i

DRY ANAEROBIC DIGESTION OF MUNICIPAL SOLIDWASTE AND DIGESTATE MANAGEMENT STRATEGIES

by

Zeshan

A dissertation submitted in partial fulfillment of the requirements forthe degree of Doctor of Philosophy in

Environmental Engineering and Management

Examination Committee: Prof. Chettiyappan Visvanathan(Chairperson)Prof. Ajit P. AnnachhatreDr. P. Abdul Salam

External Examiner: Dr. Yasumasa TojoLaboratory of Solid Waste DisposalEngineeringDivision of Environmental EngineeringHokkaido University, Japan

Nationality: PakistaniPrevious Degree: Master of Science (Honors) Agriculture

(Soil and Environmental Sciences)University of Agriculture, Faisalabad

Scholarship donor: Higher Education Commission (HEC)Pakistan-AIT Fellowship

Asian Institute of TechnologySchool of Environment, Resources and Development

ThailandDecember 2012

ii

Acknowledgements

First of all, the author would like to thank Allah, the most gracious, the most beneficent,for giving him the opportunity to achieve higher education at this level and giving him thecourage and the patience during the course of his Ph.D. study at AIT.

The author would like to express his profound gratitude to his adviser Prof. C. Visvanathanfor kindly giving valuable guidance, stimulating suggestions and ample encouragementduring the study at AIT. The author is deeply indebted to Prof. Ajit Annachhatre and Dr.Abdul Salam for their valuable comments, suggestions and support and serving asmembers of the examination committee.

A special thank is addressed to Dr. Yasumasa TOJO for kindly accepting to serve as theexternal examiner. His constructive and professional comments will be highly appreciated.A special note of appreciation is extended to Dr. Obuli P. Karthikeyan for his help andgreat interest in this research including valuable comments and suggestions during variousphases. Special thanks to Dr. Romchat Rattanaoudom for her help and guide as a seniorand a colleague.

Sincere thanks are given to Ms. Phonthida Sensai, Mr. Supawat Chaikasem, Mr.Muhammad Zeeshan Ali Khan and Mr. Amila Abeynayaka as helping friends. The authorwould also like to thank all friends, EEM staff, laboratory colleagues and technicians fortheir help, moral support and cooperation which contributed in various ways to thecompletion of this dissertation.

The author gratefully acknowledges Higher Education Commission (HEC) of Pakistan andAIT for the joint scholarship for the Ph.D. study at AIT.

The author would like to dedicate this piece of work to his beloved brother who passedaway during the course of this study. His long lasting love and prayers always inspired andencouraged author to fulfill his desires.

Deepest and sincere gratitude goes to his beloved parents (Mr. and Mrs. SheikhMuhammad Ramzan) for their endless love, encouragement and prayers. The authorwishes to express his deepest appreciation to his siblings for their prayers, patience andunderstanding throughout the entire period of this study.

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Abstract

Global solid waste generation is continuously rising. Improper disposal of the giganticamount of solid waste seriously affects the environment and contributes to climate changeby release of green house gases (GHGs). Practicing anaerobic digestion for organicfraction of municipal solid waste (OFMSW) can reduce emissions to environment andthereby alleviate the environmental problems together with production of biogas, an energysource, and digestate, a soil amendment. Dry anaerobic digestion has gained muchattention because of its advantages of lesser water addition, lower reactor volume andhigher volumetric biogas production than wet digestion. However, one of its problems isaccumulation of ammonia which is more common in digesters fed with improper C/N ratiowastes and needs to be corrected.

This study was carried out to evaluate the performance of a pilot-scale thermophilic dryanaerobic reactor for biogas production and to analyze the management options for thedigestate. This was achieved by investigating substrates of different C/N ratio to get acorrect feedstock for dry anaerobic digestion (to minimize ammonia accumulation) and byinvestigating different organic loading rates (OLRs) of the correct feedstock. Moreover,GHG emission potential of digestate was calculated (based on its characteristics) with andwithout storage and curing and different digestate management options were analyzed.

In first experiment, the effect of C/N ratio and total ammonia-N accumulation in a dryanaerobic digestion was studied effectively. Two simulations of OFMSW were prepared toattain C/N ratio 27 and C/N ratio 32 using biodegradable feedstocks such as food waste,fruit and vegetable waste, leaf waste and paper waste. Results showed that the simulationwith C/N ratio 32 had about 30% less ammonia-N in digestate as compared to that withC/N ratio 27. Moreover, a free ammonia accumulation/inhibition effect was documentedand methods to overcome the adverse effects were discussed.

In another experiment, correct feedstock from the first experiment (C/N ratio 32) was usedas substrate to improve the performance of the same reactor. The effect of different OLRs,such as 4.55, 6.30 and 8.50 kg VS/m3d, was studied on the parameters like biogasproduction, VS removal and VFA accumulation. Results showed that increase in OLRproportionally increased the gas production rate (5, 6.37 and 7.55 m3/m3

reactor vol/d for threeOLRs respectively) of reactor, but the specific methane production reduced (330, 320 and266 L CH4/kg VS). Similarly, VS removal also reduced (78, 75 and 67%) with increase inOLR. The system performed well at OLR and RT of 6.40 kg VS/m3d and 24 daysrespectively, however, purpose of treatment also determines the optimum operatingconditions.

Digestate from the reactor was characterized and its C/N ratio and GHG emission potentialwas calculated. It was found that the C/N ratio of digestate was 15-20 for most of the studyperiod, which is safe range for its application to agricultural land without further treatment.The GHG potential calculation shows that storage of the digestate for 2 months decreasedits GHG potential by 10%, hence, storage was found to be a source of GHG emission.Moreover, application of digestate directly to land has minimum net GHG emission (i.e. -11 gCO2-eq/kg digestate). Therefore, digestate should be applied to land immediately afterdigestion to minimize GHG emission from the storage system.

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Table of Contents

Chapter Title Page

Title page iAcknowledgements iiAbstract iiiTable of Contents ivList of Tables viiList of Figures viiiList of Abbreviations x

1 Introduction 11.1 Background 11.2 Objectives of the Study 21.3 Scope of the Study 3

2 Literature Review 42.1 Introduction of Dry Anaerobic Digestion 42.2 Process of Anaerobic Digestion: The Fundamentals 5

2.2.1 Hydrolysis 52.2.2 Acidogenesis 62.2.3 Acetogenesis 62.2.4 Methanogenesis 7

2.3 Inhibition of Dry Anaerobic Digestion 82.3.1 Volatile fatty acids (VFA) 82.3.2 Ammonia 9

2.4 Optimization of Factors Affecting Dry Anaerobic Digestion 112.4.1 pH 112.4.2 Solids content 112.4.3 C/N ratio 132.4.4 Temperature 142.4.5 Mixing 162.4.6 Retention time 172.4.7 Organic loading rate 18

2.5 Other techniques to Optimize Dry Anaerobic Digestion 192.5.1 Physical pretreatment 192.5.2 Chemical pretreatment 192.5.3 Biological pretreatment (inoculation) 202.5.4 Co-digestion 20

2.6 Reactor Design for Dry Anaerobic Digestion 212.6.1 Single-stage batch systems 222.6.2 Single-stage continuous systems 232.6.3 Multi-stage continuous systems 232.6.4 Design of available technologies for dry anaerobic digestion 25

2.7 Research Progress and Research Needs of Dry Anaerobic Digestion 272.8 Anaerobic Digestion and Digestate Management 30

2.8.1 Need of digestate management and digestate utilization 302.8.2 Effect of prior digestion on properties of digestate 31

v

2.9 Characteristics of Digestates 332.9.1 Characteristics of solid digestates 332.9.2 Characteristics of liquid digestates 352.9.3 Presence of organic pollutants 372.9.4 Presence of heavy metals 382.9.5 GHG emission potential of digestate 38

2.10 Management Aspects of Anaerobic Digestate 392.10.1 Separation of liquid and solid digestate 392.10.2 Direct land application of liquid digestate 402.10.3 Aerobic post-treatment of solid digestate and its effects on

quality40

2.10.4 Digestate storage and its effects on characteristics 412.11 Post Utilization Monitoring Issues of Anaerobic Digestate 42

2.11.1 Effect of digestate application on soil 422.11.2 Influence of digestate application on plant growth and health 42

2.12 Research Needs for the Dissertation 43

3 Methodology 443.1 Inoculum and Simulations of Waste 45

3.1.1 Inoculum for anaerobic digestion experiments 453.1.2 Simulations of waste 45

3.2 Experimental Set-up 463.2.1 Experimental set-up for gas formation potential test 463.2.2 Experimental set-up for pilot-scale experiments 46

3.3 Experimental Conditions 473.3.1 Experimental conditions for gas formation potential test 473.3.2 Experimental conditions for Phase I pilot experiment 483.3.3 Experimental conditions for Phase II pilot experiment 50

3.4 Digestate Management and GHG Emissions Estimation (Phase III) 513.4.1 Storage of digestate 523.4.2 Dewatering of digestate 523.4.3 Curing of dewatered digestate 533.4.4 Estimation of GHG emissions in the digestate management

system55

3.5 Analytical Methods 58

4 Results and Discussion 604.1 Gas Formation Potential of Waste 604.2 Effect of C/N Ratio and Ammonia-N Accumulation on ITDAR

(Results of Phase I Pilot Experiment)62

4.2.1 Performance of ITDAR during start-up and continuousoperations

63

4.2.2 Effect of C/N ratio and ammonia-N accumulation in ITDAR 654.2.3 Summary of the effect of ammonia-N accumulation in ITDAR 684.2.4 Energy balance of ITDAR in Phase I pilot experiment 70

4.3 Optimization of a Pilot-Scale Thermophilic Dry Anaerobic Digester(Results of Phase II Pilot Experiment)

71

4.3.1 Start-up of ITDAR in phase II pilot experiment 714.3.2 Stability parameters of ITDAR: Effect of organic loading rate 74

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4.3.3 Effect of organic loading rate on performance parameters ofITDAR

76

4.4 Digestate Management and GHG Emissions (Phase III) 794.4.1 Characteristics of raw digestate 794.4.2 Characteristics of stored, dewatered and cured digestate 814.4.3 Digestate management from perspectives of GHG emissions 83

4.5 Decentralized Dry Anaerobic Digestion of OFMSW for a Communityof 5000 People

86

4.5.1 Design of the decentralized AD system 864.5.2 Preparation of feedstock for dry AD (Pre-treatment) 864.5.3 Operation of decentralized AD system 874.5.4 Generation of methane and energy 884.5.5 Digestate management 884.5.6 Reduction of GHG emissions 904.5.7 Material flow (VS balance) 90

5 Conclusions and Recommendations 915.1 Conclusions 915.2 Recommendations 93

References 95

Appendices 110Appendix A 110Appendix B 114Appendix C 122Appendix D 127Appendix E 130Appendix F 134

vii

List of Tables

Table Title Page

2.1 Biomethanization Inhibitors and their Inhibitory Concentration 82.2 Change in TAN Inhibitory Concentration with Feed TS and Temperature 102.3 Typical C/N Ratios of Different Materials 132.4 High Gas Production Rate in Relation to High Organic Loading Rate in

Dry Anaerobic Digestion 182.5 Performance of Various Kinds of Dry Anaerobic Digesters 282.6 Effect of Digestion on Properties of Waste 322.7 Characteristics of Solid Digestate in Dry Anaerobic Digestion Systems 342.8 Characteristics of Separated Liquid Digestates from Different Digestion

Systems 362.9 Concentration of Organic Pollutants in Digesates and Composts (µg/kg

DM)37

2.10 Heavy Metal Content in Different Types of Digestates (mg/kg DM) 382.11 Regulations of Nutrient Loading on Agricultural Land 403.1 Composition and Characteristics of Simulated Feedstock 453.2 Characteristics of Substrate and Inoculum Used in Gas Formation Potential

Test 483.3 Operating Conditions of ITDAR for Phase I Pilot Experiment 493.4 Forms and Sources of GHG Contributed and GHG Avoided 563.5 Analytical Methods for Various Parameters of Anaerobic Digestion of

OFMSW 594.1 Digestion Parameters and Methane Yield of ITDAR 664.2 Surplus Energy of ITDAR During Various Runs 714.3 Percentage of VS Removal and Specific Methane Production in ITDAR 774.4 Comparison of Digestate Characteristics and Guidelines 824.5 Characteristics of Digestate at Different Stages of Management 824.6 Net GHG Emissions from All Scenarios of Digestate Management 854.7 Technical Details of Proposed AD Plant and its Comparison to Pilot Plant 874.8 Technical Data of Sand Drying Bed for Digestate Dewatering 88

viii

List of Figures

Figure Title Page

2.1 Trend of low solids and high solids anaerobic digestion plants in Europe 52.2 Main stages of anaerobic digestion process 62.3 Graphical representation of temperature ranges for anaerobic digestion 142.4 Capacity of mesophilic versus thermophilic digestion operation in Europe 152.5 General methods of mixing in dry anaerobic digestion, a) digestate

recirculation, b) biogas recirculation and c) mechanical mixer 162.6 Classes of dry anaerobic digestion by operational criteria 222.7 Comparison between one stage and two stage process in Europe 242.8 Designs of single-stage dry anaerobic digesters 262.9 Emissions from soil applied digestate to environments 302.10 Liquid-solid separation of digestate with production of useful products 392.11 Changing parameters during aerobic post-treatment 413.1 Phases of overall research study 443.2 Experimental set-up for gas formation potential test 463.3 Pilot-scale experimental setup of inclined thermophilic dry anaerobic

digester 473.4 Method steps for gas formation potential test 483.5 Operating conditions of ITDAR for Phase II pilot experiment 513.6 Possible unit processes of digestate management system 523.7 Plastic drums for storage of digestate 533.8 Sand drying bed: Top view 543.9 Sand bed for digestate dewatering, A-A cross-sectional view 543.10 Comparative scenarios of digestate management 564.1 Cumulative and specific biogas production by feedstock 1 614.2 Cumulative and specific biogas production by feedstock 2 624.3 Time course of dry anaerobic digestion with various parameters in

ITDAR64

4.4 Interaction of ammonia and VFA in ITDAR 674.5 Variation of total ammonia-N concentration and TAN/TKN ratio with

feed C/N ratio in ITDAR 694.6 pH profile of ITDAR during start-up 724.7 Profile of VFA and VFA/Alk ratio during start-up 734.8 CH4, CO2 and GPR fluctuation during start-up phase 744.9 Evolution of pH in ITDAR during continuous loading 754.10 Concentration of VFA in ITDAR during continuous loading 754.11 VFA/Alk ratio in ITDAR during continuous loading 764.12 Gas production rate of ITDAR during different OLRs 774.13 Cumulative methane per liter of reactor volume in ITDAR 784.14 Selection of operating conditions based on purpose of waste treatment 784.15 Comparison of feed and digestate regarding total solids in phase I

experiment80

4.16 TKN and C/N ratio of the digestate in phase I experiment 804.17 TS and VS content of digestate in phase II experiment 814.18 GHG emission potential of OFMSW and digestates 834.19 Net GHG emissions from all scenarios of digestate management 86

ix

4.20 Layout of conceptual decentralized AD plant for a community 894.21 Conceptual mass balance for VS of the proposed decentralized system 90

x

List of Abbreviations

AD Anaerobic DigestionAIT Asian Institute of TechnologyAPHA American Public Health AssociationBVS Biodegradable Volatile SolidsCOD Chemical Oxygen DemandDAD Dry Anaerobic DigestionDEHP Di-2-ethylhexyl PhthalatesDigrr Digestate Recirculation RateDM Dry MatterDOC Dissolved Organic CarbonDRANCO Dry Anaerobic CompostigEU European UnionFID Flame Ionization DetectorFM Fresh MatterGC Gas ChromatographyGHG Greenhouse GasGP Gas PotentialHRT Hydraulic Retention TimeMS-OFMSW Mechanically Separated Organic Fraction of Municipal Solid WasteMSW Municipal Solid WasteNP Nonyl PhenolOFMSW Organic Fraction of Municipal Solid WasteOLR Organic Loading RateOM Organic MatterORP Oxidation Reduction PotentialPAH Polycyclic Aromatic HydrocarbonPBDE Polybrominated Diphenyl EthersPCB Polychlorinated BiphenylPCDD Polychlorinated Dibenzo-p-DioxinPCDF Polychlorinated Dibenzo FuransRT Retention TimeRVS Refractory Volatile SolidsSDB Sand Drying BedSEBAC Sequential Batch Anaerobic CompostingSSHS Single Stage High SolidSS-OFMSW Source Separated OFMSWSRT Solids Retention TimeSTP Standard Temperature and PressureTCD Thermal Conductivity DetectorTS Total SolidsUASB Upflow Anaerobic Sludge BlanketVFA Volatile Fatty AcidVS Volatile SolidsWM Wet Mass

1

Chapter 1

Introduction

1.1 Background

Global solid waste generation is increasing day by day, not only because of growingpopulation, but also due to improved standard of living. About 40-60% of the householdwaste is biodegradable in Asia, most of which is usually disposed by landfilling or opendumping. Improper handling and disposal of the gigantic amount of solid waste seriouslyaffects the air, land and water environments and human health. It is also contributing toclimate change by releasing methane and carbon dioxide. There is a pressing need tomanage it from the time of creation to its safe disposal. Incineration may also not besuitable for organic waste because of low calorific value due to its high moisture content.Moreover, it causes air pollution and also requires high capital and operating cost.Therefore anaerobic digestion and composting could be alternative options to treat organicwaste.

Anaerobic digestion is widely being practiced as major treatment option for disposal oforganic municipal solid waste on par with composting technology. Anaerobic digestionmainly combines with the energy recovery benefits, green house gas mitigation andproduces stable end products, which can be further upgraded as compost for landapplication (Forster-Carneiro et al. 2008; Walker et al. 2009). In general, anaerobicdigestion systems are broadly categorized under wet (<10 % total solids) or dry (>20 %total solids), mesophilic (35-40oC) or thermophilic (> 55oC), batch or continuous andsingle or two stage systems (Fdez-Guelfo et al. 2010; Forster-Carneiro et al. 2008; Yabu etal. 2011).

Dry anaerobic digestion offers several advantages over wet digestion process like, lesserwater addition, smaller reactor volume, technical simplicity in design due to plug flowmovement of substrate and no mechanical devices required inside the reactor for mixingand easy handling of digested residues (Guendouz et al. 2010; Yabu et al. 2011). Similarly,better process conversion efficiency and maximum net energy gains are reported especiallywith the thermophilic operations of dry anaerobic digestions systems (Fdez-Guelfo et al.2010; Forster-Carneiro et al. 2008). For example, the Dranco and Kompogas processes aresingle stage, dry, thermophilic systems, which have been commercialized in Europe andother parts of the world.

Ammonia-N accumulation is, however, identified as a major issue with dry thermophilicanaerobic digestion systems, which can affect the overall methane yield. Generally, theOFMSW is characterized with the average of 4% of protein content, a major source ofnitrogen, which is removed via ammonification process and accumulated as ammonia-N(Jokela and Rintala, 2003). Also, the chances of ammonia-N accumulation are higher, ifthe feedstock is mixed up with the large portions of food processing waste and animalwaste from slaughter houses. Even though, the protein degradation process is found to bevery slow, the released ammonia-N tend to accumulate in anaerobic digesters because ofleachate recycling and there is no mechanism to remove it except by leachate removal orleaching.

2

Thus, the leaching is the only mechanism proposed to overcome this issue, through whichthe ammonia-N concentration is reduced with the external water addition. But wateraddition is not desirable in the case of dry anaerobic digestion systems. Also, ammonia-Nstripping is not considered as good option for organic wastes with the high solid contentsdue to their poor fluidity and difficulty in handling. On the other hand, it can be governedpossibly through adjusting the carbon to nitrogen (C/N) ratio of the feedstock, whichdetermines the overall ammonia-N concentration within the digester (Straka et al. 2007).Albeit, the optimum C/N ratio for anaerobic digestion is agreed to be in the range of 20 to30 (Li et al. 2011), with a higher C/N ratio, the release of lower concentrations ofammonia-N can be expected within the anaerobic systems.

But, the adjustment of feedstock C/N ratio, especially in a large scale centralized systems,is one of the major problems besides getting the insufficient feedstocks to operate at theirfull capacity throughout the year (Siles et al. 2010). In addition, the overall net energygained from the centralized system is mainly balanced with the waste collection,transportation and segregation costs. On the other hand, decentralized small scaleanaerobic systems are more attractive and easy to manipulate with the feedstockcharacteristics and quantity to attain maximum net energy gain. In addition, these canminimize the waste handling and associated pollution emissions, along with the overallwaste management costs. Hence, more detailed research in decentralized system will bemore appropriate and requisite at this juncture.

Similarly, most of the previously performed research studies on dry anaerobic digestionuse lab-scale reactor with synthetic and well-homogenized feed having the particle size ofaround 10 mm. Thus there is a need of research on full-scale or pilot-scale dry anaerobicdigesters operating at conditions closer to the field conditions and optimizing theircontinuous operation with practicable organic loading rates.

Digestate from anaerobic digester has been found to contain a considerable amount ofnutrients and organic matter and is useful for agriculture. However, still it has certainresidual GHG emission potential, and may contain organic pollutants, heavy metals andpathogens. Therefore, the stored digestate tends to emit methane to the atmosphere andhence can contribute to the climate change. Thus, it is necessary to carefully analyzevarious digestate management options in terms of net GHG emissions, so that digestate canbe managed with maximum possible GHG reduction.

Therefore, this research work is intended to optimize dry anaerobic digestion in terms offeed C/N ratio, associated ammonia-N accumulation and organic loading rates in a pilot-scale thermophilic system designed for decentralized applications. Moreover, variousoptions of digestate management have been analyzed from perspective of GHG emissionsfor improved digestate management.

1.2 Objectives of the Study

The main objective of this research is to optimize operation and performance of dryanaerobic digestion process treating OFMSW by testing feed C/N ratio, investigatingammonia-N accumulation and conducting different organic loading rates. This leads topromote dry anaerobic digestion technology, which will enable recovery of valuables(energy and digestate) from municipal solid waste, minimize the amount of waste going tolandfill and protect the environment.

3

The specific objectives of this research are given in the following:

Optimization of the methane yield of OFMSW by using different feed C/N ratio andorganic loading rates under dry and thermophilic conditions;

To characterize the digestate of anaerobic digestion and analysis of digestatemanagement options in terms of GHG emissions.

1.3 Scope of the Study

To accomplish the above objectives, scope of the study is given as follows:

1. The methane yield optimization of dry digestion was conducted in a pilot-scalethermophilic anaerobic digester, whereas gas formation potential test was conducted inlab-scale reactors with eudiometer set-up.

2. Food waste, leaf waste and waste paper were collected from restaurants, fields andoffices of AIT respectively. Vegetable and fruit wastes were collected from avegetable and fruit market (Tallad Thai) situated nearby. Waste simulations withdifferent C/N ratio were prepared at Environmental Research Station of AIT.

3. Inoculum to be used for gas formation potential test consisted of anaerobic sludgefrom wastewater treatment plant of Singha Beer Factory, Bangkok. Inoculum to beused for pilot experiment was composed of anaerobic sludge, cow dung and digestateof anaerobic digestion of municipal solid waste (MSW).

4. Solid-liquid separation of digestate was done in a pilot-scale sand drying bed.5. Characteristics of waste and digestate and operational parameters of digestion all were

analyzed in EEM laboratory, AIT.6. The GHG emitted from stored digestate, cured digestate and land applied digestate

were theoretically calculated based on their characteristics (C and N content), whichwere analyzed in EEM Lab.

4

Chapter 2

Literature Review

Anaerobic digestion is considered as an alternative option to manage and treat the organicfraction of municipal solid waste (OFMSW). This process not only treats the organic wastebut also produces clean energy (biogas). The digestion residues (digestate) obtained fromthe process can be used as soil amendment or even nutrient rich organic fertilizerdepending on its final quality. Based on the solid content of waste used in the process,anaerobic digestion is of two types which are dry and wet anaerobic digestion. Dryanaerobic digestion has got much attention due to its advantages of smaller reactor volumerequirement (higher organic loading rate), lesser water addition and lesser pretreatmentneeded with higher volumetric biogas production rate as compared to wet digestion.Moreover, high solid content of the digestate makes it simpler and easier to handle ascompared to liquid digestate of wet digestion that adds dewatering cost as well. Due to lowwater content and small reactor volume, energy requirement for heating is less for drydigester.

In this chapter, the process and problems of dry anaerobic digestion have been discussed.Moreover, optimization of factors affecting dry anaerobic digestion has been discussed andbased on this, solutions of some problems in dry anaerobic digestion have been analyzed aswell as the developments in the process have been reviewed. Furthermore, characteristicsof digestate have been presented and the present management strategies for digestate havebeen reviewed.

2.1 Introduction of Dry Anaerobic Digestion

In dry anaerobic digestion (high-solids digestion), the feedstock to be digested has totalsolids (TS) content more than 15%. In contrast, wet anaerobic digestion (low-solidsdigestion) deals with diluted feedstock having TS content less than 15% (Li et al., 2011).Dry anaerobic digestion technology emerged from research performed in 1980s thatdocumented higher biogas production rates by high-solid wastes fed without dilution.Conventionally (1990 and earlier), wet anaerobic digestion used to be the main anaerobicdigestion technology for digestion of manures in vertical reactors requiring feed materialwith less than 10% TS content (Forster-Carneiro et al., 2009). But then the trend of dryanaerobic digestion technology increased so quickly that in late 1990s, total anaerobicdigestion capacity in Europe for treating OFMSW was equally divided between the wetand dry anaerobic digestion as shown in Figure 2.1. The trend further changed and in 2006,dry anaerobic digestion and wet anaerobic digestion provided 56% and 44% of thecapacity respectively (De Baere, 2006). It became > 60% for dry digestion in 2010 asshown in the same figure.

Dry anaerobic digestion is performed with organic fraction of municipal solid waste(OFMSW) in both horizontal and vertical plug flow reactors. Apart from OFMSW, it canalso be conducted with straws and residues of crops, solid livestock waste (e.g. cow dung,horse dung), food waste and dewatered sewage sludge as substrates (Mumme et al., 2010;Kusch et al., 2008; Kim and Oh, 2011; Duan et al., 2012). According to Luning et al.(2003), both the wet and dry anaerobic digestion processes can be considered as a proventechnology for the treatment of the OFMSW because the specific gas production by the full

5

scale plants of the two processes is almost similar. Some available technology examples ofdry anaerobic digestion are BIOCEL, DRANCO, KOMPOGAS, Valorga, Linde-BRV andSUBBOR, whereas those of wet digestion are BTA, KCA, BIOSTAB, and WAASA.

Figure 2.1 Trend of low solids and high solids anaerobic digestion plants in Europe(Mattheeuws, 2011)

2.2 Process of Anaerobic Digestion: The Fundamentals

Anaerobic digestion of organics is a complex process under both dry and wet conditions,which can be divided into 4 biodegradation stages. The microbes involved in theseprocesses need different environmental conditions and have synergism. The four basicsteps of the process have been explained in Figure 2.2.

2.2.1 Hydrolysis

Hydrolysis of the complex organic matter is an important step of the anaerobicbiodegradation process. It is the first and often rate-limiting step during the anaerobicdigestion of complex organic matter. During this step, the complex organic matter (lipids,proteins and carbohydrates, etc) are hydrolyzed into simple compounds (amino acids,sugars, fatty acids, etc.) by hydrolytic microorganisms.

An approximate chemical formula for the mixture of organic waste is C6H10O4 (Ostrem,2004). A hydrolysis reaction where organic waste is broken down into a simple sugar(glucose) can be represented by the Eq. 1.

0

500

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Cum

ulat

ive

capa

city

(kt

on/y

ear)

YearWet operation Dry operation

C6H10O4 + 2H2O C6H12O6 + 2H2 Eq. 1

6

Figure 2.2 Main stages of anaerobic digestion process (Modified from Appels et al.,2008)

2.2.2 Acidogenesis

During acidogenesis, fermentation of hydrolyzed compounds into volatile fatty acids likebutyric acid, propionic acid, acetic acid, valeric acid, etc. happens which causes the drop ofpH. Moreover, neutral compounds i.e., methanol and ethanol, and ammonia are alsoformed. Furthermore, catabolism of carbohydrates causes the evolution of CO2 and H2.Acidogens consisting of facultative microbes and obligate anaerobes run this step ofdigestion process.

The specific concentrations of products formed in this stage vary with the type of microbesas well as with culture conditions such as temperature and pH. Typical reactions in theacid-forming stages are shown below. In Eq. 2, glucose is converted to ethanol and in Eq. 3glucose is transformed to propionate.

2.2.3 Acetogenesis

Acetogenesis is the third stage in which the products of acidogenesis undergo furtherdigestion and form acetic acid, hydrogen and carbon dioxide. Obligate microbes called

Hydrolysis

Acidogenesis

AcetotrophicMethanogenesis

70% CH4

HydrogenotrophicMethanogenesis

30% CH4

Volatile Fatty Acids(Propionate, Butyrate, etc), Ethanol

H2, CO2 Acetate

CH4 + CO2

Organic Waste(Carbohydrates, Proteins, Lipids)

Acetogenesis

Soluble Organics(Simple sugars, Amino acids, Fatty acids)

C6H12O6 + 2H2 2CH3CH2COOH + 2H2O Eq. 3

C6H12O6 2CH3CH2OH + 2CO2 Eq. 2

7

acetogens play their part to run this step. Environmental conditions play their part andaffect the formation of different by-products.

