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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Anaerobic Digestion: Processes, Products and Applications : Processes, Products and Applications, Nova Science Publishers,

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Anaerobic Digestion: Processes, Products and Applications : Processes, Products and Applications, Nova Science

ENVIRONMENTAL SCIENCE, ENGINEERING AND TECHNOLOGY

ANAEROBIC DIGESTION

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

PROCESSES, PRODUCTS AND APPLICATIONS

No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services. Anaerobic Digestion: Processes, Products and Applications : Processes, Products and Applications, Nova Science

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Anaerobic Digestion: Processes, Products and Applications : Processes, Products and Applications, Nova Science

ENVIRONMENTAL SCIENCE, ENGINEERING AND TECHNOLOGY

ANAEROBIC DIGESTION

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

PROCESSES, PRODUCTS AND APPLICATIONS

DANIEL J. CARUANA AND

AMANDA E. OLSEN EDITORS

Nova Science Publishers, Inc. New York

Anaerobic Digestion: Processes, Products and Applications : Processes, Products and Applications, Nova Science

Copyright © 2012 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book. Library of Congress Cataloging-in-Publication Data Anaerobic digestion : processes, products, and applications / editors, Daniel J. Caruana and Amanda E. Olsen. p. cm. Includes bibliographical references and index. 1. Sewage--Purification--Anaerobic treatment. I. ISBN:  (eBook) Caruana, Daniel J. II. Olsen, Amanda E. TD756.45.A53 2011 628.3'54--dc23 2011012588 Published by Nova Science Publishers, Inc. † New York

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Contents

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Preface

vii

Chapter I

Anaerobic Treatment of Domestic Wastewater J. A. Álvarez and M. Soto

Chapter II

Effect of Anaerobic Pretreatment by Dry Batch Technology on Aerobic Degradability of Sewage Sludge Beatrix Rózsáné Szűcs, Miklós Simon and György Füleky

Chapter III

Chapter IV

Chapter V

Real Time Identification for the Hydrogen Anaerobic Production Process Based on Multiple Differential Neural Networks and Takagi-Sugeno Fuzzy Decision Method I. Salgado, R. Fuentes, M. Alfaro, R. Cando, L. Viana and I. Chairez

1

49

77

A Scientific Review of the Agronomic, Environmental and Social Benefits of Anaerobic Digestion D. I. Massé, G. Talbot and Y. Gilbert

109

Anaerobic Digestion: Processes, Products and Applications Abdul-Sattar Nizami

133

Anaerobic Digestion: Processes, Products and Applications : Processes, Products and Applications, Nova Science

vi Chapter VI

Contents Microbial Aspects of Anaerobic Digestion for Biogas Production A. B. Chaudhari, P. C. Suryawanshi and R. M. Kothari

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Index

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149

175

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Preface Anaerobic digestion (AD) is a biological process that happens naturally when bacteria breaks down organic matter in environments with little or no oxygen. It is effectively a controlled and enclosed version of the anaerobic breakdown of organic waste in landfills which release methane. In this book, the authors present current research in the study of the processes, products and application of anaerobic activity. Topics discussed include the anaerobic treatment of wastewater; a scientific review of the agronomic, environmental and social benefits of anaerobic digestion and the microbial aspects of anaerobic digestion for biogas production. Chapter I - Anaerobic treatment has become the most frequently used method for the treatment of medium-and high-concentration effluents. The costs of construction, installation and operation of anaerobic reactors are lower than those of conventional aerobic units because the reactor does not require equipment for process maintenance and control. In fact, if the environmental conditions inside the reactor are adequate, anaerobic processes are mainly self-controlled. Additionally, the production of excess sludge is minimal, and energy balances are quite favourable due to the production of methane, even when heating is required. The use of anaerobic reactors for the treatment of low-strength wastewater, including domestic sewage and industrial effluents, has been definitively established in tropical and sub-tropical regions, where wastewater temperatures are above 20ºC. Low biomass generation and low or zero energy requirements are the main advantages of wastewater anaerobic treatment. On the other hand, at low temperature conditions (below 20ºC), the hydrolysis rate is low and a high amount of suspended solids (SS) accumulates on the biomass bed, reducing the efficiency of one-stage up flow anaerobic sludge blanket (UASB) digesters. However, in the

