or fiberglass. It has a plastic gas tight cover to capture the biogas. Biogas pressures

3.3 Suspended Growth Reactors

43

Fig. 3.4 Fixed dome biogas reactors (Source With permission from [3])

Fig. 3.5 Plug flow reactor (Source In-house)

are lower than inside a fixed dome. There are cheaper to construct and can be built to suit any substrate volume. There is no internal agitation, and the liquid flows by hydraulic head difference. The substrate continually digests as it moves through this digester. After the retention time the fully digested substrate exits the reactor (Fig. 3.5). It is called a plug flow reactor because the liquid is not mixed longitudinally as it travels through. Each plug of liquid is pushed toward the outlet when additional substrate is added. At the outlet, there is a baffle, designed to maintain a gas tight pressure but allow the effluent to flow out. In practice, the heavier portion at the bottom of the digester flows more slowly than the lighter fluid on the top creating sedimentation layers inside the reactor. Two of the most common plug flow reactors are covered lagoons and channel digesters.

44

3 Biogas Reactors

Fig. 3.6 Covered lagoon reactor (Source In-house)

3.3.3 Covered Lagoon A covered lagoon is a passive system that captures biogas under an impermeable PVC or HDPE cover, see Fig. 3.6. These covered lagoons have a depth up to 6 m and a large surface area. In warmer climates, such as found in Southeast Asia, the digester temperature is not controlled. This allows the reactor temperature to be close to the ambient temperature. Therefore covered lagoons in the tropics can produce biogas all year round. On the other hand, in cooler-climates, heating equipment is needed to maintain microbe activity that produce biogas. In Europe and North America, biogas production drops considerably during the winter months. The lagoon is made by land excavation. The bottom can be either paved with concrete or made with compacted soil layers and plastic liners. Due to its slow degradation rate, the lagoon usually has long retention times of 30–45 days. The effluent can have a solid concentration as low as 0.5–2%. Most of the solids settle and accumulate at the bottom of the lagoon. The advantages of anaerobic lagoons are their relatively low construction and operational costs. The system is not sensitive to changes in wastewater characteristics because the lagoon volume is adequately large. However, traditional covered lagoons do not have a non-degradable solid removal chamber. This can lower the active volume over time, causing a decrease in biogas production, if the wastewater contains a high proportion of non-degradable solids. Nowadays, sludge removal is added to overcome this limitation. This system is called a modified covered lagoon. Sludge removal equipment is installed to extract the sediment. Also, pumps are used to mix the liquid and increase the degradation efficiency. See Fig. 3.7. The [4] published recommendations for building covered lagoons for animal farms in the United States. There is a relationship between the retention time and the rate of organic loading depending on the average temperature in the area. For a local temperature of 25 °C, a system with a retention time lower than 35 days, the maximum organic loading rate should be less than 0.15 kgVSS/m3 · day. In an area with a temperature of 4 °C, a hydraulic retention time less than 60 days should be built with an organic loading capacity of less than 0.05 kgVSS/m3 · day. Therefore a rule of thumb for Southeast Asia which normally has a temperature higher than 30 °C, the retention time for farm and food wastewater should be designed to be approximately 35 days. For high COD wastewater, such as ethanol or industrial

3.3 Suspended Growth Reactors

45

Fig. 3.7 Schematic of a modified covered lagoon (Source In-house)

starch or palm oil, it is preferable to design a covered lagoon reactor with a higher retention time than this. A proper analysis and design calculation should be done to determine the appropriate HRT. A covered lagoon system does not monitor the wastewater distribution. Therefore, these systems are often designed to be large in order to prevent over loading problems. The pond capacity is usually limited by the size of the available space. A covered lagoon system can be used for wastewater from animal farms, industrial plants or community wastewater. The wastewater should have a total solids (TS) concentration between 0.5–3.0%. Since there is no mixing, the wastewater moves slowly through this system. Therefore, it should limit its COD loading rate to approximately 1–2 kg/m3 · day. This results in covered lagoon systems requiring more physical space. The design characteristics of the covered lagoon are as follows: • The pond volume is calculated from the solid loading rate (kgVSS/m3 · day) and the COD loading rate (COD/m3 · day) or calculated from the hydraulic retention time • The length to width ratio should be greater than 4.0 • The slope of the wall in the horizontal and vertical direction depends on the soil characteristics in that area • Inlet and outlet water flow must be designed to prevent wastewater short circuits • The water level inside the pond is governed by the effluent pipe level and so is usually kept constant • Rainwater should not result in soil erosion or spills

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• To save pumping costs, anaerobic covered lagoons should kept be at a lower level than the source of waste • The lagoon should also be located close to where the final biogas will be used, if possible • To prevent the leakage of water from the pond into an underground water source the pond should have concrete or plastic sealed floors and walls.

3.3.4 Channel Digesters A channel digester (CD) is another plug flow reactor characterized by a unidirectional flow. A CD is more popular at smaller scales compared to covered lagoon. The flow is more controllable and the wastewater treatment more efficient. The organic matter decomposes rapidly throughout its retention time. Solid sediment gradually falls along the length of the CD resulting in a solid retention time greater than the hydraulic retention time. Uniform steady flow with consistent solids sedimentation results in good quality wastewater at the end. These digesters produce wastewater with a 70–80% decrease in COD. In general, CDs have walls and floors made from reinforced concrete. The top of the pond is covered with PVC or HDPE plastic, which stores the biogas, as shown in Fig. 3.8. Channel digesters are commonly used in animal farms. In order to prevent excessive accumulation of sediment, it must be removed before it reaches 1% of the overall volume. A sludge withdrawal system is employed which prevents dredging the system like a covered lagoon. The sediment is dried and used as fertilizer. The exit water

Fig. 3.8 Channel digester (Source In-house)

3.3 Suspended Growth Reactors

47

Fig. 3.9 Channel digester for treating pig farm wastewater (Source In-house)

is circulated to a wastewater collection pond. The effluent has low organic content but is high in microbes. Some of the water from the collection pond is re-fed back to the CD inlet. This increases the amount of microbes and mixes the bacteria with wastewater causing organic to decompose faster. An example of a channel digester for treating pig farm wastewater is shown in Fig. 3.9. Comparisons between the reactors discussed so far are shown in Table 3.2.

3.3.5 Continuous Stirred Tank Reactor (CSTR) Continuous stirred tank reactors (CSTR) are reactors in which microbes are stirred to maintain their suspension in the liquid. These anaerobic tanks are popular for treating industrial, energy crops, or agro-industrial wastewater. The fluid is mixed thoroughly. A variety of mixers can be used, e.g., propellers, paddles, impellers and screws. They can be installed vertically, horizontally or tilted at angles. This agitation aims to create a uniform concentration and promotes even contact between microorganisms and substrates. Another reactor with a similar working principle is the anaerobic contact (AC) tank reactor . This reactor has an extra sedimentation tank where the fluid is pumped back into the main reaction tank, allowing microbes a longer residence time. A continuous stirred tank reactor and a schematic of an anaerobic contact tank are shown in Fig. 3.10. In Southeast Asia CSTRs are mostly used for energy crops where mixing is essential. They typically do not have thermal control, unlike CSTR’s in Europe/US.

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Table 3.2 Reactor technology comparison Technology comparison Digester system Fixed dome/floating drum Optimum load capacity Hydraulic retention time Organic removal (%) System stability Maintenance cost Operational skills Environmental impact Capital cost per digester volume

(a)

Small Medium 60–80 Medium Low Medium Medium Medium

Covered lagoon

Channel digester

Small/large High 60–90 Medium Low Low Medium Low

Medium/large Medium/low 80–90 Medium/high Medium Medium Low Medium

(b)

Fig. 3.10 (a) Continuous Stirred Tank Reactor (Source Inhouse) (b) Anaerobic contact reaction tank (Source with permission from Tauseef etal. [15])

3.3.6 Anaerobic Sequence Batch Reactor (ASBR) In anaerobic sequence batch reactors (ASBR), both organic degradation and sedimentation take place in the same reaction tank. The processes are operated batch-wise. First, wastewater is fed into the reactor where it is stirred by mechanical mixers to promote biological degradation. After, the suspended solids are allowed to settle and the clear liquid is discharged from the upper part of the reactor. This discrete

3.3 Suspended Growth Reactors

49

Fig. 3.11 The four phases of an ASBR cycle

Fig. 3.12 Anaerobic baffled reactor (Source In-house)

operation makes it an ideal reaction tank for small treatment systems, especially when the wastewater availability is fluctuating or periodic. The operation of an ASBR reaction tank is shown in Fig. 3.11.

3.3.7 Anaerobic Baffled Reactor (ABR) This tank is designed to have a number of baffles to control the wastewater flow direction and promote good mixing. The vertical flow velocity is in a range of 0.2–0.4 m/h. The baffles, see Fig. 3.12, help reduce possible microbe washout. The microorganisms move slowly in the flow direction, causing them to agglomerate in their respective sections. The baffles thus help create the physical conditions which allow different types of microbes to dominate in each section. In the first section, acidogenic bacteria dominate whereas in the last section methanogenic bacteria take control. This behavior is similar to the two-phase anaerobic process which separates acidogenesis and methanogenesis. The position and size of the baffles depend on the wastewater properties. This system can handle highly concentrated wastewater; however, its disadvantage is its relatively large footprint.

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Fig. 3.13 Anaerobic migrating blanket reactor

3.3.8 Anaerobic Migrating Blanket Reactor (AMBR) This system is based on the concept of the anaerobic baffle reactor (ABR), except with a mixer in each section. The mixing regime can be better controlled, and the reactor can handle a surge of liquid flow and/or high concentration of solids. A schematic of an AMBR is shown in Fig. 3.13.

3.3.9 Upflow Anaerobic Sludge Blanket (UASB) In a sludge blanket reactor, wastewater is fed and forced through a sediment layer located at the tank bottom. Microbes are suspended throughout the tank volume. The sedimentation layer is formed by cohesion of flocculent sludge and granular sludge. As wastewater travels upwards, through the microbes, it is gradually cleaned. The clear water layer is separated from the sedimentation layer. Sludge reaction tanks can be subdivided into the following: A upflow anaerobic sludge blanket system is made from a concrete or metal tank. The influent feed channel is located at the bottom. The wastewater travels upwards passing through a microbial sediment layer which is called the “sludge blanket”. The upward flow combined with the settling action of gravity suspends the blanket, sometimes with the aid of flocculants. Small sludge granules begin to form whose surface area is covered in aggregations of bacteria. The blanket will reach maturity at around three months. The flow conditions create a selective environment in which only those microorganisms capable of attaching to each other survive and proliferate. Eventually the aggregates form into dense compact biofilms referred to as granules. The granules clump together as shown in Fig. 3.14, with diameters ranging from 1–5 mm and a settling velocity of approximately 20–60 m/h.

3.3 Suspended Growth Reactors

51

Fig. 3.14 Granular sludge inside a UASB and EGSB tank (with permission from [5])

The ability to carry the organic load depends on the characteristics of the sludge granules. If they are compact and heavy the UASB tank can handle a higher organic load. Effective sludge granules have a Methanogenic Activity (MA) greater than 0.35 kg COD/kg VSS.d. The specific methanogenic activity determines the methaneproducing capability of the sludge for a specific substrate at the concentration level where the availability of substrate is not a limiting factor. There are many different types of sludge granules or pellets found in UASB operation: • Sorcina granule are cluster forming microbes. They are small, with diameters smaller than 0.5 mm. They are easily washed out from the reaction tank. • Spiky granule are more than 1 mm in length with a thickness of less than 0.5 mm. • Filamentous granules have a rod shape. Most of them consist of methane-producing strands. • Rod granules have a spherical shape consisting of a short line of Methanothrix. These are a species of methanogenic archaea. Its cells are non-motile, non-spore forming and rod-shaped (0.8 × 2 µm) and are normally combined end to end in long filaments, surrounded by a sheath-like structure. They are often found in reaction tanks used in the treatment of starch wastewater from sugar factories.

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Fig. 3.15 UASB reaction tank (with permission from [7])

The granules concentrate at the bottom due to gravity. The solid concentration of this sludge ranges from 40–100 kgVSS/m3 [6]. The sludge granules are prevented from sinking to the bottom by the rising flow of wastewater. At the upper part of the UASB digester, there is a gas solid separator, which is an inclined-plate tilting at an angle of 45–60 ◦ C, installed on the upper part of the tank. This accumulates rising biogas and prevents bacteria sludge from exiting. The gas solid separator is sometimes called a 3-Phase Separator. A schematic of an UASB tank is shown in Fig. 3.15. This is a popular design in Southeast Asia, especially for wastewater with low solids content.

3.3.10 Expanded Granular Sludge Blanket (EGSB) A EGSB tank was developed from a UASB. The design, equipment and flow directions are similar. The difference is the increased feeding velocity which induces turbulence and mixing in the tank. Unlike a UASB tank, the EGSB has more even concentrations of sludge and substrates. EGSB tanks are capable of treating wastewater containing toxic substances or those with high organic concentration as the influent is quickly dispersed and diluted after entering the reactor. The schematic diagram of the EGSB tank is shown in Fig. 3.16, and a typical granule can be seen in Fig. 3.14b.

3.4 Attached Growth Reactors An attached growth reactor is based on an assumption that the efficiency of anaerobic digestion depends on the retention of microorganisms in the reactor. Thus, certain media can be placed in the reactor to allow microbes to attach and grow on the

3.4 Attached Growth Reactors

53

Fig. 3.16 Expanded granular sludge blanket reaction tank

media surface. The formation of biofilm helps reduce microbe washout and makes them resilient to changes in wastewater flow and characteristics. There is evidence showing higher efficiency of attached growth systems compared to others. They are best suited to wastewater with low solids content as clogging is a major problem for these systems. The media used can be glass bead, clay, sand, plastic, polyesters, polyurethane foam, rubber, wood, ceramic or other porous materials. The attached growth system can be divided into 2 groups: fixed bed media and mobile media bed.

3.4.1 Anaerobic Filter (AF) In an anaerobic filter, the media is packed into the anaerobic tank randomly. The arrangement and gap between the media depends on the packing design. Wastewater can enter either from the top or from the bottom; however, the bottom feed is more popular as it has less clogging. The wastewater contacts the microorganisms which are attached to the media. The media usually has a high surface area in the range of

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Fig. 3.17 Upflow anaerobic filter reactor

90–300 m2 /m3 [8]. Alternatively, the media can be arranged in an orderly manner, resulted in a more uniform gap and pattern of the media. This type of digester is called an Anaerobic Fixed Film, as shown in Fig. 3.17. The anaerobic fixed film digester usually has less clogging problems as the wastewater flow is more uniformly distributed. The anaerobic filter is often designed to have an upflow feed direction and is sometimes called an upflow packed-bed reactor. In an upflow packed-bed reactor, wastewater is fed from the bottom of the tank through the fixed media. This system is capable of treating high-strength, low solid wastewater and accommodating a high organic loading rate. However, the reactor requires high construction cost and is susceptible to clogging by suspended solids. A modified version of the upflow packedbed reactor is known as an upflow expanded-bed reactor. It has the same working principle as the upflow packed-bed reactor except that the effluent is recirculated back into the reaction tank. This re-circulation increases the velocity of wastewater inside the tank to around 2 m/h, resulting in the media bed volume expanding by 20% [9]. The upflow expanded-bed reactor is also more tolerant of clogging compared to the upflow packed-bed reactor.

3.4.2 Fluidized Bed Reactor In a fluidized bed reactor, media can move freely as they are suspended by the upward-moving liquid as shown in Fig. 3.18. The flow regime is similar to the upflow packed-bed reactor [10], except the media is suspended. This system is relatively stable and more resistant to shock loads or high flow fluctuations. The media must have a sufficient high surface area for the microbes to adhere to. The most popular media are gravel, anthracites, resins or plastics.

3.5 Reactor Design

55

Fig. 3.18 Fluidized bed reaction tank (Source In-house)

3.5 Reactor Design This section outlines concepts and design principles to help choose a suitable biogas system that satisfies both technical and economic requirements. A biogas system generally has three main stages: a primary treatment, a secondary treatment and a posttreatment. As the wastewater progresses through these stages, its quality improves. The ideal characteristics of a biogas production system can be summarized as: • Effective in removing organic matter to achieve the desired effluent quality • Capable of producing large amounts of biogas • In the biogas, having a high percentage of methane and a low percentage of contaminants such as H2 S and CO2 • Highly stable and resistant to various toxins and wastewater load fluctuation • Having fast resumption capability after a prolonged downtime period. This is especially necessary for factories that have to halt production or have seasonal production • Having reasonable capital, operating and maintenance costs • Simple to operate. To select an appropriate biogas production system, a number of considerations are as follows: • Wastewater flow rates and flow patterns • Organic content, usually COD concentration, of the wastewater and other characteristics such as temperature and suspended solids. • An effluent quality level and maximum concentration of specific contaminant, if any. This may depend on national and local regulations

56

3 Biogas Reactors

Table 3.3 Comparison of biogas systems commonly used in Thailand [11] Reaction tank

Organic loading rate

Construction Operating area difficulty

Maintenance Capital cost requirements and operating expenses

Industries where commonly used

CSTR

Moderate

Lowmoderate

Moderate

Moderate

Moderatehigh

Energy crop

UASB

High

Low

High

High

Moderatehigh

Tapioca starch, food processing

Covered lagoon

Lowmoderate

High

Low

Low

Lowmoderate

Livestock waste, POME, Ethanol

ABR

Lowmoderate

High

Moderate

Low

Lowmoderate

Food processing, Tapioca starch

CD

Lowmoderate

Moderatehigh

Moderate

Low

Lowmoderate

Livestock waste

AF

High

Low

Moderate

Moderate

Moderatehigh

Food processing, Tapioca starch

Note Organic loading rate: Low (< 2 kg COD/m3 /day); Medium (2 − 4 kg COD/m3 /day); High (> 4 kg COD/m3 /day) Area requirements (For a depth of 6 m): Low (< 42 m2 /ton COD-day); Medium (42 − 84 m2 /ton COD-day); High (> 84 m2 /ton COD-day)

• Available land area for the whole system including wastewater collection, storage, digester and post-treatment system • Labor skills and operational expertise • Sufficient retention time for the active microbes in the reaction tank • Consistent and effective contact between the microbes and organic matter • Effective control over the tank conditions • Biogas plant investment and operation budget as well as the economic return from biogas, organic fertilizer and wastewater recycling • Environmental commitment to maintain a good relationship with the local community. Odor control is important for this. In Southeast Asia suspended growth systems, especially UASB, are commonly used for industrial wastes, e.g., tapioca starch, vegetable oil, ethanol, liquor, food and beverage. Each reactor type has its own advantages and limitations, as shown in Table 3.3. Crude palm oil plants usually use continuous stirred tank reactors (CSTR) or modified covered lagoons (MCL) whereas slaughter houses use either CD or UASB reactors. Ethanol plants usually use modified covered lagoon (MCL).