Acetogenesis occurs through carbohydrate fermentation in which acetate is the mainproduct and other metabolic processes also occur. The result is a combination of acetate,CO2 and H2. In anaerobic reactions, role of H2 as an intermediate compound is criticallyimportant. The H2 gas formation occurs when oxidation of long chain fatty acids intopropionate or acetate happens. However, this oxidation is inhibited by H2 in the solutionunder standard conditions. Hence, if partial pressure of H2 is sufficiently low, the abovementioned oxidation can thermodynamically proceed. Thus, for the conversion of all acids,low partial pressure of H2 gas is needed, which can be ensured if hydrogen consumingmicrobes are present in large number. Conversion of propionate into acetate has beenshown in Eq. 4. From above discussion, it can be concluded that level of H2 (measured bypartial pressure) is also an indicator of digester’s health (Mata-Alvarez, 2003).

2.2.4 Methanogenesis

This is the last step of anaerobic digestion process in which methane formation happensfrom the material produced in the previous step. Methane formation can happen frommethanol, acetic acid or hydrogen and carbon dioxide. Based on the raw material used,groups of microbes (called methanogens) responsible for this step are of two types: (i)acetoclastic methanogens which consume acetic acid mainly (Eq. 5&6, and consumemethyl alcohol as well, Eq. 7) and contribute to 2/3rd of total methanation, and, (ii)hydrogenotrophic methanogens which consume carbon dioxide and hydrogen (Eq. 8) andare responsible for 1/3rd of total methane production (Ostrem, 2004). The growth rate ofmethanogens is however slower than that of organisms responsible for other steps ofdigestion.

It has been found by Montero et al., (2010) that consumption of butyric acid, the mainprecursor of methane, is related to hydrogenotrophic methanogens during start-up phaseand to acetotrophic methanogens during stabilization phase. It was concluded thatmethanogenic population dynamics depends on the concentration of VFA (specificallybutyric acid). Thus if concentration of VFA is high, hydrogenotrophic methanogens willprevail.

CO2 + 4H2 CH4 + 2H2O Eq. 8

CH3OH + H2 CH4 + H2O Eq. 7

CH3COOH CH4 + CO2 Eq. 6

2CH3CH3OH+ CO2 2 CH3 COOH + CH4 Eq. 5

CH3CH2COO- + 3H2O CH3COO- + H+ + HCO3- + 3H2 Eq. 4

8

2.3 Inhibition of Dry Anaerobic Digestion

Dry anaerobic digestion of organic solid waste faces many inhibition problems which areharder to control (Guendouz et al., 2010). These include inhibition by volatile fatty acids(VFA), ammonia, heavy metals and metals, as given in Table 2.1 with their inhibitoryconcentrations. The main inhibitors of dry anaerobic digestion process (ammonia andVFA) have been discussed in detail as under.

2.3.1 Volatile fatty acids (VFA)

Methanogens are susceptible to high concentrations of acids in reactor, so acid conditionscan inhibit their growth. Volatile fatty acids are intermediate compounds of methanationand their high concentration can cause stress to microbes. The mainly producedintermediates during anaerobic digestion of organics are acetic, propionic, butyric andvaleric acids (Buyukkamaci and Filibeli, 2004) whereas the concentration of acetic andpropionic acid is a useful measure for performance of digester.

Table 2.1 Biomethanization Inhibitors and their Inhibitory ConcentrationParameter Concentration of inhibition (g/L)Volatile fatty acids >2 (as acetic acid)

> 6-8 (as overall volatile acids)Total ammonia nitrogen 1.5-3 (at pH>7.6)Free ammonia 0.6Sulfide 0.25 (as H2S at pH 6.4-7.2)

0.09 (as H2S at pH 7.8-8.0)Sulfide >0.1 (as soluble sulfide)Calcium 2.5-4.5

8 (strongly inhibitory)Magnesium 1-1.5

3 (strongly inhibitory)Potassium 2.5-4.5

12 (strongly inhibitory)Sodium 3.5-5.5

8 (strongly inhibitory)Heavy metals

Copper (Cu)

Cadmium (Cd)Iron (Fe)Chromium (Cr3+)Chromium (Cr6+)Nickel

0.0005 (soluble metal)0.15 a

0.151.71a

0.0030.50.002

Source: Chen et al., 2008; Polprasert, 2007; Dong et al., 2010a mole/kg dry solids

However, if the ammonia concentration in the medium is very high or substrate containshigh concentrations of proteins, accumulation of VFA will not lead to acidification due tobuffer capacity provided by ammonia (Angelidaki and Sanders, 2004). Ammoniamaintains a high level of bicarbonate to do that (Cho et al., 1995).

9

In dry anaerobic digestion, recycling of digestate or leachate is needed to control the solidcontent and to inoculate fresh waste. It causes the accumulation of VFA as well (El-Hadj etal., 2009; Li et al., 2011). Moreover, loading of reactor over its capacity also causes theaccumulation of VFA in the digester. Furthermore, during start-up of dry anaerobicdigestion, it is highly possible that VFA accumulation happens. Angelidaki et al., (2006a)concluded that inhibition could happen initially at higher TS and the magnitude ofinhibition would increase with increase in TS.

Presence of different volatile fatty acids is also affected by pH of reactor medium. Hu andYu, (2006) reported that there was not effect of pH on propionic acid production, whereaswith increase in pH, the yield of acetic acid slightly increased while that of butyric acidincreased with decrease in pH.

Proper inoculation of fresh feed (to avoid overloading), mixing of certain percentage ofpaper waste or some other slowly degradable waste (to slow down the production ofvolatile acids) and controlling the loading rate of feed, are some of strategies to minimizethe problem of VFA accumulation. For further detailed discussion, please follow sectionsnamely inoculation, co-digestion and OLR later in this chapter.

2.3.2 Ammonia

Ammonia is produced by the biological degradation of the nitrogenous matter, mostly inthe form of proteins and urea. About 60-80% of total nitrogen (especially the proteins andother organic nitrogen compounds) is converted to ammonia during anaerobic digestion oforganic waste (Bujoczek et al., 2000; Angelidaki et al., 2006b; Yabu et al., 2011).Microorganisms responsible for anaerobic digestion need low concentration of ammoniafor their growth and multiplication. However, the excess ammonia accumulates in thedigester and hinders the process. A substrate or part of substrate (in case of co-digestion)with feed C/N ratio lower than 27 causes ammonia accumulation (Kayhanian, 1999) andpH values exceeding 8.5, which is toxic to methanogens. In dry anaerobic digestion, apartfrom low C/N ratio of the feedstock, recycling of a fraction of leachate or digestate(intended to optimize solid contents and inoculate fresh waste) has also been found toincrease the ammonia concentration (Li et al., 2011).

Ammonia inhibits the digestion process by change in the intracellular pH through itsdiffusion into cells and causing proton imbalance, increase of maintenance energyrequirement, and inhibition of a specific enzyme reaction. Inhibition happens at totalammonia nitrogen (TAN) concentration range of 1200-6000 mg/L or more depending onTS content, pH, temperature and degree of acclimation of reactor medium. Table 2.2shows the effect of feed TS content and temperature on inhibitory concentration of TAN(NH3+NH4

+).

It is clear from Table 2.2 that at low TS content, inhibition occurs at a higher TANconcentration and vice versa. It is also supported by other researchers (Poggi-varaldo et al.,1997) that inhibition by TAN is expected to occur at a lower TAN concentration in dryanaerobic digestion process as compared to semi-dry and wet digestion. Similarly iftemperature is low, inhibition happens at a higher TAN concentration and vice versa. Thus,if TS content is very low and temperature is mesophilic, inhibition happens at very highTAN concentration (i.e. 6000 mg/L). If TS is very high and temperature is thermophilic,TAN can be inhibitory at very low concentration (i.e. 1200 mg/L).

10

Free ammonia (NH3) has been suggested to be the main cause of inhibition (Kayhanian,1999) since it is freely membrane-permeable. Free ammonia concentration of about 600mg/L is inhibitory for thermophilic dry digestion (Gallert and Winter, 1997). Hartmannand Ahring, (2005) reported that no inhibition was observed at free ammonia concentrationof 450-620 mg/L under thermophilic digestion. Duan et al., (2012) also reported that withthe concentration of free ammonia lower than 600 mg/L, high-solid anaerobic digestion ofsewage sludge could maintain satisfactory stability. Significant inhibition happened atTAN concentration of 3000-4000 mg/L and free ammonia concentration of 600-800 mg/Lin their study.

Table 2.2 Change in TAN Inhibitory Concentration with Feed TS and TemperatureSubstrate Digestion

typeFeedTS(%)

Temperature InhibitoryTAN(mg/L)

Inhibitiondegree

Reference

Potatojuice

Wet 4.5 Mesophilic 4000-6000 57% loss inmethanogenicactivity

KosterandLettinga,1988

OFMSW Wet 6 Thermophilic 3500-5500 6-11% loss inmethaneyield

Angelidakiet al.,2006b

Cattlemanure

Wet 7 Thermophilic 4000-5000 50% loss inspecificgrowth rateofmethanogens

Borja etal., 1996

Syntheticwaste-water

Wet - Thermophilic 4920-5770 39-64% lossin methane

Sung andLiu, 2003

Foodwaste

Dry 18.4 Thermophilic 3500 50% loss inmethane

Gallert andwinter,1997

OFMSW Dry 30 Mesophilic 2800 ProcessCease

Poggi-Varaldo etal., 1997

OFMSW Dry 24.79 Mesophilic 2000 > 50% loss inbiogas

Jiang et al.,2008

OFMSW Dry 30 Thermophilic 1200 - Kayhanian,1999

Free ammonia concentration is affected mainly by temperature, pH and TANconcentration. Thus, at 55°C, free ammonia will always be higher than at 35°C (Lin et al.,2009; De la Rubia et al., 2010) at a given concentration of TAN. Moreover, free ammoniahas been reported to be inhibitory at lower concentration (220 and 215 mg/L) undermesophilic conditions as compared to thermophilic conditions where its inhibitoryconcentration was higher (568 and 468 mg/L) (Gallert and Winter, 1997; El-Hadj et al.,2009). This also implies that inhibition happens not only by free ammonia, but also byionic ammonia, because when threshold concentration of ionic ammonia reaches, the

11

process is inhibited and at that point the free ammonia value is low and high undermesophilic and thermophilic conditions respectively.

Two ways to mitigate ammonia inhibition in dry anaerobic digestion systems are: (i)dilution of digester content and/or removal of leachate and addition of the same amount ofwater, and (ii) adjustment of feed C/N ratio where C/N ratio of 27 to 32 is effective(Kayhanian, 1999). Mitigation of ammonia accumulation has been further discussed in co-digestion section of the review.

2.4 Optimization of Factors Affecting Dry Anaerobic Digestion

2.4.1 pH

Even though, the pH and VFA are linked to each other but their relation depends on thewaste composition which may differ from the type of waste and the environmentalconditions of anaerobic digestion process. It has been determined that an optimum pHvalue for anaerobic digestion lies between 6.5 and 7.5 (Liu et al., 2008). The optimum pHrequirement of the two steps of digestion namely acidogenesis and methanogenesis isdifferent. High concentrations of acids can result as low pH as <5 during acidogenesis. Onthe other hand, low pH or high acid generation inhibits acid-sensitive methanogens.

For high-solids methane production from sludge digestion, the pH range of 6.6-7.8 wasfound functional under mesophilic conditions at moisture content of 90 to 96%. Theoptimum pH was 6.8, however the process may fail if the pH is beyond the range of 6.1-8.3 (Jiunn-Jyi et al., 1997).Reduction in pH can be controlled by the addition of lime, sodium hydroxide or sodiumbicarbonate. Chen et al., (2010) reported that alkalinity of about 2,500 mg CaCO3/L andpH above 7 was maintained by adding 0.2 g NaOH/g VS. The results of this studyindicated that it was necessary to use the chemicals, such as NaOH, to control the pH ofthe single-stage anaerobic digester treating the food waste.

As the digestion proceeds and reaches the step of methanogenesis, protein degradationincreases the ammonia concentration through release of amino groups. The producedammonia acts as a buffer and during this time, pH can reach 8 or above. After stabilizationof methanation, pH becomes stable between 7.2 and 8.2. Thus, in anaerobic digesters,ammonia is also responsible for buffering, and stabilizes pH when present up to 1000 mg/Lconcentration (Fricke et al., 2007).

El-Hadj et al., (2009) discussed about optimum pH in relation to TAN concentration bothunder mesophilic and thermophilic conditions. They found that the optimum pH values arearound 7 under mesophilic digestion of OFMSW independently of TAN concentration.However under thermophilic conditions, it can be observed that 7.5 is the optimum pHwhen TAN is higher than 1,331 mg/L and it can be present in the range 7–8 if the TAN isbelow or equal to 1,331 mg/L.

2.4.2 Solids content

Earlier, Rivard et al., (1990) identified maximum solid content of reactor medium forstable anaerobic fermentation performance as 36%. Currently, it is generally thought thatdry anaerobic digestion is possible to be conducted up to as high solid contents as 40%

12

(Ward et al., 2008). Kim and Oh, (2011) found that increase of the substrate concentrationfrom 40% TS to 50% TS resulted in a drastic decrease in performance. Thus if solidcontent of the substrate is not within suitable range, it needs to be adjusted by addition ofwater, drying or by mixing with other waste type.

It has been found that organic removal and methane yield decrease with increase in totalsolids content. Fernandez et al., (2008) reported that organic removal decreased from 80.69to 69.05% and methane yield reduced from 110 to 70 L/kg VSremoved, as total solids contentwas increased from 20 to 30% under mesophilic conditions. Similarly, increasing the totalsolid (TS) content from 20 to 26% reduced biogas yield by 35% at S/I ratio of 6.2 andNaOH loading of 3.5%, where the solid-state anaerobic digestion of fallen leaves wasperformed under mesophilic conditions (Liew et al., 2011). Moreover, Duan et al., (2012)reported that in mesophilic anaerobic digestion of sewage sludge, increasing feeding TSfrom 10 to 15% at retention time of 30 days decreased methane yield and volatile solidsremoval by 7.4 and 6% respectively. Similar results were obtained by Li et al., (2010)under similar conditions. Forster-Carneiro et al., (2008) reported similar results for dryanaerobic digestion of food waste under thermophilic conditions as well. Thus it can beconcluded from above discussion that organic removal and biogas production decreasewith increase in TS content of feed regardless of temperature.

In wastewater treatment, chemical oxygen demand (COD) is usually used as a parameter todescribe organic removal (Rao et al., 2011; Eskicioglu et al., 2011) while in case of dryanaerobic digestion volatile solids (VS) content is more appropriately used (Dong et al.,2010; Duan et al., 2012) for the same purpose. Oleszkiewicz and Poggi-Varaldo, (1997)stated 1.1 kg COD equal to 1 kg VS, and this relation can be used for inter-conversion ofCOD and VS. Thus total solids and volatile solids content is used to describe theconcentration of feedstock in dry anaerobic digestion. However, the reactor medium in dryanaerobic digesters is heterogeneous in structure, composition and size of the solids(Guendouz et al., 2010). Moreover, the biomass is mixed with the substrate and the liquidfraction needs to be extracted from the reactor medium for analysis of various parameters.Thus, it makes in-line measurements very difficult and it is impossible to directly accessthe reaction yield.

Volatile solids (VS) in case of dry digester comprise the biodegradable volatile solidsfraction (BVS) and the refractory volatile solids (RVS), which is just similar to solubleCOD and particulate COD in case of wet process. It was reported by Kayhanian and Rich(1995) that “waste biodegradability, biogas production, C/N ratio and OLR could beestimated well and correctly with the knowledge of biodegradable volatile solids (BVS)fraction of solid waste. Lignin is a type of organic material opposite of BVS and is not easyby anaerobic microbes to degrade it and is called refractory volatile solids or RVS.

For treatment through anaerobic digestion, the most suitable waste is the one with highvolatile solids in which non-biodegradable matter is low. The quality and yield of biogas aswell as quality of compost is affected by waste composition. The waste componentsconsisting of shrub and tree clippings, straw, bark, sawdust and shavings, which are lignin-rich and woody materials should be avoided to feed into the digester in more than a certainquantity. The degree of biodegradation can be measured by removal of volatile solidscontent, which is an important parameter showing activity status of some groups ofanaerobic microbes. In some study (Elango et al., 2007), 73% of VS reduction was initially

13

reported, which increased with time, but after some time of continuous feeding, it reducedgradually.

2.4.3 C/N ratio

It shows the proportion of quantity of C and N in any organic material. Nitrogen is neededby microorganisms for their multiplication by making new cells. The proper ratio ofnutrients C:N:P:S for methanation is 600:15:5:3. Therefore, a C/N ratio of 20-30 has beenreported as optimum (Fricke et al., 2007), which can supply sufficient amount of nitrogento produce microbial cells and to degrade C of the waste.

In the process of anaerobic digestion, the compounds of reduced N are not removed andtend to accumulate in the digester and inhibit the digestion process. Therefore, C/N ratio ofthe waste material is very crucial parameter. Because if the C/N ratio is high, it means thewaste has low N or high C content, so the low N content will be rapidly consumed bymethanogen, after that there will be lower biogas production due to low N. On thecontrary, if C/N ratio of the material is low, it means it has high N, which will accumulateinside digester with time that will raise pH to 8 or more, and will become toxic tomethanogens. To minimize ammonia accumulation problem in dry thermophilic anaerobicdigestion, C/N ratio ranging from 27 to 32 has been suggested by Kayhanian, (1999).

Table 2.3 Typical C/N Ratios of Different MaterialsRaw material C/N RatioHuman Excreta 8.0Duck Dung 8.0Sewage Sludge 8.6a-11.3b

Chicken Dung 10.0Pig Dung 18.0Goat Dung 12.0Sheep Dung 19.0Food Waste 15.0c

Cow Dung 24.0Water Hyacinth 25.0Fruit and Vegetable Waste 34.0d

Elephant Dung 43.0Municipal Solid Waste 40.0Rice Straw 70.0Wheat Straw 90.0Maize Straw 60.0Saw Dust >200.0Waste Paper >400.0

Source: aKymäläinen et al., 2012; bLi et al., 2011; cLiu et al., 2011; dBouallagui et al.,2009; RISE-AT, 1998.

Also if the C/N ratio is very high, it slows down degradation and hence VFA production isvery low. Similarly if the C/N ratio is very low, VFA accumulation will happen. ProperC/N ratio of the feedstock can be obtained by mixing different materials having differentC/N ratios, for example, animal manure or sewage have low C/N ratio which can be mixedwith solid waste having high C/N ratio to get optimum value. Table 2.3 gives C/N ratios of

14

typical organic materials that can be used for anaerobic digestion after mixing in properratio. This concept has been discussed in detail under the section of co-digestion.

2.4.4 Temperature

Temperature is very critical parameter of anaerobic digestion process, as the rate ofdigestion strongly depends on it. Mainly, methane production through anaerobic digestioncan be performed in two ranges of temperature, which are mesophilic range andthermophilic range. For mesophilic range, temperature of 35-40°C needs to be maintainedin the digester, whereas temperature range of 50-55°C is the range for thermophilicoperation (De Baere, 2006).

Figure 2.3 Graphical representation of temperature ranges for anaerobic digestion

A thermophilic temperature reduces the required retention time. The microbial growth,digestion capacity and biogas production could be enhanced by thermophilic digestion,since the specific growth rate of thermophilic microbes is higher than that of mesophilicmicrobes (Kim and Speece, 2002). Figure 2.3 (Mata-Alvarez, 2003) graphically illustratesthe direct relationship between the temperature and the rate of anaerobic digestion.

Thermophilic anaerobic digestion has been reported to generate about 25-50% highermethane than mesophilic digestion (Khanal, 2008; Yilmaz et al., 2008). The semi-dry thermophilic process has a gas production rate 2-3 times the mesophilic process(Cecchi et al., 1991). These results demonstrate the feasibility of construction ofthermophilic digesters for working at 11-12 days retention time when the OLR is 8 kgVS/m3d or lower providing for the highest volatile solid removal. The elasticity of thesystem permits the reduction of retention time down to 8 days by increasing the OLR to 14kg VS/m3d. Li et al., (2002) also reported that thermophilic methane fermentation wasmore effective for reducing lipids and had more higher loading capacity compared tomesophilic condition. Lu et al., (2007) confirmed that thermophilic process was morefeasible for achieving better performance against misbalance, especially during the start-upperiod in a dry anaerobic digestion process as compared to mesophilic digestion.Angelidaki et al., (2006a) also concluded that the best biodegradation results for start-upcould be achieved under thermophilic conditions. Increased destruction rate of organic

15

acids and increased downfall of pathogen removal is also possible in thermophiliccondition. Besides, thermophilic anaerobic digestion could produce high quality of residuethat could be used further as soil conditioner or fertilizer instead of placing them onlandfills.

Thermophilic process is, however, more sensitive to toxins and smaller changes in theenvironment. Oleszkiewicz and Poggi-Varaldo, (1997) reported that thermophilic process(at 55°C) was found to be superior to a mesophilic (35°C) one, both in terms ofvolatile solid (VS) reduction and specific gas production, but was somewhat less stable atshort mass retention times (MRT). Similarly, Lv et al., (2010) also reported thatthermophilic anaerobic digestion is good as it can perform accelerated hydrolysis byloosening the structure of polymers and lignocellulose in the substrate. But on the otherhand, it also causes the accumulation of propionate (short chain fatty acid) and hencedecreases methane and increases carbon dioxide in biogas. The reason is that hightemperature decreases microbial diversity. Moreover, at high temperature, solubility ofhydrogen decreases, thus it escapes from the reactor in gaseous form and hence notavailable for methane formation. Moreover, solubility of carbon dioxide also reduces withthermophilic temperature that leads to lesser methane formation (no or less methaneformation through hydrogenotrophic pathway) and removal of carbon dioxide also raisespH leading to high free ammonia (Gallert and Winter, 1997)..

Figure 2.4 Capacity of mesophilic versus thermophilic digestion operation in Europe

The distribution of mesophilic and thermophilic plants is given in Figure 2.4. Mesophilicplants are more in number than thermophilic plants and these are also mostly for wetdigestion. At the end of 2004, 75% of the capacity was provided by mesophilic plants.Then a large number of thermophilic plants were constructed in 2005. Almost 96% ofthermophilic plants are provided by dry fermentation systems. The advantages ofthermophilic operation are more important for dry systems than for wet systems, while the

0

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3500

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4500

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Cum

ulat

ive

capa

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(kt

on/y

ear)

YearMesophilic operation Thermophilic operation

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need for heating is also less for dry systems when thermophilic operation is chosen (DeBaere, 2006).

2.4.5 Mixing

Mixing is very important for efficient transfer of organic substrate to the active microbialbiomass and homogenization of reactor medium. Further, mixing enables heat transfer andthus avoids temperature gradients in both low-solids and high-solids digesters. Also, itprevents sedimentation of denser particulate material in the digester and helps to releaseproduced gas from the digester contents (Ward et al., 2008).There are many methods of mixing including mechanical mixers, recirculation of digestate,or recirculation of the produced biogas using pumps (Kaparaju et al. 2008) as shown inFigure 2.5. However, recirculation of digestate has been found the most suitable method ofmixing and compared to mechanical mixer and biogas recirculation, produced higherbiogas volumes from the substrate with TS content more than 10% (Karim et al. 2005b).Moreover, intermittent mixing has been reported to be suitable for substrate conversionand higher biogas production (Mills, 1979 and Smith et al., 1979).

Figure 2.5 General methods of mixing in dry anaerobic digestion, a) digestaterecirculation, b) biogas recirculation and c) mechanical mixer

The advantage of dry anaerobic digestion systems is that the reactors do not contain anymoving parts inside, but waste moves through the vessel as new waste is fed and digestateis removed. However, in dry digestion, a high level of mixing of fresh waste and digestedresidues is needed at the time of feeding, where the purpose is to inoculated and moistenthe fresh waste to be fed. Rivard et al., (1990) reported that no significant difference in the

Inlet

Biogas

Outlet

(b)

Inlet

Outlet

(a)

Inlet Outlet

(c)

Mechanical mixing paddle

17

performance of a poorly mixed (1 rpm) and a well mixed (25 rpm) dry digester wasobserved. On the other hand, Karim et al., (2005a) reported that mixed and unmixeddigesters performed quite similar when fed with slurry with 5% TS (wet digestion),whereas the mixing effect became important when fed with thicker slurry (with 10% and15% TS), because mixed digesters fed with 10-15% TS feed produced 10-30% morebiogas than unmixed digesters. However Stroot et al., (2001) reported that continuousmixing in high-solid digestion showed unstable performance at high OLR whereasminimal mixing showed a better performance at all OLR studied. Moreover, acontinuously mixed unstable reactor became stable (as shown by consumption ofaccumulated VFA and simultaneous rise in pH) when the mixing level was reduced. Fromabove discussion it can be concluded that either intermittent mixing or a low level ofmixing is needed for good performance of dry anaerobic digestion.

Impellers and paddles have been provided in available dry anaerobic digestiontechnologies like COMPOGAS and Linde-BRV to facilitate plug-flow and mixing ofreactor medium in the horizontal reactors. Guendouz et al., (2010) stated that one problemwith large-scale reactors is that complete mixing is never achieved. Thus, they designedand operated a laboratory-scale reactor in which complete mixing was enabled with the useof a paddle mixer.

2.4.6 Retention time

This is the ratio of volume of reactor to the flow rate of influent substrate (Eq. 9). In otherwords, the time required by the waste material to pass through the reactor is calledretention time.

Retention time (d) = Reactor volume (m3)Flow rate (m3/d)

The required retention time for completion of the anaerobic digestion reactions varies withtechnologies, process temperature, TS content and waste composition. The retention timefor wastes treated in mesophilic digester is usually higher (up to 40 days) than that ofthermophillic digesters which can be up to 8 days (Cecchi et al., 1991). Shortening theretention time decreases the reactor volume and hence saves capital cost. Increase inretention time, however, increases reactor stability. High retention time increases biogasdue to better stabilization (Poggi-Varaldo and Oleszkiewicz, 1992; Kim and Oh, 2011) onone hand and decreases it due to lower OLR on the other hand. If the concentration oforganic matter in the substrate is relatively constant, the shorter the RT the higher will bethe value of OLR as can be noted in many reports (Rivard et al., 1990; Gallert and Winter,1997; Montero et al., 2010; Mumme et al., 2010; Fdez-Guelfo et al., 2011).

One of the disadvantages of dry anaerobic digestion is that it needs high retention time (15-30 days) as compared to wet anaerobic digestion (where it can be as low as 3 days)(Nayono, 2010). Thus, feedstock with high TS content needs long RT for digestion. Kimand Oh, (2011) reported that at a fixed solid content of 30% total solids, stableperformance was maintained up to an HRT decrease to 40 d. However, the stableperformance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again.However, relatively low RT is required in dry digesters operated within thermophilicrange.

18

2.4.7 Organic loading rate

Organic loading rate is a measure of the biological conversion capacity of the anaerobicdigestion system. Feeding the system above its sustainable OLR results in low biogasyield, it happens due to accumulation of inhibiting substances such as fatty acids in thedigester slurry. In such a case, the feeding rate to the system must be reduced. Feeding atbelow average loading rate was done by Svensson et al., (2006) to build up biomass. Incontinuous systems, OLR is an important control parameter. System failures have beenreported by many plants due to overloading (RISE-AT, 1998). The amount of substrateintroduced into the digester is given by Eq. 10.

whereS = substrate concentration (kg substrate in terms of TVS)OLR = organic loading rate (kg substrate/m3 digester)

Table 2.4 High Gas Production Rate in Relation to High Organic Loading Rate inDry Anaerobic DigestionSubstrate Feed TS (%) OLR

(kg VS/m3d)GPR (m3/m3d) Reference

OFMSW 30 2-10 5 Kim and Oh,(2011)

OFMSW - - 6.80 De Baere andMattheeuws,(2011)

SS-OFMSW 20 12.1 5.3 Pavan et al.,(2000)

OFMSW+Yardwaste

18-40 11 3.70 Hamzawi et al.,(1999)

MS-OFMSW 18 9.65 5.20 Gallert andWinter, (1997)

OFMSW+Paper 30 12.6 7.14 Vermeulen etal., (1993)

OFMSW 23-30 13-15 6 Kayhanian andTchobanoglous,(1993)

In comparison, wet anaerobic digestion has a gas production rate of 1-2.5 m3/m3/d(Bouallagui et al., 2004; De La Rubia et al., 2006).

In the dry anaerobic reactor, an increase in TS content results in an equivalent decrease inrequired reactor volume, and thus, it enables a higher volumetric organic loading rate, asdescribed in Guendouz et al., (2010). It has been reported (Vandevivere, 1999) that OLR isdouble in high-solids digestion as compared to low-solid digestion. For example, thesustainable OLR for dry and wet anaerobic digestion is 12 and 6 kg VS/m3d respectively(Hartmann and Ahring, 2006). Duan et al., (2012) reported that high-solid system couldsupport 4-6 times higher OLR and obtain similar methane yield and VS reduction as

=V

Q.SOLR =

RT

SEq. 10

19

conventional low-solid system at the same SRT, thus reach much higher volumetric gasproduction rate as presented in Table 2.4.

2.5 Other Techniques to Optimize Dry Anaerobic Digestion

Most of the digestion systems need pretreatment of waste to get homogeneity in feedstockor to improve the subsequent digestion process. This section focuses on pre-treatmentprocess unique to dry anaerobic digestion. Guendouz et al., (2010) stated that lesstreatment is required in case of dry anaerobic digestion as compared to wet digestion. Thepretreatment of feedstock for dry anaerobic digestion may involve the removal of the non-biodegradable materials, protecting the downstream plant from waste components that mayphysically damage it, provision of feedstock of uniform and small particle size for effectivedigestion and removal of the materials which may decrease the quality of the digestate.Hydrolysis is improved and solubilization is increased by the help of chemical and thermalpre-treatments, which could be helpful to decrease retention time. Similarly, ultrasonic pre-treatment also has been studied in this regard. Major types of pretreatments for dryanaerobic digestion can be:

Physical pretreatment Chemical pretreatment Biological pretreatment (inoculation)

Moreover, co-digestion can also be considered as a good technique to optimize dryanaerobic digestion.