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Daniel J. Caruana and Amanda E. Olsen

last years, it has been studied different anaerobic technologies to solve these problems and improved anaerobic treatment of municipal wastewater at temperate and cold climates. In this chapter, the results of different case studies on the treatment of municipal wastewater with single-stage or two-stage high-rate anaerobic systems at temperatures ranging from 13 to 20 ºC are presented. At one-stage configuration, hydrolytic (HUSB - hydrolytic upflow sludge bed-) and methanogenic (UASB) reactor were operated at temperatures between 15 and 20ºC. A high suspended solids (SS) removal of about 82-85% from the influent was reached, most of which (81 to 88%) was eliminated by hydrolysis in HUSB reactor. Removals of 85%, 58% and 63% for TSS, total COD and BOD5 were achieved in UASB reactor, respectively, at 15ºC. At several two-stage configuration, like the hydrolytic-methanogenic (HUSB-UASB) and UASBCMSS (completely mixed sludge stabilization) systems were performed at low temperatures. Single-stage configuration organic matter removals were overcame and high hydrolysis of SS was obtained. Based on the research conducted during last decade, anaerobic treatment can be considered as a low-cost, robust, and long-term treatment or pretreatment alternative of domestic wastewater with COD higher than 250 mg/L. Chapter II - An alternative treatment exists for the stabilization of organic wastes: the semidry batch treatment. The benefit of this technology is the required smaller reactor volume and its simplicity. However, there is a lack of information about the optimum seeding ratio of treating sewage sludge with semi dry batch technology. It can be assumed that the semi dry anaerobic technology can be an alternative treatment option to stabilize sewage sludge for smaller sized waste water treatment plants. Nowadays, more and more plants use sludge composting after digestion as an aerobic post treatment step. The authors‘ goal was to evaluate the possibility of the semi dry batch treatment for sewage sludge digestion and give information about the optimization of seeding. The authors‘ main objective was to contribute to the optimization of the sequential anaerobic and aerobic treatment by investigating the effect of anaerobic stabilization on the aerobic degradability of sludge. The authors investigated the effect of seeding with 7 different substrate seed ratios. The anaerobic pretreatment of samples used in the aerobic treatment was done in a 70 dm3 reactor using 1:1.25 substrate seed ratios, established as an optimum in the authors anaerobic experiments. Digested samples were removed from the anaerobic reactor and transferred to the composting reactors every 10 days, giving the possibility to investigate the aerobic degradability of digested sludge samples.

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Preface

ix

On the basis of preliminary anaerobic experiments, the authors can state that the semi dry anaerobic batch treatments can be an alternative technology for treating sewage sludge where the seeding ratio is a very important factor of operation. A seeding ratio of 1:1.25 considered the optimum ensuring balanced process as the main target. It was concluded that the anaerobic pretreatment basically determined the process of aerobic post treatment. However, the aerobic degradation could be a more effective continuation of the stabilization after a given anaerobic pretreatment than the anaerobic one. The anaerobic stabilization of the sewage sludge could follow the aerobic way, leading to a more stabile endproduct. Chapter III - In nature, many systems show very complex behaviors. Among others characteristics, some of those plants exhibit a high degree of oscillations throughout the time or metastable locations. In particular, biological systems which are slow compared with physical or chemical ones, includes lots of examples where those metastable regions are really important to understand their dynamics. Moreover, those metastable states produce very complex mathematical models that usually are inaccuracy or fail to reproduce such real behaviors. In real life, if the mathematical model has the aforementioned failures, the process control may become into a hard task. The anaerobic digestion satisfies all aforementioned characteristics. Indeed, when this process is used to produce methane or hydrogen in continuous regimen, many metastable regions will appear. On the other hand, to obtain an accuracy model of such process may be simplified using mass balance methods. Nevertheless, no one model could reproduce the trajectories observed in real digestion process. A natural alternative to solve this problem is to use adaptive algorithms to obtain approximation models. Nevertheless, adaptive algorithms used to approximate such difficult behaviors can also show important deficiencies. Many adaptive nonparametric algorithms could not reconstruct the trajectories of such complex dynamics. The differential neural network (DNN) is not an exception. Indeed, when just one DNN is applied to achieve the approximation, the modeling error may be not so close to zero. One possible suggestion to solve this problem is to construct a set of DNN working in parallel. The members of such set will work each one on welldefined trajectories subspaces where the uncertain system evolves. How to combine the identification properties offered by the DNN and the characteristic decision capabilities provided by fuzzy methods is the main topic discussed in this chapter. The selection of which neural network is activated depends on decision achieved by a Takagi-Sugeno fuzzy system.

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Even when the proposed method works efficiently in numerical simulations, the proposed method should demonstrate its workability in real situations. That is why the authors reported the construction of low-cost embedded device that implements the designed adaptive modeling scheme. The designed device works with similar quality to that performed in numerical results. Chapter IV - According to the FAO, the intensification of the livestock sector contributes to climate change, air pollution, soil and water degradation, and biodiversity depletion. The agricultural sector dependency on fossil fuels and energy intensive fertilizers contributes to the decrease of fuel reserves and additional anthropogenic GHG emissions. These environmental and energy concerns are leading to a renewed interest in environmental biotechnologies that can produce renewable energy, minimize environmental impact and add value to livestock operations‘ by-products. This paper describes the potential contribution of on-farm biogas production in reducing GHG emissions from livestock operation, either via the production of renewable energy to substitute fossil fuel, via the reduction of fugitive GHG emissions from stored manure and land-applied manures and via the reduction in chemical fertilizer needs for forages and cash crop productions. Anaerobic digestion biotechnologies produce biogas at average rates of 0.3, 0.25 and 0.48 L g-1 VS from swine, bovine and poultry slurries respectively. The biogas produced is of high quality with a methane concentration ranging between 60 and 80%. AD treatments can be an acceptable solution to the management of phosphorous surplus by precipitating up to 25% of the phosphorous in batch or semi-batch operated bioreactors and by additionally precipitating and concentrating up to 70% of the bioreactor effluent‘s phosphorous at the bottom of the long term storage. These effluents are more balanced to meet the crop needs thus reducing the need for supplementary chemical nitrogen fertilizers. Both the recovery of green energy and the reduced needs for chemical fertilizers will substantially decrease the carbon footprint of livestock food products. On-farm biogas production contributes to achieve more sustainable livestock operations by substantially reducing other environmental impacts related to manure management. It reduces the risk of water pollution associated with animal manure slurries (eutrophication) by removing 80 to 90% of the soluble chemical oxygen demand (COD). In addition, some AD processes are effective in eliminating populations of zoonotic pathogens and parasites present in livestock manures. AD also improves cohabitation in rural regions by reducing odour emissions by 70 to 95%. The reduction in odour allows more frequent and better timing for land applications of manure. Both the timing of application and the improve nutrients