3.5 Reactor Design

57

3.5.1 Design Principles To design an effective biogas production system, the following steps can be used as a preliminary guide: 1. Collect preliminary data on wastewater characteristics, flow patterns and quantity 2. Determine the land size available for the biogas reactor and its components. Soil testing is often required to identify underneath soil and rock formations 3. Explore a number of reactor types by considering their pros and cons 4. Estimate the capital cost, maintenance cost and personnel training and skill level required. At this stage, the reactor of choice is identified. 5. Prepare the wastewater treatment requirements. The requirements should be as specific as possible. Compile the design criteria around these requirements. 6. Design specifications should be based on operational data for similar reactors treating wastewater with similar characteristics 7. Design the reactor type and size from the desired specifications 8. Design auxiliary units, including wastewater pipes, post-treatment ponds, sludge treatment equipment 9. Design the civil works in adherence to local standards 10. Integrate all components and check subsystems for hydraulic profile, head loss, control system, etc. 11. Create the final drawing and related documentation for soil preparation, civil, mechanical, electrical works and pipework. A bill of quantities must be also made. Variables Flow HRT

Volume

The flow rate (Q) is the amount of wastewater entering the treatment system per unit time. Units commonly used include, m3 /day or m3 /h. The hydraulic retention time (HRT) is the amount of time that wastewater spends inside the reaction tank. The most popular units are days (d) or hours (h). The volume of the tank is V (m3 ), and: HRT =

SRT

V Q

(3.1)

The solid retention time (SRT) is the amount of time that sediment is inside the reaction tank. In the case of a continuous stirred reaction tank the sediment retention time is equal to the hydraulic retention time. For in other reaction tanks the SRT is: SRT =

Sludge volume in tank (kg) Sludge removal rate (kg/d)

(3.2)

58

3 Biogas Reactors

The sludge volume inside the tank is given by tank volume (m3 ) × sludge concentration (kg/m3 ). The sludge removal rate is the sludge discharged with wastewater (kg/d) + excess sludge drained from the tank (kg/d) OLR

The organic loading rate (OLR) is the amount of organic substance entering the reaction tank divided by the tank volume per unit time, in kg COD/m3 .d. OLR =

Q.COD 1000.V

(3.3)

where: Q Wastewater flow rate (m3 /d) COD COD in wastewater (mg/L) V Tank volume (m3 ) ULV The upflow liquid velocity (ULV) is the speed of wastewater inside the reaction tank in the vertical direction, in m/h or m/d. ULV =

Q A

(3.4)

where: A is the cross-section of the reaction tank (m2 ) F/M The food to microorganism ratio (F/M Ratio) is the ratio between the quantity of organic substances that enter the system per unit time and the amount of microbes that are inside the tank. The ratio is measured in g COD/g biomass.d. F/M =

COD.Q X.V

(3.5)

where X is the average concentration of microbes in the tank (mg/L)

3.5.2 Designing a Suspended Growth Reactor The design of a suspended growth biogas reactor is based on the same design principle as an aerobic activated sludge reactor. Both are operated similarly. The design process is as follows: • Select the appropriate solid retention time (SRT) to obtain the desired effluent quality and treatment efficiency • Determine the amount of sediment microbes that are sufficient for the SRT • Calculate the concentration of sediment needed and the volume of the reaction tank

3.5 Reactor Design

59

• Estimate the biogas production rate and the excess solids from the system • Determine the amount of nutrients and the alkalinity needed for the organic loading rate. The HRT and OLR criteria needed for three different types of reactors are shown in Table 3.4. The kinetic rate constants which determine the reaction rate are shown in Table 3.5.

3.5.2.1

Channel Digester Design

The volume of the CD must be calculated according to the digestion and loading rates, together with the COD input and output. The fermentation ponds are long and straight. The ratio of length to width is between 4.0–6.0. The operating depth should be greater than 3.5 m. The pond must be protected against clogging, corrosion and leakage. The outlet pipe location controls the water level. Recommended digester design values are shown in Table 3.6. Example 3.1 Example of a channel digester design Design a channel digester for agricultural waste, given the requirements in Table 3.6. Calculate: • • • •

The digester volume The tank dimensions The exit COD concentration Rate of methane production.

The daily COD loading rate is: Q × CODin = (135 m3 /day × 18 kgCOD/m3 ) = 2430 kgCOD/day Assuming a hydraulic retention time of 6 days, gives a required volume: 135 × 6 = 810 m3 Letting the width be w and the length be 5 w. The depth is 3.5 m. So the volume is given by: (5w)(w) × 3.5 = 810 m3 This gives a width: w = 6.8 ∼ 7 m and a length of L = 5w = 35 m

Table 3.4 The organic loading rate for the design of a microbial suspension treatment system Reaction tank Organic loading Hydraulic Solid retention References rate (OLR) retention time time (SRT) (d) kg COD/m3 .d (HRT) (d) CSTR Anaerobic contact ASBR

1.0–5.0 1.0–8.0

15–30 0.5–5

15–30 –

[12] [12]

2–10 gTS/L/d-1.2–2.4

1–5 10–50 0.25–0.50

8–25 14–51-

[12–14]

60

3 Biogas Reactors

Table 3.5 Kinetic constants used in the design of suspended microbial systems ([12], Wastewater Engineering: Treatment and Reuse, copyright McGraw Hill) Parameter Unit Recommended range Recommended value Specific substrate utilization rate, k Domestic wastewater gCOD/gVSS.d 2–10 Glucose gCOD/gVSS.d 25–35 Long-chain fatty acids gCOD/gVSS.d 3.85–6.67 Biogas yield, Y Fermentation gVSS/gCOD 0.06–0.12 Methanogenesis gVSS/gCOD 0.02–0.06 Combined gVSS/gCOD 0.05–0.10 Decay coefficient, kd Fermentation g/g.d 0.02–0.06 Methanogenesis g/g.d 0.01–0.04 Combined g/g.d 0.02–0.04 Max. specific growth rate, µmax 35 °C g/g.d 0.30–0.38 30 °C g/g.d 0.22–0.28 25 °C g/g.d 0.18–0.24 Half-saturation constant or monod constant, K s 35 °C mg/L 60–200 30 °C mg/L 300–500 25 °C mg/L 800–1100 Methane Production at 35 °C m3 /kg.COD 0.4 3 Density at 35 °C kg/m 0.635 Constant of gas % 60–70 Energy constant kJ/g 50.1

5 30 4.65 0.10 0.04 0.08 0.04 0.02 0.03 0.35 0.25 0.20 160 360 900 0.4 0.635 65 50.1

The volume with these dimensions is: Vol = 7 × 35 × 3.5 = 857.5 m3 7 Width to depth ratio: 3.5 = 2 which is less than 2.5. 1 Flow rate per cross-sectional area: QA = ( 135 )( 3.5×7 ) = 0.23 m/h 24 The system removes 95% of the COD in water. The COD of the water exiting is therefore: CODexit = 0.05 × 18 = 0.9 kg/m3 The concentration of the volatile solids entering the system: 18/1.2 = 15 kg/m3 This give a total daily solid flow rate of: Q × TSS = 135 × 15 = 2025 kgVS /day Biogas production rate: 0.312 × 2025 = 631.2 m3 /day Methane production rate (60% methane composition): 631.2 × 0.6 = 378.7 m3 / day

3.5 Reactor Design

61

Table 3.6 Recommended criteria for the design of channel digesters Parameters Design range Hydraulic retention time (days) Organic loading rate (kgCOD/m3 · day) Operating depth (m) Length to width ratio Width to depth ratio Solid retention time (days) Wastewater volume flow (Q) COD in wastewater Non-dissolved COD/TSS System efficiency Biogas production (m3 /kgVS) Methane composition

16–20 3.75–6.25 3.5 4.0–6.0 Less than 2.5 40–60 135 m3 /day 18 kg/m3 1.2 95% 0.312 60%

Source in-house

3.5.2.2

Continuous Stirred Tank Reactor Design

Continuous stirred tank reactor (CSTR), anaerobic contact reactor and anaerobic sequence batch reactor (ASBR) are three popular reactors for industrial wastewater treatment. Their design methods are described as follows: Biological wastewater treatment systems rely on microbial growth which can be mathematically modeled. However, the models only work well under certain temperature and wastewater characteristics. In the actual systems, the operating conditions are not always stable or constant. The Monod equation (Eq. 3.6) describes the influence of the environment on the behavior and composition of microbial communities in the system. It is named after Jacques Monod, who proposed using this equation to relate microbial growth rates in water to the concentration of a limiting substrate. It describes the behavior of bacteria in a wastewater treatment system as closely as possible to reality. It is only applicable for balanced growth, when a semi-steady state inside the reactor has been established. Therefore, the model is not applicable in the lag phase of cell growth. To use the Monod equation, the concept of an ideal agitation system is required (Fig. 3.19). • The reactor tank has a volume equal to V (L), and the water flow rates in and out are constant and equal to Q (L/d) • The hydraulic retention time (HRT) and the solid retention time (SRT) are both roughly equal. Where: S0 S1

Soluble substrate concentration at inlet (mg COD/L), usually expressed as BOD or COD Soluble substrate concentration at outlet (mg COD/L) usually expressed as BOD or COD

62

3 Biogas Reactors

Fig. 3.19 Continuous stirred tank reactor model

S X H RT Y µ µmax kd Ks

Soluble substrate concentration in tank (mg COD/L) usually expressed as BOD or COD, for a well mixed tank (S = S1 ) Average concentration of the tank bacterial microbes (mg VSS/L) Hydraulic retention time (d) Biomass yield factor (−) (amount of volatile suspended solids/amount of substrate, mg VSS/mg COD) Specific microbe growth rate (d−1 ) maximum value of µ(d−1 ) Decay rate of microbes (d−1 ) Half-saturation constant or the value of S (mg COD/L)) at µmax /2

The substrate concentration in this case is the limiting factor for growth. As the substrate concentration decreases the microbe concentration increases. The death of some microbes causes a decrease in volatile solid concentration, and this process is described by the endogenous decay coefficient, kd . The empirical Monod equation for the specific growth rate is: µ = µmax

S Ks + S

(3.6)

The value of µmax is found by increasing the concentration of substrate until a saturation point is reached. At half this value, µmax /2, the substrate value is noted. This value is known as the half saturation constant, K s , and can be visualized in Fig. 3.20. The growth rate per liter of biomass or microbes is then given by: dX = µX dt

(3.7)

3.5 Reactor Design

63

Fig. 3.20 Monod equation

Inside the reactor, the rate of decay per liter of microbes is: −kd X The hydraulic retention time is the volume of the tank divided by the inlet volume flow rate V (3.8) HRT = Q0 A mass balance on the microbes equates the inflow of microbes and the microbe growth rate with the rate of decay and the microbe concentration in the effluent flow: Q X 0 + V (µX − kd X ) = Q X 1

(3.9)

If it is assumed that the inflow of microbes is so small that the concentration is effectively zero, X 0 = 0, and that the microbe concentration in the effluent is the same as that in the tank, X 1 = X , the following equation is obtained: µ=

1 + kd HRT

(3.10)

A mass balance on the substrate equates the inflow of substrate to the outflow of substrate and the conversion of substrate to microbe growth inside the tank: 

µX Q S0 − V Y Rearranging to isolate µ µ=

 = QS

Y (S0 − S) HRT.X

(3.11)

(3.12)

Equating Eqs. 3.10 and 3.12 gives the concentration of microbes in the tank (X ), from: Y (S0 − S) X= (3.13) 1 + kd · HRT Starting from the Monod equation, Eq. 3.6 and using Eq. 3.10 gives:

64

3 Biogas Reactors

Table 3.7 Operating conditions for a continuous stirred reaction tank for various waste types (with permission from [15]) Waste type HRT (d) Loading rate Biogas Methane (%) (kgVS/m3 · d) production (m3 /kgVS) Piggery waste Distillery waste Primary sludge Cattle manure Petrochemical wastewater Palm oil mill effluent Olive mill wastewater Cassava ethanol wastewater Cheese whey wastewater Tequila vinasse

10–20 10 17 24 10

1.4–4.5 2.7 1.4 2.5 -2.8 3.3

0.26–0.55 0.55 0.37 0.20–0.25 –

60–71 65 68 60 –

18

2.6–3.5

54–70

19

5

28.3 Nm3 /m3 POME digested –

65–74

5

14

0.20–0.25

55–61

0.45

10.2

–

53.9

5

6

0.54

>60

1 µmax S = + kd Ks + S HRT

(3.14)

The substrate concentration in the tank (S) can be calculated from: S=

K s (1 + kd · HRT) HRT(µmax − kd ) − 1

(3.15)

Interestingly the effluent substrate concentration is not a function of influent concentration, (S0 ). In a continuous stirred tank reactor, the solid retention time (SRT) and the hydraulic retention time (HRT) are usually equal and between 15–30 days as shown in Table 3.4. The CSTR is suitable for wastewater with high solid concentration or high COD wastewater. However, it has some disadvantages, such as significant microbe washout and relatively high effluent solids fraction. The reactor design must allow sufficient retention time for continuous decomposition of organic matter. Guidelines for the design of a continuous stirred reaction tank for various wastes are shown in Table 3.7

3.5.2.3

Anaerobic Contact Reactor Design

An anaerobic contact reactor is designed to eliminate the limitations of a CSTR. Sludge discharged with the wastewater is separated by sedimentation and circulated

3.5 Reactor Design

65

Table 3.8 Operating conditions for an anaerobic contact reactor for various waste types (with permission from [15]) Waste type Waste strength SS (g/L) Loading rate Conversion (%) (g/L) (g/L.d) Meat packing waste Dairy waste Cannery waste Sugar-beet waste Beef processing wastewater Ice cream wastewater Cheese whey

1.5 (TVS)

0.8

1.4–3.6

92.0–98.0

3.0 (COD) 20.0 (COD) 4.7 (COD) 2.7 (COD)

– 69

– –

[21] [22]

3.5.3 Designing an Attached Growth Reactor The design of an attached microbial tank depends on the type of media used and the rate of expansion of the medium. There are many different types of attached growth reactor, and each operates slightly differently. Several of these reactors will be introduced in the succeeding sections.

3.5.3.1

Anaerobic Filter/ Upflow Packed-Bed Reactor

The anaerobic filter tank is used for both upflow and downflow wastewater. Although the operating principle of both is the same, in general, the latter is more popular in industrial wastewater treatment. Both cylindrical and rectangular tanks are used. The width/diameter of the tank is between 2–8 m, and the height of the tank is in the range of 3–13 m. The medium is contained within the entire tank height and occupies 50– 70% of the total tank volume. The specific cross-sectional area of the medium is approximately 100 m2 / m3 , with the design and operating conditions for continuous flow-through reaction tank as shown in Table 3.12.

3.5.3.2

Fluidized Bed Reactor

The working principle of a fluidized bed reactor is to fluidize the medium using fast moving upward wastewater flow. As the wastewater flows upwards, it contacts highly concentrated microorganisms growing on the medium surface. This allows a high rate of organic degradation. The fluidized bed uses a higher flow velocity (20 m/h), which is achieved by circulating the wastewater back into the tank. This requires energy intensive pumps. This system takes 3–6 months to start, which is a long period of time. The reactor parameters for treating various types of wastewater are shown in Table 3.13.

72

3 Biogas Reactors

Table 3.12 Operating conditions of a anaerobic filter reaction tank for various waste types [12, 15] Wastewater Packing type Temp. (°C) COD loading HRT (d) COD removal (kg/m3 · d) (%) Guar gum Chemical processing Domestic Landfill leachate Food canning Olive mill Palm oil mill Seafood processing Distillery

Pall rings Pall rings

37 37 15–25

7.7 12–15 0.2–0.7 1.5–2.5

1.2 0.9–1.3 0.5–0.75 25–37 2.0–3.0

Tubular Cross-flow

37 35

Cross-flow Polyurethane foam – –

90–96 89

30 –

4-6 6

1.8–2.5 2

90 65–70

– –

1.2–11.4 1.3

6–15 6.6

90 65

18

1.6

80

11.8

0.4

87

12.25 0.77

6 0.5

– 67

Non-woven 53 fabric material Cassava starch Bamboo piece 19 - 30 extraction Paper mill Rashing ring 55 Poultry PVC ring 29–35 slaughterhouse

61 80–90 50–70

The price of the medium is quite high, or else expensive control systems are required to manage the level of expansion of the media layer and the loss of sediment and microorganisms. In addition, this reactor is not suitable for treating wastewater with a high proportion of solids as it will inevitably lead to blockages.

3.5.3.3

Downflow Attached Growth Reactor

The downflow attached growth reaction tank has been applied to treat many kinds of wastewater, especially highly polluted wastewater. The medium inside the tank is made of various materials such as cinder block, random plastic or tubular plastic. The height of the medium layer in the tank is between 2–4 m. The organic loading rate in the range of 1–30 kgCOD/m3 · d. The operating parameters are shown in Table 3.14. The advantage of this type of tank is that it has less clogging problems than upflow designs. Clogging problems will be less common if the design distributes water thoroughly throughout the cross-sectional area. In many wastewater treatment systems, it is found that vertical plastic packing media used for trickling filters can also be used.

3.5 Reactor Design

73

Table 3.13 Parameters for fluidized bed reactors for various wastewater Waste type COD (mg/L) Loading rate COD removal (kgCOD/m3 · d) (%) Slaughter house wastewater Dairy wastewater Brewery wastewater Cane molasses stillage Citric acid wastewater Cassava wastewater Milk wastewater Molasses wastewater Glucose wastewater Pulp wastewater Palm oil mill effluent Sugar industry wastewater

References

430–850

2–5

60–85

@35 °C [23]

200–500 860–2500

1–3 2–6

40–80 40–60

@35 °C [23] @35 °C [23]

130,000

4.65–20

85

[24]

–

42

70

[12]

–

8.2

99

[12]

– –

3–5 12–30

71–85 50–95

[12] [12]

–

10

95

[12]

– 2500–10,000

3–18 10–40

60–80 78–90

[12] [25]

98

39.5

76

[26]

Table 3.14 Parameters for downflow attached growth reactors Waste type Loading rate COD removal (%) (kgCOD/m3 · d) Slaughterhouse waste Citrus wastewater Cheese whey wastewater Sludge heat-treatment liquor Brewery wastewater Molasses wastewater Piggery slurry

References

9.2 1–6 5–22

54–70 40–80 92–97

[27] [12] [12]

20–30

58

[12]

20 2–13 5–25

76 56–80 40–60

[12] [12] [12]

74

3 Biogas Reactors

Fig. 3.21 CMU-Hybrid digesters for treating palm oil mill effluent

3.6 Palm Oil Biogas Case Study A palm oil plant in Northeastern Thailand has a capacity of 45 tons of fresh fruit branches (FFB) per hour. A wastewater treatment system must be built to match the wastewater volume and to meet the regulatory standards for effluent. The biosystem is designed to be a series of modified cover lagoons, anaerobic ponds, facultative ponds and polish ponds as shown in Fig. 3.21. The POME wastewater usually contains a high level of oil and grease and is discharged at a temperature of 80–90 °C. Thus a grease trap and cooling ponds are necessary pretreatment processes. After that, the wastewater is fed in parallel into anaerobic digesters through distributor boxes. The digesters are “CMU-Hybrid” industrial models which are designed based on the concept of a modified covered lagoon with a number of pumps for internal recirculation and mixing. It was a design originally from Chiang Mai University. Thus its flow regime is similar to a CSTR.