2.5.1 Physical pretreatment

The organic waste to be digested is also separated before it is sent to the digester. Theseparation method of this waste is either source separation or mechanical separation. Therecyclables like glass, plastics, metals or undesirables like stones can be eliminated bywaste separation. If the waste is not source separated then mechanical separation is donethat involves separation of non-digestible materials with a size larger than 40 mm, theprocess is called screening. To reduce particle size to less than 25 mm and 10 mm (forpilot-scale and lab-scale experiments respectively), shredding is done. Before feeding intodigester, shredding of waste is performed that helps to enhance the availability or surfacearea of the substrate to the hydrolyzing enzymes and hence can enhance the digestion rate.

2.5.2 Chemical pretreatment

It mainly consists of alkaline treatment. Alkalis are added to increase the pH to 8-11 duringthis process. This is particularly advantageous when using plant or crop material asfeedstock. Chemical pretreatment reduces particulate organic matter of waste into solublematter (i.e. carbohydrates, fats, proteins, or even lower molecular weight substances) andhence changes the composition of waste.

Zhu et al., (2010) tested different NaOH loadings (1%, 2.5%, 5.0% and 7.5% (w/w)) forsolid-state pretreatment of corn stover. Corn stover pretreated with 5% NaOH gave thehighest biogas yield of 372.4 L/kg VS, which was 37% higher than that of the untreatedcorn stover. Similarly, Liew et al., (2011) worked on enhancing the solid-state anaerobicdigestion of fallen leaves through simultaneous alkaline treatment. Three loadings of

20

NaOH (2, 3.5, and 5%) were tested for this purpose. About 24-fold enhancement inmethane yield than that of the control (without NaOH addition) was achieved at substrateto inoculums (S/I) ratio of 6.2 with NaOH loading of 3.5%.

2.5.3 Biological pretreatment (inoculation)

The purpose of this pretreatment is inoculation of feedstock. As the anaerobic digestion isa complex biological process and its performance is influenced by microbial diversity.Therefore, balanced active inoculum is essential for the possible degradation to be carriedout. In this view, it is very important to find an appropriate inoculum containing thenecessary microbes for the degradation process to proceed (Lopes, 2004). Anotherimportant factor is the amount of inoculums, which can be described by percentage ofinoculums in reactor medium or more appropriately by substrate to inoculum (S/I) ratio onvolatile solids basis. Digested material from an established reactor, anaerobic sludge orruminant manure is often used to seed a new reactor for reducing the start-up time. Manycontinuously running reactors inoculate the fresh material with either digested material orthe liquid fraction from the reactor, thus reducing washout of microorganisms. Very shortretention time has been reported to be the cause of microbial washout from the digester.

Forster-Carneiro, (2007) studied the effect of six different types of inoculums (25% w/w)on thermophilic dry anaerobic digestion (TS 30%) of source-sorted OFMSW performed inlab-scale digesters. The six inoculums studied were corn silage, restaurant digested wastemixed with rice hulls, cattle dung, swine dung, digested sludge and 1:1 mixture of swinedung and digested sludge. Results showed that sludge was the best inoculum with 43% VSremoval and 530 L CH4/kg VS after 60 days. Moreover, swine dung and mixture of swinedung and digested sludge also performed well. Similarly, microbial pretreatment of cornstalks was done by Zhou et al., (2012) and it was found that corn straws pretreated withcow dung and sludge produced 19.6% and 18.9% higher cumulative biogas yieldrespectively as compared to untreated straws by solid-state anaerobic digestion performedfor 60 days.

The effect of two different concentrations of inoculums (20% and 30%) was studied on dryanaerobic digestion of food waste at three different TS percentages, viz., 20%, 25% and30% (Foster-Carneiro et al., (2008). The percentage of inoculum for good biodegradationof waste and methane yield was established as 30% w/w at 20% TS in this study.

Optimum S/I ratio for dry anaerobic digestion is 1.0 or lesser. Guendouz et al., (2010) usedS/I ratio ranging from 0.19 to 0.35 in their experiments. The same S/I ratio is used forBMP test of wastes. Zhou et al., (2011) studied the effect of S/I ratio (ranging from 0.1 to3.0) on mesophilic anaerobic digestion of okara at 10% TS (higher TS compared toconventional digestion) in batch digesters. Results showed maximum methane yield at S/Ivalues of 0.6-0.9. On the other hand, methane yield decreased when the S/I exceeded 1.0,which was due to accumulation of volatile fatty acid (VFA) that significantly inhibitsfermentation. Thus VFA accumulation can be avoided by adjusting the S/I ratio and properinoculation.

2.5.4 Co-digestion

Co-digestion is mostly advantageous to adjust the C/N ratio of waste. By co-digestion inmost cases, biogas yield is improved due to synergism developed in the digester and due to

21

supply of the nutrients missing in the digestion medium by some of co-substrate.Kayhanian and Rich (1995) suggested that for dry anaerobic digestion, mixing of two orthree organic wastes could provide a nutrient sufficient feedstock. Moreover, by equipmentsharing during co-digestion, significant economic advantages are also obtained.Furthermore, adjustment of moisture content or total solids content of feedstock is alsoresulted from co-digestion. Common use of access facilities, easier and better managementand handling of mixed wastes are some of the other advantages of co-digestion. Currently,co-digestion of animal manure and OFMSW is frequently being done in the field. Co-digestion of OFMSW and sewage sludge is also an attractive option being used by manyresearchers.

Co-digestion of waste paper with the waste containing high nitrogen (food waste,vegetable waste, food processing industry waste and slaughterhouse waste) has been foundbeneficial as it can adjust C/N ratio of the medium. Mixing waste paper is useful forcontrolled dry digestion (Li et al., 2011) as it can control the accumulation of both theammonia and VFA. To get a feedstock with optimum C/N ratio for anaerobic digestion,many researchers (Wu et al., 2006; Walker et al., 2009) mixed 2-6% paper waste and 5-8%other slowly degradable waste (e.g. wood chips, straw and fallen leaves) with 86-93% ofOFMSW (food, fruit and vegetable waste). This could balance rapid production andaccumulation of VFA. Thus co-digestion could be used as tool to control VFA inhibition.

Li et al., (2011) studied co-digestion effect of undiluted cow manure, wastewater sludgethrough dry methane fermentation. Various mixtures of both substrates with different ratios(1:0, 4:1, 3:2, 2:3, 1:4 and 0:1) were co-digested at 35°C in the laboratory-scale single-stage batch reactors for 63 days. The results showed that the co-digestion with ratio of 2:3obtained highest specific biogas generation of 0.503 m3/kg VS, specific methanegeneration of 0.328 m3/kgVS and volatile solids and total organic carbon reductions of54.80% and 70.71% compared to the other co-digestion ratios and single digestions. It wasalso revealed that co-digestion resulted in 3.11- 3.99% higher methane gas yields, due tosynergistic effect. The synergistic effect is mainly attributed to more balanced nutrientsand increased buffering capacity.

Kim and Oh (2011) co-digested food waste and livestock waste in a continuously fedreactor mixed with impellers, and livestock waste content was gradually increased duringthe operation. Until an increase of 40% livestock waste, the reactor exhibited a stableperformance. But, when the livestock waste was increased to 60%, there was a significantdrop in performance, which was attributed to free ammonia inhibition. As the livestockwaste is rich in nitrogen, it caused the accumulation of ammonia in the reactor. Thus careshould be taken to select the proper ratios of co-substrate to avoid deficiency or toxicity ofsuch nutrients as nitrogen.

2.6 Reactor Design for Dry Anaerobic Digestion

The basic requirements of an anaerobic digester design are to allow for a continuously highand sustainable organic load rate, a short hydraulic retention time and to get the maximummethane yield. There are three main types of reactors (Figure 2.6) based on mode ofoperation for anaerobic digestion which are batch, one stage continuous and multistagecontinuous and are being discussed as under.

22

2.6.1 Single-stage batch systems

In batch digesters, once the feeding of substrate is done, it is sealed a duration equal to theretention time and there is no more feeding of substrate during this time until the processcompletes. After this, all the digested material is removed and new feed is added into thedigester for next cycle of digestion. In this process, the biogas production is not constant orcontinuous. There is low biogas production at the start and in the end, whereas at themiddle time, the rate of gas production is higher. To obtain a constant supply of biogas,many batch reactors in parallel are operated with feeding or loading of substrate atdifferent times.

Figure 2.6 Classes of dry anaerobic digestion by operational criteria

Batch operated dry systems are technically simple, less expensive, require less energy andoffer more control over the process than continuous system (Mumme et al., 2010) andhence are more attractive for developing countries. However, they need heavy inoculumand mixing for better stabilization of waste plus close observation of safety measures isalso necessary during the opening and emptying of the batches to avoid explosion. Biogaslosses during emptying the reactors and restricted reactor heights are the other drawbacks(Mumme et al., 2010). To overcome the problems of inoculum addition, mixing andinstability sequential batch system also known as SEBAC was developed.In batch system, the leachate collected in chambers is sprayed on top of the fermentingwastes. One technical shortcoming of such a process is that clogging can occur at theperforated floor. Although the batch systems are well suited to the demands of treatingrelatively large quantities of waste yet, biogas production and quality is variable andsomewhat unsteady (Evans, 2001). Besides, the batch systems are technically simple butthe land area required by the process is considerably large. Because of these shortcomings,batch system up to now has not been able to succeed in taking a substantial market(Bouallagui et al., 2005).

Dry Anaerobic Digestion

Single stage continuous Multi stage continuous Single stage batch

Substrate input mode

Reactor temperature

Thermophilic Mesophilic MesophilicThermophilic

Flow pattern

VerticalPlug flow

HorizontalPlug flow

HorizontalPlug flow

Example of technology available

HorizontalPlug flow No flow

Kompogas DRANCO Valorga Linde - BRV BIOCEL

23

2.6.2 Single-stage continuous systems

In continuous digestion process, the waste is fed and withdrawn from the reactorcontinuously. As the substrate is continuously added, all biochemical reactions involved inthe generation of biogas occur at a reasonably constant rate. The system receives its weightlittle by little, spread over time, so that digestion takes place uninterrupted having no endpoint. A reasonably constant rate of biogas production is resulted by this. Full-scale singlestage continuous digestion systems in Europe cover over 87% digestion capacity ofbiowaste and sewage sludge (De Baere, 2006). Industrialists prefer one-stage systemsbecause of their simpler designs and lower investment costs. The continuous inputanaerobic digestion requires less land area and its operating cost can be comparable.Importantly, the higher initial investment cost may be compensated from real state costreduction where the land is scarce.

However, a technical difficulty associated with pump has been encountered in loading thefeedstock in continuous manner (Sharma et al. 2000). Mixing is of pivotal importance inall anaerobic digestion systems, continuous systems rely on pumping for its continuousoperation (Lissens et al, 2001). Further, the continuous system requires high internalfluidity for the smooth feedstock intake and removal process. Such systems are, therefore,principally suitable for low solid wastes. For higher solid content, transport and handlingof the waste is carried out with conveyor belts, screws and powerful pumps especiallydesigned for viscous streams. Such types of equipment are very expensive (Mata-Alvarezet al., 2003).

There has been a shift of the research focus on semi-continuous mode of operation. Semi-continuous digesters are fed at continuous intervals of time, as for example on daily basis,or on more frequent intervals, with simultaneous removal of the digestate (Wang et al.,2003; Misi and Forster, 2002). These systems are suited to regularly and steadily arisingwaste stream. The biogas yield of semi-continuous processes is characteristically higherand more regular. The higher production rate is attributed to the waste that is kept in theiroriginal state, and not diluted with water (Oleszkiewicz and Poggi-Varaldo, 1997). Thedistinction between continuous and semi-continuous system is rather subjective. Most ofthe continuous digesters in large scales are not truly continuous. They are operated in semi-continuous mode (Sharma et al., 2000). The term ‘continuous system’ is used in a broadersense, which includes truly continuous and semi-continuous digestion systems where thedigesters are fed once or twice a day.

2.6.3 Multi-stage continuous systems

As anaerobic digestion process consists of four steps and each of these requires differentconditions for optimal growth and function of its respective microbial group involved inthe process. For instance, acidogens are more active at relatively low pH (i.e. 5.5–6.0)while acetogens and methanogens require stable neutral pH and are sensitive to even lowconcentrations of inhibitors (e.g., ammonia and VFA). Moreover, acetogens also needclose proximity with methanogens for efficient inter-species hydrogen transfer. However,most full-scale digestion systems in use are single-stage digesters, in which all of the fourprocesses have to be carried out simultaneously. In this way, the metabolic activities ofdifferent microbial groups are compromised and the performance of single-stage systems isoften suboptimal with a low reduction of VS. Thus, multi-stage system or moreappropriately, two-stage systems were introduced by the researchers to manage and

24

examine intermediate steps of digestion process. Hence, continuous anaerobic digestionprocess can be grouped into single-stage and multi-stage system on the basis of phase ofoperation.

Continuous operation of two reactors is generally performed in two stage digestionsystems. In the first reactor, hydrolysis and acetogenesis is done while in the secondreactor, methanogenesis is performed. The limitation of first reactor is hydrolysis rate ofcellulose whereas microbial growth is the limitation for second reactor. By dividing thedigester into two parts like this allows a better control on hydrolysis rate andmethanogenesis. In this type of systems, we can use different techniques to improve therate of reaction, for example, hydrolysis rate can be increased by microaerophilicconditions. One of the advantages of this system is that we can use the digesters as storagedevices. Moreover, for rapidly degradable waste materials (food waste, etc), a higherbiological stability is achieved.

Figure 2.7 Comparison between one stage and two stage process in Europe

In general, the two-phase system provides rapid and stable treatment increasing the rate ofhydrolysis and methanization (O’Keefe and Chynoweth, 2000). However, the claimedadvantage of the two-phase digestion has not been substantiated in real practice (De Baere,2000). Contrary to the claim, the added investment cost and operating complexity havecaused this system to be limited in a small market share. Kim and Speece (2002) concludedthat two-phase digestion system showed little benefit over single-phase during start-upperiod and no benefit were observed during the long-term period. The high digestion ratesprovided by the single phase system makes the system a viable even today.

As can be seen in Figure 2.7, the single-stage anaerobic digestion exhibits the highest trendof capacity from the period of 1990 up to the present. Some of the examples of high-solid

0

1000

2000

3000

4000

5000

6000

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Cum

ulat

ive

capa

city

(kt

on/y

ear)

YearOne-phase process Two-phase process

25

single stage systems are DRANCO, Valorga and KOMPOGAS processes which have beendiscussed in detail in the next section.

2.6.4 Design of available technologies for dry anaerobic digestion

Dry anaerobic digestion technologies used at industrial scale can be divided into threemain categories, which are batch (e.g. BIOCEL), single-stage continuous (e.g. DRANCO(thermophilic), Valorga (mesophilic) and KOMPOGAS (thermophilic)) (Figure 2.8) andmulti-stage continuous (e.g. Linde-BRV, SUBBOR) systems. In all the dry systems,because of high solid content, a part of the digested residues is recycled which is mixedwith the feed for inoculation. Due to their high viscosity, waste passes through the vesselas a slug so that fresh waste is not mixed with the partially digested waste, which is calledas plug flow. This offers the advantage of technical simplicity as no mechanical devicesneed to be installed within the reactor (Lissens et al., 2001). The designs of single-stagedry anaerobic digesters have been discussed in detail below.

BIOCEL: The system is based on a batch-wise dry anaerobic digestion. The total solidsconcentration of organic solid wastes as feeding substrate is maintained at 30–40% drymatter (w/w). The process is accomplished in several rectangular concrete digesters atmesophilic temperature. The floors of the digesters are perforated and equipped with achamber below for leachate collection. Prior to feeding, fresh biowaste substrate andinocula (digestate from previous feeding) are mixed then loaded to the digester by shovels.After the loading is finished, the digesters are closed with air tight doors. In order tocontrol the odor emission; the system is housed in a closed building that is kept at a slightunder-pressure. The temperature is controlled at 35–40ºC by spraying leachate, which ispre-heated by a heat exchanger, from nozzles on top of the digesters. Typical retentiontime in this process is reported to be 15-21 days (Ten Brummeler, 2000). A full-scaleBIOCEL plant is reported to have successfully treated vegetable, garden and fruit wasteswith the capacity of 35,000 tons/year. Approximately 310 kg of high quality compost, 455kg of water, 100 kg of sand, 90 kg of biogas with an average methane content of 58% and45 kg of inert waste are produced from each ton of waste processed (CADDET, 2000).According to Vandevivere et al., (1999) the BIOCEL plant produces on the average 70 kgbiogas/ton of source-sorted biowaste which is 40 % less than from a single stage low-solidsdigester treating similar wastes.

In the DRANCO process, feed is introduced daily into the top of the reactor by pumpingthrough the feed tubes, and the digested waste is removed from the bottom at the sametime. Part of the digested waste is used as inoculums (one part of fresh waste for six partsof digested waste) while the rest is dewatered to obtain an organic compost material. Thereare no mixing devices in the reactor other than the natural downward movement of thewaste. This process focuses on the conversion of the organic fraction of the municipal solidwastes to energy and a humus-like final product, called Humotex. The operatingtemperature is 55 oC, the total solids concentration is 32% and the residence time is around18 days. The process produces approximately 100 m3 biogas/ton input (Chavez-Vazquezand Bagley, 2002).

The Valorga system is quite different in that the horizontal plug-flow is circular in avertical cylindrical reactor, which is partially partitioned (around 2/3rd of the reactor) by acentral wall or baffle. The partition wall is connected to reactor wall at one end, while theother end is free allowing the passage of waste. The inlet is on one side of the baffle while

26

outlet is on the other side. The waste is forced to move around the baffle from inlet tooutlet that creates a plug-flow. Moreover, mixing is done by biogas injection at a highpressure at the bottom of the reactor. This biogas injection takes place every 15 minutesthrough a network of injectors. The residence time is between 18-25 days at 37oC andsolids content is kept at 30%. The Valorga process is ill suited for relatively wet wastesbecause sedimentation of heavy particles inside the reactor takes place when the totalsolids content is less than 20% (Lissens et al., 2001). Possible drawbacks of this system arethe clogging of the gas injection ports and the overall maintenance.

Figure 2.8 Designs of single-stage dry anaerobic digesters, (a) BIOCEL, (b)DRANCO, (c) Valorga, (d) KOMPOGAS

The KOMPOGAS process works similarly, except that the plug flow takes placehorizontally in cylindrical reactors. The digested material is removed from the far end ofthe reactor after approximately 20 days. The horizontal plug flow is aided by slowlyrotating impellers inside the reactors, which also serve for homogenization, degassing, andresuspending heavier particles. This process runs at 55oC and requires careful adjustment

Feed

Freshwaste

Digestate

Biogas

(b)

Feedtubes

Feed

Biogas

Digestate

Biogasrecirculation

(c)

Partitionwall

Biogas Leachate Recirculation

Leachate

(a)

Freshwaste

Digestate

(d)

Biogas

Digestate recirculation

27

of the solid content around 23% TS inside the reactor. At lower values, heavy particlessuch as sand and glass tend to sink and accumulate inside the reactor while higher TSvalues cause excessive resistance to the flow. The most significant factor of tubular reactoris its ability to separate acidogenesis and methanogenesis longitudinally down the reactor,allowing the reactor to behave as a system of two phases (Bouallagui et al., 2005).

2.7 Research Progress and Research Needs of Dry Anaerobic Digestion

Various research studies on dry anaerobic digestion have been conducted at TS content of15-30% using different substrates (Table 2.5). Only few research studies conducted on dryanaerobic digestion have been conducted in pilot-scale digesters (with size of 1-3 m3) butmost of them have been conducted in laboratory-scale digesters (2-60 L). As far as modeof reactor operation is concerned, both batch and continuous systems are found and almostall of them are single-stage. Dry anaerobic digestion studies have been performed at bothmesophilic (30-37oC) and thermophilic (55oC) temperature ranges. The studies conductedunder continuous mode are up to OLR and RT of up to 7.5-13 kg VS/m3d and 12-40 daysrespectively. Almost half of the dry AD studies given in this table focused on OFMSW assubstrate. Other than OFMSW, straws and residues of crops, solid livestock waste (e.g.cow dung, horse dung), food waste and dewatered sewage sludge have been used as feedfor high-solids digestion. Methane yield and VS removal of various studies given here cannot be compared as these vary with the waste type and waste characteristics. However,process design and experimental conditions can significantly influence the methane yieldand gas production rate.

Research conducted on dry anaerobic digestion so far, as discussed in the above part ofchapter summarizes that proper inoculation, optimum C/N ratio, controlled or minimalmixing, relatively low TS feed and high or thermophilic temperature can be helpful forsmooth start-up of dry digesters and can control VFA accumulation during start-up of drydigesters. Moreover, alkaline pre-treatment and thermophilic temperature can enhancehydrolysis of slowly degradable wastes like straws, and other wastes consisting of cropresidues. In addition, co-digestion or adjustment of C/N ratio to control both VFA andammonia accumulation can be used during continuous operation of dry anaerobicdigesters. Thermophilic temperature for dry AD could reduce the retention time and hencecould reduce reactor volume. This together with smaller reactor advantage of dry AD couldsufficiently save the capital cost.

Research on pilot and field scale in the field of dry AD needs to be done. Moreover, theeffect of mixing on performance of dry anaerobic digestion still needs to be studied athigher range of TS content.

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Table 2.5 Performance of Various Kinds of Dry Anaerobic DigestersSubstrate Feed

TS (%)ReactorType

ReactorSize

Temperature(°C)

RetentionTime (d)

OLR(kgVS/m3d)

SMPLCH4/kgVS

CH4

(%)VSReduction

Reference

Foodwastes

20 Batch Lab8L

37 33 - 367 90 Cho et al,(1995)

SS-OFMSW

20 Batch Lab5L

55 60 - - - 45 Forster-Carneiro,2008b

OFMSW 16c Batch Lab35L

30 32 - 273 64.6 26.1 Dong et al.,(2010)

Municipalsolidwaste

35 Batch withpaddlemixer

Lab40L

37 35 - 200 - - Guendouz etal., (2010)

Horsedung withstraw

15-30 Batch withpercolation

Lab57L

35 42 - 170 51-53

44 -49 Kusch et al.,(2008)

Cornstover

22 Batch Lab2L

37 40 - 223e 50-60

44.4 Zhu et al.,(2010)

Wheatstrawfromhorse bed

22 Batch Lab1L

37 30 - 150 55-60

- Cui et al.,(2011)

Cowmanure +Sludge

15-16 Batch Lab2.5L

35 63 - 328 65.17

54.80 Li et al.,(2011)

SS-OFMSW

20 ContinuousCSTR

Pilot3m3, 1m3

55 11.6 12.1 490d - 59.3 Pavan et al.,(2000)

OFMSW 20 ContinuousCSTR

Pilot3m3

55 13.5 9.2 230 68.7 - Bolzonella etal.,(2003)

29

Substrate FeedTS (%)

ReactorType

ReactorSize

Temperature(°C)

RetentionTime (d)

OLR(kgVS/m3d)

SMPLCH4/kgVS

CH4

(%)VSReduction

Reference

MS-OFMSW

18 Continuous Lab4.8L

55 19 9.65 342 b 59 65 Gallert andWinter, (1997)

simOFMSW

30 Semicontinuous

Lab5L

55 15 11.8 97b - 89 Fdez-Guelfo etal., (2011)

FW +Paperwaste

30 Contnuouswith lateralimpeller

Lab60L

35 30-100 10a 250 - 80 Kim and Oh,(2011)

OFMSW 25-30 ContinuousCSTR

Lab4.5L

55 25-40 4.42-7.5 300 e 50 80 Montero et al.,(2008)

Maizesilage +Barleystraw

- ContinuousUASS,Magneticstirring

Lab26.5L

55 - 12.7 182 44.1 88.7 Mumme et al.,(2010)

Sewagesludge

20 Continuous, CSTR

Lab6L

35 12 8.5 190 64.9 29 Duan et al.,(2012)

CSTR: Continuously stirred tank reactorSHW: Slaughter house wasteFW: Food waste,MS-OFMSW: Mechanically sorted OFMSWSS-OFMSW: Source sorted OFMSWUASS: Upflow anaerobic solid-state reactoraThe value given is of solid loading rate (i.e. kg TS/m3d).bCalculated by dividing methane production rate (L CH4/Lreactor vol..d) by OLR (g VS/Lreactor vol.d).cTS in reactor mediumdSpecific gas production (SGP)eThe unit here is LCH4/kg CODfCalculated considering 60% methane content

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2.8 Anaerobic Digestion and Digestate Management

Anaerobic digestion treats organic waste, which not only produces biogas, but also otherby-products such as liquor/liquid digestate and fiber/solid digestate/digestate. Thecombination of fiber and liquor produced by anaerobic digestion is termed as digestate. Ithas been found that these materials contain a considerable amount of nutrients and organicmatter and are useful for agriculture. Utilization of digestate as a soil amendment ororganic fertilizer can improve cop yield and soil properties and hence helps promotingclosure of nutrient cycles. In this way, the need for production of synthetic inorganicfertilizers can decrease and consequently significant amount of energy can be saved andGHG emission can be reduced leading to energy and economic benefits.

But the problem is that, digestate has potential of methane formation, NH3 and N2Oemission and may also contain pollution causing organic compounds and other hazardousmaterial such as heavy metals. Therefore, application of digestate to agricultural land needsa sound management and sometimes prior treatment to improve its quality. Also, afterapplication, careful monitoring of soil properties and plant growth is required.

2.8.1 Need of digestate management and digestate utilization

Management of digestate (anaerobically digested waste) is needed because of manyreasons. First is to keep the environment safe during and after anaerobic digestion becauseits handling and spreading may cause environmental risk, either due to leakage of nitrate torecipient waters or due to extensive gaseous losses of ammonia and the nitrous oxide aswell as because of having certain methane formation potential (Figure 2.9).

Figure 2.9 Emissions from soil applied digestate to environments

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2.8 Anaerobic Digestion and Digestate Management

Anaerobic digestion treats organic waste, which not only produces biogas, but also otherby-products such as liquor/liquid digestate and fiber/solid digestate/digestate. Thecombination of fiber and liquor produced by anaerobic digestion is termed as digestate. Ithas been found that these materials contain a considerable amount of nutrients and organicmatter and are useful for agriculture. Utilization of digestate as a soil amendment ororganic fertilizer can improve cop yield and soil properties and hence helps promotingclosure of nutrient cycles. In this way, the need for production of synthetic inorganicfertilizers can decrease and consequently significant amount of energy can be saved andGHG emission can be reduced leading to energy and economic benefits.

But the problem is that, digestate has potential of methane formation, NH3 and N2Oemission and may also contain pollution causing organic compounds and other hazardousmaterial such as heavy metals. Therefore, application of digestate to agricultural land needsa sound management and sometimes prior treatment to improve its quality. Also, afterapplication, careful monitoring of soil properties and plant growth is required.

2.8.1 Need of digestate management and digestate utilization

Management of digestate (anaerobically digested waste) is needed because of manyreasons. First is to keep the environment safe during and after anaerobic digestion becauseits handling and spreading may cause environmental risk, either due to leakage of nitrate torecipient waters or due to extensive gaseous losses of ammonia and the nitrous oxide aswell as because of having certain methane formation potential (Figure 2.9).

Figure 2.9 Emissions from soil applied digestate to environments

30

2.8 Anaerobic Digestion and Digestate Management

Anaerobic digestion treats organic waste, which not only produces biogas, but also otherby-products such as liquor/liquid digestate and fiber/solid digestate/digestate. Thecombination of fiber and liquor produced by anaerobic digestion is termed as digestate. Ithas been found that these materials contain a considerable amount of nutrients and organicmatter and are useful for agriculture. Utilization of digestate as a soil amendment ororganic fertilizer can improve cop yield and soil properties and hence helps promotingclosure of nutrient cycles. In this way, the need for production of synthetic inorganicfertilizers can decrease and consequently significant amount of energy can be saved andGHG emission can be reduced leading to energy and economic benefits.

But the problem is that, digestate has potential of methane formation, NH3 and N2Oemission and may also contain pollution causing organic compounds and other hazardousmaterial such as heavy metals. Therefore, application of digestate to agricultural land needsa sound management and sometimes prior treatment to improve its quality. Also, afterapplication, careful monitoring of soil properties and plant growth is required.

2.8.1 Need of digestate management and digestate utilization

Management of digestate (anaerobically digested waste) is needed because of manyreasons. First is to keep the environment safe during and after anaerobic digestion becauseits handling and spreading may cause environmental risk, either due to leakage of nitrate torecipient waters or due to extensive gaseous losses of ammonia and the nitrous oxide aswell as because of having certain methane formation potential (Figure 2.9).

Figure 2.9 Emissions from soil applied digestate to environments

31

Second is that it has a high water content which makes it expensive to handle, transportand spread in the field. Thirdly, it is the source of organic matter and nutrients especiallynitrogen and can be used in gardens, forests, recreation and sports ground, and fish pond asfertilizer or soil conditioner/soil amendment. Moreover, the residue from anaerobicdigestion (digestate) has the potential advantage over untreated slurries that it is consistentin nutrient content and availability. This makes it easier for farmers to calculate the correctfertilizer applications to crop requirements (Berglund, 2006). Monetary benefits are alsoobtained because the energy consumption for fertilizer manufacturing is decreased if it isproduced from on-farm anaerobic digestion plant. Furthermore, digestate can provideexport revenue, depending on quality. However, Lantz et al., (2007) reported that digestatecontains all the non-biodegradable contamination of the feedstock putting its use as anorganic fertilizer in question.