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Preface

xi

balance have the potential to improve nutrients uptake by the crops and minimize nutrient loss to the environment. The reduction in weed seeds during AD process reduces the need for herbicides and makes the bioreactor‘s effluent more acceptable for organic farming. Chapter V - Anaerobic digestion produces biogas by the conversion of organic matter under anaerobic conditions. The biogas is mainly contains methane (55-80%) and carbon dioxide (20-45%). The anaerobic digestion is completed in four successive biological processes such as hydrolysis, acidogenesis, acetogenesis and methanogenesis. In hydrolysis, monomers are produced from complex polymers by extra-cellular enzymes and further transformed into volatile fatty acids (acetic, propionic and butyric acids) and hydrogen (H2) during acidogenesis. In acetogensis, acetate, carbon dioxide and H2 are produced from volatile fatty acids and finally converted into methane in the methanogensis process. The process is widely used for the treatment of organic waste such as manure, farm waste, wastewater, industrial organic waste, municipal solid waste and agricultural residues such as crops, crops residues and grass silage. An array of anaerobic digester and configurations exists to complete the process based on: dry or wet process; batch or continuous process; number of phases or stages of digestion activities; operating temperature (thermophilic or mesophilic); retention time and organic loading rate. The substrate and its properties is an essential criteria in selecting digester type and configuration. The resultant of the process; biogas is used as a renewable energy source for combined heat and power, renewable gas for heating purposes and biofuel for transport fuel. The process also results in residues known as digestate, which is generally used as a fertilizer. Alternatively, the digestate can be separated into liquid and fibre components when treating high solid content feedstock such as organic fraction of municipal solid waste and agricultural residues. The efficiency of the anaerobic process can be increased by recycling back some of the liquid portion of digestate and remaining can be converted into liquid biofertilizer or a press juice for multipurpose usage. The solid leftover is processed into fibers and applied to land as soil conditioners or as high value insulation boards. This scheme of the generation of biomethane with value added products has now emerged into a new concept of biorefinery. This will make the anaerobic digestion process more sustainable and economically viable by increasing the GHG emissions savings, reducing technological cost with high process efficiency. Chapter VI - Biogas production through anaerobic digestion of biodegradable solid and liquid waste appears to be a sustainable approach. It involves hydrolysis of waste by a consortium of aerobes, acidogenesis by facultative microbes,

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followed by acetogenesis by both facultative or obligate anaerobes and finally methanogenesis through acetotrophic/hydrogenotrophic/ methylotrophic reactions. For this purpose, habitats of facultative and obligate anaerobes have been reviewed, classification of methanogens undertaken, their substratespecificity examined and metabolic pathways for methane production from a variety of substrates discussed through major and minor metabolic pathways, including the role of syntrophs. Together, these microbes provide efficiency of digestion and if their integrated processes are not attended in time, it leads to deterioration in methane production and at times digester failure. Finally, ecofriendly applications for biogas before and after up-gradation are considered, concluding intricacies of the process characterized by ecological and economical gains to control pollution.

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In: Anaerobic Digestions Editors: D. J. Caruana, A. E. Olsen

ISBN 978-1-61324-420-3 © 2012 Nova Science Publishers, Inc.