3.6 Palm Oil Biogas Case Study

75

Measurements from the plant gave the primary wastewater data, which consists of a flow rate (Q) of 702 m3 /day, COD in of 76,000 mg/L, BOD of 38,000 mg/L and OLR of 2.5 kgCOD/m3 /d a biogas yield of 0.50 Nm3 biogas/kgCODremoved (This number is 86% of the theoretical methane yield, see Section 2.4 and confirmed by BMP lab tests). The biogas reactor volume (Vol) is calculated from Vol = A · h =

Vol = A · h =

COD · Q 1000 · OLR

76,000 mg/L · 702 m3 /d 1000 · 2.5 kgCOD/m3 /d

Vol = 21,340 m3 Three biogas reactors with a volume each of 7,339 m3 are employed as shown in 21,340 m3 Fig. 3.21 . The HRT ( 702 ) is 31 days which is sufficient compared to the value m3 /day in Table 3.4. Thus, the actual OLR is 2.42 kgCOD/m3 /d which is less than the design criteria of 2.5 kgCOD/m3 /d. The engineering drawings are illustrated in Figs. 3.22 and 3.23. Assuming a COD removal efficiency of 85%, Effluent COD = 0.85 · 76,000 = 11,400 mg/L

CODremoved =

(76,000 − 11,400) mg/L · 702 m3 /d = 45,349 kg/d 1000

Biogas Production = CODremoved · Yield = 45,349 · 0.50 = 22,674 Nm3 /day @60% methane content The biogas production of 22,674 Nm3 /day is equivalent to 32.3 Nm3 /m3 of wastewater, comparable to the value in Table 3.7. Before utilization, the biogas must be further cleaned by passing through pretreatment units. First, a bioscrubber biologically converts H2 S to elemental sulfur or sulfates as most biogas equipment such as boilers or internal combustion engines requires a H2 S level of 20 kHz). When ultrasound is applied in a liquid phase, it generates a series of compression and rarefaction waves. In the zone of rarefaction, the liquid is unduly stretched, generating large negative pressures. Microbubbles form and subsequently implode producing strong hydrodynamic shear forces [6]. The shear force facilitates the disintegration of the complex biomass cell wall structure. The resulting size reduction significantly improves the surface area for hydrolytic enzymes [7]. Ultrasound pretreatment efficiently improves biomass digestibility without additional heat treatment. Moreover, lignin is partly removed from the biomass cell wall structure [8]. Ultrasound-assisted approaches can reduce the pretreatment time and also reduce the chemical or enzyme requirements for a more environmentally friendly approach.

4.5 Chemical Pretreatment 4.5.1 Acid Pretreatment Hemicellulose solubilizes in acid because of its non-crystalline structure and branched functional groups. Acid pretreatment aims to solubilize hemicellulose, releasing fermentable sugars from the cell wall matrix. Depending on type of biomass and condition use, some lignin might be released as a result of disrupting bonds between lignin and hemicellulose. Cellulose and most lignin is unaffected remaining mostly in the solid phase (Fig. 4.8). Sulfuric, hydrochloric, nitric, peracetic or phosphoric acids are commonly used. Normally, acid pretreatment uses dilute acid (~0.2–2.5% w/w). Heat (130–260 °C) is commonly applied to improve process efficiency. A big advantage of acid pretreatment is its cost since acids are inexpensive. The type of acid, acid concentration, temperature and time affects pretreatment efficiency. Harsh conditions further degrade released sugars to form furfural and HMF, inhibitory compounds. Since the pretreatment is normally carried out above 160 °C, the energy

4.5 Chemical Pretreatment

89

Fig. 4.8 Effect of acid and alkaline pretreatment on biomass chemical compositions

input is high. Acids at high temperatures are corrosive and require expensive reaction vessels. Finally, downstream neutralization is required before AD.

4.5.2 Alkaline Pretreatment Alkaline pretreatment efficiently removes lignin from biomass (Fig. 4.8). Alkaline solutions disrupt the internal bond of xylan increasing the porosity. The main advantage is the efficient removal of lignin from the biomass [9]. Lime (calcium hydroxide), sodium hydroxide or ammonium hydroxide are most commonly employed. Diluted alkaline causes swelling of lignocellulose and partial lignin solubilization. The crystal structure of the cellulose is reduced. Post-processing needs a huge amount of water for washing out salts of calcium and sodium, which are difficult to remove. This pretreatment technology is economically unattractive due to the high costs of alkalines and intensive water use for biomass neutralization.

4.5.3 Oganosolv Pretreatment Oganosolv pretreatment uses organic solvents, including alcohols (methanol, ethanol, ethylene glycol, tetrahydrofurfuryl alcohol and butanol), organic acids (formic and acetic acids), acetone or other ketones, phenol or cresols, and dimethyl sulfide, to selectively extract and dissolve lignin (Fig. 4.9). When combined with an acid catalyst, almost all hemicellulose is hydrolyzed to sugar. Cellulose can also be partly depolymerized depending on the pretreatment condition. This leaves the cellulose fraction (solid fraction) ready for enzyme digestion. Solvents can be recovered and recycled.

90

4 Lignocellulosic Feedstock Pretreatment for Biogas Production

Fig. 4.9 Schematic diagram for organosolv pretreatment

4.6 Physicochemical Pretreatment Physicochemical processes use a combination of physical and chemical pretreatments. The use of heat in combination with chemicals to enhance reaction rates is common. This could also be called a thermochemical pretreatment. This pretreatment is typically more effective compared with single mechanisms. However complexity is increased because of the multiple processes.

4.6.1 Ammonia Fiber Explosion, AFEX This method uses liquid anhydrous ammonia at moderate temperatures (60–100 °C) and high pressures (1700–2000 kPa) for 5 min [10]. The pressure is then rapidly reduced. Breakup of the lignocellulosic structure is a common feature of AFEX. All biomass components remain solid with no lignin removal. This pretreatment is suitable for agri-residues and herbaceous crops (not for woody biomass and others with high lignin content). AFEX does not generate inhibitory compounds thus further cleaning is not required. After AFEX, feedstocks exhibit near complete enzymatic conversion of cellulose and hemicellulose to fermentable sugars. This pretreatment process is suitable for high solids loading. Although, nearly all of the ammonia can be recovered and reused, the process is complicated and costly to implement at commercial scale.

4.7 Biological Pretreatment

91

4.6.2 CO2 Explosion The supercritical CO2 -based pretreatment is similar to the steam explosion pretreatment. Steam explosion causes significant hemicellulose destruction and generates inhibitory compounds. Water wash can remove these inhibitory compounds, but it also results in significant loss of hemicellulose sugars. In the supercritical CO2 pretreatment, the more inert CO2 molecules do not produce these inhibitory compounds while causing the same physical biomass explosion. Carbon dioxide is a cheap and abundant gas. Its critical temperature is 31.0 °C, and critical pressure is 7380 kPa. As a supercritical fluid, CO2 is able to penetrate biomass’s micropores. Quickly releasing the pressure ruptures the pores and exposes the surface area of cellulose. The process takes between 30 and 60 mins, at pressures from 20 to 25MPa with temperatures ranging from 100 to 170 °C [11].

4.7 Biological Pretreatment Biological pretreatment uses microorganisms, both bacteria and fungi, to selectively digest a given lignocellulosic under mild conditions. The microbes secrete extracellular enzymes which helps to partly digest components in lignocellulosic materials [12–15]. Various factors account for biological pretreatment performance, particle size, material moisture content, pH, pretreatment time and temperature [16]. The most effective microorganism for biological pretreatment of lignocellulosic biomass is white-rot fungi which degrades lignin along with cellulose and hemicellulose (Fig. 4.10). Brown- and soft-rot fungi degrade cellulose and hemicellulose but leave lignin unchanged. Typically, during biological pretreatment, lignin and hemicellulose (the most susceptible part of lignocellulosic biomass) are mostly degraded. The resulting biomass is more porous and less crystalline, allowing access to cellulase and hemicellulase. The biological pretreatment has low energy consumption and produces no inhibitory compounds. Chaitanoo et al. [17] found that the filamentous fungus, trichoderma longibrachiatum, effectively digested the outer layer of farm-derived lignocellulosic bedding material (rice husk) resulting in a two fourfold increase in methane yield during AD (Fig. 4.11). The scanning electron microscope images of a pretreated sample showed a near-complete destruction of the external cell surface revealing a large fraction of the smooth thorn-like inner cellulose crystalline structure. This pretreatment could be an environmentally friendly and low-cost approach to enhance AD efficiency. However, the time required for this biological pretreatment is quite long with respect to chemical or thermochemical pretreatment processes [1, 18]. Moreover, sterilization might be required after, to limit microbial contamination. Consequently, the biological pretreatment process is not popular in commercially.

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Fig. 4.10 White-rot fungus grown on lignocellulosic biomass (Shutterstock No: 1905632008)

Fig. 4.11 Improvement of methane yield during anaerobic digestion by Trichoderma longibrachiatum (Source In-house)

4.8 Pretreatment Summary As described throughout this chapter the purpose for pretreating lignocellulose biomass is to destroy its lignocellulosic rigid structure and allow it to digest. This allows enzymes or microbes to access and digest material, especially during the hydrolysis process. Table 4.2 shows the strengths and weaknesses of each pretreatment process. Different processes affect the material composition differently; see Table 4.3. There is no clear winner. Therefore, given a specific biomass material, it is necessary to select the optimal pretreatment process. Ultimately, the result is cheaper biogas production costs.

4.8 Pretreatment Summary

93

Table 4.2 Advantages and disadvantages of lignocellulose pretreatment processes (with permission from [19]) Pretreatment process Strengths Weaknesses

AFEX

• Degradation of lignin and hemicellulose • Low power consumption • Cellulose size is reduced • Degrades lignin and hemicellulose • Reasonable investment cost • High yield of glucose • Efficient degradation

CO2 explosion

• Efficient degradation

Biological

Milling Steam explosion

Wet oxidation

Ozone reaction

Organosolv

Strong acid

Diluted acid

• Slow decomposition rate

• High energy use • Can produce toxic substances for the biogas reactor

• Not suitable for materials with high lignin content • High investment costs • Does not affect lignin and hemicellulose • High pressure is needed

• Reasonable investment cost • Produces less toxic substances • Effective in separating lignin • The catalyst is expensive • Produces less toxic substances • Low power consumption • Reduces the amount of lignin • High cost • Does not produce toxic substances • Effective lignin and • High investment cost hemicellulose degradation • Organic solvents requires careful management • High glucose yield • High costs and the process must be carefully controlled • Reaction occurs at room • Highly toxic substances temperatures and pressures • Reduces corrosion problems • A large variety of products compared to strong acids are produced • The output composition is of • Low sugar production low toxicity

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Table 4.3 The impact of pretreatment processes on chemical composition and structure of waste (with permission from [19]) Parameter

Mechanical grinding

Steam explosion

LHW

Acid

Alkaline

Oxidants

AFEX

CO2 explosion

Increase surface area for the reaction

High

High

High

High

High

High

High

High

Decrease cellulose crystallinity

High

–

No information

–

–

No information

High

–

Solubilization – of hemicellulose

High

High

High

Low

–

Medium

High

Lignin separation

-

Medium

Low

Medium

Medium

Medium

High

–

Toxic by products

–

High

Low

High

Low

Low

Low

–

Change in lignin structure

–

High

Medium

High

High

High

High

–

References 1. Sun Y, Cheng J (2002) Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour Technol 83(1):1–11 2. Ruiz Hector A, RodrÃguez-Jasso Rosa M, Fernandes Bruno D, Vicente Antonio A, Teixeira Jose A (2013) Hydrothermal processing, as an alternative for upgrading agriculture residues and marine biomass according to the biorefinery concept: a review. Renew Sustainab Energy Rev 21:35–51. https://doi.org/10.1016/j.rser.2012.11.069 3. Tomoko S, Kengo M, Shuji H, Satoshi O, Takafumi S, Kozo N (2009) Ozone pretreatment of lignocellulosic materials for ethanol production: improvement of enzymatic susceptibility of softwood. Holzforschung 63(5):537–543. https://doi.org/10.1515/HF.2009.091 4. Anette S, Anne T (1998) Optimization of wet oxidation pretreatment of wheat straw. Bioresour Technol 64(2):139–151. https://doi.org/10.1016/S0960-8524(97)00164-8 5. Hu Z, Wen Z (2008) Enhancing enzymatic digestibility of switchgrass by microwave-assisted alkali pretreatment. Biochem Eng J 38:369–78 6. Kuttruff H (1991) Ultrasonics fundamentals and applications. Elsevier Science Publishers Ltd., Essex, England 7. Subhedar PB, Gogate PR (2016) Biomass fractionation technologies for a lignocellulosic feedstock based biorefinery. Chapter 6—Use of ultrasound for pretreatment of biomass and subsequent hydrolysis and fermentation. Elsevier, pp 127–149. https://doi.org/10.1016/B978-0-12802323-5.00006-2 8. Bussemaker MJ, Zhang D (2013) Effect of ultrasound on lignocellulosic biomass pretreatment for biorefinery and biofuel applications. Ind Eng Chem Res 52:3563–80 9. Venkatesh C, Pradeep V (2013) An overview of key pretreatment processes employed for bioconversion of lignocellulosic biomass into biofuels and value added products. Biotech 3:415– 431. https://doi.org/10.1007/s13205-013-0167-8 10. Teymouri F, Laureano-Perez E, Alizadeh H, Dale BE (2005) Optimization of the ammonia fiber explosion (AFEX) treatment parameters for enzymatic hydrolysis of Corn Stover. Bioresour Technol 96(18):2014–2018. https://doi.org/10.1016/j.biortech.2005.01.016

References

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11. Gu T, Michael HA, Ahmed F (2013) Supercritical CO2 and ionic liquids for the pretreatment of lignocellulosic biomass in bioethanol production. Environ Technol 34(13–14):1735–1749. https://doi.org/10.1080/09593330.2013.809777 12. Xi-Yu C, Chun-Zhao L (2012) Fungal pretreatment enhances hydrogen production via thermophilic fermentation of cornstalk. Appl Energy 91(1):1–6 13. Ge X, Matsumoto T, Keith L, Li Y (2015) Fungal pretreatment of Albizia chips for enhanced biogas production by solid-state anaerobic digestion. Energy ‘I&’ Fuels 29:200–204. https:// doi.org/10.1021/ef501922t 14. Azin M, Moravej R, Zare D (2007) Production of xylanase by trichoderma longibrachiatum on a mixture of wheat bran and wheat straw: optimization of culture condition by taguchi method. Enzyme and Microbial Technol 40:801–805. https://doi.org/10.1016/j.enzmictec.2006.06.013 15. Mehdi D, Heidi S, Wensheng Q (2009) Fungal bioconversion of lignocellulosic residues; opportunities ‘i’ perspectives. Int J Biol Sci 5:578–595 16. Jian S, Ratna S-S, Mari C, Noura H (2009) Effect of microbial pretreatment on enzymatic hydrolysis and fermentation of cotton stalks for ethanol production. Biomass Bioenergy 33(1):88–96. https://doi.org/10.1016/j.biombioe.2008.04.016 17. Chaitanoo N, Aggarangsi P, Nitayavardhana S (2021) Improvement of solid-state anaerobic digestion of broiler farm-derived waste via fungal pretreatment. Bioresour Technol 332:125146. https://doi.org/10.1016/j.biortech.2021.125146 18. Agbor VB, Cicek N, Sparling R, Berlin A, Levin DB (2011) Biomass pretreatment: fundamentals toward application. Biotechnol Adv 29:675–85 19. Alvira P, Pejo ET, Ballesteros M, Negro M (2010) Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: a review. Bioresource Technol 101(13):4851–4861. https://doi.org/10.1016/j.biortech.2009.11.093

Chapter 5

Biogas System Operation

5.1 Introduction After pretreatment, as discussed in the previous chapter, the next phase is to operate and control the digester. Automatic monitoring and control of the anaerobic digestion system does not happen in the majority of systems nowadays. Automatic control of the digester is desirable but difficult to implement. As sensors become cheaper and more robust, their use in biogas systems will increase. The long-term objective is to allow AD processes to stably operate at maximum capacity. Biogas production consists of four steps, each relying on different types of microorganisms. The accumulated intermediates of one process may inhibit other processes. Compounds such as VFAs, ammonia and hydrogen can act as inhibitors. It is necessary to control certain parameters so inhibitors do not overwhelm the digester. Microorganism growth across all steps needs to be promoted. A lack of control can lead to AD failure or low biogas yields, both of which can lead to plant closure. Restarting a failed digester is very time-consuming and difficult. This is especially an issue for wastewater treatment systems where the biogas is of secondary importance. In this case, failure of the treatment system can lead to factory closure.

5.2 Plant Commissioning The first step in any new digester is to commission the project. This is done through four steps. Commissioning a plant involves testing all mechanical and electrical equipment, ensuring operators are trained in operation and safety and beginning the biological start-up sequence. These steps are broken down as shown in Fig. 5.1. Each plant, depending on the reactor design, will have a slightly different commissioning process. This section is a general overview, applicable to any particular plant design.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_5

97

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5 Biogas System Operation

5.2.1 Mechanical and Electrical Completion (MEC) The first step for a new plant is to test all mechanical and electrical components before the feedstock is added. This step is called the Mechanical and Electrical Completion (MEC). The aim of commissioning is to verify the correct operation of the plant according to its design parameters. During the MEC Commissioning all equipment is functionally tested. However, the plant tuning and the final setting can be performed during the following biocommissioning phase. Step 1. Verify and test the operation of all mechanical pumps and valves, inspect the electrical connections and test the software sequences. The plant operators should be present, for training in all the basic procedures for the technical operation of the plant. The following tests may form a part of the overall test procedure. 1. Hydraulic sealing test—to test for leaks throughout the digester • Feed the digester with the necessary quantity of water • Verify any level reduction in water and examine connections for leaks. 2. Pressure test—to test the digester sealing • • • •

Pressurize the digester to the biogas production pressure Observe any pressure reduction Pressurize every pipe and connector Observe any pressure reduction. Reseal/fix as required.

3. Equipment mechanical test • Visual inspect each piece of equipment (mixer, pumps, valves, etc.) for vibration or excessive motion. 4. Control cabinet power • Supply electric power to the control cabinet • Inspect all breakers and alarm signals in control panel. 5. Equipment electrical test • Ensure wiring to all electrical equipment is safe • Electrical activation of pumps (automatic control or in manual mode) • Check electrical absorption. Step 2. Commissioning finishes when the components successfully pass their defined operating parameters. Once this happens the plant is ready for the biological load and start-up and for biocommissioning.

Fig. 5.1 Plant commissioning test procedure

5.2 Plant Commissioning

99

Table 5.1 Inoculum for anaerobic wastewater treatment systems Reaction tank Initial sludge from other Manure initial quantity digester(s) (% of tank volume) (kg/m3 ) Suspended growth reactors Attached growth reactors

5–10 20–30

5–10 10–20

5.2.2 Biological Start-Up Biological start-up is when the plant is ready for operation and the inoculum is pumped into the tank. The digester is fed with highly concentrated microbes (inoculum) and some wastewater, see Table 5.1. This can be either sludge from other plants and/or animal manure inoculum. Initially, the digester is gradually fed with wastewater at a low organic loading rate to create a condition in which the microbes can utilize the substrate. Due to the abundance of food, the bacteria grow rapidly using oxygen as they do so. The oxygen in the water decreases along with the oxygen in the tank. The digester transitions from aerobic to anaerobic conditions. After that, the organic loading rate is increased to reach the desired value. The initial microbe seeding is discussed next.