The post treatment in anaerobic digestion to manage digestate can be liquid digestatetreatment (aeration, nitrogen removal, precipitation of heavy metals) or separation of liquidand solid fraction of digestate from each other (dewatering, fiber separation, and sandremoval) to solve the problem of its handling and transport (Bauer et al., 2009). If heavymetal content and other pollutions are within safe limit, the solid digestate can becomposted or spread on agricultural land or used as landfill cover. In such case, it may taketwo to four weeks for stabilization of its organic contents before its use for the mentionedpurposes. The liquid fraction is either used directly as a fertilizer in agriculture, recycledback to the anaerobic digestion process for dilution and inoculation of new waste stream(especially in dry digesters), treated in a wastewater treatment plant or discharged intosewage. In case of dry anaerobic digestion, most of the liquid is recycled to the system formoistening and inoculating the new waste and dewatered digested material is matured tocompost. Other than agricultural use, it can also be utilized for other purpose. For example,Teater et al., (2011) reported that solid digestate (AD fiber) from a CSTR digesting dairymanure was a suitable biorefining feedstock (for production of ethanol) as compared toswitchgrass and corn stover.

2.8.2 Effect of prior digestion on properties of digestate

During digestion, properties of waste change considerably as shown in Table 2.6. Totalsolids, organic carbon, volatile organic compounds (odor), GHG emissions potential,pathogens and weed seeds in the waste decrease because of digestion. pH of the digestermedium increases. Organic N is transformed to NH4-N, so N availability to plantsincreases, if the digestate is applied to agricultural land. Moreover, fluidity, homogeneityand infiltration properties of waste are also improved by digestion, which further increasesthe nutrient availability of digestate (Lantz et al., 2007). However, the increasedconcentrations of NH4-N and high pH also increase the loss potential of N in NH3 formthrough volatilization. There is no much effect of fermentation on P and K availability.Digestion also improves handling and solids separating characteristics if manure is used asfeedstock, and reduces attractiveness of the manure to rodents and flies. Some of theseeffects of digestion on the properties of waste have been presented in Table 2.6.

Tambone et al., 2009 studied the transformation of organic matter during anaerobicdigestion of mixtures of energetic crops, cow slurry, agro-industrial waste and OFMSW.The anaerobic digestion process proceeded by degradation of more labile fraction (e.g.carbohydrate-like molecules) and concentration of more recalcitrant molecules (lignin andnon-hydrolysable lipids).

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Table 2.6 Effect of Digestion on Properties of WasteParameter Feedstock Before

DigestionAfterDigestion

% Change Reference

pH Primarysludge +OFMSW

3.5 7.5 +114 Gomez et al.,2007

Cattle slurry 7.2 8.4 +16.7 Mokry et al.,2008

Pig slurry 7.0 8.1 +15.7 Mokry et al.,2008

Cattlemanure

6.9 7.6 +10.1 Gomez et al.,2007

TotalSolids

OFMSW 90 g 43.4 g -52 Rao andSingh, 2004

Primarysludge +OFMSW

60 g/L 23.6 g/L -60.66 Gomez et al.,2007

Cattlemanure

263 g/L 122.6 g/L -53.38 Gomez et al.,2007

Cattle slurry 9.9 % 7.1 % -28.28 Mokry et al.,2008

Pig slurry 7.6 % 4.9 % -35.52 Mokry et al.,2008

VolatileSolids

OFMSW 79.65 33.10 -58.44 Rao andSingh, 2004

OFMSW 82.32 % 40.95 % -50.25 Eliyan, 2007Primarysludge +OFMSW

55.2 g/L 16.5 g/L -70.10 Gomez et al.,2007

Cattlemanure

226.2 g/L 105.4 g/L -53.40 Gomez et al.,2007

Cattle slurry 86.9a %TS 75.35 %TS -13.25 Mokry et al.,2008

Pig slurry 78.5a %TS 73.26 %TS -6.66 Mokry et al.,2008

Total N OFMSW 1.4 g 1.06 g -24.28 Rao andSingh, 2004

Cattle slurry 4.1 kg/t 4.5 kg/t +9.75 Mokry et al.,2008

Pig slurry 4.1 kg/t 4.5 kg/t +9.75 Mokry et al.,2008

NH4-N Cattle slurry 1.7 kg/t 2.5 kg/t +47 Mokry et al.,2008

Pig slurry 2.0 kg/t 3.5 kg/t +75 Mokry et al.,2008

Manure 70 % of TN 85 % of TN +21.42 Berglund,2006

aCalculated by the formula: VS = OM X 1.8/1.72

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2.9 Characteristics of Digestates

Digestate is the liquid-solid suspension that is produced by anaerobic digestion of foodwaste, OFMSW, manures and wastewater. It is combination of solid part and liquid phaserespectively called solid digestate or fiber and liquid digestate or liquor. The proportion ofsolid and liquid part depends upon the type and nature of feedstock used for digestion aswell as type of digestion process (dry/wet). It has been found to contain certain proportionof organic matter and plant nutrients like N, P, K, Ca, Mg, etc. that make it suitable to beused as soil amendment or fertilizer. Liquid part contains high N percentage and solid partcontains high P content. The presence of heavy metals (Cd, Cr, Pb, Ni, Hg, Cu, Zn) andorganic pollutants on the other hand decreases the possibility of digestate to be used inagriculture. Some other parameters are also used for characterization of digestates such asmoisture content, total solids and volatile solids content, calorific value of dry fiber, pH,etc. Physical characteristics of digestates are described by such parameters as bulk density,cellulose structure/grain size, or presence of plastic, rubber, metals, glass, ceramics, sandand stone. Biological parameters to describe the digestate are presence of pathogens orweed seeds and biological stability of the digestate.

2.9.1 Characteristics of solid digestates

In case of dry anaerobic digestion, most of the liquid digestate is recycled to inoculate andmoisten the fresh waste. Thus solid fraction of digestate is the major part of digestate thatneeds to be managed in dry anaerobic digestion. Characteristics of solid digestate in dryanaerobic digestion systems have been shown in Table 2.7. Total solids, volatile solids, saltcontent, pH, nutrients (N, P, K, Ca, Mg, etc.) and C/N ratio are assessed to describe thecharacteristics of solid digestate. Sometimes, density, pore volume and water holdingcapacity are also discussed. The pH of the solid digestate varies between 7.5 and 8.5. Saltcontents vary greatly in composts and digestates.

Through a more consistent choice of the materials of origin, the compost producers canobtain a more constant salt content in the final product, because this is also influenced bymaterial of origin. The salinity of biosolids from an animal slurry and cattle manuredigester has been reported as 0.0469 and 2 dS/cm respectively. Organic matter isprincipally influenced by the maturity of the products (Fuchs et al., 2008). Similarly, othercharacteristics also vary depending on the type of feedstock.

Nutrient contents in the digestates are influenced principally by the materials of origin. Theresults of analysis of 100 samples of digestates and composts representative of differentsystems and taken from Swiss market show that the nutrient content of the digestates varygreatly among the digestates. The median values of total N, P, K, Ca, Mg, Fe and Na havebeen found to be 1.53%, 0.36%, 1.25%, 4.66%, 0.68%, 0.89% and 0.13% of dry matterrespectively (Fuchs et al., 2008) in this research, while the contents of total N, P and K inthe anaerobic digestion digestate of OFMSW have also been reported as 1.09%, 0.65% and0.65% respectively (Eliyan, 2007). These differences in nutrient content of differentdigestates depend upon the materials of origin.

In a continuously fed dry anaerobic digester, if the moisture content is not maintained byaddition of lost water through digestate and biogas removal, the TS content of the reactormaterial (digestate) increases. This increase in TS content of digestate is more rapid, if theOLR is increased as compared to a constant OLR.

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Table 2.7 Characteristics of Solid Digestate in Dry Anaerobic Digestion Systems

Substrate pHMC(%)

TS(%)

VS(% TS)

TN(% TS) C/N

NH4-N(% TS)

P(%TS)

K(%TS)

Ca(%TS)

Mg(%TS) Reference

OFMSW - 92.77 7.23 76.261.06

12.07 - - - - -Rao andSingh, 2004d

Farm manure(Spring)

- 70.55 29.45 89.66 1.46 29.06 0.23 0.28 1.05 - -Schafer et al.,2006Farm manure

(Autumn)- 73.32 26.68 88.95 1.38 30.27 0.17 0.27 1.46 - -

OFMSW - 73.00 27.00 40.95 1.09 20.87 - 0.65 0.65 - -Eliyan, 2007

Digestatesfrom market

8.5a 46.9053.10

52.64b 1.53 19.11c 0.06 0.36 1.25 4.66 0.68Fuchs et al.,2008

Animal slurry 8.1a 68.10 31.90 66.30 3.30 11.16c 1.05 3.16 - - -Jorgensen andJensen, 2008

Cattle manure 7.5 - - - 2.38 - 0.36 1.47 1.91 2.61 0.76Sanchez et al.,2008dPoultry

manure8.5 - - - 1.49 - 0.22 2.33 1.19 4.34 0.61

Food waste +Energy crops+Animalmanure

7.9 90.40 9.60 77.00 4.40 9.72c 2.00 - - - -Menardo etal., 2011d

aFrom analysis of 1:2 water suspensionbCalculated by the formula: VS = OM X 1.8/1.72cC = VS/1.8dCharacteristics of raw (un-separated) digestate

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For instance, Mumme et al., (2010) reported an increase in TS concentration from 15.8 to22.1% continuously since day 39 (during feeding steps with higher OLR of 12.7 and 17 kgVS/m3d). Moreover, compaction of solid-state bed was also observed during the sametime. The increase in TS content of reactor medium has been found to decrease VSremoval of the digester as proved in section 2.4.2. Similarly, increase in TS of digestatealso increases remaining methane formation potential of digestate. Both the TKN andNH4–N accumulate in the solid residues of a continuously operated digester, which is justsimilar to that in liquid digestate.

Fuchs et al., (2008) analyzed one hundred compost and digestate samples representative ofthe different composting systems and qualities and concluded that the respiration rate,enzymatic activities and phytotoxicity varied greatly which depend on maturity andmanagement process. Mature composts showed less respiration rates and enzymaticactivity. In Europe humic substances are described as parameters for quality of composts inaddition to low contents of pollutants (Binner et al., 2008). Unhurried degradation withlong lasting biological reactivity of the feedstock is important for high compost quality thatleads to the formation of more humic acids. Anaerobic pretreatment seems to be positivefor the development of humic acids during the following rotting period. In contrast,intensive supply of oxygen mineralizes the metabolic products quickly and completely anddischarges into air. Pure sewage sludge or sewage sludge with low amount of yard wasteslead to a low development of humic acids.

Tambone et al., (2010) studied a total of 23 samples of digestates, ingestates, composts anddigested sludge to assess their amendment and fertilizing properties. The results shown thatdigestates differed from ingestates and also from compost, although the starting organicmix influenced the digestate final characteristics. In amendment properties, compost anddigestate were better than digested sludge and all of these were better than ingestate.

As to fertilizer properties, digestion produced digestate with very good fertilizingproperties because of the high nutrient (N, P, K) content in available form. Thus, thedigestate appears to be a good candidate to replace inorganic fertilizers, also contributing,to the short-term soil organic matter turnover.

2.9.2 Characteristics of liquid digestates

It has been found that the liquid digestate resulting from fermentation of farm manures,agricultural biomass, OFMSW and their combination from biogas plants provide liquidfertilizer. Typically, the liquid digestates resulting from liquid-solid separation of digestedresidues contain low total solids content (3-6 % of farm manure). Moreover, they are richin nutrients such as N (> 6% of TS), NH4-N (> 2% of TS), K (> 4% of TS), Ca and Mg.Because of high N content of liquid digestates, their C/N ratio is very low (< 6), whichmakes them suitable for direct application to the field. The required C/N ratio for anorganic fertilizer (or amendment) to be applied on soil is < 20 (Wood, 2008). However,high pH (about 8.5) of liquid digestates makes them more prone to be lost to atmospherethrough volatilization, when applied in the field especially at high ambient temperature.Thus special soil management practices need to be adopted to avoid this loss of N. Thecharacteristics of liquid digestates from different digesters as investigated by differentenvironmentalists have been shown in Table 2.8.

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Table 2.8 Characteristics of Separated Liquid Digestates from Different Digestion Systems

SubstrateMC(%)

TS(%)

VS(% TS)

TN(% TS)

C(%TS) C/N

NH4-N(% TS)

P(%TS)

K(%TS) Reference

Animal slurry 94.77 5.23 - 7.27 - - - 2.24 -Moller et al.,2002

Farm manure(Spring)

94.33 5.67 70.55 6.53 35.27 5.4 2.12 1.39 6.00Schafer et al.,2006Farm manure

(Autumn)96.17 3.83 71.00 6.53 23.49 3.6 2.61 1.33 8.36

Manure withbiowaste

99.48 0.52 86.54 30.77 48.10c 1.6 21.15 2.12 -Paavola andRintala, 2008

SS-OFMSW 96.10 3.90 66.40 13.85 36.88c 2.7 9.85 1.22 3.64 Palm, 2008

Farm manure 93.50 6.50 73.25a 6.92b 40.69c 5.9 3.84b 2.69 6.92Mokry et al.,2008 d

aCalculated by the formula: VS = OM X 1.8/1.72bFeedstock is animal slurrycCalculated by the formula: C = VS/1.8dData of raw (un-separated) digestate

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Comparison of liquid and solid digestate shows that solid digestates have much higher C/Nratio (20-30) because of lower N content as compared to liquid digestates and thus need tobe treated and stabilized by composting before they can be applied on agricultural soils asamendment. Moreover, solid digestates also have higher TS and P content as compared toliquid digestate.

2.9.3 Presence of organic pollutants

Digestates are so-called ’secondary fertilizers’ or ’waste fertilizers’ which are used inagricluture because these can provide a little amount of plant nutrients and organic matterto soil, these however, contain some pollutants in varying concentrations mostly in therange of µg/kg, that can contaminate soil and food. Organic pollutants present in thedigestates are PCBs (polychlorinated biphenyls), PAHs (polycyclic aromatichyrdrocarbons), NPs (nonylphenols), DEHPs (di-2-ethylhexyl phthalates), PBDEs(polybrominated diphenyl ethers), PCDD/Fs (polychlorinated dibenzo-p-dioxins anddibenzofurans) and some other organic compounds. The most important pathways oforganic pollutants in the digestate are deposition from air and direct application ofpesticides to the material of origin. Some of these compounds are degraded in soil afterapplication, so that we can measure concentration in soil to determine their degradationand persistence. Soil concentrations of pollutants may be 3 to 10 times lower than those indigestates (Stab et al., 2008).

As shown in the Table 2.9, PCBs have less concentrations in most of digestates orcomposts, but other pollutants like PAHs, NP and DEHP are high in concentration.Usually, these concentrations are low in green waste composts and high in biowastecomposts and composted digestates. This may be because of different levels ofcontamination of input materials. Areas having more heating through oil in winter can gethigher concentration of PAHs from atmosphere.

Table 2.9 Concentration of Organic Pollutants in Digesates and Composts (µg/kgDM)Pollutant Anaerobic

digestiondigestate

Compost Biowastecompost

Digestate ofSS waste

Composteddigestate(median)

PCB 10 20 33 32 25.6PAHs 1430 3010 2.659 5925 2680NP 4770 30 560 - 324DEHP 29700 30100 1.400 1114 1760Organiccompounds/Pesticides

430 130 - 114 -

PBDE - - 13 2.7 26.3PCDD/Fs - - - 3.2 ng I-

TEQ/ kg dw6 ng I-TEQ/kg dw

Reference Kordel andHerrchen,2008

Kordel andHerrchen,2008

Stab et al.,2008

Kupper, etal., 2008

Riedel andMarb, 2008

Note: SS stands for source separated

Estimated flux of organic pollutants (PCB and PAH) to Swiss agricultural soil wasdominated by application of manure and aerial deposition (Brandli et al., 2008). However,

38

if surface specific loads (loads per hectare) were considered, source-separated compost anddigestate were the most important inputs by more than a factor of 25 and 20 for PCBs andPAHs. Total PAH loads accounted for 33% of the input from aerial deposition. Loads ofPCBs were low and are even expected to decrease over the next decades due to the banningof PCBs.

2.9.4 Presence of heavy metals

Digestates are so-called ‘recycling fertilizers’ or ‘low price fertilizers’ which are used inagricluture because these can provide plant nutrients and organic matter to improve soil.They however, contain some pollutants in the form of heavy metals such as Cd, Cr, Cu,Hg, Ni, Pb, Zn, etc., that can contaminate soil and food. The most important pathways ofheavy metals in the digestate are deposition from air and direct application of pesticides tothe material of origin. The heavy metal contents in digestates are influenced principally bythe materials of origin (Fuchs et al., 2008). The contents of heavy metals determinedmostly are low except for Cu, Zn as shown in Table 2.10.

2.9.5 GHG emission potential of digestate

The treatment process of anaerobic digestion removes certain amount of carbon from thewaste while the remaining carbon still remains in the digested residues (digestate). Thusthe digestate can act as source of GHGs if not managed properly. Rico et al., (2011)collected four anaerobic effluents from the digester (digesting dairy manure) at differentHRTs and analyzed to measure their residual methane potentials. They reported residualmethane potential of digestates in the range of 12.7 to 102.4 L/g VS. These methanepotentials were highly influenced by the feed quality and HRT of the previous CSTRanaerobic digestion process. Menardo et al., (2011) also reported that the residual methanepotential of digestate (taken from digesters with feed of animal manure, energy crops andfood industry waste) was very variable (2.88-37.63 NL/kgVS) and depended on the OLR,HRT and feedstock quality during digestion. Mumme et al., (2010) also reported similarresults.

Table 2.10 Heavy Metal Content in Different Types of Digestates (mg/kg DM)Digestate Type Cd Cr Cu Hg Ni Pb Zn Reference

OFMSW 0.64 13.88 55 0.035 12.16 0 105Eliyan, 2007

Cattle manure - 8.05 128 - - - 555Sanchez et al.,2008

Composteddigestate

0.36 23.00 72 0.097 11.50 26.8 179Riedel and Marb,2008

Biowastedigestate

0.28 12.00 40 0.100 10.00 7.0 160 Persson, 2008

Animaldigestate

0.30 9.30 113 0.080 9.70 4.1 375 Palm, 2008

Similar to the digestion of original waste, specific methanogenic activity (SMA) ofdigestate from MSW digester is also linearly linked to moisture content. Hyaric et al.,(2011) found that low moisture content is detrimental to SMA of digestate. The SMA testfor digestate was performed at mesophilic temperature at four different moisture contents(65-82%) and it was found that SMA is highest at 82% moisture content.

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2.10 Management Aspects of Anaerobic Digestate

2.10.1 Separation of liquid and solid digestate

Moller et al. (2000) suggested that environmental problems may occur on livestock farmsbecause of presence of huge amount of nutrients in the animal slurry than required bycrops of the locality, so this problem can be mitigated by separation of slurry into nutrientrich liquid fraction and solid part and the nutrient rich part then can be easily transported tothe agricultural farm having less animals. Solid digestate or fiber rich part of manure that isrich in organic matter and P makes 10-20 % of total volume of manure or digestate(Jorgensen and Jensen, 2008; Jensen et al., 2008; Bauer et al., 2009), while the remaining80-90% proportion consists of liquid fraction.

It has been found that separation of liquid and solid parts also separates N and P(Tronheim, 2005; Palm, 2008) because solid part contains about 80 % of organic N and Pand liquid fraction contains inorganic N (NH4-N) and potassium, and this separation alsoreduces pollution, odor, and transport cost (Tronheim, 2005). Bauer et al., (2009)separated fermentation residues (into solid and liquid digestate) of two plants digestingenergy crops and found that solid digestate contained higher dry matter, volatile solids andcarbon, raw ash and phosphate in relation to the mass as compared to liquid digestate,whereas nitrogen and ammonia nitrogen were slightly enriched in the solid digestate. Onlythe potassium content decreased slightly in the solid digestate. Palm (2008) stated that thisliquid part can give a 90 % yield of that given by mineral fertilizer. Figure 2.10 shows thesequences of separation in anaerobic digestion. Here it can be mentioned that if the type ofanaerobic digestion is dry, most of liquid part will be recycled to digester while the contentof solid part will be high, and we will get a lot of solid digestate that needs to be managedmost probably through composting and this will be the focus of this study.

AnaerobicDigester

Organicwaste

Biogas

Digestate

Liquid NFertilizer

LiquidDigestate

SolidDigestate

P RichCompost

Recycle

Figure 2.10 Liquid-solid separation of digestate with production of useful products

Mayer (2008) gave another concept of separation of animal slurry before digestion, 40 %of liquid part according to him should be digested only to produce biogas and liquidfertilizer because of having high energy and water content and being odor intensive and 60% solid structural part with low energy and water content should be composted. Liquid-solid separation can be done by various methods. Jorgensen and Jensen, (2008) reporteddifference in results of various parameters by use of different separation methods likedecantation, mechanical separation and chemical separation of animal slurries and their

40

digestates. Mechanically, liquid-solid separation can be done by use of screw extractorseparator, rotary screen separator (Bauer et al., 2009; Gioelli et al., 2011), filter press andcentrifugation.

2.10.2 Direct land application of liquid digestate

The C/N ratio of digestate should be less than 20 to be fit for land application (Wood,2008). Palm (2008) stated that this liquid part of digestate, if applied to soild, can give a 90% yield of that given by mineral fertilizer. The digestate may also be spread directly ontofarmland as slurry. However, its application has been limited to maximum amount of 170kg N/ha/y by the EU Nitrate Directive in 1999 (Al Seadi et al., 2001).

Load of the nutrients is one of the concerns in recycling anaerobic digestion digestate onfarmland because of danger of nitrate leaching and overloading of phosphorous leading tosurface and ground water pollution. Lack of stability of fresh organic waste is also aconcern. Storage and application of digestate, are therefore, important in this case. Cost ofdigestate fertilizer increases because of additional storage and it also increases emissionduring storage. To regulate this nutrient loading, limits have been set in different countriesas given in the Table 2.11.

Table 2.11 Regulations of Nutrient Loading on Agricultural LandCountry Nutrient Load

(kg N/ha/y)Required Storage

Austria 100 6 monthsDenmark 170 (cattle)

140 (pig)30 kg P/ha/y7 ton DM/ha/y

9

Italy 170-500 3-6UK 250-500 4Modified from Al Seadi et al., 2001

2.10.3 Aerobic post-treatment of solid digestate and its effects on quality

Aerobic post-treatment of the anaerobically digested waste or digestate bring aboutchanges in pH, solids content and C/N ratio as shown in Figure 2.11. In this study, aerationwas carried out intermittently for 5 h on a daily basis for the treatment. The figure showsthat pH increased between 6.7 and 7.8 during the aeration period.

Figure 2.11 also shows that aeration brings about decrease in VS (%TS) possibly due toaerobic decomposition of organic matter remaining at the end of anaerobic decomposition.The decrease in C/N may be an indication of the breakdown of organic matter during thisperiod. Beyond the 40th day, the C/N ratio becomes considerably less variable dueprobably to the depletion of most of the readily biodegradable organic compounds. pH andC/N therefore seem to be good indicators for assessing compost maturity and stability.Figure 2.11 also indicates that the compost was fully stabilized on the 40th day.

Thus, an improved aeration frequency can bring about stability in a shorter period. At theend of the 70 days, the composition of the stabilised compost was found to be pH 7.8; TVS= 45% and C:N = 12.7 that are recommended values for a compost to use.

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Figure 2.11 Changing parameters during aerobic post-treatment (Abdullahi et al.,2008)

2.10.4 Digestate storage and its effects on characteristics

Storage of digestate has a possible influence on its quality. If the digestate is stored forlong, it can undergo decomposition process that can lead to decrease in TS, VS,transformation of nutrients into their different states especially of N and P and loss ofmethane.

Paavola and Rintala (2008) demonstrated that during 3-11 months of storage, averagereductions of nitrogen, TS and VS were 0-15%. Soluble chemical oxygen demand (SCOD)increased slightly from 6.5 to 7.5 g/l after 3 months storage, while after 9-11 months itdecreased from 8.3-11 to 5.6-8.4 g/l. The concentrations of total P and PO4-P in theseparated liquid fractions decreased 40–57% after 3 months storage and 71-91% after 9months storage compared to the initial concentrations. The methane potential losses during9-11 months storage corresponded only 0–10% of the total methane potential withoutstorage.

Menardo et al., (2011) reported that generally, digestate storage is in uncovered tanks fromwhich several gases, such as CO2, NH3, N2O and CH4, are lost to the atmosphere.Greenhouse gases (GHGs), such as N2O, CO2 and CH4, affect the global environment andclimate while NH3 contributes to general atmospheric pollution. For these reasons, someEuropean Countries have required that digestate be stored in covered tanks (Palm, 2008).

Gioelli et al., (2011) reported that every day, biogas plants produce huge volumes ofdigestate, which can be handled in raw form or after its separation mechanically. Effluentsare usually stored in uncovered tanks, placed aboveground in Italy, which can make thememitters of biogas into the atmosphere. They estimated the amount of biogas emitted fromnon-separated digestate as well as digested liquid fraction to atmosphere during theirstorage. The investigation was performed on two biogas plants (1 MWel.) in northwest

42

Italy. For the residual biogas recovery, a floating system was used, whereas NH3 emissionmeasurement was done with a set of three wind tunnels. The results showed significantloss to the atmosphere of each of the gases. About 19.5 and 7.90 m3 biogas MWhel.-1 werereleased to atmosphere everyday from the storage points of non-separated digestate anddigested liquid fraction respectively. It seemed to depend on surface area of the digestateexposed to temperature and atmosphere. Mixture of different crops and manures was usedas feedstock.

2.11 Post Utilization Monitoring Issues of Anaerobic Digestate

2.11.1 Effect of digestate application on soil

Application of digestate to soil may have some positive effects for soil and plant growthbecause it contains considerable quantity of plant nutrients and organic matter that cansupport soil and plant health. Digestate application has been found to improve physicalcharacteristics of soil like soil structure, infiltration, and water holding capacity; chemicalcharacteristics like increased pH in acidic soils, availability of nutrients like N, P and Kand biological characteristics of soil.

Berglund (2006) stated that to improve the poor soil structure by spreading digestate rich inorganic matter on arable land, one of the large-scale biogas plants was built, and ley cropswere introduced intended for anaerobic digestion in cereal-based crop sequences ofLaholm.

Wang et al., 2008 carried out field experiments to study the effect of different treatments ofanaerobic digestion residues on yield and quality of Chinese cabbage (Brassica rapa L.var. shanghaisis) and nutrient accumulation in soil. The soil pH was improved, and theavailable N, P and K accumulated in the soil were increased by the application of theanaerobic fermentation residue. Fuchs et al. (2008) performed two field experiments toevaluate the influence of composts and digestates on soil fertility and plant growth andconcluded that the N-mineralization potential from the most of the digestates applied tosoil was high, in comparison to young composts. Field experiments revealed that digestatesincreased the pH-value and the biological activity of soil to the same extent as composts.According to them, the potential for nitrogen immobilization is affected by maturity,composition of the composted materials (e.g. digestate) and management of thecomposting process. Two parameters predict the risk of nitrogen immobilization in soil:the NO3-N and the humic acids contents (maturity). Compost should be howevercomplemented with mineral N and the biogas residues with P (Svensson et al., 2004).

2.11.2 Influence of digestate application on plant growth and health

Digestates affect plant growth positively and negatively. Provision of plant nutrients,suitable soil characteristics and tolerance against plant diseases, are positive aspects ofdigestates for improving plant growth, yield and quality. Svensson et al. (2004) reportedthat application of anaerobic digestion residues to soil resulted in less yield than mineralfertilizer but improved yield and quality than compost. So, digestates should be used incombination with mineral fertilizer for improved yield and quality of crops. Wang et al.(2008) concluded from the field experiments that the yield and quality (Vitamin C andnitrate content) of Chinese cabbage (Brassica rapa L. var. shanghaisis) were significantlyimproved. The highest yield of 1364.31 kg/667 m² was achieved at the application of low

43

amount of anaerobic fermentation residue. It has also been found in another study that lowapplication rate of digestates with a big time gap between application and plantingenhances their benefits as soil amendment as confirmed in lab by increased seedgermination with dilution of digestate and incubation (Abdullahi et al., 2008).

One of the major negative effects of digestate on plants is its toxicity. Digestates have beenfound to be more phytotoxic than composts if applied directly on soil without anytreatment. However, phytotoxic effects can be mitigated by post-treatment (composting) ofdigestate (Fuchs et al., 2008; Abdullahi et al., 2008).

2.12 Research Needs for the Dissertation

From this review, it can be noted that ammonia toxicity, VFA accumulation andincomplete mixing are among the major problems of dry anaerobic digestion. In dryanaerobic digestion, ammonia inhibition occurs at a lower TAN concentration as comparedto wet digestion. A high C/N ratio feed (having low N) causes a relatively slow microbialgrowth and low biodegradation rate due to deficiency of nitrogen (and consequently lessproduction and accumulation of ammonia and VFA) and hence may alleviate the problemof both ammonia and VFA accumulation in dry digestion. This can be done by increasingor adding the fraction of low N (or high C/N ratio) waste. However, there is a maximumlimit of feed C/N ratio (i.e. 30) for the digestion process beyond which digestion is notfeasible. Thus the effect of feed C/N ratio higher than its maximum established limit (i.e.30) on the performance of dry anaerobic digestion should be investigated in an attempt toreduce the accumulation of ammonia.