Chapter I

Anaerobic Treatment of Domestic Wastewater J. A. Álvarez1 and M. Soto2

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1

Department of Environment. AIMEN Technology Center. Rúa Relva, Torneiros, Porriño, Pontevedra, Galiza, Spain 2 Department of Physico-Chemical and Chemical Engineering I. University of A Coruña. Rúa da, Fraga, A Coruña. Galiza, Spain

Abstract Anaerobic treatment has become the most frequently used method for the treatment of medium-and high-concentration effluents. The costs of construction, installation and operation of anaerobic reactors are lower than those of conventional aerobic units because the reactor does not require equipment for process maintenance and control. In fact, if the environmental conditions inside the reactor are adequate, anaerobic processes are mainly self-controlled. Additionally, the production of excess sludge is minimal, and energy balances are quite favourable due to the production of methane, even when heating is required. The use of anaerobic reactors for the treatment of low-strength wastewater, including domestic sewage and industrial effluents, has been definitively established in tropical and sub-tropical regions, where wastewater temperatures are above 20ºC. Low biomass generation and low or zero energy requirements are the main advantages of wastewater anaerobic

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2

J. A. Álvarez and M. Soto treatment. On the other hand, at low temperature conditions (below 20ºC), the hydrolysis rate is low and a high amount of suspended solids (SS) accumulates on the biomass bed, reducing the efficiency of one-stage up flow anaerobic sludge blanket (UASB) digesters. However, in the last years, it has been studied different anaerobic technologies to solve these problems and improved anaerobic treatment of municipal wastewater at temperate and cold climates. In this chapter, the results of different case studies on the treatment of municipal wastewater with single-stage or two-stage high-rate anaerobic systems at temperatures ranging from 13 to 20 ºC are presented. At one-stage configuration, hydrolytic (HUSB - hydrolytic upflow sludge bed-) and methanogenic (UASB) reactor were operated at temperatures between 15 and 20ºC. A high suspended solids (SS) removal of about 82-85% from the influent was reached, most of which (81 to 88%) was eliminated by hydrolysis in HUSB reactor. Removals of 85%, 58% and 63% for TSS, total COD and BOD5 were achieved in UASB reactor, respectively, at 15ºC. At several two-stage configuration, like the hydrolytic-methanogenic (HUSBUASB) and UASB-CMSS (completely mixed sludge stabilization) systems were performed at low temperatures. Single-stage configuration organic matter removals were overcame and high hydrolysis of SS was obtained. Based on the research conducted during last decade, anaerobic treatment can be considered as a low-cost, robust, and long-term treatment or pretreatment alternative of domestic wastewater with COD higher than 250 mg/L.

Abbreviations ABR: AF: AH: BOD: CMSS: COD: EGSB: HRT: HUSB: SRT: TSS, VSS: UASB: VFA:

anaerobic baffled reactor anaerobic filter anaerobic hybrid biological oxygen demand completely mixed sludge stabilization chemical oxygen demand expanded granular sludge blanket Hydraulic retention time Hydrolytic upflow sludge blanket sludge retention time total and volatile suspended solids Upflow anaerobic sludge blanket volatile fatty acids

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Anaerobic Treatment of Domestic Wastewater

3

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1. Introduction Decentralised, sustainable sanitation concepts focus on treatment and recycling of resources present in domestic wastewater such as bio-energy generated from transformation of organic material, plant nutrients (nitrogen, phosphorus as main nutrients but also potassium and sulphur) and water (produced after advanced treatment) (Kujawa-Roeleveld and Zeeman, 2006). Therefore, the selected treatment technology should meet the relevant long term sustainability commandments which were reported by Lettinga et al. (2001a) as follows; (i) little if any use of mineral resources and energy, (ii) enabling production of resource/energy from wastes, (iii) pairing high efficiency with long term of lives, (iv) applicable at any place and at any scale, and (v) plain in construction, operation and maintenance. It was reported that concentrated black water and low concentrated grey water streams in domestic wastewater represent a reuse potential; the concentrated streams – energy and nutrients-, the diluted streams – energy and clean water (Elmitwalli and Otterpohl, 2007; Zeeman et al., 2008). Anaerobic wastewater treatment concept simply offers benefits in the light of very desirable development and implementation of sustainable and non-vulnerable methods and technologies compared to conventional aerobic methods (Lettinga, 2001b). A comparison between municipal wastewater aerobic and anaerobic treatment operation and process is showed in figure 1. Applications of anaerobic processes are recently being extended to the treatment of dilute industrial wastewaters, and even anaerobic sewage treatment is thought to be achievable under certain conditions. It was reported that anaerobic treatment of low strength wastewater seems to be an ideal and reasonable solution for environmental protection (Leitao et al., 2006). Although anaerobic treatment application for sewage has been considerably limited compared to its application for the industrial wastewaters, sewage treatment by high-rate anaerobic systems has been widely reported over the last two decades. High-rate anaerobic treatment is an attractive process for domestic sewage because of its low construction, operation and maintenance costs, small land requirement, low excess sludge production, and opportunity of biogas production (Elmitwalli et al., 2002). Anaerobic processes have been traditionally operated at mesophilic conditions with an operating temperature of 35–37ºC although the temperature of certain wastewater currents might be either considerably warmer such as pulp and paper industry effluents or cooler such as brewery industry effluents and domestic

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J. A. Álvarez and M. Soto

sewage. Treating these effluents with their initial temperatures would often be beneficial in order to reduce resources and costs owing to no heating or cooling requirements (Kettunen et al., 1997; Lettinga et al., 1999; Trzcinski et al., 2010). Since all the high-rate processes have the ability to separate hydraulic and solid retention times effectively, relatively low hydraulic retention times (HRT) are allowed due to the accumulation of a high biomass concentration in the system (Hickey et al., 1991). Lettinga et al. (1999) reported that immobilization of viable biomass is an important factor for the temperature susceptibility. In spite of their dilute characteristics, many industrial and domestic wastewaters were reported to be treated successfully owing to the presence of large amounts of anaerobic sludge even at low operational temperatures (Banik et al., 1997; Van Lier et al., 1997b). However, even though wastewater treatment with high-rate anaerobic systems, especially when artificial heating can be eliminated, has indicated significant benefits in reducing the cost and energy needs, there are certain drawbacks of anaerobic sewage treatment at low operating temperatures which should be clarified.