5.2.2.1

Initial Microbe Seeding

At the start-up of a biogas plant, addition of inoculum starter or active seed sludge derived from other anaerobic wastewater treatment systems is typically required. These must have similar characteristics and components to the current wastewater system. The microbes are conditioned quickly into the treatment system. In case that there is an existing anaerobic lagoon, the bottom sludge can be ideally used as the inoculum. If the sludge is unavailable it is possible to use animal manure to initially charge the system. The system may take longer to start compared to direct addition of seed sludge as the microbial community has to be adapted to the new environment. There needs to be sufficient quantity of microbes to decompose the initial organic matter. Typically the inoculum is fed for approximately 10% of the digester volume. The initial quantity of microbes used to charge the system is shown in Table 5.1. This table is a general guide only. In contrast, livestock waste already contains microbes and therefore there is no need for inoculum or start-up sludge. For other plants, at the beginning, the digester may require continuous microbial re-filling. Proper and adequate microbe management at the beginning reduces the time to get the system operational. Table 5.2 lists the microbes and their conditions of use from various biogas reactors.

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5 Biogas System Operation

Table 5.2 Microbes suitable for different substrates Hydrolytic microbes

Substrates

pH

Temperature (°C)

References

Clostridium thermocellum

Cellulose powder

7

55

[1]

Clostridium thermocellum, Clostridium cellulolyticum

Fermented corn trees and pig manure

7.9–8.4

37

[2]

Clostridium saccharolyticum, Clostridium acetobutylicum, Clostridium thermocellum, Clostridium straminisolvens, Clostridium chartatabidum

Pig manure

–

–

[3]

Ruminococcus obeum

Fermented corn (63%), green rye (35%), chicken manure (2%)

7.7

41

[4]

Ruminococcus albus

Fermented corn trees and pig manure

7.9–8.4

37

[2]

Acetivibrio sp.

Fermented corn (63%), green rye (35%), chicken manure (2%)

7.7

41

[5, 6]

Halocella cellulosilytica

Organic waste

–

50–55

[7]

Genus fibrobacter Uncultured fibrobacteres

Pig droppings

–

–

[3]

Bacteroides graminisolvens

Waste from cattle

–

Medium temperature range

[8]

Spirochaeta xylanolyticus

Seaweed

7.3

37

[9]

Thermotoga lettingae

Fermented corn (63%), green rye (35%), chicken manure (2%)

7.7

41

[4]

Petrotoga mobilis

Agricultural waste

–

55

[10]

5.2.2.2

Nutrients

At start-up normally nutrients are not added because they are present in sufficient quantities in substrate. It is important to test the substrate beforehand as each is unique. For example, energy crops and ethanol wastewater require the addition of nutrients. Another strategy is to use animal manure as inoculum. More information on nutrients can be found in Sect. 2.5.3. Although there is no requirement for trace element addition, the small amount of trace element could potentially increase the biogas yield. For example, in-house research shows that biogas from ethanol wastewater, when iron, nickel and zinc nutrients were added a 72% and 54% increase in methane yield were observed at OLR’s of 7.42 and 11.2 kg COD/m3 /day, respectively.

5.2 Plant Commissioning

101

5.2.3 Biocommissioning Procedure The biocommissioning phase consists of the following steps: 1. 2. 3. 4.

The digestate is fed into the digester. Usually with a small initial quantity Inoculum/manure feeding. The digester is fed with inoculum/manure The biogas is monitored for quality (% methane) and quantity The influent and effluent are monitored for: COD, pH, ALK and VFA. Depending on the data, the feed rate is adjusted accordingly 5. If biogas quality is not satisfactory, it can be flared.

The biocommissioning ends when: 1. The biogas achieves a certain quality or certain designated methane percentage 2. Biogas is produced for a 24 h continuous period 3. The pH of the effluent is between 6.8 and 7.2.

5.2.4 Performance Test The final phase of commissioning is the performance test (PT) which takes place over several months, depending on the HRT of the plant. The performance test is specific to each plant but in general this test ensures the plant operates as designed. The following are possible system designated targets: • During the length of time the PT is performed (e.g., 45 days) the biogas yield must be a certain quantity for a given amount of COD removed. This is conditional on the COD of incoming wastewater • The biogas must contain a certain percentage of methane. The COD removal efficiency must be maintained for a certain number of days, again conditional on the quantity of incoming COD. There are some basic considerations to ensure smooth performance testing. • During the digester filling and emptying procedures, attention should be paid to any pressure fluctuations. These can be a sign of blockages • At all times all operating equipment should be easily accessible • All sources of ignition should be avoided • The operator should be physically on site for all material transfer procedures. If a problem is suspected, always reduce feeding. For each new incoming truck of substrates, the operator should know the origin/quality of the substrates. Substrates origin and tonnage should be kept in the operation log. Operator must be careful not to introduce sand, grit, contaminants or over-feed with the substrate.

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5 Biogas System Operation

5.2.5 Early Treatment Issues Basic treatment systems differ based on the nature of wastewater supply. For example, wastewater from a starch factory will have starch particles grouping together. These starch agglomerates are difficult to disintegrate. If they enter the reaction tank, they will accumulate inside. They must first be precipitated. Therefore, the primary treatment for starch industries is usually sedimentation tanks to precipitate starch. In the seafood industry, the waste includes raw materials, such as fish bones and fish scales. These materials can accumulate inside the tank. Or they gather inside the pump, clogging it. In addition, the wastewater contains animal fats which must be removed before entering the anaerobic treatment system. Therefore, the primary treatment system must consist of fine sieves, grease traps and fat skimming methods. For efficient anaerobic system operation, any initial treatment process must adjust the wastewater to match with the needs of the treatment system. Initial treatment system failure results in contaminated wastewater entering the anaerobic system. An obvious problem due to a malfunctioning initial treatment system is the COD of the wastewater becoming higher. An increasing inlet COD must be investigated immediately. Components of the initial treatment system should be inspected, as follows: Inspection of waste sieve • Broken sieve • Sieve clogging. Inspection of the fat traps • • • •

Ensure the sediment is properly drained Check the sediment level in the tank Is the grease sweeper operating correctly Is the fat pumped out correctly.

First stage inspection of the precipitation tank system • Check the sediment level in the tank • Is the sediment draining adequately • Precipitation of suspensions in the wastewater.

5.3 Digester Operation and Control Once the digester is commissioned then it can begin steady-state operation. Several digester parameters should be monitored on a daily basis. The operator should know at all-time the state of the digester. An important concern for an operator should be the properties of the incoming substrates and the digester operating condition as discussed below.

5.3 Digester Operation and Control

103

Feedstock: Organic material/wastewater entering the biogas system should have an optimal carbon-to-nitrogen (C/N) ratio for efficient anaerobic digestion. The appropriate C/N ratio ranges between 20 and 30. Normal wastewater or agricultural crop waste contains a high C/N ratio; therefore codigestion with other high N material such as animal manure or addition of urea fertilizer is a general practice. The low C/N ratio ( 85% < 0.3 60–65 < 30 20–30

Acceptable range 25–40 6.6–7.4 2000 2000 70–85% 0.03–0.5 55–60 < 200 < 100

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5 Biogas System Operation

Table 5.4 Optimal OLRs for various feedstocks Feedstock

Type of digester

OLR

HRT (day)

Reference

Wastewater from palm oil industry

Covered lagoon (commercial scale)

< 2.0 kg COD/m3 day

> 25

In house research

Wastewater from pig farm

High suspension solids-up flow anaerobic sludge blanket (H-UASB) (commercial scale)

4.6 kg COD/m3 day

4

In house research

Municipals solid waste Dry fermentation (commercial scale)

2.0–2.4 kgVS/m3 day

30

In house research

Wastewater from cassava starch industry

CSTR (lab scale)

2.58 kg COD/m3 day

19

In house research

Broiler farm-derived waste

Semi-continuous solid state anaerobic digestion (lab scale)

6.1 kg VS/m3 day

30

[12]

Layer chicken manure

Channel digester (pilot scale)

2.2 kg COD/m3 day 1.3 kg VS/m3 day

10

In house research

Organic solid poultry slaughterhouse waste

CSTR (commercial scale)

2.5 kg VS/m3 day

25

[13]

Livestock manure

CSTR (commercial scale)

2.1 kg VS/m3 day

29 ± 7

[14]

5.3.1 Operational Problems In order to control biogas production from complex biochemical reactions [16], it is necessary to measure parameters capable of conveying the microbial operating conditions in the reactor. This allows for early detection of process disturbances. The fact that most biogas plants are manually operated because of a lack of onlinemeasurements and limited knowledge about the anaerobic digestion process makes it necessary to develop new optimization and control strategies. Problems that arise inside the reaction tank need to be quickly solved. It could be a problem in the anaerobic digester or from the pretreatment system. One of the most encountered operational problems is the process imbalance. This has three major causes: hydraulic overloading, organic overloading or the presence of inhibitory concentrations of toxic materials in the reactor such heavy metals or ammonia. Organic overloading results from a COD input exceeding the degradation capacity of the microorganisms. Under-loading the biogas plant enables stable operation, but this results in relatively low productivity and low economic returns. Hydraulic overloading occurs when excessive influent flows into the reactor in a short period of time. This not only lowers the organic degradation efficiency but also causes changes in the liquid flow pattern in the reactor and, in extreme cases, might induce undesirable bacteria washout. Hydraulic overloading can be prevented by placing an equalization tank or a buffer tank in front of the anaerobic digester. Organic overloading is found when the influent flow rate is too high and/or there is a surge of organic concentration in the influent. Organic overloading can lead to many consequences such as pH changes, accumulation of VFA, lower biogas yield

5.3 Digester Operation and Control

109

Table 5.5 Measurement and monitoring checklist for biogas systems Parameter

Units

Design criteria

Measurement frequency

Reason for measurement

Inlet flow rate

m3 /h

–

Hourly

Basic control of wastewater supply

pH

–

> 6.0

Daily

Adjust pH

COD

mg/L

–

Daily

Control the organic matter entering the system

Temperature

°C

< 40

Daily

Control temperature and biogas production rate

pH

–

6.8–7.2

Daily

Adjust pH

COD

mg/L

< 1000

Daily

Check system performance

Volatile fatty acids

mg/L

< 250

Daily

Control toxicity, monitor the operation of the system

Alkalinity

mg/L

1000–2000

Daily

Check the system buffer for stability

Solid suspension

mg/L

< 500

Monthly

Check the quantity of bacterial sludge

Influent

Liquid inside reactor

Sludge layer height

m

–

n/a

Measure sediment quantity

Sludge concentration

mg/L

< 0.4

Monthly

Measure sediment quantity

F/M ratio

–

0.4–0.8

Monthly

Suitability of nutrients supplied to the system

< 0.4

Daily

System suitability for bacterial function

−100 to −300 mV

Daily

Confirms anaerobic conditions

< 0.4

Daily

System suitability for bacterial function

VFA/Alk ORP

mV

Alk/COD Effluent pH

–

6.8–7.2

Daily

pH Suitability

COD

mg/L

< 1000

Daily

Monitor system performance

Volatile fatty acids

mg/L

< 250

Monthly

Monitor system performance

Alkalinity

mg/L

1000–2000

Monthly

Check the water buffer potential

Solid suspension

mg/L

< 500

Monthly

Sediment exiting the system

SV30

mg/L

< 10

Monthly

Sediment exiting the system

Overall system analysis OLR

kg COD/m3 day

2–10

Daily

Suitability of nutrients supplied to the system

Biogas production rate

m3 -biogas/kg COD eliminated

0.6

Hourly

Check system performance

Biogas composition

% CH4

50–70%

Hourly

Check biogas quality

110

5 Biogas System Operation

Fig. 5.2 Factors affecting production and stability of biogas system

or system failure. To prevent this, an equalization tank is necessary and/or a real-time monitoring system for flow rate, pH, VFA and COD should be employed. When there is a sign of overloading, measures such as decreasing the influent flow rate, temporary halt of feeding or addition of alkaline chemicals can be used to counter the problem. Lastly, some toxic chemicals, such as disinfectants, washing chemicals, acidic or alkaline chemicals, can be found in the feed stream. However, early detection of specific harmful chemical is difficult to achieve, so regular monitoring of the digester performance is recommended. In case that there is a known species of toxic material, chemical precipitation or oxidation might be employed to remove it from the feed. For suspended growth systems such as CSTR and UASB, bacteria washout is often found. Maintaining the correct amount of bacteria in the system is very important. If there is too much bacteria flowing out, less organic matter is degraded. A solution to this is maintaining the constant feed flow rate and/or the installation of a 3-phase separator. This equipment can keep the bacteria trapped in the system for a longer time and prevents bacterial washout. The bacterial volume can be measured by the amount of suspended solids in the reactor. The sludge should therefore be regularly collected and analyzed for the volatile solid content. It is also important to observe the sludge concentration distribution inside the tank. In a UASB the sludge concentration is higher at the tank bottom before decreasing and leveling off with height. At 0.5 m, the suspended solids should be around 45,000 mg/L, with it reducing to approximately 3000–5000 mg/L at a height of 3.5 m. If the sludge concentration is high throughout, the tank is overloaded and some sludge needs to be withdrawn. However, if the sludge has a low concentration throughout, it shows that the system has undergone a washout. If the sludge concentration is too low, the cause of sludge decrease needs to be found. Low concentration of sludge is often related to operational problems, including high organic loading rate, low pH or high water flow rate. The monitoring of microbial communities is not typically done for operational purposes since the monitoring technology is expensive and is only available off-line.

• Biogas yield decreases • COD removal plunges rapidly • VFA/ALK increases

• Producing too small quantity of biogas from the system

• Harms the living microbes and reduces the concentrations • Causes unsuitable conditions for microbe growth

• The growth rate of microorganisms is low

3. Toxic substances

4. Organic under-loading

(continued)

• Limits the OLR below 2 kg COD/m3 day by decreasing the influent flow rate • If VFA/ALK is high, adds alkalinity, e.g., caustic soda or lime, to maintain the pH value of the system • Recirculates the effluent from the post-treatment system to dilute the concentration in the reactor and to maintain the alkalinity within the system • Adds more inoculum or sludge from other biogas systems • Reduces/stops the influent feeding • Investigates the possible species of toxic substances in the wastewater • Consults an expert or designer for the solution • Increases the flow rate of influent • Recirculates the effluent back to the digester to maintain the concentration of organic matters in the system

• % CH4 decreases • % COD removal decreases • VFA/ALK increases • pH decreases

2. Organic overloading

• Reduces/stops the influent feeding • Controls the flow rate of wastewater so that it does not exceed the designed value • Adds more inoculum or sludge from other biogas systems

• % CH4 decreases • % COD removal decreases • Solids in the effluent increases • VFA/ALK increases

1. Hydraulic overloading

Correcting measures

Impact

Effects

• Microbes in the system are washed out causing lower solid retention time than the design value • The flow pattern and condition become unsuitable for microbial growth • The wastewater retention time becomes lower than the design value • There is an imbalance between organic matter and microorganisms in the system • Causes unsuitable conditions for microorganisms to grow

Problems

Table 5.6 Troubleshooting common problems found in a biogas system

5.3 Digester Operation and Control 111

8. Too low concentration of biomass (VSS too small)

• The balance between microorganisms and food is poor • Sludge production is too low

• COD removal efficiency is lower • The effluent SS might be high, indicating microbe washing out

• Excessive sludge needs to be withdrawn frequently • Increased costs of sludge management

• Low utilization of biogas • Low financial return

• Inadequate quantity of biogas

6. Low biogas production

7. Too high concentration of biomass • The balance between (VSS too large) microorganisms and food is poor • Sludge production is too high

• COD removal plunges rapidly • Rapidly declining pH • Biogas production decreases

• Causes unsuitable conditions for methane-producing microorganisms to grow

5. VFA/ALK > 0.4 or low pH < 6.8

Impact

Effects

Problems

Table 5.6 (continued) • Re-balances the VFA/ALK by adding caustic soda or lime • Recirculates the effluent from the post-treatment system to maintain the alkalinity within the system • Investigate possible causes of low biogas production • Investigates possible causes of low biogas production • Analyze important parameters, e.g., feed flow rate, COD, pH and VFA/ALK • Increases the sludge removal • Analyze important parameters, e.g., feed flow rate, COD, pH and VFA/ALK to determine causes of the problem • Decreases or stops the sludge removal • Add some sludge from other biogas facilities to increase biomass in the system • Checks the COD:N:P ratio of the influent • Check if there is any toxic substance in wastewater

Correcting measures

112 5 Biogas System Operation

References

113

The most common problems found in a biogas system are summarized in Table 5.6. The effects, impacts and correction measures for each problem are also provided. Any biogas system based on living microorganisms is susceptible to environmental changes. Therefore, the system operators need to promptly respond to problems which may arise. Often the operators cannot take action due to lack of responsive sensors to provide an early warning. Most biogas systems in Thailand livestock farms are not equipped to actively control the biogas production process. Optimization and control of such plants is a challenging problem due to the underlying highly nonlinear and complex digestion processes. Biogas systems of industrial wastewater, especially large industries, have begun to install online biogas sensors and controllers, allowing real-time monitoring.