Moreover, most of the previously performed research studies on dry anaerobic digestionuse lab-scale reactor (Table 2.5) with synthetic and well-homogenized feed having theparticle size of around 10 mm. Thus there is a need of research on full-scale or pilot-scaledry anaerobic digesters operating at closer to the field conditions and optimizing theircontinuous operation with practicable organic loading rate.

Furthermore, literature also shows the digested organic waste (digestate) still has certainresidual GHG emission potential, which depends on retention time and loading rate ofprevious digestion. Also the stored digestate tends to emit methane to the atmosphere (ifnot stored properly) and hence can contribute to the climate change. On the other hand, ithas certain amount of nutrients and organic matter, which could be useful if applied onagricultural soils. Therefore, it is necessary to carefully analyze various digestatemanagement options based on their net GHG emission reductions under various scenarios,so that the best scenario can be selected for digestate management and utilize its economicvalue.

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Chapter 3

Methodology

The research work of this dissertation consists of three phases as shown below in Figure3.1. In the beginning, gas formation potential of simulated OFMSW was investigated inlab-scale set-up. The Phase I and II of this research work, consisting of two separate pilot-scale experiments, mainly focused on optimization of dry anaerobic digestion in terms ofC/N ratio and organic loading rates respectively. Various operational parameters ofdigestion (pH, alkalinity, VFA, ammonia) were continuously analyzed to run the digestionprocess smoothly and their effect on methane production and VS removal were noticed.Similarly, characteristics of digestate from the reactor were also determined continuously.Moreover, in Phase III, various options of digestate management were analyzed fromperspective of GHG emissions.

Figure 3.1 Phases of overall research study

Phase III

•Testing simulated substrateswith different C/N ratio

•Investigation of AD operationalparameters

•Characterization of digestate(TS, VS, C, N)

AD Optimization – C/N ratio

•Testing OLRs of feed of selectedC/N ratio

•Investigation of AD operationalparameters

•Characterization of digestate(TS, VS, C)

•GHG emission potential of AD feed, raw digestate,stored digestate and cured digestate

•GHG emission reduction options for a betterdigestate management

Digestate Management and GHG Emissions

Phase IIPhase I

AD Optimization – OLR

Gas formationpotential test

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3.1 Inoculum and Simulations of Waste

3.1.1 Inoculum for anaerobic digestion experiments

Inoculum for gas formation potential test was brewery sludge from up-flow anaerobicsludge blanket reactor (UASB), Singha beer factory, Thailand. Inoculum for start-up ofPhase I experiment was prepared by blending 50, 25 and 25% (w/w) of cow dung,anaerobically digested food waste and anaerobic brewery sludge from the same UASB asmentioned above. Thailand. Mixed inoculum was characterized with the TS and VScontent of 13% and 65 %TS, respectively. Inoculum for start-up of Phase II experimentwas prepared by blending 25, 69 and 6% (w/w) of cow dung, anaerobically digestedOFMSW and anaerobic brewery sludge from the same UASB. Mixed inoculums had TSand VS content of 9.67% and 67.48 %TS respectively.

3.1.2 Simulations of waste

The OFMSW was simulated using food waste, vegetable and fruit waste, leaf waste andoffice papers collected from their respective sources. The components were individuallysize reduced up to 25mm using mechanical shredder before mixing. Three feedstocks wereprepared with different composition. Feedstock 1 and 2 were used for Phase I experimentwhile Feedstock 3 was used for Phase II experiment. Mixing composition was varied toachieve the C/N ratio of 27 and 32 for the study purpose. The composition andcharacteristics of three feedstocks are given in Table 3.1.

Table 3.1 Composition and Characteristics of Simulated FeedstockDetail Unit Feedstock 1 Feedstock 2 Feedstock 3

Composition

Food waste % FWa 42 40 45Vegetable waste % FW 45 27 33Fruit waste % FW 5 20 15Leaf waste % FW 5 8 -Paper waste % FW 3 5 7

Characteristics

Moisture % 79-84 75-85 81-86TS (range) % 16-21 15-25 14-19VS (range) %TS 79-90 80-90 84-88C (avg.) %TS 51.30 52.10 51.20TKN (avg.) %TS 1.92 1.63 1.61C/N (avg.) - 26.72 31.96 31.80aPercentage based on fresh weight.

In Feedstock 3, composition was set to achieve C/N ratio of 32 but without the use of leafwaste. Leaf waste was not used in Feedstock 3 to reduce feed TS and to avoid the observedblockage of pump. To ensure the homogenous feedstock supply for anaerobic digestionsystem, it was prepared in bulk quantities and stored at 5oC. Every day, required amount of

46

feedstocks were sub-sampled and freeze-thawed under ambient temperature for loadinginto the reactor. The simulations were also characterized in different time intervals duringthe long time storage. The average total solid (TS), volatile solid (VS) and carbon (C)contents of the Feedstocks 1 & 2, and Feedstock 3 were 20.2%, 85%TS, 51.5%TS and17%, 86%TS, 51%TS respectively.

3.2 Experimental Set-up

3.2.1 Experimental set-up for gas formation potential test

Gas formation potential test (also called GP21 test) was conducted for Feedstock 1 and 2.The experimental set-up for it has been shown in Figure 3.2. Laboratory glass bottle of 500mL with a thick Teflon/silicon (0.01”/0.05”) septum in its arms were used as reactionvessels for waste. The reaction vessel was connected to a long cylindrical approximately1L glass tube called eudiometer by plastic tube that was used for measurement of volumeof biogas produced. The eudiometer had barrier solution that was free to move by thepressure of produced biogas to another 1L plastic bottle (called reservoir tank) connectedto the other end of eudiometer. The reaction vessel was placed in a water bath to attain therequired temperature of 35 ºC (Heerenklage and Stegmann, 2005).

Figure 3.2 Experimental set-up for gas formation potential test

3.2.2 Experimental set-up for pilot-scale experiments

A stainless steel inclined thermophilic dry anaerobic digestion system (ITDAR) wasdesigned with the total and working volume of 0.69 and 0.55 m3, respectively. The internaldiameter of the reactor was 0.6 m and the total height was 2.4 m. It was placed in aninclination of about 30o from the ground level to facilitate easy flow of the waste as shownin Figure 3.3. The ITDAR was externally connected with the waste feeding hopper, screw

4 cm

79 c

m

GC

1

2

3

4

5

6

7

8

9

4 cm

79 c

m

GC

1

2

3

4

5

6

7

8

9

1. Sample + inoculum2. Reaction vessel, 500 mL3. Water bath4. Liquid sampling5. Plastic tube6. Gas collection tube

(Eudiometer tube ~ 1L)7. Gas sampling8. Barrier solution9. Reservoir tank, 1L

47

bed pump, water circulating jacket, wet gas meter (to measure flow of biogas) and digestedresidue collection opening for continuous operation. Hot water tank (50 L) with immersionheater was also provided beside and connected with the water circulation jacket tomaintain the temperature of the ITDAR at 55oC. Temperature sensor inserted in the reactorwas connected with automatic temperature controller to monitor and control thetemperature. Once in a day, simulated feedstock for the given organic loading rate was fedand almost equal portion of the digested residue was removed from the ITDAR duringdifferent trials.

Figure 3.3 Pilot-scale experimental setup of inclined thermophilic dry anaerobicdigester

3.3 Experimental Conditions

3.3.1 Experimental conditions for gas formation potential test

The gas formation potential of Feedstock 1 and 2 was determined in the laboratory scaleGP21 test set-up. The basic approach of this test was incubating a small quantity of thesubstrate with anaerobic inoculums at 35ºC for 21 days and measure the biogas andmethane generation, usually by simultaneous measurements of gas volume in eudiometerand gas composition by a Gas Chromatograph (GC). The objective was to find out thepotential of one gram VS of waste material to produce methane gas. The results werepresented according to standard conditions of temperature and pressure, i.e., STP.

Particle size of the waste was reduced to < 10 mm and waste was mixed by blending thewaste sample in a blender or pulverizer in the laboratory. There was no addition of waterduring mixing. After getting a homogeneous waste sample, active anaerobic inoculum was

48

mixed with it. Three replications of this batch test were run with a blank or control run toaccomplish the experiment. A reference set-up was also run using cellulose as substrate tocheck the inoculums activity. The sequential procedure of conducting GP21 test has beendescribed in the Figure 3.4.

Figure 3.4 Method teps for gas formation potential test

Based on this method (Heerenklage and Stegmann, 2005) and the characteristics ofsubstrates and inoculum (Table 3.2), the substrate to inoculums ratio (S/I ratio) on kg VSbasis achieved in the GP21 reactors was 5.55 and 8.10 for feedstock 1 and 2 respectively.

Table 3.2 Characteristics of Substrate and Inoculum Used in Gas Formation PotentialTestCharacteristics Feedstock 1 Inoculum Feedstock 2 InoculumTS (%) 15.84 5.25 25.11 5.36VS (%TS) 81.94 44.70 79.31 45.79

3.3.2 Experimental conditions for Phase I pilot experiment

In this experiment, the effect of C/N ratio and ammonia-N accumulation was studied ondry anaerobic digestion of OFMSW. Two simulations of OFMSW with different C/N ratio(i.e. feedstock 1 and 2) were used in pilot-scale ITDAR for this experiment. The detail ofexperimental conditions is given in the following subsections.

a) Start-up operation

The ITDAR was initially loaded with the 410 kg of feedstock 1 and 180 kg of inoculumsource i.e., 70 and 30 % (w/w), respectively, to its working volume. With this composition,the S/I ratio (i.e. substrate to inoculums ratio) was 5.2 on kg-VS basis. The temperature of

Add 50 g of sample (WM) + 50 mL anaerobicsludge + 200 mL water to all reaction vessels

Flush the reaction vessels with N2 gas afterconnecting with eudiometer filled with barrier

solution

Incubate the reaction vessels at 35 ºC for 21days after lag phase

Shake the reaction vessels occasionally andtake reading of biogas for 21 days regularly

49

the reactor was increased from 35°C to 55°C with a gradual increment of 2°C per day toavoid reactor upset and was maintained at 55°C throughout the study. The system wasoperated in a batch mode, without loading any additional feedstock, for first 14 days anddenoted as start-up phase. During the initial start-up phase, the system pH was neutralizedusing commercial caustic soda (NaOH) for quick onset of methanogenesis in ITDAR. Theamount of NaOH required for the pH adjustment was calculated based on the simplelaboratory tests using 100 mL of digestate from the ITDAR and pH meter. The reactorcontents were mixed at the rate of about 1 Ldig/Lreactor vol.d (one liter digestate recirculatedper liter reactor volume per day. So Digrr = 1 means that whole contents in reactor isrecirculated for one time in a day) to mix up for homogenous distribution of reactorcontents in these periods.

From day 15 onwards, ITDAR was operated in a continuous mode by loading with thedesigned feedstocks under different organic loading rate (OLR) as detailed in belowsections. Table 3.3 provides the details of OLR, solid retention time (SRT) and digestaterecirculation rate (Digrr) during continuous operation of ITDAR. As the reactor volumewas fixed, the increase in waste loading rate i.e., the OLR, in subsequent runs thusdecreased the SRT. During these periods, the system was continuously monitored for thefluctuations in process parameters such as biogas, methane, ammonia-N, volatile fattyacids (VFA), alkalinity and pH as well as other digestate parameters (TS, VS, C and N).

Table 3.3 Operating Conditions of ITDAR for Phase I Pilot ExperimentRun Duration

(d)OLR

(kg VS/m3.d)Solid retention

time (d)Digrr

(Ldig/Lreactor vol.d)Feedstock 1 (avg. C/N ratio 27)

Start-up 1-14 - 14 1.001 15-38 0.65a 153 0.052 39-67 1.60 89 0.103 68-99 2.60 54 0.19

Feedstock 2 (avg. C/N ratio 32)4 100-148 4.00 45 0.345 149-170 10.65 13 2.126 171-215 4.35 29 2.407 216-245 7.70 21 2.908 246-280 7.30 19 2.96

aTable gives average values of OLR, retention time and recirculation rate for each run.

b) Continuous operation with feedstock 1 (C/N ratio of 27) - Run 1 to 3

As detailed in Table 3.3, the feedstock 1 with the C/N ratio of 27 was loaded into ITDARunder different OLR of 0.65 to 2.60 kg VS/m3.d in three consecutive runs (1 to 3). Eachrun was preceded until the biogas yield attained to its steady state, with no furtherincrement, in ITDAR.

The SRT of 153 days was given during run 1 to avoid the reactor upset with lower Digrr. Insubsequent runs the step-up increase in OLR was studied with decreasing SRT andincreasing Digrr. Every time, one part of the fresh feedstock was mixed up with the twoparts (wt/wt basis) of digestate collected from the ITDAR before loading. Based on massbalance calculation, specified amount of digestate was removed from the ITDAR. Feedingand digestate withdrawal was done once a day. To balance the moisture loss due to evapo-

50

transportation from the ITDAR, water was sprinkled over the feedstock before its feedingat regular intervals to maintain the TS content for dry digestion.

c) Continuous operation with feedstock 2 (C/N ratio of 32) - Run 4 to 8

From day 100th onwards, the reactor was loaded with the feedstock 2, which had a higherC/N ratio of 32. The OLR was varied between 4 and 10.7 kg VS/m3.d in different runs (4to 8). During these periods, the ITDAR was shifted from partial mixing mode to completemixing mode by increasing the Digrr to achieve the higher OLR operation and maximizethe biogas yield (Digrr less than 1 Ldig/Lreactor vol.d was considered as partial mixing mode,for example, Run 1-4, because whole contents in reactor are not recirculated even once in aday. Digrr more than 1 Ldig/Lreactor vol.d was considered as complete mixing mode, forexample, Run 5-8). Otherwise, the reactor operations followed same as detailed in abovesubsection (b).

3.3.3 Experimental conditions for Phase II pilot experiment

In this experiment, optimization of thermophilic dry anaerobic digestion by testing theeffect of different OLRs was studied. The simulation of OFMSW, which performed well inprevious experiment and was found optimum in terms of C/N ratio (i.e. C/N ratio 32 orfeedstock 3), was used in pilot-scale ITDAR for this purpose. The study included start-upoperation and continuous operation. The detail of experimental conditions is given in thefollowing subsections.

a) Start-up operation

The ITDAR was initially loaded with the 329 kg of feedstock 3 and 213 kg of inoculumsource i.e., 60 and 40 % (w/w), respectively, to its working volume. With this composition,the S/I ratio (i.e. substrate to inoculums ratio) was 3.04 on kg-VS basis. The inoculumconsisted of a mixture of cow dung, anaerobic sludge from a WWTP and anaerobicdigestate of the same reactor. Around 70% (w/w on fresh mass basis) of inoculumconsisted of anaerobic digestate, which was taken from previous run of the samethermophilic reactor. Therefore, thermophilic temperature (55°C) was maintainedthroughout the study, which was achieved by gradual increase of temperature during first 3days of start-up. The system was operated without loading any additional feedstock, forfirst 50 days and it is considered as start-up phase. During the initial start-up phase, thesystem pH was neutralized using commercial caustic soda (NaOH) just like Phase Iexperiment. The reactor contents were mixed at the average rate of about 1.85 Ldig/Lreactor

vol.d (liter digestate recirculated per liter reactor volume per day) (range: 1-3 Ldig/Lreactor

vol.d) to mix up for homogenous distribution of reactor contents in these periods.

b) Continuous operation with feedstock 3

From day 51 onwards, ITDAR was operated in a continuous mode by loading the reactorwith designed OLRs. As shown in Figure 3.5, the feedstock 3 was loaded into ITDAR withdifferent OLRs of 4.5, 6.3 and 8.5 kg VS/m3/d in three consecutive runs (1 to 3). Each runcontinued until the biogas yield attained to its steady state, just like previous experiment.The step-up increase in OLR was studied with decreasing SRT from 30 to 18 days. TheDigrr (digestate recirculation rate) was kept constant, at value of 2.8 Ldig/Lreactor vol.d, in thisstudy. Every time for feeding, one part of the fresh feedstock was mixed up with the two

51

parts (wt/wt basis) of digestate collected from the ITDAR. Based on mass balancecalculation, specified amount of digestate was removed from the ITDAR. Feeding anddigestate withdrawal was done once a day. To balance the moisture loss due to evapo-transportation from the ITDAR, water was sprinkled over feedstock before feeding) atregular intervals to maintain the TS content for dry digestion just like previous experiment.

Figure 3.5 Operating conditions of ITDAR for Phase II pilot experiment

During this period, the system was continuously monitored for the fluctuations in processparameters such as biogas, methane, ammonia-N, volatile fatty acids (VFA), alkalinity andpH as well as other digestate parameters (TS, VS, C).

3.4 Digestate Management and GHG Emissions Estimation (Phase III)

Management of digestate from anaerobic digestion of municipal solid waste is animportant issue due to wide variation in its characteristics. The digestate characteristicsdepend on origin of feedstock, type of feedstock and type of digestion process. Literatureshows the digestate has certain amount of plant nutrients and organic matter and can beused as organic fertilizer or soil conditioner. However, it is important to indicate thatdigestate contains significant amount of methane formation potential, and hence cancontribute to the climate change. Moreover, it can also contain some organic pollutants andheavy metals. Therefore, it is important to develop a proper digestate managementstrategy.

Run 1 OLR 4.55 kgVS /m3.d

Run 2 OLR 6.4 kgVS /m3.d

Run 3 OLR 8.5 kgVS /m3.d

Digestate characteristics analysis, biogas volume and composition

Optimizationof anaerobic

digestion

Dry digestionFeed TS: 14-19%Feed C/N ratio:32

Run 1 RT 30 daysRun 2 RT 24 days

Run 3 RT 18 days

Start-Up Constant Digrr : 2.8 Ldigestate/Lreactor vol..d

52

Depending on its characteristics, it could be either applied directly on the agriculture landor treated by aerobic process of curing or composting. During its handling andmanagement, the digestate needs to be stored as well as dewatered. Thus, the main purposeof Phase III is to find environmentally suitable options for digestate management. Thepossible unit processes involved in digestate handling and management have been shownin Figure 3.6. The unit processes shown in dotted boxes are the main sources of GHGemissions from digestate management system. The processes used for management ofdigestate in this study have been detailed in the following sections.

Figure 3.6 Possible unit processes of digestate management system

3.4.1 Storage of digestate

The digestate after being withdrawn from the anaerobic digester was temporarily stored inhigh density polyethylene (HDPE) plastic drums. The drums were kept covered and werekept in shade to avoid NH4 volatilization as shown in Figure 3.7. The size of each drum is150 L. All the withdrawn digestate of each run was collected in these drums and at the endof the run it was dewatered by the sand drying bed.

3.4.2 Dewatering of digestate

Liquid-solid separation or dewatering of the digestate was done, using simple filtrationsystem of sand drying bed (Figure 3.8 and 3.9).

Digestate

Curing

Storage

Dewatering

Solid DigestateRich in OM & P

Liquid DigestateRich in N & K

Composting

Land Application

53

Sand drying bed (SDB) is simple, easy to operate and needs low energy for its operationthan mechanical dewatering systems. One circular SDB with a radius and height of 30 and80 cm respectively was used. The detailed design with top and cross-sectional views of thebed has been presented in Figures 3.8 and 3.9 respectively. It consists of a 15 cm thicklayer each of coarse, medium and fine gravels. The first layer (15 cm depth) of the bedfrom bottom consist of course gravels, followed by other layers of medium and then finegravels (15 cm depth each). A perforated central pipe of 2.5 cm diameter with screeningnet, placed longitudinally in the center collects the filtrate percolated through the gradedgravel. The slope to the pipe is (6/30*100) 20 percent. Approximately 25 cm layer ofdigestate can be placed over the bed, whereas freeboard is 10 cm.

Figure 3.7 Plastic drums for storage of digestate

Digestate dewatering by use of sand bed mostly happens by two mechanisms, i.e., drainageor downward percolation of water and water evaporation from the surface of digestate.

The dying period (retention time) of digestate was different for different runs. It was in therange of 2-5 days achieving a TS content of more than 50%. It depended on thecharacteristics of digestate and weather conditions. The removal of dewatered digestatefrom SDB was done with a shovel and the dewatered digestate was used for curing andfurther analysis.

3.4.3 Curing of dewatered digestate

Curing is aerobic treatment of digestate passively (without the use of energy). In this study,it was done by spreading the digestate with high TS (55%) in thin layer of 5 cm in a tray. Itwas done for a period of 30 days under ambient conditions (at 30-33°C). Dewatering ofdigestate was needed to increase its TS before curing.

54

Figure 3.8 Sand drying bed: Top view

Figure 3.9 Sand bed for digestate dewatering, A-A cross-sectional view

A

60 cm

A

54

Figure 3.8 Sand drying bed: Top view

Figure 3.9 Sand bed for digestate dewatering, A-A cross-sectional view

A

60 cm

A

54

Figure 3.8 Sand drying bed: Top view

Figure 3.9 Sand bed for digestate dewatering, A-A cross-sectional view

A

60 cm

A

55

3.4.4 Estimation of GHG emissions in the digestate management system

In this part of study, different integrated digestate management unit processes (calledscenarios) were analyzed for their net GHG emissions. The best scenario with the leastGHG emissions was recommended for proper digestate management. Digestatemanagement unit processes studied here include digestate dumping, digestate applicationon land, digestate storage, aerobic treatment (i.e. curing), etc. The detail of methodology isgiven in the following subsections.

a) Goal and functional unit

The goal of this part of study is to evaluate various digestate management systems fromperspective of GHG emissions. A fixed reference point for the GHG emission evaluation isdefined as 1 kg of digestate treated or managed. The digestate used for this estimation istaken from run 4 of the digestion experiment and its initial moisture content is 89.5%.

b) Process description and comparative scenarios

The digestate is produced continuously throughout the year, but applied to agricultural landafter appropriate treatment. Also, its land application time is not continuous, but only afterharvesting of previous crop (e.g. during spring and autumn). So it needs to be stocked-upor stored for several months. Thus, digestate can be directly applied to land only if itsnutrient ratio (i.e. C/N ratio < 20) and application season are correct. If those conditions arenot suitable, aerobic treatment and storage are needed respectively.

Several digestate management scenarios have been evaluated for their GHG emissionswhich are possible in the field as given in Figure 3.10. In scenario 4 and 5, the process ofcuring is done which is passive (without the use of energy) aerobic treatment of digestate.Dewatering of digestate is needed to increase its TS before curing.

Dumping of digestate is needed, if it has high concentration of heavy metals or otherpollutants and is unfit for application to agricultural land. However, curing of suchdigestate is still needed before dumping to minimize its GHG emission potential.

c) System boundaries and sources of GHG emitted or avoided

1. The evaluation includes the GHG emission only from the unit processes of digestatemanagement system as explained in various scenarios above. Therefore, accounting ofGHG emissions from the use of energy (operation of the facilities and processes as wellas transportation of digestate) is not included in the estimation.

2. Since the digestate was stored for 60 days, thus storage in every scenario means storagefor 60 days in this study.

3. Due to very thin layer, no formation of anaerobic microenvironments is assumed andhence CH4 emission during dewatering and curing is assumed to be negligible.

4. Nitrous oxide emission from land applied digestate has been considered only, whereasduring digestate storage and curing, it is assumed to be negligible. CH4 emission fromland application of digestate has been reported minimal and has not been considered inthis study. The forms and sources of GHG emitted or avoided are given in Table 3.4.

56

Figure 3.10 Comparative scenarios of digestate management

5. In each case of digestate management, CO2 produced from biological degradation (e.g.in digestate land application, storage, dewatering, curing and dumping) has not beenaccounted as GHG source. The reason is that the material to be treated is of biogenicorigin.

Table 3.4 Forms and Sources of GHG Contributed and GHG AvoidedScenario GHG contribution GHG avoided1 CH4 from dumped digestate Nil2 N2O from land application of digestate Fossil CO2 for fertilizer

manufacture3 CH4 from storage and N2O from land

applicationFossil CO2 for fertilizermanufacture

4 CH4 from storage and N2O from landapplication

Fossil CO2 for fertilizermanufacture

5 CH4 from storage and dumping ofdigestate

Nil

Dumping of digestate (baseline)All the digestate is dumped to a dumpsite

Scenario 1

Direct application of digestate to agricultural landAll the digestate is spread over agricultural land

Scenario 2

Scenario 3

Digestate storage and land applicationStorage of all the digestate followed by its land application

Scenario 4Storage, curing and land application of digestateAll the digestate is stored followed by its curing and landapplication

Scenario 5Storage, curing and dumping of digestateAll the digestate is stored and then cured before its dumpingto dumpsite.

57

d) Calculation methods

i) CH4 emission potential of dumped digestate (as GHG source)

Estimation of GHG emissions has been done by mass balance method, which calculatesmethane formation potential on the basis of weight of carbon in the material. The methodis based on a model given by Bingemer and Crutzen, 1987 (p. 2181) as cited by Kumar etal. (2004) and used by IGES, (2011). The detail is given in the following steps:

1. The characteristics of waste and digestate material (TS, and C) used for this estimationwere practically found by lab analysis to calculate methane formation potential.

2. Methane potential of material (g/kg waste) was then converted to GHG emission (gCO2-eq/kg waste) by multiplying with a factor of 25 (IPCC, 2011).

1000)1())12/16((

/

OXRFDOCDOCMCFTSM

materialofkgmethaneofgemissionMethane

F

25)/(/2 kgmethaneofgkgequivalentgCO

whereM = Total mass of material (1 kg)TS = Total solid content (fraction, e.g. 0.25)MCF = Methane correction factor (fraction). The fraction depends upon the

method of disposal and depth available at landfills. The IPCC documentindicated the value 0.4 for open dumps of <5 m depth and hence used forcomputation.

DOC = Degradable organic carbon (fraction, e.g. 0.44)DOCF = Fraction of DOC dissimilated. The model is described as 0.014T+0.28,where T = Temperature in C (Tabasaran, IPCC document 1996). It is assumed thattemperature remains constant at 35C in the anaerobic zone of the landfill. Thevalue is computed as 0.77 and adopted.F = Fraction of methane in landfill gas (default is 0.5)R = Recovered methane (kg) in this case, methane is released directly to

atmosphere, hence the value is zeroOX = Oxidation factor (default is 0). It accounts for the methane that is oxidized

in the upper layer of waste mass where oxygen is present. Oxidation mayreduce the quantity of methane generated that is ultimately emitted.However, there is no internationally accepted factor and can be assumed aszero.

1000)12/16(5.077.044.04.025.01

/

materialofkgmethaneofgemissionMethane

= 22.6 g methane/kg materialkgequivalentCOgkgequivalentgCO /565256.22/ 22

58

ii) Calculation of N2O emission from land applied digestate

The land applied fertilizer, compost or biosolids emits N2O equal to 0.01 of applied N asstated by IPCC (Brown et al., 2010). However, Moller et al.,(2009) used N2O emissionfactor equal to 0.013-0.017 of applied N to soil. Here we have used N2O emission factor0.013 of applied N in digestate.

The total amount of N provided by 1 kg of digestate was calculated from the N content ofeach type of digestate (Table 4.5). The amount of N2O to be emitted from 1 kg of digestatewas then calculated using the factor 0.013 of applied N. The N2O to be emitted was thenconverted to its equivalent amount of CO2 by multiplying with a factor of 298 (IPCC,2011).

iii) Calculation of methane emission during storage

It is equal to the difference of methane emission potential of digestate before and afterstorage. The methane emission potential was calculated by the same method as used byIGES, (2011).

iv) Calculation of avoided GHG by land application of digestate

Land application of raw digestate, stored digestate and stored-cured digestate can somehowreplace the use of synthetic fertilizer and hence can reduce the emission of fossil carbon byenergy use for production of chemical fertilizer. The total amount of N and P provided by1 kg of digestate was calculated from the N and P content of each type of digestate. The Nand P content of each type of digestate were measured by lab analysis and are given inTable 4.5. The avoided or saved GHG by land application of digestate was then calculatedby emission factors 8.9 kg CO2-eq/ kg N and 1.8 kg CO2-eq/ kg P (Moller et al., 2009).

3.5 Analytical Methods

Simulated feedstocks, inoculum sources and digested residues (as solid samples) werecharacterized for their physico-chemical characteristics such as TS, VS, Carbon (as TOC)and Nitrogen (as TKN) by standard methods given in Table 3.5. These measurements wereperformed once to twice a week to interpret the process performance.

Liquid portion of the digested residues (liquid digestate) was collected by simplecentrifugation (5000 rpm for 20 min). These extracted samples were used for analysis ofpH, oxidation reduction potential (ORP), alkalinity, VFA and total ammonia nitrogen i.e.TAN (NH3 + NH4

+). The pH and ORP of digestate were analyzed on daily basis, whereasall other parameters of liquid digestate were analyzed once to twice a week. Biogasproduction was measured using the wet gas flow meter (TG 05, Ritter) connected withreactor gas outlet port. The biogas composition was analyzed once to twice a weekdepending on sensitivity of the reactor conditions.