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2. Domestic Wastewater Domestic wastewater is defined as all wastewater produced during different human activities mixed together. It is the liquid component of waste removed from residences, business, and institutions. However it may also include liquid waste from industrial establishments in many areas. Sewage, on the other hand, consists of household waste liquid from toilets, baths, showers, kitchens, sinks and further that is disposed of via sewers. A lot of sewage also includes some surface water from roofs or hard-standing areas. Municipal wastewater therefore may include storm water runoff due to combined sewer system. Greywater is the wastewater generated from domestic activities such as dishwashing, laundry and bathing. Greywater comprises 50–80% of residential wastewater generated from all of the house‘s sanitation equipment except for the toilets. Blackwater, on the other hand, is a term used to describe wastewater containing fecal matter and urine. Besides, blackwater contains pathogens, organic matter and nutrients requiring decomposition before they can be released safely into the environment. Thus, separation of blackwater and greywater is accomplished nowadays within all ecological buildings. In recent years, concerns over dwindling reserves of groundwater and overloaded or costly sewage

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Anaerobic Treatment of Domestic Wastewater

5

treatment plants have generated much interest in the reuse or recycling of greywater, both domestically (for flushing toilets) and for use in commercial irrigation. It was reported that treated greywater can be reused in the household, used for irrigation or returned to nature – discharged to surface water or percolated to groundwater. However special precautions are needed when grey water will be used for irrigation if no disinfection is applied (Kujawa and Zeeman, 2006). Domestic wastewater flow and composition changes significantly in time and from one place to another. The cyclical nature of human activities leads to a variable sewage production over the day. COD concentrations are given as 250, 430, and 800 mg/L for low, medium, and high strength untreated domestic Flare Influent Settler 1

Aerobic treatment (activated sludge)

Influent

Sludge

Anaerobic treatment (UASB)

Effluent

Anaerobic sludge

A

B Biogas: 45 kg COD

CO2: 35 kg COD Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

Sales

2

Return sludge

Effluent: 10 kg COD

Waste water: 100 kg COD

Primary sludge: 25 kg COD

Effluent: 30 kg COD Sludge: 25 kg COD

Secondary sludge: 30 kg COD

Flare Anaerobic sludge Influent

Generator Biogas

Settler Effluent

Sales

Generator Biogas

Aeration

Biogas: 55 kg COD Anaerobic treatment (UASB)

Aerobic treatment (activated sludge) Return sludge

Effluent Settler

C

CO2: 15 kg COD Effluent: 10 kg COD Sludge: 20 kg COD

Aerobic excess sludge

Figure 1. Municipal wastewater aerobic and anaerobic treatment operation (Fdz-Polanco et al., 2009). A: Conventional activated sludge (Aerobic); B: Anaerobic process; C: Combined anaerobic/aerobic process.

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wastewater. Sewage is relatively more concentrated in the countries where water consumption is limited. Total COD concentration is observed in the range of 1.52.0 g/L in Jordan whereas total COD concentration is observed in the range of 0.2–0.7 g/L in European countries. High flow variations are due to the number of inhabitants and dwellings connected to the sewer system, specific characteristics of the sewerage (type, material, length, maintenance, infiltration, use of pump stations), as well as climate, topography and commercial/industrial contributions (Kujawa and Zeeman, 2006; Leitao et al., 2006; Gomec, 2010). The concentration of organic matter can vary by a factor of 2-10 in a few hours and the flow rate can fluctuate by a factor of 4 depending mainly on the size of population (the larger the population, the smaller the fluctuation) and the type of sewer. For instance, combined sewer systems are usually not preferred because of the fact that the seasonal fluctuations in flow, concentration and composition reduce the efficiency of treatment plants. Besides, strong fluctuations are also reported in sewage temperature (Lens et al., 2001). Raw domestic sewage is characterized by a high fraction of suspended and colloidal COD. The high fraction of particulate COD, moderate biodegradability of various COD fractions, low strength character with various concentrations and its relatively low temperature make sewage to be categorized as a ‗complex‘ wastewater. Presence of fatty compounds, proteins, and detergents, among other barely known compounds, make domestic sewage quite complex and impose limitations to anaerobic process with respect to COD removal efficiency, and also in terms of the applied maximum organic and hydraulic loading rates (Lettinga et al., 2001a; Foresti, 2002).