References 1. Syutsubo K, Nagaya Y, Sakai S, Miya A (2005) Behavior of cellulose degrading bacteria in thermophilic anaerobic digestion process. Water Sci Technol 52:79–84. https://doi.org/10. 2166/wst.2005.0501 2. Wirth R, Kovacs E, Maroti G, Bagi Z, Rakhely G, Kovacs KL (2012) Characterization of a biogas-producing microbial community by short-read next generation DNA sequencing. Biotechnol Biofuels 5(41). https://doi.org/10.1186/1754-6834-5-41 3. Liu FH, Wang SB, Zhang JS, Zhang J, Yan X, Zhou HK, Zhao GP, Zhou ZH (2009) The structure of the bacterial and archaeal community in a biogas digester as revealed by denaturing gradient gel electrophoresis and 16S rDNA sequencing analysis. J Appl Microbiol 106(3). https://doi. org/10.1111/j.1365-2672.2008.04064.x 4. Schluter A, Bekel T, Diaz NN, Dondrup M, Eichenlaub R, Gartemann K-H, Krahn I, Krause L, Kromeke H, Kruse O, Mussgnug JH, Neuweger H, Niehaus K, Puhler A, Runte KJ, Szczepanowski R, Tauch A, Tilker A, Viehover P, Goesmann A (2008) The metagenome of a biogas-producing microbial community of a production scale biogas plant fermenter analysed by the 454 pyrosequencing technology. J Biotechnol 136:77–90 5. Lutz K, Nelcy D, Rob E, Karl-Heinz G, Holger K, Heiko N, Alfred P, Kai R, Andreas S, Jens S, Rafael S, Andreas T, Alexander G (2008) Taxonomic composition and gene content of a methane producing microbial community isolated from a biogas reactor: genome research in the light of ultrafast sequencing technologies. J Biotechnol 136:91–101. https://doi.org/10. 1016/j.jbiotec.2008.06.003 6. Jaenicke S, Ander C, Bekel T, Bisdorf R, Droege M, Gartemann K-H, Junemann S, Kaiser O, Krause L, Tille F, Zakrzewski M, Puhler A, Schlueter A, Goesmann A (2011) Comparative and joint analysis of two metagenomic datasets from a biogas fermenter obtained by 454pyrosequencing. PLoS ONE 6(1). https://doi.org/10.1371/journal.pone.0014519 7. Goberna M, Insam H, Franke-Whittle IH (2009) Effect of biowaste sludge maturation on the diversity of thermophilic bacteria and archaea in an anaerobic reactor. Appl Environ Microbiol 75(8):2566–2572. https://doi.org/10.1128/AEM.02260-08 8. Nishiyama T, Ueki A, Kaku N, Watanabe K, Ueki K (2009) Bacteroides graminisolvens sp. nov., a xylanolytic anaerobe isolated from a methanogenic reactor treating cattle waste. Int J Syst Evol Microbiol 59(8). https://doi.org/10.1099/ijs.0.008268-0 9. Pope PB, Vivekanand V, Eijsink VGH, Horn SJ (2013) Microbial community structure in a biogas digester utilizing the marine energy crop Saccharina latissima. 3 Biotech 3:407–414. https://doi.org/10.1007/s13205-012-0097-x 10. Weiss A, Jerome V, Freitag R, Mayer HK (2008) Diversity of the resident microbiota in a thermophilic municipal biogas plant. Appl Microbiol Biotechnol 81:163–173. https://doi.org/ 10.1007/s00253-008-1717-6

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11. Vongvichiankul C, Deebao J, Khongnakorn W (2017) Relationship between pH, oxidation reduction potential (ORP) and biogas production in mesophilic screw anaerobic digester. Energy Procedia 138:877–882 12. Ninlawan C, Pruk A, Saoharit N (2021) Improvement of solid-state anaerobic digestion of broiler farm-derived waste via fungal pretreatment. Bioresour Technol 332:125146 13. Salminen E, Rintala J (2002) Anaerobic digestion of organic solid poultry slaughterhouse waste—a review. Bioresour Technol 83(1):13–26 (reviews issue). https:// doi.org/10.1016/S0960-8524(01)00199-7. https://www.sciencedirect.com/science/article/pii/ S0960852401001997. ISSN 0960-8524 14. Gooch C, Labatut R (2014) Evaluation of the continuously-mixed anaerobic digester system at synergy biogas, May 2014 15. Alkaya E, Demirer GN (2011) Anaerobic acidification of sugar-beet processing wastes: effect of operational parameters. Biomass Bioenergy 35:32–39 16. Nguyen D, Gadhamshetty V, Nitayavardhana S, Khanal SK (2015) Automatic process control in anaerobic digestion technology: a critical review. Bioresour Technol 193:513–522

Chapter 6

Processing Biogas Effluent

6.1 Biogas Effluent Treatment After the wastewater effluent exits the biogas system, it usually enters a post-treatment system in order to remove the remaining organic substances and nutrients. This is necessary as most countries have regulatory standards for wastewater effluent. The treated wastewater can be recirculated and reused in factories or animal farms for watering plants, cleaning agricultural equipment, washing pig pens or chicken cages, etc. Treated wastewater can be discharged into public waterways if its quality meets environmental standards. Biogas reactors eliminate organic and inorganic substances, measured in BOD and COD, from wastewater. However, nutrients such as nitrogen and phosphorus still remain in the effluent. These nutrients can negatively impact the environmental causing blue baby disease, eutrophication or algal bloom. It is necessary to remove them from wastewater before releasing into rivers or lakes. Therefore in Southeast Asia countries, a series of stabilization ponds are usually used to treat the remaining organic matter and nutrients in the digester effluent. Nitrogen compounds can be removed by many processes, ammonia stripping, assimilation by algae, nitrification–denitrification and sedimentation, [1]. At high pH, ammonium ions are shifted to the form of free ammonia (NH3 ) which is volatile and can be stripped off by natural aeration. At a pH of 9.0, approximately 50% of ammonia is in the form of NH3 . All most all ammonia species are NH3 at a pH higher than 11.0. In maturation ponds where photosynthesis take places, the ammonia removal efficiency can be as high as 90%. In contrast, the ammonia removal is only 30–50% in facultative and aerated ponds. Nitrogen removal by algae is also important as nitrogen constitutes 6–12% of dry weight algae mass, [2]. In the presence of oxygen, ammonia is eliminated by nitrification which transforms ammonia (NH+ 4) − ) and nitrate (NO ). This is carried out by two groups of bacteria— into nitrite (NO− 2 3 the first one is an ammonia-oxidizing bacteria (AOB), such as Nitrosomonas spp. and Nitrosospira spp., which oxidizes ammonia to nitrite, see Eq. 6.1. The second group is nitrite-oxidizing bacteria (NOB), such as Nitrobacterspp. and Nitrospira spp., which converts nitrite to nitrate, Eq. 6.2. For denitrification, a group of heterotrophic © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_6

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− bacteria can use both nitrate (NO− 3 ) and nitrite (NO2 ) to produce nitrogen, Eqs. 6.3 and 6.4. The nitrogen removal process is shown in the following equations:   3 + O2 → NO− + (6.1) NH+ 4 2 + 2H + H2 O 2

  1 O2 → NO− + 3 2

(6.2)

      1 1 2 − + CH3 OH → NO2 + CO2 + H2 O 3 3 3

(6.3)

NO− 2 NO− 3

NO− 2

        1 1 1 1 CH3 OH → N2 (gas) + CO2 + H2 O + OH− (6.4) + 2 2 2 2

Phosphorus is removed from the biogas effluent by a number of mechanisms including consumption by algae and bacteria as well as chemical precipitation. It is reported that phosphorus accounts for 1.0% of algae biomass, [1]. If a stabilization pond is employed to treat the wastewater containing phosphorous and there is 100 mg/L of algae in the effluent, it can be assumed that approximate 100 × 1% = 1 mg/L of phosphorus is assimilated by algae. If the wastewater initially contained 10 mg/L of phosphorous, the removal efficiency of the phosphorus is around 10%. Most of phosphorus compounds are removed by chemical precipitation especially when the pH is high. This process usually occurs in shallow ponds such as facultative and maturation ponds in which photosynthesis takes place during the day time and carbon dioxide is consumed by algae and autotrophic microorganisms. At a pH higher than 9.0, phosphates are precipitated to form hydroxyapatite or struvite, giving a removal efficiency of 60–80%, [3].

6.2 Stabilization Pond A stabilization pond is a biological wastewater treatment system which uses bacteria to eliminate organic matter and nutrients. Stabilization ponds can be categorized into three operational types: anaerobic pond, facultative pond and aerobic or maturation pond. Usually there are a series of ponds with the final ones acting as sedimentation ponds before releasing the final effluent into public water sources. The stabilization pond has low construction and maintenance costs. The operating principle is not complicated, but it requires a large land area. A common series of ponds are illustrated in Fig. 6.1 with the design criteria shown in Table 6.1.

6.2 Stabilization Pond

117

Fig. 6.1 Stabilization ponds for biogas digester effluent

6.2.1 Anaerobic Pond An anaerobic pond is usually used as the first unit of the post-treatment system. It is also used to treat wastewater with high organic matter, such as effluent from palm oil mills, dairy farms and slaughter houses. With high organic loading, the oxygen consumption rate in the pond is always higher than the oxygen absorption from the atmosphere, resulted in anaerobic conditions. The anaerobic biodegradation process, hydrolysis, acidogenesis, acetogenesis and methanation, resembles those in a biogas reactor. The only difference is that the effluent BOD concentration is much lower; therefore, the biogas production rate is small. However, many anaerobic ponds are covered with plastic sheets in order to capture methane and reduce odors. The sizing of an anaerobic pond depends on its organic matter degradation rate. Compared to aerobic degradation, anaerobic digestion is relatively slow and requires long retention times to complete the process. In Southeast Asian countries, the tropical climate favors anaerobic decomposition and increases the degradation rate. The pond should have a depth of >2.0 m in order to allow anaerobic microorganisms to grow. The pond sizing depends on data from experiments or from practical experience. The best data is usually obtained from existing stabilization ponds which treat similar wastewater. The design criteria are given in Table 6.1. These criteria take into account the three main factors, the organic loading rate, temperature and hydraulic retention time. The empirical equation for calculating the BOD removal efficiency of an anaerobic pond was developed from the treatment of municipal wastewater with anaerobic reaction tanks in tropical areas [4]: Se =

 n Kn

Se S0

S0 (HRT + 1)

where Se = BOD value of wastewater released (mg/L) S 0 =BOD value of incoming water (mg/L) Se = BOD removal efficiency S0 K n = 6, n = 4.8 H RT = hydraulic retention time (days)

(6.5)

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6.2.2 Facultative Pond A facultative pond is usually 1.5–3.0 m in depth and can be divided into three zones according to the depth, the upper layer (aerobic zone ), middle layer (facultative zone) and the bottom (anaerobic zone). Organic matter decomposition occurs in each of the three zones. The upper layer is exposed to sunlight and free oxygen where algae and aerobic bacteria degrade the organic matter. The algae play an important role in oxygen production which is used for respiration by aerobic microorganisms. Bacteria, on the other hand, consume oxygen and organic substances producing carbon dioxide which is necessary for algae growth. Thus, there is a balance between oxygen and carbon dioxide in this layer. The middle layer is a mixed zone where organic decomposition occurs by facultative bacteria. These work in both anaerobic and aerobic conditions. As oxygen comes from photosynthesis by algae, different water depths can have different oxygen conditions. During the day time when the light intensity is high, oxygen is supplied by algae and dominates the layer. However, at night, oxygen is depleted and the layer can turn anoxic or anaerobic. In the bottom zone, heavy and inert materials accumulate, sunlight is limited and atmospheric oxygen cannot diffuse, anaerobic microorganisms dominate the microbial community. These produce methane, carbon dioxide and hydrogen sulfide as degradation products. The H2 S produced can be oxidized in the upper aerobic layer, so the odor problem is not too serious. All of these degrading mechanisms are natural and require no equipment. As it takes a long time for organic matter to decompose, the area required for facultative ponds is relatively large. A model for facultative ponds is written below, assuming that the pond is completely mixed, a first-order equation describes the rate of removal [5]. 1 Se = S0 1 + k T (HRT)

(6.6)

where Se = BOD value of wastewater effluent (mg/L) S0 = BOD value of incoming wastewater (mg/L) k T = BOD removal constant: = 1.2(1.085)T −35 where T is the temperature in ◦ ( C) H RT = hydraulic retention time (days)

6.2.3 Aerobic and Maturation Pond An aerobic pond is a wastewater treatment system where the dissolved oxygen is adequate to degrade organic matter. A natural aerobic pond is is a dug-out pond of only 0.2–0.6 m in depth. Thus, oxygen either from photosynthesis of algae or natural aeration at the wastewater surface can penetrate through the entire depth. The pond is employed when it is required to significantly lower the organic content in the

6.2 Stabilization Pond

119

Table 6.1 Post-treatment system design (Environmental Engineering Association of Thailand) Pond type Anaerobic pond Facultative pond Aerobic pond Maturation pond Hydraulic retention period (days) Pond depth (m) Organic loading rate (g BOD/m 3 -day) BOD removal efficiency (%)

20–50

5–30

4–6

5–20

2.0–5.0 100–400

1.0–2.5 20–100

0.2–0.6 45

1.0–1.5 –

50–85

80–95

80–95

60–80

wastewater. Alternatively, an aerobic pond can be a facultative pond equipped with mechanical aerators which provide oxygen to the pond. The reaction rate of aerobic degradation is higher than anaerobic degradation, an aerated pond operate at a higher loading rates compared to an ordinary facultative pond. Therefore, a smaller land area is required. The main drawback is the operating costs, including electricity, labor and maintenance, which could offset the land saving especially if the land price is not high. In Southeast Asian countries, an aerobic pond is used when the land area is limited and expansion is not possible. A maturation pond is a shallow pond with 1.0–1.5 m of water depth. It is the last treatment pond and is sometimes called a polishing pond. An important function of the pond is to kill pathogens, if any, by sunlight before releasing to the environment. A maturation pond can be a good alternative to disinfecting biogas effluent by adding chlorines. Since, the retention time for each pond is short, it is common in construct a series of maturation ponds. The relationship between hydraulic retention time and the BOD removal by a series of ponds is given by the following relationship: 1 Si = S0 (1 + k T (HRT1 ))(1 + k T (HRT2 ))(1 + k T (HRT3 )) . . . (1 + k T (HRTi )) (6.7) where Si = BOD of wastewater effluent at pond i (mg/L) S0 = BOD of incoming wastewater (mg/L) k T = BOD removal constant: = 2.6(1.19)T −20 where T is the temperature in (◦ C) HRTi = hydraulic retention time of the i-th pond (days) Example 6.1 Design a stabilization pond system This system is to be used for treating wastewater from a biogas production system, with a volume flow rate of 4000 m3 /day, with a BOD of 300 mg/L, and a temperature of 30◦ C For the anaerobic pond, see Table 6.1, a 4.5 day hydraulic retention period is selected at a depth of 3 m.

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The pond volume: = 4000(4.5) = 18,000 m 3 Pond surface area: = 18,000/3 = 6000 m 2 Organic loading rate: = 4000(300)/6000 = 200 g B O D/m 2 − day BOD removal efficiency: = 50% From Eq. 6.5:

0.5 =

1 6(0.5)4.8 (t

+ 1)

So t = 4.56 days which is close enough to the selected HRT. The BOD of wastewater leaving the anaerobic pond is 150 mg/l. Facultative pond BOD removal efficiency = 80% The BOD removal constant is k T = 1.2(1.085)T −35 = 1.2(1.085)30−35 = 0.798 From Eq. 6.6:

0.2 =

1 1 + 0.798t

which gives t = 5 days. Therefore the facultative pond volume: = 4000(5) = 20,000 m3 The depth = 1.5 m giving a surface area of the pond = 20,000/1.5 = 13, 334 m2 The organic loading rate: = 4000(150)/13, 334 = 45 gBOD/m2 − day which is acceptable, according to the design parameter of Table 6.1. With 80% removal efficiency, the BOD of wastewater leaving the facultative pond is only 30 mg/L. For the maturation pond, a 5 day hydraulic retention period is selected, then: S0 = 30 mg/L k T the BOD removal constant: = 2.6(1.19)30−20 = 14.8 From Equation: 6.7: Si 1 = ⇒ Si = 0.40 mg/L (30) (1 + 14.8 × 5) The final effluent BOD is acceptable. Maturation pond volume = 4000(5) = 20,000 m3 with a depth of 1.5 m.

6.3 Constructed Wetland

121

Fig. 6.2 A series of free water surface ponds for post-treatment

6.3 Constructed Wetland An artificial wetland is a pond created to simulate the condition of a natural wetland. Plants, such as reed, vetiver, and hermit and floating plants such as duckweed, water mimosa or water hyacinth grow in wetlands. Normally, it can handle wastewater with a high impurity levels. An artificial wetland can be divided into two types according to the flow characteristics:

6.3.1 Free Water Surface (FWS) This type of system uses compacted soil or high-density poly-ethylene (HDPE) to flow wastewater horizontally. The pond has different depths to achieve a completely natural cleaning process. The system is usually divided into three zones, which maybe located in the same pond or several separated ponds, as shown in Fig. 6.2. The first zone is a shallow pond which employs water plants with roots in the soil such as vetivers and cattails, to help filter suspended solids and organic matter. The water plants in the second zone, such as lotuses and algae, are grown in the water body and are exposed to atmosphere and sunlight. Their photosynthesis reactions result in more dissolved oxygen (DO) which helps accelerate organic matter decomposition. Due to the availability of free oxygen, nitrification also takes place in this zone. The last zone is a shallow pond with diversified aquatic plants which helps filter the remaining solids and nitrogen.

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Fig. 6.3 Subsurface artificial wetland Table 6.2 Artificial wetland design criteria Design Artificial wetland FWS Range General value Min. required space (m2 /m3 -day) Max. water depth (m) Min. length to width ratio Hydraulic retention period (days) Hydraulic load rate (m3 /day) Organic loading rate (g BOD/m2 -day) Nitrogen load rate (g/m2 -day)

Artificial wetland FS Range General value

20–70 – – 4–15 1.5–5.0 < 11

– 0.5 2:1 – – –

12–17 0.3 – 0.9 5–10 6–8 8–12

– – – – – –

–

6

–

6

6.3.2 Subsurface Flow (SF) This type of artificial pond contains materials such as crushed stone or gravel as a base for plants to attach and grow. The bottom can be paved with a waterproofing material to prevent groundwater contamination. Normally, this media layer is 0.3– 0.6 m high. The wastewater flowing through this artificial pond is always be kept below the surface of the media layer (Fig. 6.3). The SF ponds have advantages over FWS because the filter layer has more surface area for bacteria to grow. Its removal efficiency is superior to FWS ponds, and hence, it requires less pond area for the same level of treatment. Another advantage is the absence of mosquitoes and insects which are not able to lay eggs into the water. Nevertheless, the SF wetland has higher construction costs since it may require a significant quantity of costly media. To design an artificial wetland, it is the best to use experimental data from existing systems. If not available, general design criteria can be used instead. The kinetic equation for SF and the efficiency of the BOD and nitrogen removal follow a firstorder plug flow model. The design criteria are shown in Table 6.2.

6.4 Nitrogen and Phosphorus Removal

123

6.4 Nitrogen and Phosphorus Removal The nitrogen removal in the post-treatment ponds can be explained by the following mechanisms: 1. Intake by algae: The algae assimilate nitrogen compounds into their cells for approximately 6–9% of their dry weight. When the algae are separated from wastewater, the nitrogen is removed at the same time. 2. Adsorption to soil: Nitrogen compounds can be physico-chemically adsorbed into soil. 3. Nitrification and denitrification: Nitrification occurs in aerobic condition changing ammonia nitrogen to nitrite and nitrate. The oxides of nitrogen can be removed by denitrification which takes place at the sedimentation layer where the oxygen is limited. This mechanism can eliminate up to 1 g/m2 /day of nitrogen which is one of the main mechanisms of nitrogen removal. The nitrification and denitrification nitrogen removal models are as follows: (1) Nitrification Ce = exp(−k T (HRT)) C0 As =

Q ln( CC0e ) nk T d

(6.8)

(6.9)

where Ce = Ammonia in the water out (mg/L) C0 = Ammonia in the inlet water (mg/L) H RT = Hydraulic retention time (days) As = Surface area of artificial ponds (m2 ) Q = Wastewater flow rate (m3 /day) d = Water depth (m) n = Porosity which is = 0.65–0.75 (for FWS) or 0.28–0.45 (for SF) k T = Nitrification reaction constant for FWS = 0.2187(1.048)T −20 k T = Nitrification reaction constant for SF = k N H (1.048)T −20 where kNH = Nitrification reaction constant: kNH = 0.01854 + 0.3922(r z)2.6077 where r z = Proportion of the filter layer depth that has been replaced by the root of the plant (0–1). (2) Denitrification: The formulas are identical to the nitrification model: Ce = exp(−k T (HRT)) C0 As =

Q ln( CC0e ) nk T d

(6.10)

(6.11)

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where Ce = Nitrate in the water out (mg/L) C0 = Nitrate in the inlet water (mg/L) HRT = Hydraulic retention time (days) As = Surface area of artificial ponds (m2 ) Q = Wastewater flow rate (m3 /day) d = Water depth (m) n = Porosity k T = Nitrification reaction constant = 1.0(1.15)T −20 .