59

Table 3.5 Analytical Methods for Various Parameters of Anaerobic Digestion ofOFMSW

Parameters Method/Equipment Interference Reference

Parameters of solid samples (solid waste, inoculum and digestate)

TS (%) Oven drying at 105 ºC Loss of volatile OMduring drying

APHA, 2005

VS (%TS) Furnace drying to ash at 550 ºC Loss of volatileinorganic salts like(NH4)2CO3

APHA, 2005

Organic C (%) - Walkley andBlack, 1934

TKN (%) Digestion of sample with H2SO4

and use of Kjeldahl apparatusfor distillation

- APHA, 2005

P (%) HClO4 + HNO3 digestionand colorimetric method

- Olsen andSommer,1982

Parameters of liquid samples (Liquid digestate or leachate)

pH Glass electrode method Sodium if pH>10 andtemperature

APHA, 2005

ORP Electrode method - APHA, 2005TAN (mg/L) Distillation method - APHA, 2005NH3-N (mg/L) Calculation method - Siles et al.,

2010Alkalinity(mg/L asCaCO3)

Titration method - Lahav andMorgan,2004

VFA (mg/L) Titration method - Lahav andMorgan,2004

Parameters of biogas samples

Biogascomposition

SHIMADU-GC14 A Gaschromatograph with TCDdetector

- APHA, 2005

60

Chapter 4

Results and Discussion

The results and discussion chapter has been divided into four sections. In the first section,biogas and methane formation potential of waste simulations have been discussed. In thenext section, effect of C/N ratio and ammonia-N accumulation on dry anaerobic digestionis discussed. In this part of study, effect of two feedstocks with different C/N ratio (27 and32) and its associated accumulation of ammonia-N have been investigated on the operationand performance of a pilot-scale thermophilic dry anaerobic digester. In third section, theeffect of different organic loading rates on the stability and performance of the same pilot-scale thermophilic dry digester has been discussed. For this purpose, a third feedstock withC/N ratio 32 (for detail, please refer to section 3.1.2) was fed to the digester, because feedwith C/N ratio 32 performed well in the earlier experiment. In the final section,characteristics of digestate have been described and various digestate management optionsfrom perspective of GHG emissions have been discussed.

4.1 Gas Formation Potential of Waste

In this initial part of the thesis, the potential of prepared waste simulations to producebiogas was measured experimentally. For this purpose, GP21 test set-up was used. Sludgefrom anaerobic process was used as a standard inoculum in this study. Similarly, to makesure that inoculum is active, cellulose was used as a standard substrate, because its biogasproduction potential is known in literature. Thus to standardize the test conditions,experimental runs of cellulose (cellulose + inoculum) were conducted in parallel with thewaste simulations (waste + inoculum). Moreover, blanks (inoculum alone) were also runfor correction. Two separate sets of experiments were conducted for Feedstock 1 and 2.The results have been discussed below.

The inoculum (alone) used for Feedstock 1 produced lesser biogas (107 NmL) ascompared to that used for Feedstock 2 (125 NmL). The two inoculums used for Feedstock1 and Feedstock 2 are hereby called as inoculum 1 and inoculum 2 respectively. The sametrend of biogas production was observed from 1 g cellulose by inoculum 1 and 2 (176 and402 NmL) as shown in Figure 4.1 and 4.2. The inoculum used in both the cases, had TSand VS content of around 5.5% and 45%TS respectively. The specific biogas production ofinoculum 1 and 2 was 91 and 102 NmL/g VS respectively. These results show thatinoculum 2 was more active as compared to inoculum 1. The results similar to those ofinoculum 2 were obtained from the GP21 test study conducted by Heerenklage andStegmann, (2005), where the active inoculum produced around 100 and 400 NmL biogasfrom inoculum (alone) and 1 g cellulose respectively.

The cumulative biogas production of Feedstock 1 and 2 (having different TS and VScharacteristics) was almost similar, i.e., 2365 NmL (Figure 4.1 and 4.2). Therefore, thespecific biogas production of Feedstock 1 and 2 was different (i.e. 348 and 225 NmL/gVSadded respectively) as found in this experiment. The percentage of methane variedbetween 65-70 % in biogas samples collected at different time intervals from reactionbottles.

61

Figure 4.1 Cumulative and specific biogas production by feedstock 1

The test results indicated that the biogas production with maximum percentage of methanecontent was recorded between 12th and 18th day of incubation at 35oC. The methaneproduction potential of the Feedstock 1 and 2 was thus 230±5 and 160±8 mL CH4/gVSadded, respectively. Guendouz et al. (2010) also reported 187 mL CH4/g VSadded withMSW as feedstock for assay which correlates with the present study results.

The expected methane yield for source-sorted OFMSW is more than 350 mL CH4/g VS. Inboth tests, the reason for low methane production could be high substrate-to-inoculum (S/I)ratio (i.e. 5.55 and 8.10 for feedstock 1 and 2 respectively) or in other words overloading.In literature, the optimum S/I ratio for anaerobic digestion has been described as < 5 andfor BMT test, the usually maintained S/I ratio is 1 or less (Guendouz et al., 2010; Nizami etal., 2012). Methane production is less in case of Feedstock 2 as compared to 1 as S/I ratioin case of Feedstock 2 is higher than 1 as stated above. Also, it may be because of higherpercentage of slowly degradable fractions (paper and leaves) in Feedstock 2 than 1. It isimportant to note that, originally, the GP21 test is designed to be run for 21 days. But thegas production continued beyond 21 days in our experiments, therefore the observationswere made for longer time than its designed duration.

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In this set of experiments, we followed GP21 test procedure (Heerenklage and Stegmann,2005), which considers simple weight of substrate and inoculums. Therefore, the achievedS/I ratio on kg-VS basis was not in optimum range. Thus research outcomes of GP21 testattained are that i) Simple weight ratio of substrate and inoculum should not be used tostandardize the experimental conditions of GP21 test, but weight ratio on kg-VS basisshould be used. ii) Low S/I ratio (i.e. 5.5) provided better methane production results ascompared to high S/I ratio (i.e. 8.1).

4.2 Effect of C/N Ratio and Ammonia-N Accumulation on ITDAR (Results of Phase IPilot Experiment)

The phase I pilot-scale experiments mainly focused on optimization of dry anaerobicdigestion in terms of C/N ratio. Two simulations of OFMSW with different C/N ratio,namely Feedstock 1 and 2 respectively, were prepared for this purpose. The simulationswere used to operate the pilot-scale reactor under thermophilic conditions. The phase Ipilot experiment consisted of a startup and continuous loading phases. Results of GP21 testshowed that low S/I ratio (5.5) performed better than high S/I ratio (8.1). Therefore, lowS/I ratio was used for start-up of the reactor in both phase I and phase II pilot experiments

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(i.e. 5.2 and 3.04 respectively, please refer to section 3.3.2 and 3.3.3). The detailed resultshave been discussed in the following sections.

4.2.1 Performance of ITDAR during start-up and continuous operations

The performance of ITDAR for treating the OFMSW with two different C/N ratios (27 and32) was evaluated under various organic loading rates using the mesophilic inoculumsources. The overall recital during initial start-up phase, in association with the OLR andDigrr (digestate recirculation rate) were measured using parameters like pH, alkalinity,VFA and methane yield as depicted in Figure 4.3(a-e). The results have been discussed inthe following subsections.

a) Monitoring of ITDAR during start-up phase

Initial pH of the substrate contents within the ITDAR was acidic (5.5–5.8) due toaccumulation of very high VFA concentrations of 37,857 mg/L, which was mainlyreleased from the rapid degradation of readily available biodegradable components in thesimulated feedstock 1 (Table 4.1; Appendix C, Table C-1). The biogas yield was higherwith the higher CO2 and lesser CH4 contents during this period. Therefore, NaOH wasadded between 3rd and 6th day to attain the pH of near natural range to support with themethanogenic activity in ITDAR. With the continuous pH maintenance in ITDAR, itshowed the percentage increase in methane content of biogas samples i.e., increased from3% to 33%, on 10th day with the alkalinity value of 23,000–27,000 mg/L as CaCO3. Also,the ammonia-N contents were measured in the range of 1800–2100 mg/L in ITDAR. TheORP of liquid digestate, which was considered as a good indicator of anaerobic condition,was measured as -260 mV and -450 mV in ITDAR before and after pH adjustment. Lessthan -300 mV of ORP found to be favoring the methanogenesis process in anaerobicsystem was reported by Alkaya and Demirer (2011).

Therefore, it was very clear that the ITDAR attained its methanogenesis stage within 14days of batch operations with the continuous pH adjustment, which can be visualized fromthe stable pH, ORP and methane contents.

b) Monitoring of ITDAR during continuous operation phase (Run 1 to 8)

The pH variation in ITDAR during different runs (1–8) has been depicted in Figure 4.3c. Itwas observed that, the near neutral pH of 7 and above was maintained throughout the studyperiod (of 280 days) even with the increasing OLRs (Figure 4.3a) in ITDAR.

The results showed that the system was able to get naturally buffered, which means that theacid producers and acid consumers were equally dynamic, and produced specific methaneyield of 219 L/kg VSadded on an average from different trials (Figure 4.3d and Appendix C,Table C-2). In other sense, the ITDAR was continuously acting like an activemethanogenic reactor and the waste fed into the system was comparatively lesser than thereactor contents, hence the system pH and associated methane yield was not getting muchaffected with the increasing OLRs. Generally, the methanogenic system pH was reportedto be in between 6.8 and 7.8 (Lahav and Morgan, 2004).

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Figure 4.3 Time course of dry anaerobic digestion with various parameters in ITDAR

Further, the VFA concentrations were high (5000–38,000 mg/L) and average alkalinitywas 31,000 mg/L as CaCO3 (range 7700–69,000 mg/L as CaCO3) during different runs(Table 4.1; Appendix C, Table C-1). Polprasert (2007) reported the VFA concentration of6000–8000 mg/L as inhibitory. However, the digestion process in ITDAR was not really

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getting affected with the presence of VFA concentrations higher than that of inhibitorylevels. The reason is that VFA to alkalinity (VFA/Alk) ratio (a good indicator of digesterfailure) during various runs found to be in the range of 0.31–0.56 (Figure 4.3e), except forrun 1 (1.21). Therefore, the methane yield for the run 1 was comparatively lower than thesuccessive runs and correlated with the previous report by Khanal (2008). He stated theVFA/Alk ratio of ≤ 0.4 and 0.8 for successful and faultier reactor functioning, respectively.The average concentration of 2325 mg/L ammonia-N was recorded in extracted liquid ofdigestate in ITDAR. High ammonia-N concentration has also been reported to act as bufferagainst the acidification effect of VFA (Lahav and Morgan, 2004). The detailed data of allthese operational parameters has been given in Appendix C, Table C-1.

From overall study, a maximum specific methane yield of 327 L/kg VSadded and minimumof 121 L/kg VSadded was recorded from runs 3 and 5, respectively (Appendix C, Table C-2). The possible reasons could be that the lower OLR with higher SRT and higher OLRwith lower SRT, respectively, for run 3 and 5. Further, the higher Digrr and suddenoverloading of reactor and consequent drop in pH could also be the possible reasons for thelower methane yield during run 5 (Figure 4.3b). The specific methane yield for thecentralized DRANCO system was reported to be in the range of 210 to 300 L/kgVSadded

(De Gioannis et al., 2008). The present study results, in terms of specific methane yield,were in line with the performance of centralized units. But, on the other hand ITDAR canuphold the overall net energy gain by reducing the collection and transportation costsfound to be advantageous for using it in decentralized level.

Further, the understanding of relationship between the pH, ammonia-N and VFAaccumulation with the different feedstock characteristics was considered as important toimprove the reactor performance. Hence, the following sections primarily emphasized therelationship between feedstock characteristics, ammonia-N accumulation and VFAinteractions in ITDAR (Figure 4.4, Note: VFA data for Day 1-120 has not been included toclearly show the probable interaction, however, this data is provided in Table C-1 ofAppendix C and in Table 4.1).

4.2.2 Effect of C/N ratio and ammonia-N accumulation in ITDAR

Table 4.1 and Figure 4.4(a–e) depict the important parameters of digestion viz., pH, VFA,VFA/Alk ratio, ammonia-N, free ammonia, methane yield and VS removal (Appendix C,Table C-1 and Table C-2), which can directly affect the performance of the ITDAR.

a) Effect of feedstock 1 (C/N ratio of 27) in ITDAR – Run 1 to 3

Feedstock 1 with the C/N ratio of 27 was used in the start-up of ITDAR and in runs 1–3.The maximum concentration of 3200 mg/L of ammonia-N concentration was recordedduring run 1. Later, the average concentrations subsequently reduced up to 3040 mg/L and2671 mg/L in run 2 and 3, respectively. The pH was lower during run 1 and increased tonear neutral range during run 2 and 3. Therefore, the escape of ammonia-N as gas duringrun 1 was comparatively lesser than the other two runs as noticed from the free ammoniaconcentration levels. As stated, the average concentration of free ammonia was 99 mg/L inrun 1, whereas it was around 328 and 284 mg/L during run 2 and 3, respectively. It wasreported that the free ammonia can severely affect the anaerobic system underconcentrations of 200–700 mg/L in thermophilic anaerobic systems by various authors(Hansen et al., 1998; Straka et al., 2007; Nakakubo et al., 2008; El-Hadj et al., 2009; Yabu

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et al., 2011). Therefore, it is very clear that the ITDAR possibly inhibited with the freeammonia toxicity during the trial runs 1–3 with the feed C/N ratio of 27.

Table 4.1 Digestion Parameters and Methane Yield of ITDAR

RunVFA

(mg/L)VFA/Alk

ratio pHTotal

Ammonia-N(mg/L)

Freeammonia

(mg/L)

Specificmethane

production(L/kgVS)

VSloss(%)

Feedstock 1 (avg. C/N ratio 27)Start-up 37857a 1.41 6.29 1943 25 9.60 -

1 35920 1.21 6.95 2753 99 176 46.522 27567 0.52 7.53 3040 328 286 64.163 18025 0.51 7.49 2671 284 327 70.20

Feedstock 2 (avg. C/N ratio 32)4 11725 0.31 7.75 2360 432 218 54.365 11417 0.35 7.35 1895 164 121 35.376 9625 0.39 7.50 2161 227 222 53.657 7469 0.39 7.29 1791 221 203 52.538 6010 0.56 7.34 1758 135 203 54.02

aTable gives average values for each run.

In addition, the VFA/Alk ratio was also higher i.e., 1.21 during run 1 and it was around 0.5for run 2 and 3. Very high concentration of unconsumed VFA from start-up phase carriedto run 1 could be the possible reason for the higher VFA/Alk ratio during run 1. Asdiscussed earlier, the higher VFA/Alk ratio in run 1 affected over the entire methaneproduction rate in ITDAR.

In addition to the ammonia-N concentrations, VFA/Alk ratio higher than its optimum value(i.e. ≤ 0.4) was also found to be affecting the overall methane yield while feeding thereactor with C/N ratio of 27.

b) Effect of feedstock 2 (C/N ratio of 32) in ITDAR – Run 4 to 8

Feedstock 2 with the C/N ratio of 32 was used for runs 4–8 to reduce the free ammoniatoxicity. Using this range of C/N ratio has been reported (Kayhanian, 1999) as a one of thebest tools to mitigate ammonia-N inhibition in dry thermophilic anaerobic digesters.During trial 2 with the higher C/N ratio, the ammonia-N concentration further reducedfrom 2360 to 1758 mg/L (avg.) in ITDAR during different runs. The sudden change infeedstock composition (with the C/N ratio of 32) from run 3 (trial 1) with increasing OLRaffected the overall reactor performance during run 4 (trial 2) as shown in Figure 4.4 (b-d).The reason was that higher C/N ratio helped system to increase alkalinity and pH value(8.00), which increased free ammonia concentration (657 mg/L) in ITDAR. The detailedexplanation with data of Appendix C, Table C-1 is given here. When feeding of feedstock2 was started, the change in alkalinity and pH was not very significant (alkalinity changedfrom 32000 to 35250 mg/L and pH changed from 7.59 to 7.75) as per data from day 98 today 127 in the appendix. This duration can be regarded as acclimatization time for the newfeedstock (i.e. Feedstock 2) for ITDAR being a biological system. After that, the increasein alkalinity and pH was quite significant (alkalinity changed from 35250 to 68750 mg/Land pH changed from 7.75 to 8.00) during day 127-141, which increased free ammoniafrom 411 to 657 mg/L and directly affected methane yield. This change in parameters by

67

feedstock 2 is considered as a sudden change, which is just in 14 days (from day 127 to141), where the retention time of that run was 45 days.

Figure 4.4 Interaction of ammonia and VFA in ITDAR

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The higher concentration of free ammonia in ITDAR directly influenced the overallmethane production as can clearly be observed from the Figure 4.4b and d during run 4(day 135-145). The effect of free ammonia or ammonia-N on the inhibition of biogasproduction was instantaneous and the systems succeeded in recovering from the inhibitiononly in few cases (Chen et al., 2008). As can be noted, the feed composition variations andsudden shift in OLR severely affected the performance of ITDAR as evidenced from thefree ammonia accumulation and methane yield. In large scale centralized systems, thedetrimental effect due to the compositional and OLRs variations could be magnified andmay require longer time for the system to get recovered.

Whereas, in a decentralized system like ITDAR, the effect was shortened by altering theOLR (in run 5), which in turn lowered the system pH and reduced the free ammoniaconcentration (reduced from 432 to 164 mg/L) in ITDAR. Moreover, the lesser SRT andhigher feed C/N ratio reduced the protein solubilization rate and hence produced lesserammonia-N concentration within the system, which was found to be advantageous. Strakaet al. (2007) also found that the dilution of waste biomass mixture by addition of activatedsludge in thermophilic anaerobic digestion system reduced the ammonia-N inhibitioneffect. On the other hand, the VFA/Alk ratio of the system was also affected and became0.35 during run 5. The specific methane yield from run 5 was calculated as 121 mg/L(lowest yield among 8 runs) mainly due to lesser SRT. During run 6 and 7, the average freeammonia concentration was calculated as 227 and 221 mg/L, respectively. It was evenlower (135 mg/L) during run 8. But, specific methane production rate was almost similar inall the three runs i.e., around 200 mg/L.

Run 8 performed better among the other runs with higher OLR and lesser SRT. The pHwas near neutral, VFA/Alk ratio was around 0.56 and ammonia-N concentration wasaround 1758 mg/L in run 8. Also, long time run stabilized the system performance andassociated parameters in this case.

Although the process control is easy in a decentralized system by judiciously altering theoperating conditions (like OLR, C/N ratio, etc.), but the change should be performed verycarefully and gradually to avoid its effect on performance of biological system. It isevident in run 4 of this study where sudden change in C/N ratio affected the performanceof the system.

4.2.3 Summary of the effect of ammonia-N accumulation in ITDAR

Total ammonia-N concentration decreased with the increase in C/N ratio of the feedstock1–2 as depicted in Figure 4.5. The average ammonia-N concentration in the digestate was2820 and 1990 mg/L for the feedstock 1 (run 1–3) and 2 (4–8), respectively. Hence, thedecrease of ammonia-N concentration in ITDAR was calculated as 30% with the use offeedstock 2 (C/N ratio of 32). It can be noted from the Figure 4.4b, that free ammoniaaccumulation occurred for short duration (3–5 days) on days 41, 91 and 187 up to thelevels of 370–425 mg/L (Appendix C, Table C-1), but there was no effect observed inspecific methane yield. Whereas, the accumulation was around 400–660 mg/L andsustained for a longer duration (10–30 days) especially during run 4 and run 7. It affectedthe overall methane yield, which means that the digestion process was inhibited ormoderately inhibited. As the digestion process proceeds, the pH becomes first stable andthen it starts to increase steadily with the increasing alkalinity. As free ammonia is afunction of ammonia-N, pH and temperature, thus it increased with the increasing pH (7.5–

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8.0) at most of the above said time periods. It has been reported that a free ammoniaconcentration of 700 mg NH3-N/L causes inhibition in methane production underthermophilic conditions (Yabu et al., 2011). Hartmann and Ahring (2005) also proved thatthere was no inhibition at free ammonia concentration of 450–650 mg/L underthermophilic digestion. In contrast, it has been reported that both gaseous ammonia or freeammonia (Hansen et al., 1998) and total ammonia-N (El-Hadj et al., 2009; Nakakubo et al.,2008) cause the inhibition. Chen et al. (2008) highlighted that there was conflictinginformation reported in the literature about the sensitivity of acetoclastic andhydrogenotrophic methanogens. Similarly, there was no clarity in inhibiting concentrationsof ammonia-N or free ammonia in anaerobic digestion system for the particular substrates.

In our case with the ITDAR operations, a drop in methane yield was observed with theincreasing free ammonia concentrations. At free ammonia concentration of 400-660 mg/L,methane production was found to be inhibited. These results also correlated with thereported literature. For example, El-Hadj et al. (2009) found that the free ammoniaconcentrations of 215 and 468 mg/L reduced 50% of the methane yield under mesophilicand thermophilic conditions respectively.

Figure 4.5 Variation of total ammonia-N concentration and TAN/TKN ratio with feedC/N ratio in ITDAR

The subsequent accumulation of VFA after accumulation of ammonia-N in digester hasbeen reported in other studies as well, but most of them have pointed out that it happenedbecause of free ammonia (Chen et al., 2008). In our study, accumulation of both ammonia-N and free ammonia has been found to not directly affect the accumulation of VFA.Nakakubo et al. (2008) also found that the acetic and propionic acids did not show anyindication with the process imbalance at high ammonia-N concentrations. To support withthis, the pre adapted culture to ammonia-N showed better activity up to 700 mg/L of freeammonia concentration, while 100–150 mg/L affected the unadapted culture and inhibitedthe methane yield as reported by Hansen et al. (1998). They have further stated that theinteraction between free ammonia, volatile fatty acids and pH will lead to an ‘‘inhibitedsteady state’’, which is a condition where the process is running stable but with a lowermethane yield. Similar condition was found to be in the case of ITDAR operations underdifferent OLRs and later recovered with the changing operational sequences.

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Total ammonia-N to total nitrogen ratio (TAN/TKN ratio) is considered as a good indicatorfor estimating the percentage conversion of total nitrogen into ammonia-N during theanaerobic digestion. As stated, the calculated percentage of ammonia-N with the feedstock1 was around 0.77, whereas, it was 0.40 for the feedstock 2. Kayhanian (1999) suggestedthat using feedstock C/N ratio from 27 to 32 promotes steady digester operation atoptimum ammonia-N levels. In the present study, feedstock 2 with C/N ratio 32 producedan overall less concentration of ammonia-N and free ammonia as compared to feedstock 1.Moreover, there was no accumulation of ammonia-N under commonly used retention time(20–30 days) for digesters. Thus the feedstock with C/N ratio 32 was found to be betterthan that with C/N ratio 27 for dry thermophilic anaerobic digestion to minimize or avoidammonia-N inhibition.

In our experiment, Feedstock 2 had low %TKN (1.62%), which produced TAN 1993mg/L. Using these two values, mathematical calculation for Feedstock 1 having high%TKN (1.92%) was done, by which it should produce TAN 2350 mg/L (Expected TAN).But in actual experiment, it produced even higher TAN, i.e., 2821 mg/L. Thusexperimental value of TAN production for waste with high %TKN (or low C/N ratio) washigher than its calculated value (Expected TAN). The reason may be that the system wasbiological but not stoichiometric. The presence of high amount of %TKN in case offeedstock 1 (C/N 27) as compared to C/N 32 might have resulted a relatively rapid growthof microbes, which in turn resulted in the more conversion of organic nitrogen into TAN.Thus experimental TAN was higher than Expected TAN in case of feedstock 1. However,it needs to be proved experimentally in future studies.

From the results of this experiment, it can be concluded that ammonia nitrogenaccumulation (one main operational problem of dry anaerobic digestion) can be thusmitigated by use of correct feed mixture (i.e. by adjusting composition or C/N ratio of thefeed). ii) This method to mitigate ammonia accumulation is good for dry anaerobicdigestion as the other methods (dilution of ammonia by addition of water as well asstripping of ammonia) are not desirable and/or suitable for dry digestion. iii) Moreover,adjusting the feed composition can be easily managed for a decentralized dry AD systemcompared to centralized system.

4.2.4 Energy balance of ITDAR in Phase I pilot experiment

Table 4.2 compiled the net energy gains obtained from two different trials of ITDAR andmost of the runs produced surplus energy. The net energy gain from the reactor isdetermined by the quality of produced biogas. The quality of biogas, in turn, depends onthe operational conditions and feedstock composition. Please refer to Appendix E formethodology of energy balance calculations.

Average energy consumptions during different runs were distributed as 75% formaintaining the reactor under thermophilic conditions, 12% for the shredding, 2.5% for thewaste loading and withdrawal and 9.6% for the digestate recirculation. From the energyconsumption percentile, it was evident that the continuous temperature maintenance of theITDAR upset with the maximum percentage of net energy gained.

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Table 4.2 Surplus Energy of ITDAR During Various Runs

In run 1, however, the net energy production was negative. The reason could be that thevery high retention time of 153 days that increased the energy consumption formaintaining thermophilic conditions. Similarly, runs 6 and 7 had only a small surplusenergy (i.e. 20% and 23% for run 6 and 7, respectively). The reason could be that the runs6 and 7 had relatively high retention time compared to run 5 and 8 and thus energyconsumption was higher. Moreover, with the increasing Digrr during runs 4 to 8 of ITDARoperations resulted with the increasing energy consumption rate compared to that of runs1–3. But, overall, it can be concluded that the decentralized system (ITDAR) can producesurplus energy in the range of 50–73% and considered to be economically viable than thecentralized systems.

4.3 Optimization of a Pilot-Scale Thermophillic Dry Anaerobic Digester (Results ofPhase II Pilot Experiment)

In phase II pilot experiment, optimization of ITDAR treating OFMSW was performed bytesting different organic loading rates (OLRs). The C/N ratio of OFMSW, whichperformed well in the earlier experiment, i.e., 32, was used in this study. The study wasstarted with a start-up phase (batch mode of operation) followed by continuous operation.In continuous operation, effect of various organic loading rates on the stability andperformance of ITDAR was evaluated at a constant recirculation rate. The results havebeen discussed in the following sections.

4.3.1 Start-up of ITDAR in phase II pilot experiment

For start-up phase, 40% of the reactor’s working volume was filled with inoculum, whichconsisted of a mixture of digestate from thermophilic anaerobic reactor, anaerobic sludgeand cow dung. The remaining 60% of working volume was filled with Feedstock 3 (pleasesee detail of Feedstock 3 in section 3.1.2). The operating temperature in the start-up phasewas in thermophilic range (55°C), which was reached in 3 days by gradual increase. In thefirst 50 days (start-up phase), the reactor was not fed and only mixing of the reactorcontent was done at the rate of 2.4 Ldig/Lreactor vol.d. Under these conditions, the pH wasinitially 7, which started to decrease and reached 6.36. Therefore, small quantities of

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1 14.42 1.33 0.29 0.26 22.34 -68.012 28.61 1.27 0.28 0.23 10.15 58.283 30.64 1.03 0.23 0.27 6.79 72.86

Feedstock 2 (avg. C/N ratio 32)4 22.33 0.87 0.19 0.27 6.17 66.385 14.71 0.93 0.21 0.69 3.70 62.436 15.90 1.49 0.33 1.99 8.89 20.127 11.53 1.12 0.25 1.36 6.10 23.428 17.97 1.24 0.27 1.45 6.55 47.07

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NaOH were added to the reactor periodically during days 5-25 to maintain pH at nearneutral range. It can be noted as small peaks during days 11-26 in Figure 4.6. From day 28onwards, pH did not drop again and started increasing slowly, therefore, NaOH was notadded anymore. It became stable at around 8.2 during days 42-50 (Appendix D, Table D-1).

Figure 4.6 pH profile of ITDAR during start-up

At first, the concentration of VFA increased after loading the reactor and reached to itsmaximum (19000 mg/L) on day 25. However, once the pH became stable, VFAconcentration started to drop due to its utilization. The concentration of VFA dropped from19000 to 6300 mg/L only in 20 days (Figure 4.7 and Appendix D, Table D-1). The reasonis that there was no waste feeding throughout the star-up phase (day 1-50). The evolutionof VFA/Alk ratio was different from the VFA concentration. Increase in VFA/Alk ratioafter reactor loading was not observed, rather there was a continuous decrease, whichindicates that alkalinity started to develop and increase just after loading. It may bebecause a part of inoculum used for this start-up was taken from the system with the sameconditions (treating OFMSW under thermophilic conditions). After day 30, VFA/Alk ratiostarted to decrease in the same way as that of VFA. This may be attributed to decrease inVFA concentration.

Methane content in biogas and gas production rate (GPR) was lower in the beginning. Thereason might be unfavorable conditions for methanogenesis, i.e., pH lower than 6.8 andVFA more than 6000-8000 mg/L (Polprasert, 2007). However, methane content and GPRstarted to increase slowly as the system progressed towards stability. On the contrary,carbon dioxide content was higher at the start (Figure 4.8 and Appendix D, Table D-2),which is a sign of acidification. But it decreased slowly, as alkalinity and pH increased andVFA concentration got utilized.

From the above discussion, it can be concluded that pH and VFA concentration of thereactor were not stable until day 30, thus, methane content was low, carbon dioxide contentwas high and GPR was unstable. However, after day 35, the reactor conditions were stableand, therefore, both the methane content in biogas and GPR increased.

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Figure 4.7 Profile of VFA and VFA/Alk ratio during start-up

Overall, this start-up took relatively longer time for attaining stable conditions than aprevious study (Zeshan et al., 2012) in the same reactor. The reason may be the use ofhigher recirculation rate (i.e. 2.4 Ldig/Lreactor vol.d) as compared to previous study (where itwas 1 Ldig/Lreactor vol.d). Suwannoppadol et al., (2011) also recommended that during start-up phase low mixing rate should be used. In the end of start-up phase, GPR decreased,which may be because most of the accumulated VFA was utilized by microorganisms asdepicted by drop in VFA and rise in pH. This point shows the end of start-up phase inbatch mode where continuous reactor loading should be started.

It was found from the results of this study that the start-up phase ended (at around day 50)when most of the accumulated VFA was utilized by microorganisms and its concentrationdecreased from 19000 mg/L at day 25 to 5700 mg/L at day 54 and further decreased to4700 mg/L at day 68. With this decrease in VFA concentration, pH increased from 7.2 to7.8 and the reactor stability conditions reached its maximum by achieving the lowestVFA/Alk ratio of 0.32. Similarly, volatile solids concentration (VS/TS) of digestate wasalso found at its minimum (i.e. 0.56-0.57) at this point. Thus the end of start-up phase wasmarked by the lowest values of VFA concentration, VFA/Alk ratio and VS/TS of digestate.