3. Anaerobic Digestion The processes involved in anaerobic digestion are catalyzed by intra- or extracellular enzymes and available soluble and particulate organic matters are used. Composite particulate materials are converted into carbohydrates, proteins, and lipids by the extracellular process called disintegration. Complex organic compounds such as proteins, carbohydrates, and lipids are then transformed into simple soluble products such as amino acids, sugars, and long chain fatty acids and glycerine, by the action of extracellular enzymes excreted by the fermentative bacteria. This step is commonly known as hydrolysis or liquefaction which can be a rate-limiting step in the overall anaerobic treatment processes for waste

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containing lipids and/or a significant amount of particulate matter (Lens et al., 2001). The catabolism is initiated by fermentative bacteria producing acids and alcohols that are then readily utilized as substrates by acetogenic bacteria. The methanogens obtain energy from converting acetate, carbon dioxide, and hydrogen to methane at the final stage. Thus, very efficient cooperation and mutual dependence can occur within anaerobic granules providing an ideal environment for the establishment of syntrophic relationships. In a complex multistep process such as anaerobic digestion, the kinetic characteristics of the slowest step govern the overall rate of anaerobic degradation. If the digestion process is to be optimised, the rate-limiting step must be enhanced. Because change and/or retardation of the rate limiting step may clue in the presence of a toxicant before the toxicity severely affect the overall process. Hutnan et al. (1999) reported that considering easy biodegradable materials (containing short-chain VFA, monomeric saccharides, etc.) the limiting step of anaerobic degradation is generally the methanogenic step. On the other hand, during the anaerobic digestion of complex materials (e.g. wastes/wastewaters which are mainly composed of cellulosics, lipids, and proteins), the limiting step of the process is often the hydrolytic step.

3.1. Anaerobic Biomass The development of the microbial population that is appropriate to treat wastewater has vital importance in anaerobic reactors. Generally, the reactors are inoculated with a suitable seed source in order to shorten the start-up period. Verstraete and Vandevivere (1999) also suggested inoculation (e.g., the granular sludge) for an effective start-up of anaerobic reactors. It was reported that an anaerobic reactor, operated at ambient temperature, could reach a stable phase in a short time even if mesophilic sludge is used at start-up and adapted to low temperatures by gradual temperature reductions (Akila and Chandra, 2007). O‘Flaherty et al. (2006) reported that mesophiles have been indicated to grow under low temperature conditions and show psychro-tolerant tendencies. Anaerobic reactors are commonly inoculated with digester sludge. After 2–3 months of operation, provided the temperatures are high enough, a very concentrated sludge bed (40–100 kg volatile suspended solids per m3) develops near the bottom. This developed sludge is very dense and may be granular in nature with a high settling velocity. Granular sludge development is dependant on

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the characteristics of the wastewater and on the seed sludge used when starting the reactor (Ligero et al., 2002; Aiyuk et al., 2006). Though always desired, the formation of anaerobic sludge granules cannot be guaranteed on each type of wastewater. Sludge granulation is possible when the sludge residence time (SRT) reaches a time period of several months. Since the success of specific anaerobic reactors relies on the formation of active and settleable sludge granules, granulation mechanisms as well as the microbial composition of the granules have been studied by several researchers in order to provide a full understanding of the syntrophic associations between the hydrogenproducing acetogens and the hydrogen-consuming methanogens (Lens et al., 2001). Granular sludge is an aggregate and much attention has been focused on the microbial structure and composition of granular sludge. Different types of anaerobic bacteria compose the granular sludge and play an important function in the occurrence of the syntrophic association in granules. It was reported that studies should focus on the syntrophic association between H2-producing acetogenic bacteria and methanogens in granular sludge (Jianrong et al., 1997). Methanogens are found at large numbers and belong to Archaea. Two functional groups of methanogens (aceticlastic and hydrogenotrophic) have been well described in terms of physiology and phylogeny. Aceticlastic methanogens are in the order of Methanosarcinales (Methanosaetaceae and Methanosarcinaceae) and hydrogenotrophic methanogens are in the orders of Methanobacteriales, Methanomicrobiales, and Methanococcales. Organisms assumed to be aceticlastic were more abundant than organisms assumed to be hyrogenotrophic in most anaerobic systems. Generally, the granules are composed of Methanosaeta spp. (formerly Methanothrix) rather than Methanosarcina spp. in high rate anaerobic sludge bed reactors and aceticlastic Methanosaeta is the key structural element in all anaerobic granules (O‘Flaherty et al., 2006). These filamentous microorganisms play an important role in sludge granulation. Different morphologies from filaments longer that 1000 units to short filaments of 5–10 units could be observed for this bacterium which is the dominant aceticlastic species under low substrate concentrations in the case when sewage is used as the feed (Alves et al., 2000). In a well-operated anaerobic reactor, the developed granular sludge has often high methanogenic activity (Hickey et al., 1991). Usually, the granules have a complex layered structure. On the outer surface, mainly fermentative bacteria and hydrogenotrophic methanogens exist and the inner layer is occupied by aceticlastic methanogens and H2-producing bacteria (Jianrong et al., 1997).