6.4.1 Phosphorus Most phosphorus entering stabilization ponds is in the form of inorganic substances. The mechanism for phosphorus removal are: 1. Uptake by algae: Approximately 1 percent of the dry weight of algae cells contain phosphorus. Therefore, the removed algae cells also remove phosphorus, equivalent to a concentration of approximately 1 mg/L. 2. Adsorption to soil: Phosphorus compounds can be physico-chemically adsorbed to soil which contain aluminum, iron oxides and hydroxides. 3. Precipitation: Phosphorus can react with aluminum, iron and calcium to form phosphates which can precipitate from the wastewater. Some phosphorus crystallizes in the form of calcium hydroxyapatite which is one of the main mechanisms of phosphorus removal, [6]. Normally, the dissolution of phosphates depends on the pH value. It was found that phosphorus removal is difficult in anaerobic conditions since it enhances the dissolution of phosphates. Example 6.2 Artificial wetland design example An FWS artificial wetland is designed for nitrogen removal. The wastewater flow rate is 400 m3 /day at a temperature of 22 ◦ C. The total nitrogen (TN) which is the sum − of nitrogen in nitrate (NO− 3 ), nitrite (NO2 ), ammonia (NH3 ) and organic nitrogens is 30 mg/L. The effluent is required to contain not more than 3 mg/L of TN. The wetland is a free water surface (FWS) type with a water depth of 0.5 m and a porosity of 0.65. The ammonia nitrification reaction constant, k T = 0.2187(1.048)T −20 = 0.24   400 ln 30 3 As = = 11,808 m2 (0.65)(0.24)(0.5)

Vol = As d = 11808(0.5) = 5904 m3

6.5 Sludge Treatment

t=

125

(0.65)(11,808)(0.5) n As d = = 9.5 days Q 400

6.5 Sludge Treatment Sludge from biogas reactors can be bulky and difficult to dispose due to its high moisture levels. The sludge total solids range between 20 and 40% by weight. Therefore, drying is required to minimize the sludge volume. Dry sludge can be disposed by landfilling or utilized as a soil conditioner. Simple and low-cost sludge treatment technologies include:

6.5.1 Sludge Drying Bed A sludge drying bed is constructed with brick blocks or concrete with multiple layers of gravel and sand.The bed height ranges between 0.45 and 0.80 m, as shown in Fig. 6.4. The drying bed is a simple dewatering unit which relies on the evaporation by wind and sunlight. It usually takes 5–7 days for the sludge to be dried in good weather. However, the drying bed has a major drawback as it is open to rainwater which prolongs the drying process. Some have clear roofs to prevent such problems. Wastewater is fed onto the top layer of the bed and seeps through the media layers. The filtered wastewater is collected at the bottom by hole-drilled collection pipes. This reclaimed wastewater can be recirculated back to the biogas reactor. After drying, the sludge is removed from the bed by manual scraping (Fig. 6.5). New sand needs to be filled to maintain the sand thickness of 30 cm. This sand refilling is conducted 2–3 times a year or when the sand layer is too thin. In addition, there should also be regular maintenance of the floor drainage system to prevent clogging.

Fig. 6.4 A drying bed for treating sludge from a biogas reactor (Source In-house)

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6 Processing Biogas Effluent

Fig. 6.5 Sludge drying bed (Source In-house)

6.5.2 Sludge Lagoon A sludge lagoon is a sedimentation pond with a depth of 0.75–1.25 m. It can store a large quantity of sludge. Similar to the sludge drying bed, it depends on evaporation by natural forces, like wind and sunlight. However, it might take longer time for the sludge to dry because of the larger depth. Therefore, the plant usually has more than one lagoon. These lagoons have some environmental disadvantages, e.g., foul odors and insect infestation; therefore, they are not widely used.

6.6 Sludge Utilization Biogas sludge can be used as an excellent organic soil amendment since it contains nutrients such as nitrogen, phosphorus, potassium, which are essential for plant growth or soil quality improvement. Unlike fresh manure or agricultural waste, the digested biogas sludge is more stable, homogeneous, odorless and easier to handle. The sludge comes from a natural process, beneficial to soil and plants, unlike chemical fertilizers. Moreover, the use of organic sludge in agriculture helps create a circular economy which has many environmental benefits. Depending on the crops and soil, there can be an issue with nutrient overloading so local regulations should be followed before using biogas sludge.

6.7 POME Biogas Post-treatment Case Study

127

6.7 POME Biogas Post-treatment Case Study A palm oil plant in Northeastern Thailand has a capacity of 45 tons of fresh fruit branches (FFB) per hour. The design of the biogas reactor is explained in Chap. 3. For the post-treatment processes, a series of anaerobic ponds, facultative ponds and maturation ponds are used, which are shown in Fig. 6.6. The engineering design of each pond is as follows: (1) Anaerobic Pond The flow rate (Q) is 702 m3 /day, and the influent COD is 11,400 mg/L. Providing that the BOD is only 50% of the COD, this gives the BOD of 5700 mg/L. The design OLR is 0.30 kgBOD/m3 /day. The total anaerobic pond volume (Vol) is calculated from Vol = A · h =

Vol = A · h =

BOD · Q 1000 · OLR

5700 mg/l · 702 m3 /d 1000 · 0.30 kgBOD/m3 /d

Vol = 13, 338 m3 The engineering design is illustrated in Fig. 6.7. The lagoon is covered by HDPE sheets in order to capture any methane gas possibly formed inside. Two dug-out ponds (No. 9 and No. 10 in Fig. 6.6) with reinforced concrete liners are designed in a trapezoidal prism shape with the side slope (height:width) of 1:1.5. The width: length: height (W × L × H ) of the pond is 40:80:4 m, which gives the volume of 10,096 m3 . The wastewater level in the pond is 3 m from the bottom while the freeboard distance is 1 m. Thus the wastewater volume in each pond is 7082 m3 , and the total volume is 14,164 m3 . The actual OLR can be calculated as OLR =

5700 mg/L · 702 m3 /d = 0.28 kgBOD/m3 /d 1000 · 14,164 m3

The actual OLR is lower than the design OLR. So the pond sizing is acceptable. Assuming that the treatment efficiency is 70%, the effluent BOD is 1710 mg/L. (2) Facultative Ponds The facultative ponds are designed to further remove organic content. The design OLR is 0.1 kgBOD/m3 /day. The pond volume can be calculated (Fig. 6.8); Vol = A · h =

1710 mg/L · 702 m3 /d 1000 · 0.10 kgBOD/m3 /d

Fig. 6.6 Post-treatment ponds for POME biogas effluent

128 6 Processing Biogas Effluent

Fig. 6.7 Cross-sectional profile of an anaerobic lagoon for POME biogas effluent treatment

6.7 POME Biogas Post-treatment Case Study 129

Fig. 6.8 Cross-sectional profile of a facultative pond for POME biogas effluent treatment

130 6 Processing Biogas Effluent

Fig. 6.9 Cross-sectional profile of a maturation pond for POME biogas effluent treatment

6.7 POME Biogas Post-treatment Case Study 131

132

6 Processing Biogas Effluent

Vol = 12,004 m3 Four facultative ponds (No. 11, No. 12, No. 13 and No. 14) are arranged to suit the land shape. The width: length: height (W × L × H ) of each pond is 40:80:3.5 m and the side slope of 1:1.5. As the wastewater depth is only 2.4 m, the pond volume is calculated to be 5814 m3 each and the total volume of all facultative ponds is 23,256 m3 . The actual OLR is 0.05 kgBOD/m3 /day and the HRT of the ponds is 33.1 d. The biological breakdown rate, k T = 1.2(1.085)T −35; . The k T is 0.80 day-1 at 30 ◦ C, the effluent BOD can be estimated from: 1 Se = S0 1 + kT t

(6.12)

Se = S0 /(1 + k T .t) Lp = 1710/(0.80 day−1 x33.1 days + 1) = 62.2 mg/L The BOD treatment efficiency of facultative ponds can be as high as 96.4%. (3) Maturation Ponds The maturation ponds are designed to polish the final wastewater quality and also to disinfect any pathogens. The design HRT is 5 d. The pond volume can be calculated as (Fig. 6.9); Vol = Q · HRT = 702 × 5 = 3510 m3 Three maturation ponds (No. 15, No. 16 and No. 17) are arranged to fit with the land shape. The width: length: height (W × L × H ) of each pond is 25:80:2.5 m and the side slope of 1:1.5. As the wastewater depth is only 1.4 m, the pond volume is calculated to be 1986 m3 for each one. The total volume of all maturation ponds is 5958 m3 . The actual HRT of the ponds is 8.49 d. The effluent BOD can be estimated from Se = S0 /(1 + k T · t) which gives the final BOD of 8.0 mg/L, passing the local regulatory standard of 20.0 mg/L.

References 1. Von Spearing M (2007) Waste stabilization ponds, volume III of Biological wastewater treatment series. IWA Publishing, ISBN 1 84339 163 5 2. Arceivala SJ (1981) Wastewater treatment and disposal. Marcel Dekker, New York 3. Cavalcanti PFF, van Haandel A, Lettinga G (2001) Polishing ponds for post-treatment of digested sewage part 1: flow-through ponds. Water Sci Technol 44(4):237–245. ISSN 02731223. https://doi.org/10.2166/wst.2001.0229 4. Mullick MA (1987) Wastewater treatment processes in the Middle East. Book Guild Publishing Ltd. ISBN 978-0863322334

References

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5. Ho LT, Van Echelpoel W, Goethals PLM (2017) Design of waste stabilization pond systems: a review. Water Res 123:236–248. https://doi.org/10.1016/j.watres.2017.06.071 6. Jang H, Kang SH (2002) Phosphorus removal using cow bone in hydroxyapatite crystallization. Water Res 36:1324–1330

Chapter 7

Biogas Utilization

7.1 Introduction Biogas can be used in a variety of applications, Fig. 7.1. Before use, it is necessary to clean the biogas of corrosive gases that could damage equipment. In raw form, its lower heating value is 20–25 MJ/Nm3 . Biogas can be upgraded to biomethane, which increases the heating value close to 35 MJ/Nm3 . This involves removing most of the impurities and carbon dioxide from the raw biogas. The upgraded gas is referred to as biomethane or bioCNG depending on where it gets used. Biomethane has been extensively covered in the literature and other books, [1] and so will not be discussed here. This chapter is only concerned with the raw or moderately cleaned biogas and its utilization. Biogas can be utilized in applications that produce heat, electricity, transport and chemical products. Globally, a large portion of biogas (64%) is used to generate renewable electricity with most of the remaining (27%) used in heating applications. Other uses are as a feedstock for biomethanol and biohydrogen production. This last application is minuscule in quantity but is growing fast and will be briefly discussed at the end of this chapter.

7.2 Biogas Composition Biogas produced from an anaerobic fermentation system is a mixture containing methane and carbon dioxide with trace amounts of other gases, the most harmful being hydrogen sulfide (H2 S). General biogas properties are shown in Table 1.1. The percentage of methane in raw biogas depends on many factors including the feedstock, system operation, temperature and storage time. The biochemical composition of the feedstock is one of the important factors effecting the quantity of methane and biogas yield, as shown in Table 7.1.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_7

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Fig. 7.1 Applications for biogas end use Table 7.1 Approximate biogas yield and methane composition according to the type of raw material (Source in house) Raw material Methane volume (%) Biogas yield (m3 /ton fresh feedstock) Liquid cattle manure Liquid pig manure Cattle manure Pig manure Poultry manure Organic waste Napier grass Corn silage

55–65 60–70 55–65 55–65 55–65 55–65 50–60 45–55

20–30 25–35 40–50 55 –65 75–85 90–110 70–100 150–200

Biogas can also be collected from mature landfills using a series of strategically placed wells. The organic waste in a landfill that has been buried undergoes the same conversion process as that inside an AD. Insertion of perforated pipes allows collection of the landfill biogas. The piping system may be kept under slight negative pressure, facilitating migration of the gas to the wells. Landfill gas is similar to a batch production process. The quality of the gas varies as a consequence of the variability of waste materials that gets deposited. Other contaminants such as siloxanes can be present in landfill gas and are problematic at very low levels. Landfill biogas therefore requires increased capital and operational costs for gas clean-up measures to ensure reliable operation of power generation equipment.

7.3 Biogas Cleaning

137

7.3 Biogas Cleaning A significant problem with biogas can be the presence of hydrogen sulfide in high levels. Hydrogen sulfide is harmful directly as a gas by itself when it can combine with water vapor to form sulfuric acid, as shown in Eq. 7.1. H2 S + H2 O −→ H2 SO4

(7.1)

If it is combusted it can form the corrosive gas, sulfur dioxide. 2H2 S + 3O2 −→ 2SO2 + 2H2 O

(7.2)

This acid attacks metallic components. In addition to corrosion, the combustion of sulfur, which can be present in high levels, also equates to sulfur dioxide emissions rivaling those from coal-fired steam plant without flue gas desulfurization. This negates a lot of the environmental drivers for biogas use. So for a sustainable solution, almost any technology requires some form of sulfur capture when operated with heavily sulfur-laden biogas. Removing hydrogen sulfide adds cost and complexity. Even small quantities of H2 S can cause problems over time. For internal combustion engines H2 S is limited to 200 ppm by the manufacturer. Sulfur is a particular issue for fuel cells, poisoning catalysts. This limits the sulfur tolerable in a fuel cell to less than 1 ppm. There are a number of biological, chemical and physical methods of reducing the concentration of hydrogen sulfide, none being ideal. Authur and Anna [2] discussed methods to reduce hydrogen sulfide; • • • • • • • • •

Oxygen Dosing Iron Chloride Dosing Iron Sponge Iron Oxide Pellets Activated Carbon Water Scrubbing Absorption by sodium hydroxide (NaOH Scrubbing) The use of microorganisms (Biological Removal) Air Stripping and Recovery.

Each of these technologies has its own unique advantages and disadvantages, from different capital costs to operating efficiencies, see Table 7.2.

7.3.1 Bioscrubber A bioscrubber is the most common method of removing hydrogen sulfide from raw biogas. It is widely used to treat biogas for genset use throughout Southeast Asia.

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Table 7.2 Comparison of H2 S reduction methods Method Removal Capital cost efficiency Biological removal Iron chloride dosing Water scrubbing Activated carbon Iron sponge NaOH scrubbing

Operating cost

Complexity

High

Moderate

Low

Moderate

Moderate

Low

Moderate

Low

High High High High

High High Moderate Moderate

Moderate Moderate Moderate High

High Moderate Moderate Moderate

It uses a biological process to completely remove the hydrogen sulfide. Good operation of the scrubber depends on a stable flow of biogas at the beginning with a stable concentration of hydrogen sulfide. Bacterial consortium which consumes H2 S produces a variety of enzymes that accelerate their metabolism. They also possess an ability to adapt themselves to survive in a changing environment. Bacteria which have the ability to remove hydrogen sulfide are usually identified as sulfur-oxidizing bacteria. The hydrogen sulfide is usually oxidized into elemental sulfur or insoluble sulfuric compounds. Sulfur-oxidizing bacteria can be obtained from many sources. As an example, Thiobacillus thioparus is a sulfur-oxidizing bacteria group that has the ability to oxidize sulfur compounds (hydrogen sulfide, sulfide, sulfur and thiosulfate). This type of bacteria is autotrophic so can synthesize their own food. They derive energy from chemical reactions. It grows well in a wide pH range of 5–9. The appropriate temperature range for its growth is 20–37 ◦ C which is common in Southeast Asia countries. Inside the reactors, supporting media from natural materials such as peat, fiber, compost and tree bark can be used; however, their lifetimes are relatively short. On the other hand, synthetic materials, e.g., ceramics, activated carbons and plastic packing, can also be used, although their costs are higher. The bioscrubber operating principle is to drive raw biogas through a packed column bed, either upwards or downwards, while ensuring stable temperature, moisture and pH to promote the growth of sulfur-oxidizing bacteria. The biogas flow rate and H2 S concentration are considered to be most influential factors in the design of a fixed bed-volume reactor (Fig. 7.2).

7.3.2 Moisture Removal Biogas from the digester is usually saturated with moisture. At higher temperature and humidity, as found in Southeast Asia, the biogas holds a larger amount of moisture. Sometimes to prevent condensation, the moisture must be removed. This helps prevent corrosion and equipment malfunction. If the gas temperature is lower than the

7.4 Storage and Transportation of Biogas

139

Fig. 7.2 Biofilter installation at a livestock farm (Stock Photo ID: 1844345287)

dew point, water vapor condenses and can potentially combine with CO2 , NH3 and H2 S to form bases or acids. Moisture also lowers the biogas heating value, causing problems with certain combustion process. The maximum allowable moisture varies according to the requirements of the end use system. Moisture is usually removed with industrial chillers.

7.4 Storage and Transportation of Biogas 7.4.1 Biogas Storage Biogas produced from anaerobic digestion systems is commonly stored in the digester head space. This is the volume directly above the digester. This storage volume can be designed to match the biogas daily production rate. The consumption and use of the biogas also needs to be balanced with the storage. Extra storage capacity can be used as buffering space to ensure a smooth delivery of biogas to its end use application without interruption. Shortages can result from feedstock changes or operational difficulties. Generally extra storage facilities are expensive but necessary in plants that need high buffering because of safety and redundancy requirements. Biogas storage systems are classified according to the pressure and volume of the storage tank as shown in Table 7.3. (a) Storing gas at low pressure In this case the biogas produced is stored at low pressures (0.005–0.5 kPa). This biogas is produced in a digester that has a plastic

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Table 7.3 Biogas storage arranged by pressure Pressure level Pressure Low Medium High

0.005–0.5 kPa 0.5–5 kPa 0.5–30 MPa

Application Single membrane Double membrane/fixed dome High pressure tanks

covering. Lagoons which produce and store low pressure biogas use a plastic sheet covering. These sheets are polyvinyl chloride (PVC) or high-density polyethylene (HDPE) with thicknesses between 0.8 and 2.0 mm. These digesters are discussed in detail in Sect. 3.3.3. (b) Storing gas at medium pressures A fixed dome or CSTR with double membranes generally store gas at medium (0.5–5 kPa) pressures. The pressure develops inside the digester, and external compression is not required. An advantage of medium pressures is they facilitate transportation of biogas to its end use without a need for a blower. (c) Storing gas at high pressures To store gas at high pressures (0.5–30 MPa), a high strength gas storage tank must be used. The metal can be iron, alloy steel or stainless steel. Before storage, the gas must be cleaned, dried and compressed into compressed biogas. High quantities of gas can be stored in small volumes. The general application would be fuel for transportation. Compressors and tanks must be stored in a location with adequate ventilation in case of leaks or accidents. There must be pressure relief valves or overflow protection valves, back flow protection, gas filters, alarms and detailed emergency plans.