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Moreover, from the comparison of start-up phases of the two pilot experiments, it wasfound that low digestate recirculation rate (i.e. 1 Ldig/Lreactor vol.d) should be used to achievestable reactor conditions in less time. The results of continuous loading phase have beenpresented in the following sections.

Figure 4.8 CH4, CO2 and GPR fluctuation during start-up phase

4.3.2 Stability parameters of ITDAR: Effect of organic loading rate

i) pHpH is very basic parameter to describe the stability of anaerobic digestion. With OLR of4.55 kg VS/m3/d, the system stabilized its pH at around 7.75 with a range of 7.5-8 asshown in Figure 4.9. When the OLR was increased from 4.55 to 6.4 kg VS/m3.d, pH felldown to 7.58 and regulated to an average of 7.67 (7.33-7.96). As a result of furtherincrease in OLR to 8.5 kg VS/m3/d, a drastic decrease in pH was observed and pH droppedto the value of 6.89 (Appendix D, Table D-1). Therefore, NaOH was added during days150-155 to control pH. The decline in pH in the starting days of each of the first two runsand most of the last run is linked to destabilization of the system as a result of increase inOLR. The reason is that when organic loading rate is increased, the acidogens also increasetheir activity and produce high amount of VFA, as they are fast growing. But, on the otherhand, methanogens owing to their slow specific growth rate can not utilize all the alreadyproduced VFA and need more time to build the required population size. Thus initial andtemporary decrease in pH is due to accumulation of VFA as a result of this imbalance inthe microbial groups, which is recovered until methanogens build their sufficientpopulation. The decrease of pH is more pronounced while working with higher OLR, i.e.,8.5 kg VS/m3.d. The reason is that the imbalance between acidogenic and methanogenicactivity is more pronounced.

ii) Volatile fatty acids (VFA)The concentration of volatile fatty acids in the digestate of ITDAR was quite stable at anaverage value of 5100 mg/L (range: 4400-5700 mg/L) while operating at OLR of 4.55 kgVS/m3/d (Figure 4.10 and Appendix D, Table D-1). When OLR was increased to 6.40 kgVS/m3/d, VFA concentration started to increase and reached a maximum value of 6500mg/L with an average value of 5400 mg/L in this run. Finally, at OLR of 8.50 kg VS/m3/d,

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the VFA concentration increased to 7500 mg/L because of increased organic loading rate.This trend shows the destabilization of the reactor caused by increase in OLR.

Figure 4.9 Evolution of pH in ITDAR during continuous loading

It is important to note that at the start of each OLR, the VFA started to accumulate, whichis related with imbalance of activity of microbial groups and initial temporarydestabilization of reactor as a result of increase in OLR as discussed above in the case ofpH. Similarly, at the end of each of first two OLRs, the concentration of VFA declined,which is a sign of stability of the system. However, this drop in VFA concentration forOLR of 8.50 kg VS/m3/d was not observed probably because the reactor at this stage wasoperated for duration equal to only one cycle of SRT.

Figure 4.10 Concentration of VFA in ITDAR during continuous loading

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Concentration of VFA and pH (Figure 4.9 and 4.10) are reverse of each other. A low pHsupports the production of VFA (acidogenic activity), which in turn suppresses VFAconsumption (methanogenic activity). This could be explained by our results also.

iii) VFA to Alkalinity ratio (VFA/Alk ratio)VFA/Alk ratio is a good indicator of digester functioning. With OLR of 4.55 kg VS/m3/d,this parameter remained between 0.35-0.45 for most of the time (Figure 4.11). This is agood range of VFA/Alk ratio for a working digester. But at OLR 6.4 kg VS/m3/d, theaverage value of VFA/Alk ratio increased to 0.53, which is still acceptable for an operatingdigester. The detailed data is provided in Appendix D, Table D-1. However, at OLR of 8.5kg VS/m3/d, VFA/Alk ratio increased to very harmful range (0.67-0.78), because atVFA/Alk ratio of 0.8, significant pH reduction and digester failure happen (Khanal, 2008).The trend of VFA/Alk ratio almost followed the trend of VFA concentration (Figure 4.10),except at the beginning (day 54), where VFA concentration increased but VFA/Alk ratiodid not follow it. It was because the system had high buffering capacity or alkalinity.Hence, rise in VFA concentration did not show any adverse effect on this ratio and hencesystem performance.

Figure 4.11 VFA/Alk ratio in ITDAR during continuous loading

4.3.3 Effect of organic loading rate on performance parameters of ITDAR

i) Gas production rateGas production rate (GPR) increased in OLR 6.4 and 8.5 kg VS/m3/d with an averagevalue of 6.37 and 7.55 L/Lreactor vol./d respectively as compared to OLR 4.55, where it was5.01 L/Lreactor vol./d. Figure 4.12 shows biogas production rate of ITDAR as L/Lreactor vol./dduring different stages of continuous loading phase. The biogas contained 47-49% methanein all the runs (variation is not significant, Appendix D, Table D-2), therefore, the trend ofmethane production rate for three OLRs was also similar to GPR, i.e., 2.4, 3.07 and 3.6L/Lreactor vol./d respectively. It can be noted that at the end of OLR 4.55 and 6.4 kg VS/m3/d,the GPR becomes stable. This is related with stable pH and VFA concentration of thesystem at the mentioned time.

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77

The increase in GPR was almost linear with increase in OLR during first two runs. But,during run 3 (i.e. OLR 8.5 kg VS/m3/d), the GPR did not increase with the same rate asthat of OLR. This could be explained by drastic increase in VFA/Alk ratio (or drop inalkalinity) during that run. However, this effect of overloading could be alleviated byfurther acclimatizing the reactor under those conditions.

ii) VS removal and Specific methane production (SMP)The VS removal was the highest in OLR 4.55 kg VS/m3/d. Thus, in terms of digestatequality, the best results on kg VS basis were shown at OLR of 4.55 kg VS/m3/d as shownin Table 4.3 and Appendix D, Table D-2. These results are similar to those obtained byMontero et al., (2008). They obtained VS removal of 80% in a thermophilic systemoperating at OLR of 4.42-7.50 kg VS/m3/d and 25-30% TS. The VS removal decreased asOLR was increased.

Figure 4.12 Gas production rate of ITDAR during different OLRs

The highest methane production per unit weight of volatile solids added (also calledspecific methane production, SMP) occurred at OLR of 4.55 kg VS/m3/d (330 L CH4/kgVS added) whereas it was 20% lower for OLR 8.50 kg VS/m3/d. However, methaneproduction by all the runs of this study (provided in Appendix D, Table D-2) is in line withthe methane yield values found in literature.

Table 4.3 Percentage of VS Removal and Specific Methane Production in ITDAROLR

(kg VS/m3/d)VS Inlet(kg VS)

VS Outlet(kg VS)

VS Loss(%)

SMP(L CH4/kg VS)

4.55 121.60 26.86 77.90 3306.40 116.90 28.66 75.48 3208.50 62.95 20.70 67.00 266

The SMP reported by various authors through dry anaerobic digestion of OFMSW atthermophilic conditions is in the range of 230-340 L CH4/kg VS added (Gallert andWinter, 1997; Pavan et al., 2000; Montero et al., 2008; Bolzonella et al., 2003). Similarly,SMP reported for a mixture of maize silage and barley straw under thermophilic dryanaerobic digestion was 182 L CH4/kg VS added (Mumme et al., 2010).

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iii) Cumulative methane yieldCumulative methane yield per liter reactor volume for each OLR tested has been sorted out(Figure 4.13) for the equal amount of organic waste fed to the system. As shown in thefigure, OLR 4.55 kg VS/m3/d produced maximum cumulative methane that is 75 LCH4/Lreactor vol.. At OLR of 6.4 and 8.5 kg VS/m3/d, it is only 57 L CH4/Lreactor vol..Apparently, the relationship between GPR and OLR shown in Figure 4.12 was almostlinear. But the relationship of cumulative methane yield with each OLR tested (Figure4.13) provided better information about the performance of the process.

Figure 4.13 Cumulative methane per liter of reactor volume in ITDAR

Figure 4.14 Selection of operating conditions based on purpose of waste treatment

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RT in AD: 18 dOLR: 8.5 kg VS/m3/d

RT in AD: 30 dOLR: 4.55 kg VS/m3/d

Dry AnaerobicDigestion

VS removal: 67%Cumulative methane

yield: 57 L CH4/Lreactor vol

VS removal: 78%Cumulative methane

yield: 75 L CH4/Lreactor vol

79

Based on our results, the best operating conditions for AD (RT and OLR) can be selected.But it also depends on the purpose of treatment (Figure 4.14). As we know anaerobicdigestion can be used as a single treatment for waste or in combination with aerobicprocess.

In case of dual treatment (anaerobic + aerobic), time spent for anaerobic digestion shouldbe minimized, so that the saved time could be used for aerobic treatment. Thus RT of 18days (OLR 8.5 kg VS/m3/d) should be preferred for digestion step. But if the purpose isonly a single treatment (i.e. dry anaerobic digestion), then option of higher retention time(RT 24 d or 30 d) should be chosen to get maximum VS removal and its conversion tomethane.

4.4 Digestate Management and GHG Emissions (Phase III)

Proper management of digestate is needed as it has certain GHG emission potential. So, ittends to emit methane to the atmosphere, if not stored properly. Moreover, it has certainamount of nutrients and organic matter, which could be useful if applied on agriculturalsoils. Therefore, to protect the environment and to make use of digestate’s economic value,careful digestate management should be performed. There are several options for digestatehandling and management, for example, digestate dewatering, digestate storage, digestateapplication to the land, digestate composting, digestate curing or dumping. Butcharacteristics of digestate are the key to determine the best and correct option formanagement of digestate. Thus, in this section of thesis, characteristics of digestate havebeen first described and based on the characteristics, the proper digestate managementoptions have been suggested and finally their effect studied on its nutrient content andGHG emission potential.

4.4.1 Characteristics of raw digestate

Digestate was removed from the reactor every day before feeding of fresh wastethroughout the reactor operation period. The freshly withdrawn digestate (raw digestate)was analyzed for moisture, TS and VS content twice a week. Moreover, digestate was alsocharacterized for carbon and nitrogen content to calculate its C/N ratio. Characteristics ofdigestate have been discussed in this section (Figure 4.15, 4.16 and 4.17; Appendix C,Table C-3, Appendix D, Table D-3).

With digestion the feed TS decreased, so digestate TS was lower than feed TS. DigestateTS content was high in the beginning (16-20%) and started to decrease as the digestionproceeded. During run 3, it started to increase again almost continuously and reached upto18.5% at the end of run 4 as shown in Figure 4.15 and given in Appendix C, Table C-3.This increase in digestate TS corresponds to the increase in feed TS (23-25%) combinedwith increase in OLR. Similar observations were made by Mumme et al., (2010). Alsothere was no replenishment of moisture lost through biogas, which is higher inthermophilic process. Only little changes were observed for the VS content of TS (i.e.digestate VS/TS), which has been in the range of 0.6-0.7 throughout the study.

The practical consequence of this increase in digestate TS was that it was difficult by thescrew bed pump to feed and re-circulate the feedstock and reactor material respectively.Thus TS content in the reactor was adjusted by mixing of water with feedstock (makingfeed TS 17-20%) starting from run 6 onwards as shown in Figure 4.15 and thereby the

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digestate TS was kept constant at 12-13%.Thus adjustment of TS in the reactor helped inits smooth operation.

Figure 4.15 Comparison of feed and digestate regarding total solids in phase Iexperiment

TKN in digestate dropped at the beginning and was low during day 20-90. After day 90, itstarted to increase and became almost constant for rest of the study period in the range of1.8-2.0%. The significant decrease in TKN at the start of the experiment compared to TKNin feed suggests that the TKN might have been utilized in the build-up of big size ofmicrobial biomass sufficient to sustain the reactor. Once sufficient microbial biomass wasdeveloped, consumption of TKN might have been decreased, and thus its concentrationincreased in reactor. Conversely, C/N ratio of the digestate was higher in the beginning.After day 95, C/N ratio was low in the range of 15-20 (Figure 4.16, Appendix C, Table C-3) and remained in the same range for the rest of study period. This finding is in agreementwith increase in TKN concentration.

Figure 4.16 TKN and C/N ratio of the digestate in phase I experiment

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Based on this property of digestate (i.e. C/N ratio 15-20), further intensive treatment, forinstance, composting, is not required. The digestate can be directly applied to agriculturalfields as Wood, (2008) stated that C/N ratio of organic material fit for agricultural landapplication should be <20.

Total solids content of digestate ranged from 5-7% during continuous loading stage ofphase II pilot experiment, whereas VS/TS ratio was in the range of 0.55-0.61 (Figure 4.17and Appendix D, Table D-3). Thus there was no significant difference in the digestate solidcontent among different runs of this experiment. However, as compared to digestate solidcontents in the phase I pilot experiment, the solid content in this study was significantlylower at a comparable recirculation rate, OLR and temperature. For example, average TSwere 12.34% and 5.71% and average VS/TS ratio 0.65 and 0.57 in digestates of runs 6, 7and 8 of phase I and all runs of phase II experiment respectively). The reason may be theuse of relatively low feed TS in this study (16.65%) as compared to phase I experiment(19.75%). Similar results have been reported by various researchers (Forster-Carneiro etal., 2008; Duan et al., 2012) where removal of solids decreased with increase in total solidscontent of the feed. Similarly, Fernandez et al., (2008) reported that increase in feed TSfrom 20 to 30% caused a drop in VS removal and gas yield. Also, TS of 20% was found toshow the best performance as compared to 15, 25 and 30% (Li and Wang, 2011).

Figure 4.17 TS and VS content of digestate in phase II experiment

From these results, it was found that decreasing the total solids inside a dry anaerobicdigester improved system performance in terms of removal of volatile solids.

4.4.2 Characteristics of stored, dewatered and cured digestate

The digestate characteristics have been compared with the Thai and Indian guidelines fororganic soil amendment fit for land application (Table 4.4). Only the moisture contentvalue of digestate is beyond the guidelines, which can be reduced by dewatering it usingsimple sludge drying bed. Based on this comparison, the digestate can be directly appliedto agricultural land without doing any further treatment. Wood, (2008) also stated that C/Nratio of organic material fit for agricultural land application should be < 20.

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Table 4.4 Comparison of Digestate Characteristics and Guidelines

Type of digestate pH Moisture(%)

Organicmatter(%TS)

N(%TS)

C/Nratio

Reference

Digestate 7.75 88-90 37.28 1.94 18.35 This study

Thai Guidelines* 5.5-8.5 ≤ 35 ≥ 35 ≥ 1 ≤ 20 Rattanaoudom,2005

Indian Guidelines 5.5-8.5 - > 26 > 1 12-25Gautam et al.,2010

*Given by Land Development Department, Thailand.

i) Digestate storageDigestate is produced continuously throughout the year from digestion unit. However, itsland application time is not continuous, but only after harvesting of previous crop (e.g.during spring and autumn). So it needs to be stocked-up or stored for several months.Therefore, in this study, digestate collected from the reactor was stored in the HDPEstorage tanks for a period of 2 months to simulate field conditions and effect of storage onvarious characteristics of digestate was observed. The change in concentration of solidsand nutrients with storage has been presented in Table 4.5.

Results show that there was very little change in concentration of solids (TS and VS)during storage, thus, the removal of organic matter was not significant. But theconcentration of nutrients was significantly affected. The concentration of N and P indigestate decreased by 32.3 and 33.8% respectively after 2 months storage. This may beattributed to the prevailing conditions during storage (i.e. high pH, high moisture and highambient temperature), which were suitable for the microorganisms responsible for this lossof nutrients. Similar results were reported by Paavola and Rintala, (2008), who reported40–57% decrease in the concentrations of total P and PO4-P in the separated liquid fractionof digestate after 3 months storage.

Table 4.5 Characteristics of Digestate at Different Stages of ManagementType of digestate TS (%) VS (%TS) C (%TS) N (%TS) P (%TS)Digestate 10.45 64.12 35.60 1.94 0.62Stored digestate (60 d) 9.34 63.91 35.50 1.47 0.46Dewatered digestate 54.46 52.80 29.33 1.47 0.46Stored-cured digestate 54.46 42.00 23.30 1.33 0.41

It can be concluded that storage time of raw digestate should be reduced to avoid nutrientloss. Moreover, if storage is required, it should be done for dewatered digestate, asdewatered digestate (low moisture) could lessen microbial activity and hence nutrient loss.

ii) Digestate curingAs concluded before, the digestate in this study did not need any further treatment to beused as soil amendment because its C/N ratio was <20. But because of N loss duringstorage, its C/N ratio increased to 24.15, thus a slight treatment called curing wasperformed for 30 days. Curing is aerobic treatment of digestate or other organic materialwithout any use of energy. Dewatering of digestate is needed to increase its TS contentbefore curing, an aerobic process. The stored digestate was dewatered using sand dryingbed and the dewatered digestate was used for curing purpose. The characteristics of storeddigestate after dewatering and curing (solid contents and nutrients) have been provided in

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Table 4.5. With curing of digestate, the organic content (C and VS) decreased significantly(20.55%) and hence, C/N ratio of the digestate decreased to 17.52. This range of C/N ratiois good for an organic material to be applied on agricultural land as soil amendment.

4.4.3 Digestate management from perspectives of GHG emissions

Data of digestate characteristics (Table 4.5) was obtained by lab analysis of digestatesamples at each stage. Based on these data and the methods in section 3.4.4-d-(i), GHGemission potential of digestate was calculated at different stages of digestate management.Comparison of different digestate management options with regards to GHG emissionswas then made possible. The results regarding this have been presented and discussed inthis section.

i) GHG emission potential of digestate

The GHG emission potential of digestate was estimated using equation given in section3.4.4-d-(i) and compared with that of OFMSW (the original substrate before digestion) asshown in Figure 4.18. Compared to digestate (139 g CO2-eq/kg waste), the GHG emissionpotential of OFMSW is very high (i.e. 568 g CO2-eq/kg waste). Thus, with digestion, GHGemission potential of OFMSW decreases by about 75%.

Figure 4.18 GHG emission potential of OFMSW and digestates

Among different types of digestates, the (raw) digestate has maximum GHG emissionpotential, whereas it is about 10% less for stored digestate and about 42% less for curedstored digestate. As the waste treatment process proceeds, the GHG emission potentialdecreases. The loss of GHG emission potential is more in case of curing of stored digestate(i.e. 42%). The reason is that apart from C loss during storage, good aerobic conditionswere provided during the curing process by increasing TS of digestate through dewatering,which led to sufficient loss of carbon in the form of CO2.

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The GHG emission potentials of various types of digestate shown in Figure 4.18correspond to the GHG emissions, only if they are dumped to the shallow dumpsite.However, the emissions can be minimized if they are managed well in some better way.For this purpose, various digestate management options in the form of 5 different scenarioshave been considered and compared with regard to GHG emissions and have beenpresented in the following section.

ii) Comparison of GHG emissions from different digestate management scenarios

As digestate has certain residual GHG emission potential, it tends to emit methane to theatmosphere, if not stored properly and hence can contribute to the climate change.Therefore, it is necessary to carefully analyze various digestate management options basedon their net GHG emission reductions under various scenarios, so that the best scenario canbe selected for digestate management. In this part of the study, various digestatemanagement options were considered to assess the reduction in GHG emissions. Results ofGHG emissions from each scenario of digestate management have been presented in Table4.6.

Scenario 1: The baseline scenario shows that the net GHG emission is 190 g CO2-eq/kgdigestate. All the emission is from dumpsite or landfill. The emission comes directly frombiodegradation of digestate in CH4 form. Since no flaring, collection and recovery ofmethane is considered in scenario 1, thus all the produced methane is released intoatmosphere and contributes to GHG emissions. There is no GHG saving here in thisscenario as well.

Scenario 2: In this scenario, GHG emission is mainly in N2O form, which is equal to 7.85 gCO2-eq/kg digestate from the land applied digestate. But here the N and P as nutrientsprovided by the land applied digestate is the most important factor, which replaces the useof chemical fertilizer and hence the GHG from fertilizer manufacturing is avoided. Largeravoidance of GHG emissions from fertilizer production (-19 g CO2-eq/kg digestate) thanN2O emission (8 g CO2-eq/kg digestate) from the land applied digestate results in thenegative net GHG emission as shown in Table 4.6.

Scenario 3: In this scenario, GHG emission is in both CH4 and N2O forms. Storage ofdigestate for 2 months is the major contributor of CH4 from the stored digestate which isequal to 20 g CO2-eq/kg digestate. Moreover, N2O from land applied digestate furthercontributes to GHG emission. However, the GHG saving from the fertilizer substitution byapplication of digestate is lesser than scenario 2. It is because of loss of nutrients duringstorage. For instance, GHG saving from fertilizer substitution is mainly through N contentof digestate, but during storage, ammonia is lost through its volatilization from the storeddigestate due to favorable condition of pH. Since the GHG emissions from storage andland applied digestate (25 CO2-eq/kg digestate) is larger than GHG savings (-13 CO2-eq/kgdigestate), the net GHG emission is positive in this scenario. It can be concluded from theresults of scenario 3 that if land application is the fate of digestate, its storage time shouldbe minimized as much as possible to reduce the CH4 emission and maximize the GHGsavings from fertilizer substitution by digestate.

Scenario 4: In this scenario, although, curing of digestate has reduced the GHG emissionpotential of stored digestate as shown earlier in Figure 4.18. But still the net GHGemissions here are just like scenario 3, which are during storage (before curing) and land

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application (after curing) in CH4 and N2O forms respectively. Thus, in case, if the storeddigestate is to be applied to soil, curing is less advantageous. However, curing isrecommended if the nutrient ratio of digestate is not suitable for land application (e.g C/Nratio > 20).

Table 4.6 Net GHG Emissions from All Scenarios of Digestate ManagementScenario GHG emission

(g CO2-eq/kgdigestate )

GHG saving by fertilizersubstitute (g CO2-eq/kg

digestate)

Net GHGemission

(g CO2-eq/kgdigestate )CH4 N2O N P

Scenario 1 190 0 0 0 190Scenario 2 0 7.85 -18.04 -1.17 -11Scenario 3 20 5.32 -12.22 -0.77 12Scenario 4 20 4.70 -10.80 -0.67 13Scenario 5 129 0 0 0 129

Scenario 5: In this scenario, curing has reduced the GHG emission potential of digestatedown to 109 g CO2-eq/kg digestate. However, before curing, there is CH4 emission duringstorage equivalent to 20 g CO2-eq/kg digestate. Since dumping of cured digestate is to bedone, there is no N2O emission and also no GHG savings from this scenario. If thedigestate is unfit to be applied on land because of presence of heavy metals or otherpollutants beyond safe levels, then its dumping is needed. In such case, this scenario isrecommended, because before dumping, GHG emission potential of digestate can beminimized by curing.

iii) Summary of GHG emissions from digestate management options

Net GHG emissions from all scenarios of digestate management were calculated. Scenario1 produces maximum GHG emissions as there is no management or treatment of digestate,but only direct landfilling. Scenario 2 performs best out of all other cases in terms ofgreenhouse gas emission (Figure 4.19). This is because there is no storage and productionof CH4 and its nutrient content has been fully utilized for GHG savings, so GHG emissionis minimized more than 100% here in scenario 2 as compared to scenario 1.

Scenario 3 and 4 performed almost similar to each other because GHG emission sources(digestate storage and land application) are common in both scenarios. About 92-93% ofGHG emissions have been however reduced by scenario 3 and 4 as compared to scenario1. Scenario 5 reduces 32% of GHG emissions as compared to base scenario, which ishowever better than direct dumping of digestate. Moreover, GHG emissions in scenario 5can be further reduced to about 43% as compared to base scenario by avoiding storage ofdigestate.

Some researchers have used the possibility of reduction of N2O emission from land applieddigestate by better agricultural management practices (e.g. digestate application to soil byplacement method or application during peak season of nutrient uptake, etc.), that leads todifferent results. Similarly, some studies also include the carbon sequestered into soil asGHG savings that may lead to difference in results. Sequestered carbon is the carbonapplied to soil in the form of digestate and not released as CO2 from the soil for 100 years.Also difference in characteristics of digestate (nutrient and carbon content) can producedifferent results. From the results of this study, the order of preference to manage the

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digestate from perspectives of GHG emissions is that scenario 2 is the best. Scenario 3 and4 are better than scenario 5, while scenario 1 is the worst.

Figure 4.19 Net GHG emissions from all scenarios of digestate management

4.5 Decentralized Dry Anaerobic Digestion of OFMSW for a Community of 5000People

Based on findings of this thesis work, a decentralized anaerobic digestion system isdesigned for treating the OFMSW collected from a community of 5000 people. Suchdecentralized AD system will be developed for reducing the burden on central system ofwaste management. It is also easy to manage the process at decentralized level. The detailsregarding design, feedstock preparation, start-up, continuous operation, methane andenergy generation, digestate management, reduction of GHG emissions and VS balance ofthe proposed system have been given in the following sections.

4.5.1 Design of the decentralized AD system

Around 2000 kg/d OFMSW from the mentioned size of community will reach thedecentralized facility (Appendix B). From the results of pilot study given in section 4.3.3,the selected organic loading rate of 6.5 kgVS/m3/d (RT 24 days) will be applied for thedecentralized system. Based on the operating conditions and results of our pilotexperiments, the volume of reactor becomes 62.5 m3, which has been designed for theamount of OFMSW generated by the community. The design data has been summarized inTable 7.4 (for detailed calculations, please see Appendix B). The AD plant will beprovided with pretreatment facilities to pre-treat the organic solid waste coming from thecommunity.

4.5.2 Preparation of feedstock for dry AD (Pre-treatment)

- Since the solid waste for the decentralized AD system will be source-separated waste, thepre-treatment will consist of only shredding, which is required to facilitate pumping,

190

-11

12 13

129

-50

0

50

100

150

200

250

Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5

Net

GH

G e

mis

sion

(g

CO

2-eq

/kg)

Scenario

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digestion, etc. Moreover, mixing of different kinds of waste will also be done to adjust C/Nratio.- The OFMSW received from the community will be consisting of a mixture of food waste,vegetable waste and fruit waste, with C/N ratio in the range of 18-25. Based on thefindings of this thesis, the feedstock C/N ratio 32 is helpful to mitigate ammoniaaccumulation problem in dry AD. Thus feedstock C/N ratio should be adjusted by mixingit with high C/N ratio material (e.g. waste paper, saw dust, etc.).- In our pilot experiments, various feed mixtures were prepared and used to achieve theabove objective, the details of which are given in Appendix B, Table B-2. Therecommended material from our experimental results is that 5-7% of total waste shouldconsist of waste paper. However, the composition may be varied depending on the C/Nratio of feedstock received from the community. Layout of the decentralized digestionsystem to be established for treating OFMSW of the community has been shown in Figure4.20. For this purpose, a shredder-cum -mixer will be used as shown in the Figure.

Table 4.7 Technical Details of Proposed AD Plant and its Comparison to Pilot Plant

DetailValue/Specification of the reactor

Pilot-scale Real-scaleAmount of waste (kg/d) 22 2000Reactor size (m3) 0.69 62.5Dimensions of reactor(m)-Diameter-Height

0.62.4

39

Placement/Orientation Inclined at 30° with ground Inclined at 30° with groundMaterial of the reactor Stainless steel Stainless steelSize of the pump (L/min) 200 1450Model of the pump Allweiler AE1N-200 Allweiler AE1N-1450Digestate (Liq + Solid), kg/d 15.4 1400Biogas production (L/d) 3530 200,000

4.5.3 Operation of decentralized AD system

Start-up phase

- The system will be started-up by loading a mixture of waste and inoculum with substrate-to-inoculum (S/I) ratio of ≤ 3. The inoculum should consist of a mixture of variety ofanaerobic materials (e.g. anaerobic sludge, anaerobic digestate from a working digester,and cow dung).-At start-up, the reactor will be purged with natural gas to remove oxygen and createanaerobic condition.- Low mixing rate (recirculation rate, i.e. 1 Ldig/Lreactor vol./d) will be used during start-up.- The starting reactor temperature will be same as ambient temperature. It will be thenincreased slowly and gradually (i.e. 2°C/day) to reach the designed temperature (i.e. 55°C).

Continuous operation mode

- After start-up, the continuous feeding of reactor should be started with a low OLR. Thenthe OLR should be progressively increased based on system’s capacity to reach thedesigned OLR (6.5 kg VS/m3.d).

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- Feeding and digested residue withdrawal will be performed once a day. Based on resultsof pilot experiments, the chosen retention time will be 24 days. The feedstock will bestored in feed storage tank for few days (less than a week) before feeding, becausenecessary pretreatment processing (shredding and mixing with waste paper) could beperformed in the preceding week for smooth and un-interrupted running of the plant in thecoming week.- Recirculation rate of 2.5 Ldig/Lreactor vol./d will be maintained during continuous operationof the reactor as used in pilot experiments (please refer to Table 3.3 and Figure 3.5).- The pH, ammonia, VFA and alkalinity of the liquid extract of digestate and biogascomposition, should be regularly measured to prevent reactor upset and/or to cure it bymethods like decreasing the organic loads or treating with alkali.

4.5.4 Generation of methane and energy

-Methane yield of 96 m3 CH4/d (or 200 m3 biogas/d) will be generated by the plant.-About 33% of the produced biogas from the proposed decentralized system will be used toprovide energy for running of the plant itself, whereas remaining 67% will be surplusenergy (Appendix B) that can be provided to the community.-Conversion of biogas to electricity is an environmental friendly and clean process. Thesystem will be provided with immediate conversion facility (Gas engine) of biogas intoelectricity. Moreover, biogas storage tank (100 m3) will also be installed for temporarystorage of biogas.-The amount of power produced from this system will be 14 kW (Appendix B).