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The anaerobic bacteria are generally divided into three thermal groups; psychrophiles, mesophiles, and thermophiles with optimum temperatures at 45ºC, respectively (Van Lier et al., 1997b). Numbers and activities of most bacteria are strongly dependent on the temperature and just after the temperature changes, the activities of bacteria are significantly suppressed resulting in VFAs accumulation until the bacteria recovery from temperature shock. Luostarinen et al. (2007) refer temperature as an important factor in anaerobic treatment of domestic wastewater: the higher the temperature, the higher the conversion rates.

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4. Evolution of Anaerobic Treatment: High Rate Anaerobic Reactors Anaerobic processes have been extensively applied for the digestion of primary and secondary sludge in wastewater treatments plants based on conventional aerobic systems such as the activated sludge and trickling filter systems (Mc Carthy, 1982). From 1950 to 1980, several anaerobic reactor configurations were developed for industrial wastewater treatment (Mc Carthy, 1982). In any case, anaerobic digestion was considered to be feasible for highstrength wastewater and only for temperature conditions above 25ºC (Kalogo and Verstraete, 2001). More recently, the anaerobic process proved also to be suited for the treatment of low-strength domestic sewage (Foresti, 2006). But the successful full-scale application and operation of anaerobic reactors is still restricted to tropical regions, where sewage temperatures generally exceed 20ºC (Gomec, 2010). Classic anaerobic sewage treatment systems consisted in tanks, which are known as decanter-digester, in which settleable solids were retained and digested at the bottom by the anaerobic sludge. Typical of these classic systems are septic tank and Imhoff tank. The treatment efficiency of organic material and suspended solids in raw sewage generally is of the order of 30 to 50% (Van Haandel et al., 2006). The basis for the anaerobic treatment of high capacity with high efficiency was established by the provision of the two essential requisites: intense contact and large bacterial mass retention (Van Haandel et al., 2006), these requisites are converted in absolutely necessary conditions when the anaerobic system works under psychrophilic conditions (Lettinga et al., 2001a).

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High-rate anaerobic treatment has emerged as a viable alternative for the treatment of many industrial and municipal wastewaters due to their ability to separate hydraulic and solid retention times effectively. Since high biomass concentration can be accumulated in the system, relatively low hydraulic retention times are allowed in high-rate anaerobic treatment processes. Retention of a high concentration of active sludge ensures a good treatment performance, for instance total COD reductions up to 80–90% are reported. Even at temperatures below 20ºC, average total COD removal of 70% can be expected. Low construction cost, small land requirements, and high loading rates are other benefits of these systems. Operation and maintenance costs are lower as well as energy consumption and equipment requirement. As compared to aerobic and physicalchemical treatment processes, lower sludge production occurs and the biogas produced can be used for energy production (Hickey et al., 1991; Schink, 2002; Kujawa-Roeleveld and Zeeman, 2006; Leitao et al., 2006). In the case when the rate-limiting step in the overall digestion process is the hydrolysis of the retained particulates, relatively longer solid retention time (SRT) is required depending on the applied process temperature (Lettinga et al., 2001a). According to Iza (1991), three fundamental aspects on which the concept of highrate anaerobic reactors are based can be summarized as: (a) biomass can accumulate within the reactor by means of settling, attachment to solids (fixed or mobile) or by recirculation; (b) contact between biomass and wastewater is improved that overcomes problems of diffusion of substrates and products from the bulk liquid to biofilms or granules; (c) due to adaptation and growth, the activity of the biomass is enhanced. Besides, efficient operation can primarily be determined by effective separation of the biomass from the liquid phase. Slowly growing microorganisms can be retained by ensuring that the mean solids retention time becomes much longer while keeping the mean hydraulic retention times short. These configurations thus allow high organic loadings in small reactor sizes, and the long solid retention times provide generally good process stability (Aiyuk et al., 2006). Typical high-rate anaerobic reactors are classified as anaerobic contact process, fixed film or anaerobic filter reactor (AF), downflow stationary fixed film reactor, fluidized bed reactor, expanded granular sludge bed reactor (EGSB), internal circulation reactor, anaerobic baffled reactor (ABR), hydrolytic up flow sludge blanket reactor (HUSB), upflow anaerobic sludge bed reactor (UASB) and anaerobic hybrid reactor (AH) (Aiyuk et al., 2006). Seghezzo et al. (1998) reported that UASB and EGSB reactors are the most powerful anaerobic treatment system for low strength wastewater such as sewage. However, application of ABR for the treatment of domestic sewage has improved

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in recent years (Bodkhe et al., 2009). Several other high-rate anaerobic process applications for the treatment of sewage, not covered in this chapter, are reported in literature like anaerobic filters, anaerobic hybrid reactors, traditional anaerobic digesters, anaerobic fluidized bed reactors, etc (Seghezzo et al., 1998; Elmitwalli et al., 2002; Martin et al., 2009). A review on anaerobic reactor design concepts for the treatment of domestic wastewater was studied by Van Haandel et al. (2006). According to Leitao et al. (2006), there are four indications for evaluating the robustness and stability of a UASB and therefore other anaerobic reactors: COD removal efficiency, effluent variability, pH stability, and recovery time. These aspects are discussed for UASB, ABR and EGSB reactors in the following sections.