7.4.2 Biogas Piping System The important parameters in piping are the gas flow rate, temperature and pressure. The diameter of the pipe is a function of the biogas flow rate. The pipe material and thickness is a function of the gas temperature and pressure. Generally, the initial gas temperature is the temperature inside the biogas digester, which is usually not more than 50 ◦ C even during summer days with thermophilic digestion. The gas temperature increases after passing through the blower. The pipe temperature depends mainly on the pressure that the biogas is compressed to. A safety factor of 1.5 should be used so the pipe can comfortably handle a temperature 1.5 times higher than the operating temperature, as specified in ASME B31.8 [3]. For transporting biogas, pressure relief valves should be installed to protect the pipe from pressure surges. Popular pipe materials include plastics, such as high-density polyethylene resin (HDPE), chlorinated polyvinyl chloride (CPVC) or unplasticized polyvinyl chloride (UPVC). Metal pipes can also be used, although

7.5 Biogas Applications

141

they are not as common due to their cost. Some general guidelines for different piping materials: • Polyvinyl chloride (PVC) pipe without UV protection is not able to withstand moderate temperatures • Pipes used at high temperature locations, such as those near the blower should use high temperature materials. For selecting the pipe diameter, there are trade offs, large diameter pipes are expensive but have a low pressure drop along the pipe length. Smaller diameter pipes are cheaper to purchase but are more expensive to operate with higher pressure drops. The final pressure and flow rate required by the end use equipment determine the piping design. The final design should support a flow rate of at least 1.3 times the average flow rate or 1.1 times the maximum flow rate. High biogas velocities cause friction, erosion and noise. The gas speed should be limited because of dust, which is always present in pipeline gas, causes abrasion and wear. A maximum velocity of about 20 m/s is therefore recommended to avoid pipe erosion. The main digester biogas extraction pipe should be at least 50 cm above the water level to prevent blockage from foam or scum. There should be several points of biogas extraction from the digester. In addition, various safety devices should be installed in the biogas piping system, including: • • • • •

Over pressure relief valve Check valve Pressure transducers Pipe moisture drainage system Flame trap or reverse flame protection device.

7.5 Biogas Applications The composition of natural gas does not have a single defined value since it is a function of geographical source. Similarly, the composition of biogas depends on a number of factors particularly the production process and the nature of the feedstock material. Therefore, any power generation technology must be capable of coping with variable biogas compositions in order to be useful. Statistics on biogas global production from usage are shown in Table 1.4 A number of technologies are compatible with biogas. These are small-scale power generation technologies that would normally be fueled with natural gas, but which are also capable of handling biogas. The calorific value of biogas is lower than that of natural gas as there is carbon dioxide present. From Table 1.1, it can be seen that biogas with methane concentrations of 50–70% has combustion enthalpies of 20–26 MJ/m3 (Natural gas is around 35.6 MJ/m3 .) . For technologies that can use biogas the issue is how comparable its performance is to its conventionally fueled variant. If a technology directly replaces

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Table 7.4 Biogas properties required for different technologies Technology

Required biogas properties

Power range

Efficiency (% LHV)

Industrial burner

– H2 S < 1000 ppm, pressure 8–25 kPa, no contaminated water, CH4 > 50%

–

99%

Domestic stove

– H2 S < 10 ppm, pressure 0.8–2.5 kPa

–

99%

Internal combustion engine

H2 S < 100 ppm, pressure 0.8–2.5 kPa, no moisture and no siloxanes, CH4 > 50%

5 kW–6 MW

24–37%

Microturbines

Calorific value > 13 MJ/m3 Gas pressure 520 kPa, no moisture and no siloxanes

25–500 kW

17–30%

Fuel Cells

PEM: CO < 10 ppm, eliminate H2 S

1–250 kW

40–45%

PAFC: H2 S < 20 ppm, CO < 10 ppm, Halogens < 4 ppm

200 kW–2 MW

38–45%

MCFC: H2 S < 10 ppm in fuel (H2 S < 0.5 ppm to stack), Halogens < 1 ppm

250 kW–2 MW

45–60%

SOFC: H2 S < 1 ppm, Halogens < 1 ppm

250 kW–5 MW

50–65% 22%

Stirling engines

H2 S < 1000 ppm, pressure: 1–14 kPa

1–30 kW

Upgraded to biomethane

H2 S < 4 ppm, CH4 > 80–95%, CO2 < 2%, no siloxanes and no particulates

–

natural gas with biogas this usually means a lower power output and lower efficiency. One solution is to increase the fuel throughput to maintain heat input. This is possible with turbines, burners and internal combustion engines. This causes an increase in parasitic loads, as higher fuel flow rates are required. Specific combustion equipment requires a specific biogas grade. Normally a manufacturer would specify the minimum quality of gas required for their equipment. If not, a rough guide to biogas quality for different applications is shown in Table 7.4.

7.5.1 Biogas Flaring When excess biogas is generated, that cannot be stored, it becomes necessary to have a flare system. This helps prevent methane release into the atmosphere. A flare system can be either open flare or a closed flare. An open flare system (Fig. 7.3) is an external combustion system. The flame is visible but the combustion efficiency is lower than the closed type. The open flare gas combustor should be located at a sufficient height and distance (>30 m) so that the heat radiated causes no damage. A closed flare combusts biogas internally inside a flare shroud, Fig. 7.4. Only hot vapors escapes. Combustion is more complete and more secure than open systems. Open and closed flares have different advantages, limitations and suitable conditions for use. These are shown in Table 7.5. Both flare types should be located far away from the gas storage area to prevent possible explosions. The exit vent should be at least 6 m above the ground.

7.5 Biogas Applications

143

Fig. 7.3 Open flare gas system (Stock Photo ID: 2035273256)

Fig. 7.4 Closed flare gas system (Shutterstock: ID: 1361985284) Table 7.5 Comparison between open and closed gas flares Open flare system Closed flare system • Poor combustion performance • Poor pollution control • 20–70% lower cost than closed systems • Suitable for combustion of gases at low flow rates

• Good combustion performance • Good pollution control • Can be used for combustion of gases at high flow rates • The height of the shroud should be 6–10 meters • Works in all conditions

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Fig. 7.5 Biogas burner (Stock Photo ID: 341516294)

7.5.2 Thermal Energy The basic form of energy production from biogas is thermal energy. It comes from direct combustion in a burner. The heat can be transferred to a hot gas, hot water/steam or hot oil. Complete combustion should occur in well-designed burners which mix the fuel and air thoroughly and allow sufficient time for combustion. A biogas burner is just a natural gas burner designed and modified to be used directly with biogas. Usually the combustion occurs just outside of a burner head which mixes the biogas and air. The burner manufacturer normally gives precise specifications on biogas quality required; however a rule of thumb is to limit the concentration of H2 S to not more than 1000 ppm, the relative humidity of not more than 60% and a methane concentration of more than 50%. Biogas burners can be divided into 3 categories depending on the fuels used: (1) A biogas burner is used directly with biogas, see Fig. 7.5. The burner nozzle contains a baffle diffuser to create a rotating air and biogas mixture. This swirling flame creates good fuel and air mixing. This type of burner is available in a wide variety of thermal capacities/biogas flow rates. Its operation and control are not complicated. (2) Should biogas be temporarily unavailable, then an alternative fuel is needed to operate the boiler. A burner that can run on two or more different fuels is known as a dual fuel system, see Fig. 7.6. Dual biogas/oil burners are capable of burning biogas or oil. It uses a high pressure oil pump which injects the oil through a capillary tube with a high rotational velocity. This helps the oil and air mix well together. The biogas flows around the outside of the fuel injector. There are many sizes and thermal capacities to choose from. The disadvantage of this type of burner is that the fuel

7.6 Electric Power Generation

145

Fig. 7.6 Dual biogas oil burner (Stock Photo ID: 2140597087)

injector can often get clogged with dust or other impurities, so they must be cleaned or filtered thoroughly. (3) The third type of biogas burner is one that can operate in three modes: biogas only, oil only or biogas mixed with oil. For mixed fuel combustion, one of the fuels must flow steadily while the other can have an adjustable flow rate. This burner works well when the rate of biogas production varies. Since it uses both types of fuel simultaneously the control is more difficult. These burners require more care and maintenance than the others. A popular biogas oil combustor is the rotary cup combination burner. It uses centrifugal forces to distribute liquid fuel into capillaries. Fuel is fed inside a rotating cone with the centrifugal forces forcing the oil to the edge of the cone. When compressed air is fed into the cone it breaks up the oil into micro droplets. The minimum oil content is set at approximately 30% of the heat capacity of the burner.

7.6 Electric Power Generation Biogas can be directly converted into electricity, using power generators. The most common method is to connect an internal combustion engine (ICE) with a generator. In Southeast Asia, this accounts for 80% of all biogas utilization. Others include Stirling type engines, fuel cells or gas turbines. Conventional Rankine cycles, including using solid biomass, are unsuitable for small biogas plants. Steam turbine technology is not commonly used for small-scale power generation. At small scales, its economics and efficiency are low. Examples of an internal combustion engine/generator are shown in Fig. 7.7 with details provided in Table 7.6.

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Fig. 7.7 Biogas Akricomp BGA 095, 150 kWel CHP unit (with permission) Table 7.6 Biogas Akricomp CHP unit specs Parameters Units CHP type—Biogas 50% CH4 Electrical output at Cosφ = 1 kWel Rated current at Cosφ = 1 A Electrical efficiency % ◦ Thermal output at 180 C kWth exhaust temperature Electric/thermal ratio at 180◦ C % Lean burn turbocharged biogas motor Construction type Displacement L Rated thermal input—100% load kWth Gas consumption at 50% CH4 Nm3 /h Length × width × height m

Type 100 75 108 33.1 98

100 144 35.9 120

Type 150 120 173 36.2 151

150 217 36.6 174

0.77

0.83

0.79

0.88

5R 130 × 140 9.3 227 279 45.3 55.7 2.9 × 1.3 × 2.15

5R 130 × 140 9.3 331 410 66.3 82.0 4.1 × 1.5 × 2.1

7.6.1 Internal Combustion Engine Traditionally, ICEs are the most developed power generation option for biogas, see another example in Fig. 7.9. The air-fuel ratio must be modified from natural gas to adapt for biogas. Combustion is intermittent, and peak combustion temperatures approach 1800 ◦ C. The high combustion temperatures drive the formation of NOx in the combustion products. Exhaust clean-up processes are used to reduce emissions. Another issue is the build up of biogas contaminants in the engine oil. Sulfur build up makes the oil more corrosive, which reduces engine life. The technology has

7.6 Electric Power Generation

147

Fig. 7.8 Jenbacher type 6, Biogas CHP generator (with permission)

historically been the natural choice for small-scale power generation since it was the only cheap, well-known option. A large spark ignition engine can generate 1–4 MW. The exhaust gas or engine body can be used to produce steam. This coproduction system, electricity and heat, is known as a combined heat and power (CHP) cycle. Combined heat and power is the production of electricity and heat together. It tries to make better use of the fuel’s capacity, resulting in a cycle efficiency higher than electric production only. This is limited in SE Asia by the limited uses for low grade heat. By contrast, in Europe where heating the AD is common, CHP is a most natural application for biogas. The heat recovered from the generator is used to heat the digester and adjacent facilities. INNIO is a company producing power generation equipment and internal combustion engines. It has the Waukesha and Jenbacher engine brands. These engines can generate heat and power using biogas. Jenbacher has a series of engines from its J2 to J6 capable of running on biogas and producing electric power from 330 kW– 4MW, [4]. The type 6 engine shown in Fig. 7.8 has an electric power output from 2 to 4.4 MW depending on which model is selected. The power output and efficiency from biogas is almost identical to that from natural gas as the control system is design to flexibly operate on a range of gases including coal mine gas, coke gas, wood gas and pyrolysis gas (Fig. 7.9). Pilot Injection Natural Gas engines (PING) This engine operates on a diesel cycle but can be run on combined diesel and gas. They are often used for tractors and other heavy duty vehicles. The biogas is mixed with combustion air. The advantage of this type of engine is that it can be used only with diesel in the event that there is insufficient biogas. This is suited for biogas plants with seasonal production. In addition, greener, biodegradable fuels such as biodiesel or vegetable oil can be used. Unlike diesel, these fuels do not contain sulfur and release less carbon monoxide.

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Fig. 7.9 Internal combustion engine (Stock Photo ID: 180736850)

7.6.2 Gas Turbine In principle the gas turbine cycle is simple, Fig. 7.10. Air is pressurized by a compressor. Heat is supplied from fuel combustion inside a combustion chamber. This causes the gas to expand and increase its kinetic energy. The hot gases are expanded through a turbine producing work. The majority of this work is used to drive the compressor, and the rest can be used to drive an electric generator. Gas turbines become more cost effective as the output and efficiency increase. Exhaust temperatures of around 600 ◦ C are common, which means meaningful quantities of heat can be recovered for cogeneration applications or used to preheat the incoming air. As biogas is produced at low pressures, a compressor or booster is required to increase the pressure for fuel delivery. This adds a parasitic load. The parasitic load from the compression stage is higher than that for a natural gas-fueled system as the biogas flow rates are higher. Small-scale turbines and microturbines produce power in the range of 28–200 kW. They are derivatives of the turbocharger. Microturbines are attractive for a number of reasons including low emissions, compact size and potential low cost. Kevin and Mike [5] ran tests on a microturbine with biogas purity ranging from 50 to 70% and

7.6 Electric Power Generation

149

Fig. 7.10 Gas turbine operating principle COMPRESSOR

INJECTOR

COMBUSTION CHAMBER

GAS TURBINE

JET EXHAUST NOZZLE

Fig. 7.11 Gas turbine (Stock Vector ID: 1259213491)

found the microturbine worked well. Manufacturers include Capstone Green Energy, Elliott/Bowman and Ansaldo Energia. All of Capstone Green Energy’s (https://www. capstonegreenenergy.com/) microturbines can run on biogas with a power output ranging form 65 kW–1 MW, depending on the model selected. These specifications listed are based on high pressure natural gas. Biogas with a lower heating value would have a lower power output and lower exhaust temperature. Ansaldo Energia (https:// www.ansaldoenergia.com/) can custom fit a 100 kW turbine to run on biogas with an efficiency of 30%. The biogas must have a minimum of 40% CH4 and a Wobbe Index between 18–25 MJ/m3 . It can even operate at H2 S levels as high as 1500 ppmv (Fig 7.11).

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7.7 Upgrading to Biomethane Biogas can be upgraded by eliminating impurities and reducing the volume of carbon dioxide. The gas remaining contains a very high percentage of methane (80–95%). Biogas thus improved is generally referred to as biomethane, a topic with extensive existing literature [1]. Biomethane can be transported through the existing natural gas grids and used for all intensive purposes as natural gas. It can be compressed and stored in pressure vessels to be used as a fuel for transportation. There are various technologies for upgrading biogas to biomethane. The advantages of biomethane include the following: 1. It is a renewable energy source unlike natural gas and oil 2. It releases less pollution when combusted in comparison to diesel and gasoline 3. Biogas, the raw material needed for biomethane, is not made in one single location. It is dispersed throughout a country. This eliminates the need for longdistance gas transportation as in the case of natural gas, promotes local business and is a more reliable and resilient system due to its dispersed geography. 4. The biomethane production process separates out carbon dioxide gas. This gas, which is also a green gas, can be used to manufacture products such as methanol, bioplastics and carbon nanotubes, among others. Biomethane gas can be used in a variety of applications:

7.7.1 Transportation Fuel Biomethane can be used as a substitute for compressed natural gas (CNG) in transportation. When used in this application it is sometimes referred to as BioCNG. Currently, there is no specific global standard for determining the exact constitution of biomethane in transportation applications. There is a plethora of different standards for biomethane quality depending on the type of engine its location. Figure 7.12 compares the distance that cars can run when using different types of biofuels. The input is standardized by producing the biofuel from energy plants grown on 1 ha (10,000 m2 ) of farmland. In addition, biomethane potential is even higher if organic waste is used as a raw material instead of energy crops. None of the other biofuels are producible from waste. Heavy duty vehicles can be modified to work with biomethane. They can be used as a dual fuel engine. This requires less engine maintenance while having the same propulsion capability as pure diesel engines. Vehicles powered by biomethane have lower carbon dioxide emissions and lower particulate matter emissions and soot compared to cars with pure gasoline or diesel engines. Biomethane’s overall green credentials depend on the source of raw materials used in its manufacture and the electric source used to compress the biomethane.

7.7 Upgrading to Biomethane

151

Fig. 7.12 The distance traveled using different biofuels (with permission from [6])

7.7.2 Injection into a Gas Pipeline System Biomethane can be injected and distributed through a regional or national natural gas grid. To inject the biomethane, there must exist standards that specify the properties and constitution of the biomethane, such as ASME B31.8. Currently, biogas from certain production sources, such from landfill waste which has a high nitrogen content, is not yet suitable for injection into a natural gas pipeline system. The limitation of injecting biomethane is the cost of gas upgrading and the economic and regulatory hurdles for installing a pipeline connection into the main network.

7.7.3 Other Possible Biogas Applications 7.7.3.1

Fuel Cells

A fuel cell is a device that converts energy from a chemical reaction directly into electrical energy. The chemical compounds are not stored within the device, but are continuously supplied in the form of a fuel and oxidant (usually air). The reaction products are continually removed. It has a physical structure consisting of an electrolyte layer, an anode and cathode, see Fig. 7.13. Hydrogen is the preferred power source for fuel cells because of its high reactivity. Generally, fuel cells can run on other fuels only if they are first chemically converted to hydrogen either by a separate fuel processor or, in some cases, in the fuel cell stack itself.

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Fig. 7.13 Fuel cell operating principle

For use in most fuel cells, the biogas must be reformed to hydrogen. Fuel cells require stringent purification of biogas. There are a number of different technologies which can reform biogas. Many fuel cells have different characteristics and are at differing stages of development and commercialization. They all work on the same basic principles of electrochemistry which is the oxidation of fuel and reduction of oxidant. Fuel cells differ by their electrolyte material and the temperature. Fuel cells are named after the type of electrolyte and temperature used, low temperature fuel cells (PEM) (Fig. 7.14), medium temperature (PA) or high temperature fuel cells (MC, SO). Polymer-Electrolyte-Membrane Fuel Cell (PEM fuel cells) This type of fuel cell is sensitive to impurities in the fuel gas, including carbon dioxide. In order to be usable for the PEM, the biogas needs to be processed to a hydrogen-rich gas. This is done via a steam reforming. Steam reforming is a commercialized and industrially available technology. State-of-the-art steam reforming is used by the petrochemical industry on an industrial scale. During steam reforming, a catalytic conversion of methane with steam to hydrogen, carbon monoxide and carbon dioxide occurs. On a smaller scale, the issue is that its thermal energy efficiency is less than 40% ([7]). PEM operating temperatures are around 80 ◦ C. [8] observed that a PEM can work stably using hydrogen-rich gas containing more than 60% vol. hydrogen. However, the electrical efficiency was only 22.9% which was due to the lower hydrogen content compared with pure hydrogen. If a PEM is fed with pure hydrogen, the electrical efficiency is in the range of 40–60%. The market leaders Ballard (www.Ballard.com) can supply megawatt scale stationary power units that run on pure hydrogen. A PEM fuel cell delivered in 2001 to a sewage treatment plant in Japan was the worlds first example of a PEM running on biogas. Phosphoric Acid Fuel Cell (PA fuel cell) The electric production efficiency is low compared to other fuel cells, but the advantage is that the PA fuel cells are less sensi-

7.7 Upgrading to Biomethane

153

Fig. 7.14 PEM fuel cell (Stock Vector ID: 1142997314)

tive to carbon dioxide and carbon monoxide in the fuel gas. There are several hundred 200kW cogeneration units in commercial use, delivered by ONSI/International Fuel Cells Inc (IFC). Several are used with biogas. These generators have been shown to be reliable, but are too expensive to be competitive with conventional technology (ICE). They have achieved negligible market penetration. IFC’s development plan centers on cost reduction which will be technically challenging given the relatively late stage of development of this technology. Molten Carbonate Fuel Cell (MC fuel cells) Molten Carbonate (MC) fuel cells are composed of a porous nickel-based anode, a porous nickel-oxide-based cathode and molten carbonate salts as an electrolyte within a porous lithium aluminate matrix. It is a high temperature fuel cell, operating at temperature ranges from 600 to 700 ◦ C . This means that reforming of methane to hydrogen may occur within the fuel cell (so-called internally reforming MCFCs). No precious metals are required as the fuel catalyst, so carbon monoxide is not a poisoning element, but can be used as a fuel. The operation of a MCFC is not restricted to hydrogen availability alone and allows the utilization of a variety of hydrocarbon fuels, natural gas, syngas and landfill gas, as well as biogas. A major advantage of MCFCs is that waste heat is at a temperature that is more suitable for combined heat and power applications. Alternatively, the waste heat can be used to generate steam for electricity by conventional means. The disadvantage of MC fuel cells is their sensitivity to hydrogen sulfide. The concentration of H2 S must be reduced to very small values, 1–10 ppmv, before using in a MCFC.