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4.1. Upflow Anaerobic Sludge Bed (UASB) Reactor The UASB reactor system, which was developed by Lettinga and his coworkers in 1970‘s, has received widespread acceptance and has been used successfully in the treatment of several types of wastewaters (Hickey et al, 1991). UASB reactor, in which biomass recirculation is not required, has a capability to retain a high concentration of active suspended biomass which explains the great success of this reactor. UASB reactors are fed in upflow mode and they act first as settling devices. The retention of both biomass and influent suspended solids is crucial for the subsequent biological processes that take place in the system. Compared to other anaerobic reactor configurations, UASB reactors have the advantage of avoiding any support media for biomass attached growth, an important characteristic in the treatment of wastewaters consisting of high influent suspended solids (De Sousa and Foresti, 1996; Seghezzo et al., 2006). A biological reaction zone and a sedimentation zone are the two main zones found in a typical UASB reactor. The organic compounds in the influent are converted to methane and carbon dioxide in the reaction zone during the passage of the flow upward through a bed of highly active sludge. The gas-solid-liquid separator at the top of the reactor helps the gas produced and the sludge buoyed by entrapped or attached gas bubbles to be separated from the liquid effluent (Hickey et al, 1991). Modern ‗high rate‘ anaerobic treatment systems such as UASB reactors are based on sludge immobilization retaining as much viable sludge as possible in the reactor. The wastewater passes through the anaerobic biomass in which dissolved

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substrate is digested and the particulate matter firstly is captured and next digested. Speece (1996) reported that one of the most important issues in the design process of anaerobic reactors for a particular wastewater is the selection of the most appropriate method of immobilizing the biomass. Lettinga et al. (1997) also reported that whatever reactor system is considered, in all cases ‗immobilization of the proper balanced anaerobic bacterial consortia‘ is the key for the success. In the UASB reactor, the anaerobic microorganisms agglomerate to form biogranules by a process of impulsive aggregation of bacteria to dense compact granules with good settling characteristics (Aiyuk et al., 2006; Sabry, 2008). UASB reactor has been considered the most appropriate anaerobic system to treat domestic wastewater because of its simplicity, low investment and operation costs and the long favourable experience in the treatment of a wide range of wastewater types. UASB reactor application for sewage treatment at low operational temperatures has been studied in Netherlands since 1976 (Lettinga et al., 1981). However, this first application of UASB reactor for domestic sewage treatment gave contradictory results. The performance was not as high as expected and bad odours produced in some plants made its application undesired. But two decades later, in tropical regions, UASB reactor configuration has been also reported to be the process most employed for domestic sewage (Verstraete and Vandevivere, 1999; Foresti, 2002). Thereupon, full-scale anaerobic sewage treatment applications have increased rapidly after satisfactory results were obtained with COD and BOD removal efficiencies higher than 75% at an average temperature of 25ºC at the first pilot-plant UASB reactor treating rather dilute sewage in Colombia (Cali). The main drawback of anaerobic treatment of domestic sewage in the highrate systems is the long start up period when seed sludge is not available due to low growth rate of methanogenic microorganisms (Elmitwalli and Otterpohl, 2007). Thus, development of the microbial population that is appropriate to the treated wastewater has vital importance in the UASB reactors. Results reported in the literature for the conventional UASB reactors treating domestic sewage usually indicate COD removal efficiencies below 70% and suspended solids in the final effluent around 60–100 mg/L (Chernicharo et al., 1999). Since suspended solids represent the major COD fraction in domestic wastewater, high removal of total COD will be achieved by high removal of suspended solids which occurs by settling and by filtration through the sludge bed and/or filter media. Suspended solids and colloidal particles represent around 70%–80% and 30%–20% of particulate matter respectively in domestic sewage.

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Entrapment, sorption, and straining are three mechanisms of filtration. Influent concentration, temperature, reactor height, hydraulic retention time (HRT), upflow velocity and particle size and density are the main factors affecting the physical removal of suspended solids in anaerobic high rate systems. The up-flow mode of operation in the UASB reactor system improves the physical removal of suspended solids by the gravity settling and by entrapment mechanism (Sabri, 2008). Since particulate organics are physically removed by settling, adsorption and entrapment firstly in the UASB reactor, the rate-limiting step in the overall digestion process is the hydrolysis of these retained particulates requiring relatively long retention times depending on the applied process temperature. Long SRT requirement is also reported by Kujawa-Roeleveld and Zeeman (2006) in order to provide adequate hydrolysis and methanogenesis. The retention of solids was improved with a better design of the settling zone with tilted plates resulting in enhanced reactor performance, mainly when operating at short HRT (