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Today MC fuel cells are being installed on commercial basis in sizes from 100kW to several MW. The main producer of MCFC technology is Fuel Cell Energy (https:// www.fuelcellenergy.com) of the US. They produce fuel cells that are capable of running on biogas. In Riverside city, California, they have installed a 1.4 MW fuel cell plant that uses biogas from a wastewater treatment process to generate power and heat for the facility. Solid Oxide Fuel Cell (SO fuel cells) SOFCs enjoy the same advantages as MCFCs, but are simpler since all of the components are solid state. A dense layer of ceramic, the solid electrolyte, is sandwiched between the anode and cathode. The electrodes are made out of specific porous conducting material. These fuel cells have high operating temperatures, 750–1000 ◦ C . These temperatures provide challenges from a materials standpoint. They have high electricity efficiency and allow methane to be reformed within the fuel cell. The ideal fuel is hydrogen but they are suitable for use with biogas. The power generation from biogas-SOFC is high, even when the methane content of biogas is below the normal combustibility limit [9]. Bloom energy (www. bloomenergy.com) and Elcogen (https://elcogen.com/) have commercially available SO fuel cells for sale. PEM, MC and SO fuel cells are the technologies most likely to succeed in the power market. The latter two types due to their suitability for use with natural gas and biogas and their utilization of waste heat. PEM fuel cells are relatively cheap and well suited in transportation.

7.7.3.2

Stirling Engine

Stirling engines are closed cycle regenerative heat engines. They are related to the internal combustion engine, but the key difference is the external supply of heat to the engine. There are very few in commercial use due to low practical efficiencies and economic and technical challenges. It is mentioned here because they use an external combustor as a heat source. This makes them relatively fuel flexible and are potentially suitable for a wide range of renewable applications including biogas. New Zealand-based WhisperTech produce a 1 kW Stirling engine for home use. Other manufacturers include Sigma in Norway, Solo in Germany and the American Stirling Company.

7.7.3.3

Chemical Production

Carbon dioxide can be produced from biogas. It is a by-product from biomethane production. This gas is important to the chemical industry as it is used for the production of polycarbonate, dry ice and surface coatings. The brewing and food processing industry use carbon dioxide. It also is finding uses in new areas, such as growing algae for biogas. Algae are considered very suitable for biofuels because they use less

7.7 Upgrading to Biomethane

155

Fig. 7.15 Biofuel production from algae (with permission from [10])

space compared with land based crops. However, there are still economic constraints on the production of fuels from algae which requires more research and development to allow large-scale commercialization (Fig. 7.15).

7.7.3.4

Biomethane to EV Charging Station

In Thailand and other Southeast Asian countries, large domestic gas pipelines do not exist. This limits take-off options for biomethane. Compressed natural gas vehicle filling stations are therefore an important option. Unfortunately CNG vehicles are in decline throughout the region, see Table 7.7. From 2016 to 2019, CNG in transportation use declined 30%. The abrupt upward trend in electric vehicle (EV) use is creating another future take-off option. In Thailand, the target is for EVs to have 30% of the new car market share by 2036. There will be a need for EV charging stations. This off-take option involves converting existing/retiring compressed natural gas filling station to electric vehicle charging facilities. Electricity can be directly drawn from electricity grid; however if biomethane is used as the electricity source, this ensures 100% renewable electricity for EV charging. This is an important issue for environmentally aware consumers who purchase EVs. It allows for biogas with minimal upgrading to be used productively. Connection to the grid is an extremely challenging task but a charging station does not need to be grid connected. Also, in many cases, the electricity grid infrastructure needs to be improved or expanded to accommodate electric vehicle charging capacity.

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Table 7.7 Thailand CNG demand in transportation (ton/day) Year CNG in transportation (tons/day) 2014 2015 2016 2017 2018 2019 2020 2021

7.7.3.5

11,504 11,155 10,227 8890 8067 7152 5144 4141

Syngas Production

Biogas is a raw material that can be used to produce biohydrogen. The process involves splitting the hydrogen molecule (H2 ) from the methane (CH4 ) molecule. A preliminary step is to produce a syngaswith a steam/methane reformer, see Eq. 7.3. Syngas is a mixture of hydrogen and carbon monoxide. This important gas is also the raw ingredient for producing liquid biofuels such as biomethanol. It can be produced from any fossil fuel in a catalytic reaction. Methane is the most common fuel used. The reforming unit is a capital-intensive unit, far more expensive than a biogas upgrading plant. Despite the long-term R&D and commercialization of these technologies a significant effort is still ongoing to seek a reduction of the capital and energy requirements of the syngas production. CH4 + (H2 O, CO2 , O2 )  xCO + yH2 (Hro = 247 kJ/mol)

(7.3)

The most common syngas production processes are: • Steam reforming (SR): Steam reforming is a catalytic and energy-efficient technology for producing a H2 -rich syngas from light hydrocarbons like natural gas, LPG or Naphta. This is the most common technology for producing syngas. This technology reacts light desulfurized hydrocarbons (S content under 50 ppb) with steam as shown in Eq. 7.4: CH4 + H2 O  CO + 3H2 (Hro = 206 kJ/mol)

(7.4)

In SR, methane reacts in highly endothermic reactions with steam over Ni-based catalysts at temperatures between 800 and 1000 ◦ C and a pressure in the range of 2–3 MPa. To maximize H2 production the SR step is followed by a Water Gas Shift (WGS) reaction for CO conversion, Eq: 7.5. CO + H2 O  CO2 + H2 (Hro = −41 kJ/mol)

(7.5)

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157

• Partial oxidation (POx): Partial oxidation is a non-catalytic technology with a unique possibility of utilizing heavy hydrocarbon feedstock. It produces a COrich syngas at temperatures between 1100 and 1400 ◦ C. Its energy efficiency is lower than SR. POx technology is based on partial combustion of fuels; in the case of CH4 , the reaction is represented by Eq: 7.6. CH4 + 1/2O2  CO + 2H2

(7.6)

The ratio of H2 and CO ideally should be 2; however, due to side reactions, the carbon monoxide and hydrogen may oxidize to carbon dioxide and water, resulting in loss of product gases. This process is mainly used for producing syngas from heavy hydrocarbons, such as petroleum coke. These are pre-heated and then mixed with oxygen within a burner; after ignition, reactions occur inside a high temperature combustion chamber. An effluent is produced that contains differing amounts of soot. Exit gas temperatures are typically between 1200 and 1400 ◦ C. The syngas has to be cooled and cleaned. The high temperature (1400–1100 ◦ C) heat recovery in POx is not very efficient. Currently the main uses of POx are: 1. H2 production for refinery applications 2. Synthesis gas production from coal 3. Electric energy production from petroleum coke. This makes POx unsuited for a small-scale biogas plant. • Autothermal reforming (ATR): ATR combines gaseous phase combustion reactions and catalytic steam/CO2 reforming reactions; it is much less applied than SR and POx but it is the optimal choice for integration with large-scale methanol production plants. ATR combines non-catalytic partial oxidation and catalytic steam and CO2 reforming in a single reactor, see Eq: 7.7. The process was developed in the late 1950s. CH4 + 3/2O2  CO + 3H2 O (Hro = −519 kJ/mol)

(7.7)

Autothermal reforming reactions are carried out in a temperature range of 950– 1400 ◦ C at elevated pressures of 3–5 MPa. For methanol production, a particular ratio called the stoichiometric module or S module indicates the gas suitability. CH4 + CO2  2CO + 2H2 (Hro = 247 kJ/mol) 7.7.3.6

(7.8)

Hydrogen from Syngas

Syngas can be purified to H2 with pressure swing adsorption (PSA) technology. A study was carried out on biohydrogen production [11], starting from a farm in

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Fig. 7.16 Production of liquid fuels by the Fischer-Tropsch process (with permission from [12])

Serbia, producing biogas via anaerobic digestion, biogas upgrading, syngas production and finally biohydrogen production. According to a life cycle energy assessment approach, results obtained show that biohydrogen production via biogas steam reforming has a negative energy balance. This is primarily due to the large heat requirements from the SR. Future improvements in catalysts and cost reductions for small-scale reformers are required to make biogas to biohydrogen a viable route. Hydrogen is predicted to be the main energy carrier of the future. It has a high energy yield (120–140 MJ/kg) compared with fossil fuels (44–46 MJ/kggasoline ) and can be used to produce electrical energy directly via fuel cells. In addition, it is abundantly available in the universe, odorless, non-poisonous and its combustion does not generate pollutants.

7.7.3.7

Gas to Liquid Fuel

One challenge for biogas, especially if produced in a rural location, is transportation. Gas is difficult and expensive to transport over long distances. If it could be converted into a liquid the economics of transportation improve. The conversion of biogas into higher value liquid fuels is based on reforming. Methane is reacted with various oxidants to produce syngas as described in Sect. 7.7.3.5. Synthesizing liquid fuels from solid and gaseous fuels are topics undergoing extensive research. One widely used process is the Fischer-Tropsch Synthesis (FTS) which intakes solid or gaseous fuels and transforms them into liquid fuels using metal catalysts such as iron (Fe) or cobalt (Co). There are three types of reactions: Gas to Liquid (GTL), Biomass to Liquid (BTL) and Coal to Liquid (CTL) (Fig. 7.16). Another liquid that is producible from biogas is biomethanol. According to IHS, global methanol demand reached 117 million metric tons in 2020, 90% of which is produced from natural gas. Methanol from natural gas reforming entails a significantly high carbon footprint leading to exploration for renewable alternatives.

References

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Biomethanol can be blended with gasoline at 10–20 wt%. There are car manufacturers in China and India already producing vehicles with engines compatible with methanol. Methanol fuel cells have advantages over hydrogen fuel cells in feed handling and lower operating temperatures. Methanol is used in biodiesel production . To produce methanol, synthesis gas is produced from biogas, see Sect. 7.7.3.5. Distributed methanol production can make sense for waste to energy applications. The logical alternative to a syngas-based process would be one in which methane is directly converted to final products, in the case of methanol: CH4 + 1/2O2  CH3 OH (Hro = −126 kJ/mol)

(7.9)

These direct reactions generally suffer from impractically low net yields of desired products. These direct processes are also inevitably accompanied by the undesired complete oxidation of methane to CO2 + H2 O. Therefore, large-scale applications for methanol production are exclusively based on the indirect synthesis route via the intermediate step of syngas production. The smallest capacity of steam methane reformers (SR) is around 2500 ton/d. Below this capacity, large steam reformers become progressively more expensive and thus show no economies of scale. Just for comparison, a relatively large POME biogas plant might be capable of producing 80 ton/d biomethanol.

References 1. Koonaphapdeelert S, Aggarangsi P, Moran J Biomethane: production and applications, 1st edn. Springer Nature 2. Authur W, Anna L (2006) Biogas upgrading and utilization. Technical report 3. ASME (2020) Gas transmission and distribution piping systems. https://www.asme.org/codesstandards/find-codes-standards/b31-8-gas-transmission-distribution-piping-systems 4. INNIO (2012) Jenbacher and Waukesha gas engines (Online), April 2022. https://www.innio. com/en 5. Pointon K, Langan M (2002) Distributed power generation using biogas fuelled microturbines. Technical report, The Energy Technology Support Unit 6. Behera Bk, Varma A (2019) Green gaseous fuel technology. Springer International Publishing, Cham, pp 205–264. ISBN 978-3-319-96538-3. https://doi.org/10.1007/978-3-319-96538-3_4 7. Schmersahl R, Scholz V (2005) Testing a PEM fuel cell system with biogas fuel. Agric Eng Int: CIGR EJournal 7 8. Mumme J, Scholz V, Schmersahl R (2007) Farm-based biogas production, processing, and use in polymer electrolyte membrane (PEM) fuel cells. Ind Eng Chem Res 46:8946–8950 9. Saadabadi SA, Thattai At, Fan L, Lindeboom REF, Spanjers H, Aravind PV (2018) Solid oxide fuel cells fuelled with biogas: Potential and constraints. Renew Energy 134: 194–214. ISSN 0960-1481. https://doi.org/10.1016/j.renene.2018.11.028. https://www.sciencedirect. com/science/article/pii/S0960148118313478 10. Adeniyi OM, Azimov U, Burluka A (2018) Algae biofuel: current status and future applications. Renew Sustain Energy Rev 90:316–335. https://doi.org/10.1016/j.rser.2018.03.067

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11. Cvetkovic SM, Radoicic TK, Kijevcanin M, Novakovic JG (2021) Life cycle energy assessment of biohydrogen production via biogas steam reforming: case study of biogas plant on a farm in Serbia. Int J Hydrogen Energy 46(27):14130–14137. ISSN 0360-3199. https://doi.org/10.1016/j.ijhydene.2021.01.181. https://www.sciencedirect.com/ science/article/pii/S0360319921003517 12. Ail SS, Dasappa S (2016) Biomass to liquid transportation fuel via Fischer Tropsch synthesis— technology review and current scenario. Renew Sustain Energy Rev 58:267–286. https://doi. org/10.1016/j.rser.2015.12.143

Chapter 8

Designing a Biogas Plant—Case Study

8.1 Introduction This case study comes from Nam Hong Power Co., Ltd., which is a company based in Krabi province in the south of Thailand. It produces palm oil in its factories. The raw material is fresh palm fruit which comes from Nam Hong’s own plantation, and it supplements this production by purchasing palm fruits from neighboring plantations. This palm oil mill has a capacity of about 60 tons of fresh fruit branch (FFB) per hour. The factory operates 16 h a day, 300 days a year. The production process involves steaming the fruit to extract the oil. This leaves solid waste, called the kernel which is incinerated to produce steam and power. The steam produced is used to process the incoming FFB. The liquid effluent enters a settling tank where the crude palm oil is extracted. The leftover liquid, palm oil mill effluent (POME), is treated by the biogas system. The maximum production rate is 500 m3 of wastewater per day (Fig. 8.1). Previously, this POME was treated by a series of open ponds. Since they are uncovered, any gases produced escaped into the environment. This caused odor in the local community as well as releasing large quantities of methane gas and carbon dioxide. Methane gas has a global warming potential (GWP) 25 times that of carbon dioxide. The CO2 equivalent annual emission from this system was 28,700 tCO2e . At the same time in 2013, the Thai government was promoting the use of biogas technology as both a wastewater treatment system and a way to generate additional income. Nam Hong Palm Oil decided to green light the project as the construction costs were partially supported by the Ministry of Energy. The subsidiary, Nam Hong Power Co., Ltd., was established to produce electricity for the national grid using biogas. ERDI was contracted to design and commission a digester with the capability of handling 500 m3 of POME wastewater per day, with a COD content of 60,000 mg/L. The system specifications were for a COD removal efficiency of 85% giving a biogas production rate of 16,000 m3 /day. The available site for the biogas system was 2.2 ha. After the production process the POME enters a settling tank where the oil is skimmed from the top. The leftover wastewater is sent to an oil trap where after a few hours, oil is again skimmed off. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 P. Aggarangsi et al., Biogas Technology in Southeast Asia, Green Energy and Technology, https://doi.org/10.1007/978-981-19-8887-5_8

161

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Fig. 8.1 Nam Hong palm oil mill (with permission)

From here it is pumped to the anaerobic reactor of the biogas system. The anaerobic reactor is a CMU hybrid system, which is a modified covered lagoon with internal recirculation and mixing, which was introduced in Sect. 3.6. The biogas is cleaned and dried through a biofilter and a de-humidifier before entering one of three Jenbacher 1.0 MW gensets. The electricity generation is timed to coincide with high electric prices during peak demand hours. Around 20–35.2 MWh are produced each day (Fig. 8.2). The nutrient-rich treated wastewater and sludge can be recirculated back to the plantation or donated to nearby farmers for use as fertilizer.

8.2 Plant Design A CMU hybrid, which is a trademarked covered lagoon with mechanical stirring, was selected as the digester. It is suitable for POME due to its agility and flexibility in handling large quantities of wastewater. Palm oil mill productions are based on fresh palm fruit which is highly dependent on the season. A CMU hybrid is based on the modified covered lagoon. It has a large lagoon volume and a reasonable construction cost. Unlike CSTR reactors, the CMU hybrid can store wastewater for an extended time period and thus stabilize biological processes. This is important during the low season where incoming wastewater can be as low as 10% of designed input. This low turndown capacity suits the variable palm fruit harvest. On the other hand during the peak season there are weeks where the input flow rate approaches the

8.2 Plant Design

163

Fig. 8.2 Inlet and outlet digester mass flows

maximum. CMU hybrid reactors can handle this due to jet agitation. This innovation uses the input flow rate to induce a mild-to-medium scale mixing across the volume. This action can significantly enhance biological activity and results in higher COD removal efficiency at significantly smaller digester volume compared with typical covered lagoon digesters. Another feature of the CMU hybrid is its pumping system for sludge removal which give operators control of the sludge level and distribution. Figure 8.3 shows the digester under construction. The digester size is an important design criteria. The maximum OLR for stable operation of this particular hybrid digester was designed to be 2.0 kgCOD /m3 day. Based on experience, biogas digesters operating with this OLR can deliver a COD removal efficiency of 85%. The inlet flow rate is 500 m3 /day with a maximum COD content of 60,000 mg/L. The optimal depth for this digester is about 4–5 m. This

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Fig. 8.3 Nam Hong hybrid digester construction (Source in house) Table 8.1 Parameters for digester sizing Parameter

Limit

Wastewater quantity Max. Inlet COD Digester COD removal efficiency Methane content in biogas Volumetric safety factor Max. OLR in digester Methane production efficiency

500 m3 /day 60,000 mg/l > 85% >55% 30%