4.5.5 Digestate management

Table 4.8 Technical Data of Sand Drying Bed for Digestate DewateringDetail Value/Specification

Dimensions of sand drying beds (m)-Width-Length

616

Thickness of bed layers (cm)-Sand-Coarse sand-Fine gravel-Medium gravel-Coarse gravel

207.57.57.515

Type of sand drying bed Conventional and openNumber of sand drying bed 3Under-drainage system-Pipe material-Pipe type-Pipe diameter-Pipe spacing-Pipe slope

PVCPerforated

15 cm2 m2%

Layer of digestate over bed 20 cm

89

Figure 4.20 Layout of conceptual decentralized AD plant for a community

90

- Storage of digestate will be minimized as much as possible to avoid the loss of carbonand nutrients as observed from the results of this thesis (please refer to sections 4.4.2(i) and4.4.3 (i).- Dewatering of digestate will be performed by use of simple sand drying beds, so that itsweight is reduced and it can be transported and managed easily for land application. Thetechnical details about the design and construction of drying bed for the proposed systemhave been summarized in the Table 4.8. For detailed calculations about design of bed, andweight reduction due to dewatering, please see Appendix B.-The dewatered solid digestate has coarse cracked surface and dark brown in color. If C/Nratio of solid digestate is within safe range (< 20), the digestate will be directly applied toagricultural land, as it stops methane formation and nutrients are utilized by plants. If C/Nratio of solid digestate is > 20, it should be further treated and then it can be utilized as soilamendment.

4.5.6 Reduction of GHG emissions

Managing waste sector is one option for mitigation of global effects of GHGs. Byconstruction and proper operation of this decentralized AD system, methane emission toatmosphere will reduce by 280,000 L of CH4 per day which is equal to 5 ton CO2-eq/d(Appendix B). Thus there is significant effect of introduction of this system in terms ofreduction of GHG emissions.

4.5.7 Material flow (VS balance)

Typical mass balance for volatile solids of the proposed system is given in Figure 4.21. Itis based on the results of run 2 of phase II pilot experiment. It describes that the conversionefficiency obtained will be up to 80% of VS added to the system. The daily amounts offeedstock to be fed and residues to be withdrawn are generalized in this figure.

Figure 4.21 Conceptual mass balance for VS of the proposed decentralized system

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Chapter 5

Conclusions and Recommendations

This study investigated the optimization of thermophilic dry anaerobic digestion ofOFMSW either by testing different feed C/N ratio or by testing different organic loadingrates. Two separate pilot-scale experiments were conducted for this purpose. In the firstexperiment, the effect two feed compositions (with C/N ratio 27 and 32) and its associatedammonia-N accumulation was studied on performance of dry anaerobic digestion. In thesecond experiment, effect of different organic loading rates (4.5-8.5 kg VS/m3/d) onstability and performance parameters of dry anaerobic digestion was studied. In addition,digestate characteristics were analyzed continuously for both experiments. Variousdigestate management options were also analyzed from the perspective of GHG emissions.The conclusions drawn from three phases of the study have been presented in the followingsection.

5.1 Conclusions

Conclusions of phase I (Dry digestion optimization by testing different feed C/N ratio)

1. The system accumulated 30% less ammonia-N by the use of feed having C/N ratio 32as compared to 27. Ammonia nitrogen accumulation in dry anaerobic digestion can bethus mitigated by use of correct feed mixture (i.e. by adjusting composition or C/Nratio of the feed).

2. The ITDAR performed well with the various OLRs and produced 200-300 L ofCH4/kg VS. However, it was observed that the free ammonia concentration of 400-660mg/L adversely inhibited the steady state methane production in ITDAR.

3. Adverse effects of ammonia inhibition reduced with the practice of altering the C/Nratio, higher OLR and different recirculation rates, which have been provedadvantageous and suggested to overcome the ammonia-N inhibition in ITDAR. Thealterations made during this study are easy to manage in a decentralized system andthus can help in better process control.

4. Moreover, ITDAR can produce surplus net energy in the range of 50-73%. Therefore,the system can be effectively implemented at a decentralized level to recover the netenergy from OFMSW.

Conclusions of phase II (Dry digestion optimization by testing different OLRs)

1. Comparison of start-up phases of the two experiments shows that low digestaterecirculation rate (Digrr) should be used during start-up of the reactor to achievereactor stability conditions in less time. Low Digrr (1 Ldig/Lreactor vol./d) in firstexperiment achieved stable reactor conditions in shorter time as compared to highDigrr (2.4 Ldig/Lreactor vol./d) of second experiment.

2. Gas production rate increases linearly with organic loading rate, provided that properacclimatization is performed for every increased OLR.

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3. The OLR 4.55 kg VS/m3/d achieved higher methane yield per kg VS added than otherruns. However, OLR of 6.4 kg VS/m3/d also achieved comparable methane yield at alesser RT (24 days).

4. Similarly, VS removal was the highest (77.9%) by OLR of 4.55 kg VS/m3/d whereasOLR of 6.4 kg VS/m3/d also achieved a good VS removal (i.e. 75.5%).

5. Based on these results, the best conditions with reasonable methane yield and VSremoval for thermophillic dry anaerobic conditions were OLR 6.4 kg VS/m3/d withRT of 24 days. As the methane yield and VS removal decreased with increase inOLR, therefore, if the objective is partial digestion or dual treatment of waste(anaerobic + aerobic), then the condition with the lowest RT (i.e. 18 days) and highestOLR should be preferred.

Conclusions of phase III (Digestate management and GHG emission reduction)

1. Control of total solids content of the reactor helped in smooth operation of the reactor.Moreover, by comparing the characteristics of digestate from two experiments, it canbe concluded that by increasing TS in feed, the VS/TS ratio of digestate increases, andhence the removal of solids decreases in dry anaerobic digestion.

2. The digestate didn’t require aerobic treatment (or composting), because the C/N ratioof raw digestate was optimum for its land application during most of the study period.

3. It can be concluded that storage time of raw digestate from anaerobic reactor should bereduced to avoid nutrient and carbon loss.

4. Results of the GHG emissions from digestate show that raw digestate has themaximum GHG emission potential and should not be dumped or landfilled as itsdumping contributes the most to GHG emissions.

5. The study reveals that land application of digestate is very good digestate managementstrategy as 93 to more than 100% GHG emissions from digestate can be avoided by itsland application. However, C/N ratio of digestate and time of application to land needto be considered. Thus the scenarios which include land application are better than theothers whereas base scenario is the worst.

6. Avoid storage of digestate after digestion or otherwise, storage time should beminimized as much as possible. Results from this study showed that the digestateemitted 10% of its total GHG potential in 2 months of storage.

7. The results of this study can help the anaerobic digester operators to manage thedigestate efficiently from perspective of GHG emission reduction. It is important tonote that the results are based on digestate characteristics from our pilot-scaleanaerobic reactor. However, these may be different from other studies based ondifferent digestate characteristics which depend on type of AD substrate and type ofdigestion process.

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5.2 Recommendations

Following are the recommendations for future studies based on the extensive experimentalwork of this dissertation:

1. More combinations of wastes consisting of other sources of substrates should beevaluated for mitigation of ammonia accumulation. For example, feedstock withhigh C/N ratio (e.g. straw, crop residues, saw dust, industrial by-products likedistiller grains from cassava ethanol production, waste paper, etc.) should be mixedwith low C/N ratio materials (e.g. kitchen waste, vegetable waste, manures, etc).

2. Combination of only two types of waste (for example, (i) food waste + waste paper,or (ii) manure + paper) should also be investigated as it will be helpful inmanagement of waste supply and easy feed preparation as compared to using 4-5types of waste.

3. Effect of recirculation rate on both the start-up and continuous operation phases ofdry anaerobic digestion should be further studied. From this study, it was found thatlow digestate recirculation rate should be used during start-up of the reactor. It willhelp to achieve reactor stability conditions (linked to VFA accumulation and fallingpH) in less time in dry anaerobic digestion. However, more research in this areashould be performed.

4. Reactor design modification in a way to avoid the use of pump for mixing purposeshould also be investigated, because beyond certain limit of TS, the pump getsblocked and reactor material can not be pumped. Moreover, the modified design tobe studied should provide alternatives means of mixing.

5. Effect of feed total solids percentage on performance of thermophilic dry anaerobicdigestion needs to be studied. In our study, TS for the two pilot experiments wasdifferent that affected the digestion performance. However, this was not a focuspoint of our research. Moreover, most of the research already performed on thisissue deals with dry digestion at mesophilic temperature.

6. Based on the findings of this thesis, it is recommended that the feedstock C/N ratio32 is helpful to mitigate ammonia accumulation problem in dry anaerobicdigestion. Thus the recommended material from our experimental results is that 5-7% of total waste should consist of waste paper, depending on the C/N ratio ofmain portion of feedstock received. This C/N ratio should be used as a base forfuture studies dealing with mitigation of ammonia accumulation.

7. Optimum operating conditions for dry anaerobic digestion also depend on thepurpose of treatment. In case of dual treatment (anaerobic + aerobic), time spent foranaerobic digestion should be minimized, so that the saved time could be used foraerobic treatment. Thus low retention time (e.g. 15 days) with high OLR (e.g. 8.5-10 kg VS/m3.d) should be used for digestion step. But if the purpose is only a singletreatment (i.e. dry anaerobic digestion), then option of higher retention time (RT 24d or 30 d) should be chosen to get maximum VS removal and its conversion tomethane.

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8. Although the process control is easy in a decentralized system by judiciouslyaltering the operating conditions (like OLR, C/N ratio, etc.), but the change shouldbe performed very carefully and gradually to avoid its effect on performance ofbiological system. Future study should also investigate the difference in reactorperformance by sudden and gradual change in feed C/N ratio.

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Appendix A

Experimental Set-up Pictures

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Figure A-1 Waste material used in this study

Figure A-2 Set-up for GP21Test (Front view)

Fruit and vegetable

Waste

Waste material used (5 fractions)

Mechanical shredder

(25 mm size)

Shredded waste

Five fractions

Food waste

VegetablesFruit waste

Leaf waste

Paper wasteCloser to real field conditions

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Figure A-3 Set-up for GP21Test (Side view)

Figure A-4 Pilot-scale inclined thermophilic dry anaerobic reactor

Hot watertank

Reactor

Feedinghopper

Wet gasmeter

Withdrawalport

Screwpump

Biogastube

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Figure A-5 Sand drying bed for dewatering of digestate

114

Appendix B

Calculation of Proposed Decentralized AD Plant

115

Assumptions

Size of the community: 5,000Total waste generated by the community: 5 t/d (Waste generation in low-income countriesis 1.0 kg/p/d (Bogner et al., 2007)Weight of organic waste: 60 % of total waste = 0.60 x 5 = 3 t/d,By applying the average waste collection efficiency i.e., 65-70% (Hoornweg and Bhada-Tata, 2012) of South Asian and East Asian regions, the resulted waste quantity: 2.0 t/d or2000 kg/dTS of waste: 18 % (please refer to Table B-2)VS of waste: 84 %TS (please refer to Table B-2)

Density of pretreated waste: 1000 kg/m3 (please refer to Table B-2 of Appendix B)Reactor working volume: 75 % of total volumeOperating conditions (loading rate and retention) and methane yield used for designcalculations have been taken from results of run 2 of phase II pilot experiment as given inTable B-1.

Table B-1 Operating Conditions Versus Methane Yield of Phase II Pilot ExperimentDetail Run 1 Run 2 Run 3OLR (kg VS/m3/d) 4.55 6.40 8.50RT (d) 30 24 18Methane yield (L/kg VS) 330 320 266

1. Calculations for volume of reactor

Method 1:

Working volume of reactor = VS (kg) to be added per dayVS load (kg VS/m3.d)

VS to be added per day = 2000 kg/d x 18/100 (TS) x 84/100 (VS) = 300 kg VS/d

Working volume = 300 (kgVS/d) / 6.4 (kgVS/m3/d) = 46.88 m3

Reactor total volume: 46.88 x 100/75 = 62.5 m3

Method 2:

VS to be added per day = 2000 kg/d x 18/100 (TS) x 84/100 (VS) = 300 kg VS/d

Volume of organic waste = weight/density = 2000 (kg/d) / 1000 kg/m3 = 2.0 m3/d

Working volume of reactor (L ) = Flow rate (L/d) * RT (d)

= 2000 x 24 = 48,000 L = 48 m3

OLR at this volume = 300 (kg VS/d)/ 48 m3 = 6.25 kg VS/ m3.dWorking volume of reactor at OLR of 6.4 kg VS/m3.d = 48 x 6.25/6.4 = 46.88 m3

Total volume: 46.88 x 100/75 = 62.5 m3

116

The dimensions of the digester: 3 m (dia) x 9 m (height)Daily amount of waste to feed: 2000 kg/d

2. Preparation of correct feed mixture for dry anaerobic digestion

The main purpose of this research was to prepare a correct feed mixture (in terms of C/Nratio and TS) for dry anaerobic digestion, which can reduce the problem of ammoniainhibition. To achieve that purpose, various feed mixtures were prepared, which consistedof different percentage of different types of waste (i.e. food waste, fruit and vegetablewaste, waste paper and leaves). The detail of feed mixtures and their characteristics isgiven in Table B-2.

Table B-2 Composition and Characteristics of Various Feed MixturesDetail Unit Feedstock 1 Feedstock 2 Feedstock 3

Composition

Food waste % FWa 42 40 45Vegetable waste % FW 45 27 33Fruit waste % FW 5 20 15Leaf waste % FW 5 8 -Paper waste % FW 3 5 7

Characteristics

Moisture % 79-84 75-85 81-86TS (range) % 16-21 15-25 14-19VS (range) %TS 79-90 80-90 84-88C (avg.) %TS 51.30 52.10 51.20TKN (avg.) %TS 1.92 1.63 1.61C/N (avg.) - 26.72 31.96 31.80Density (avg.) kg/m3 1045 1000 1020aPercentage based on fresh weight.

Feedstock with C/N ratio 32 performed well and had 30% lesser ammonia accumulationthan C/N ratio 27. Thus, feedstock with C/N ratio 32 is recommended for the proposeddecentralized AD system, which can be prepared by mixing kitchen waste with 5-7% ofwaste paper as shown in Table B-2.

3. Calculation for methane or biogas yield

Specific methane production = 320 L/kg VS (From pilot experiment, please see Table B-1)

Methane production = volatile solids x specific methane production

= 300 (kgVS/d) x 320 (L/kg VS)= 96000 L CH4/d or 96 m3 CH4/d= 96 m3 (CH4/d) x 100/48.12 (biogas/methane) = 200 m3 biogas/d= 8.33 m3 biogas/h

117

The flow rate of 0.569 m3 biogas/h is required to convert the biogas into electricity for agas engine of 1 kW (Prakash et al., 2012). Thus the flow rate of biogas produced from theproposed decentralized AD plant is sufficient to generate electricity.

4. Energy and electricity generation

Energy balance of the pilot system has been calculated (Table B-3) based on themethodology given in Appendix E. Results of run 2 have been considered to calculate theelectricity generation for real scale proposed decentralized AD plant.

Table B-3 Surplus Energy of ITDAR During Various Runs of Experiment 2

Energy production: 18.21 MJ/kg VS (from pilot experiment 2, run 2, Table B-3)Energy available: 67% of produced (33% spent on operation of the plant)

= 18.21 (MJ/kg VS) x 0.67 = 12.2 MJ/kg VSInput of proposed plant = 300 kg VS/dEnergy produced = 12.2 (MJ/kg VS) x 300 (kg VS/d) = 3660 MJ/d = 3.66GJ/d

We know that 1 GJ/d = 11.57 kW (1 J/s = 1 W)3.66 GJ/d = 3.66 x 11.57 = 42.35 kW

If efficiency of gas engine is 33% (Surroop and Mohee, 2012), then

Electricity Generation = 42.35 x 33% = 13.97 kW

Thus electricity of 14 kW will be generated by use of a 14 kW gas engine.

Table B-4 Technical Detail and Availability of Gas EngineDescription SpecificationRanked power 14 kW (continuous operation)Voltage 400 VCurrent 27 AApproximate weight 800 kgElectrical efficiency 34.80%Biogas consumption < 0.5 m3/kWhSuppliers Address 1. EMAC International

http://www.sino-cummins.com2. http://www.alibaba.com/

Biogas flow rate for 1 kW gas engine: 0.569 m3/h (Prakash et al., 2012)

Run

Energyproduction

(MJ/kgVS)

Energy Consumption (MJ/kg VS)Surplusenergy

(%)ShreddingFeeding

andwithdrawal

RecirculationHeating andmaintainingthermophilic

conditions1 20.60 1.29 0.28 2.09 4.149 62.022 18.21 1.16 0.26 1.68 2.76 67.773 15.98 1.21 0.27 1.18 2.50 67.75

118

Biogas flow rate for 14 kW gas engine: 0.569 x 14 = 7.96 m3/h. This flow rate can besupplied by the proposed system, as it has flow rate of 8.33 m3/h. The detail of thecommercially available gas engine is given in the Table B-4.

5. Use of the produced biogas

The produced biogas can be used for various purposes like cooking, lighting and electricityproduction. The extent of use of the produced biogas for various purposes has been givenin the Table B-5.

Table B-5 Use of Produced Biogas (8.33 M3/H) for Various PurposesPurpose of use Specifications 3Biogas flow

required (m3/h)Number of uses ata time

Stove burners 10 cm 0.28 30Mantle Lamps 60 watts equivalent 0.195 42House electricity 11000 watt/house 20.569 151 Electricity consumption of a house in US and Australia is 900-1100 watt (Elert, 2003)2 For 1000 watt power production3 Prakash et al., (2012)

6. Technical details of digestate management

Digestate withdrawn from the reactor will not be stored because storage causes emission ofGHGs as found from the results of this study (please refer to sections 4.4.2(i) and 4.4.3 (i).As the C/N ratio of digestate is within safe range to be applied on agricultural land (pleaserefer to Figure 4.16 and Table 4.4), therefore, it will be sent to use for agricultural purposes.But before sending, its weight will be reduced by dewatering for easy management.Technical details of dewatering have been given below.

i) Design of sand drying bed for dewatering digestate from proposed system

Sand drying bed will be used to dewater digestate. The advantages of sand drying bed arelow cost and simple.

Quantity of digestate: 1400 kg/dSolid content of raw digestate: 6.5%Dewatering time: 10 daysDepth of digestate layer on bed: 20 cm

Calculation for area of sand drying bed

Area of sand drying bed can be calculated from the loading of dry solids.

Typical dry solids loading: 120 kg dry solids/m2-yr (Tchobanoglous et al., 2003)For a drying time of 10 d: 120/365 x 10 = 3.28 kg dry solids/m2

Thus area needed for 3.28 kg dry solids = 1 m2

Digestate produced as dry solids from proposed system= 1400 (kg/d) x 10 d x 6.5% dry solids= 910 kg dry solids

119

Area for 910 kg dry solids = 910 (kg dry solids)/3.28 (kg dry solids/m2)= 277 m2

Standard dimensions of drying beds are 6 m (wide) x 6 to 30 m (long)

Therefore, 3 drying beds with dimensions of 6 m (wide) x 16 m (long) will beconstructed to fulfill the digestate dewatering requirement of proposed system.

Other considerations

-Starting from the bottom to top, the bed will consist of 15 cm coarse gravel layer and7.5 cm thick layer each of medium gravel, fine gravel and coarse sand. At the top,there will be a 15-25 cm layer of sand.

-Digestate will be placed on bed in 20 cm layer.

-The bed will be equipped with lateral drainage lines under it, which will consist ofperforated 15 cm plastic pipes spaced 2 m apart and sloped at 2 percent.

ii) Calculation for reduction of weight (%) by digestate dewatering

Total amount of raw digestate: 1400 kg/dSolid content of raw digestate: 6.5%Solid content of dewatered digestate: 50%Leachate solid content: 2.0%

TS in raw plant residues or digestate = 1400 * 0.065= 91 kg

Total weight of dewatered residues = XTotal weight of leachate = Y

We know that, X + Y = 1400 kg or X = 1400 – Y Eq. (i)

Weight of TS added = Weight of TS removed

or 91 = 0.50 *X + 0.02 * Yor 91 – 0.02Y = 0.5Xor X = (91 – 0.02Y)/0.5 = X Eq. (ii)

Comparing Eq. (i) and (ii), we haveX = 131 kgY = 1269 kg

Weight reduction = (1400 - 131) * 100 = 90.6 %1400

7. Size of the pump

The size of the pump used in pilot-scale ITDAR is AE1N-200. The number 200 meanstheoretical delivery of the pump is 200 L/min. But in practice, its delivery (flow rate) wasobserved as 25 L/min probably due to viscosity of the reactor material. The pump head

120

(vertical height to deliver digestate to the reactor) for the real scale reactor is just 3 timesthat of pilot reactor as given in Table B-6. But, to meet the high delivery (L/min) due tohuge size of real plant, much higher size (not just 3 times like head) of pump should beused.

Calculations for selecting the size of pump

As the same operating conditions and feed composition will be replicated, almost the sameviscosity of the reactor material will achieve in real scale reactor. Also the practical flowrate of real scale pump can be calculated from pilot observation as follows:

The practical flow rate of pilot pump (200 L/min) = 25 L/minIf we select a pump size of 1450 L/min for the proposed real-scale reactor,Its practical flow rate will be: = 25/200 x 1450 = 181 L/minTime taken to recirculate whole reactor material for 1 time

= 47000 (L)/181 (L/min)= 260 min

Recirculation time required to achieve the recirculation rate of 2.5 Ldig/Lreactor vol./d= 2.5 x 260 min = 650 min

Recirculation frequency: 4 times/d (in every 6 h) = 650/4 = 160 min ≈ 3 h

Table B-6 Comparison of Pump System Between Pilot and Real ReactorDetail Pilot-scale plant Real proposed plantSize (m3) 0.7 (0.6 m dia x 2.4m height) 62.5 (3 m dia x 2.4m height)Pump head (m) 1.7 5Pump size (L/min) 200 1450Observed flow rate(L/min)

25 180

Time for 1 cycle ofrecirculation (min)

22 260

Full Model Number AE1N-200 AE1N-1450Company Allweiler Allweiler

The pump (size 1450 L/min) needs to run for 3 hours in every 6 hours to achieve therequired recirculation rate (or recirculation every alternate hour i.e. 1h on and 1h off).Thus, this is the minimum pump size (1450 L/min) for the proposed reactor (working vol.47 m3). Because if pump size smaller than this is used, it needs to be turned-on longer timethan turned-off, which is not good. However, if we want to reduce turned-on time thanturned-off time, we need to select a pump size bigger than size 1450 L/min.

This pump size (1450) will also allow pumping of 3 times grain size and 2 times fiberlength of the material as compared to pump size 200.

8. Calculation of GHG emission reduction

Landfill methane production = 140 L/kg waste(Bogner and Spokas, 1993)

Total methane emission, if the community landfills all the waste,= 2000 kg waste /d 140 *L CH4/kg waste = 280000 L CH4 = 280 m3 CH4/d

121

= 280 m3/d x 0.000717 tons/m3 (Density of methane)= 0.2 tons CH4/d = 0.2 (ton CH4/d) x 25 (tons CO2-eq/ton CH4)= 5 ton CO2-eq/d

9. Plant capital and operating and maintenance cost (Things to consider)Considerations for plant (2000 kg/d OFMSW (Dry thermophilic anaerobic digestion

plant) capital as well as O&M costs are given in Tables B-7 and B-8.

Table B-7 Considerations for Calculating Capital Cost of the Proposed AD PlantDetail Specification/Characteristics/ RemarksSite/General

-Land area (for all digester set-up andbuilding)-Building (store room for equipment,office, etc)

Feed preparation system-Shredder or comminuter-Mixer-Feed storage tanks 2 m3 (5 tanks)

Reactor system-Digester 3m (dia) x 9m (length) steel vessel

Pumping and recirculation system-Screw bed pump AE1N-1450 (Allweiler)

Heating system-Steam generator

Biogas conversion to electricity-Biogas storage tank-Gas engine

100 m3

14 kWDigestate management

-Sand drying bed-Digestate storage tanks

6 m (wide) x 16 m (long) (3 beds)2 m3 (5 tanks)

Miscellaneous-Pipes and valves, other spare parts,tools, etc.-Bins, containers, etc.

Table B-8 Considerations to Calculate Operating and Maintenance Cost of AD PlantDetail Specification/RemarksStaff requirement -1 Plant Manager

-1 Process control operator-2 Maintenance technicians-2 General labor

Utilities and Fuels -Fuel-Electricity-Water-Natural gas for start-up

Maintenance -Equipment-Site works

Miscellaneous -Lab analysis cost-Wastewater treatment cost

122

Appendix C

Data of Phase I Pilot Experiment

123

Table C-1 Operational Parameters of Anaerobic Digestion During Phase I PilotExperimentRun Time

(day)pH VFA

(mg/L)Alkalinity

(mg/L)VFA/Alk ratio TAN

(mg/L)Free Ammonia

(mg/L)1 5.43 49130 23450 2.10 1766 26 5.98 35640 27360 1.30 1928 7

13 6.92 28800 35000 0.82 2134 6518 6.68 37930 28860 1.31 2600 4624 7.14 34250 28800 1.19 2480 12329 6.9 37750 28860 1.31 2990 8735 7.12 33750 32850 1.03 2940 13941 7.61 30900 65500 0.47 3206 42747 7.48 28500 52700 0.54 3115 31853 7.39 26500 48450 0.55 2944 24956 7.53 25250 49050 0.51 2947 33463 7.49 24000 46350 0.52 2986 31270 7.41 19125 38950 0.49 2811 24877 7.40 20000 37400 0.53 2779 24083 7.36 20750 36000 0.58 2912 23291 7.70 15500 31350 0.49 2618 41698 7.59 14750 32000 0.46 2233 286

105 7.45 13750 28500 0.48 2205 212119 7.71 12750 33750 0.38 2566 416127 7.75 10750 35250 0.30 2352 411133 7.82 11100 44400 0.25 2436 486141 8.00 10750 68750 0.16 2400 657147 7.78 11250 36250 0.31 2200 407154 7.45 11250 35100 0.32 1862 179160 7.44 10500 33600 0.31 1883 177168 7.30 12500 30450 0.41 1939 136175 7.51 12000 30250 0.40 2114 230182 7.35 10750 30250 0.36 2163 168187 7.72 10000 27025 0.37 2254 372196 7.47 8750 22425 0.39 2310 231203 7.31 8375 19425 0.43 2086 149210 7.49 7875 19800 0.40 2037 213217 7.33 8375 19000 0.44 1897 141224 7.41 7800 19250 0.41 1820 161233 7.84 6700 22650 0.30 1850 383240 7.55 7000 16500 0.42 1680 198246 7.50 5150 15062 0.34 1708 182253 7.25 8150 11700 0.70 1512 95261 7.00 5750 7700 0.75 1897 69264 7.37 5500 9100 0.60 2135 173271 7.47 5500 14000 0.39 1540 154

124

Table C-2 Performance Parameters of Anaerobic Digestion During Phase I PilotExperimentRun Time

(days)GPR

(L/Lreactor vol./d)% CO2 % CH4 Methane Yield

(L CH4/kg VS)VS Removal

(%)15 0.16 58.55 34.31 76 33.3816 0.35 59.93 31.87 70 33.3818 0.27 59.72 32.71 72 33.3819 0.28 56.22 33.23 107 49.5320 0.24 51.31 36.50 112 49.0022 0.26 37.26 46.35 131 49.0023 0.12 39.33 48.10 200 49.5324 0.31 44.74 46.22 197 49.5425 0.46 35.06 42.68 175 49.5326 0.39 39.77 49.74 197 49.5327 0.44 37.69 50.08 160 39.5528 0.54 37.69 50.08 167 39.5529 0.51 37.51 51.51 167 39.5430 0.60 37.58 49.96 170 39.5531 0.83 34.3 54.53 167 39.5532 0.77 34.89 50.80 264 55.7933 1.00 32.66 56.58 250 55.7934 0.85 35.32 54.46 278 55.7935 1.09 32.53 56.48 259 55.7636 0.94 35.32 54.52 279 55.7937 1.01 32.39 54.27 259 55.7938 1.06 32.38 54.23 273 55.7939 1.00 33.00 51.17 275 56.2240 1.16 34.00 54.60 262 56.2241 0.99 36.96 52.94 268 56.2142 1.22 32.76 56.48 249 56.2243 1.07 35.17 54.43 280 56.2244 1.16 36.71 53.08 262 56.2245 1.34 32.52 56.63 274 61.4346 1.14 32.00 50.43 308 61.4347 1.38 36.62 52.97 289 61.4348 1.67 36.14 51.01 274 61.4349 1.83 35.58 54.72 269 61.4350 1.67 38.58 52.32 285 61.4351 1.96 36.72 53.78 280 65.4653 2.18 38.64 52.07 294 65.4854 2.32 38.04 52.22 278 65.4656 2.84 35.79 54.02 282 65.4658 3.27 35.45 54.42 300 65.4662 3.19 40.04 50.50 297 64.2564 3.50 40.41 50.02 261 64.25

125

Run Time(days)

GPR(L/Lreactor vol./d)

% CO2 % CH4 Methane Yield(L CH4/kg VS)

VS Removal(%)

69 3.00 36.95 52.78 307 69.5676 3.18 34.92 55.26 457 96.7378 3.27 38.40 52.33 457 96.7382 3.61 41.28 49.91 257 65.0785 3.73 43.78 47.34 265 67.1888 3.84 35.33 51.80 237 65.0090 4.01 32.61 57.27 292 65.0093 5.32 36.79 53.71 335 66.5995 6.05 34.11 56.08 309 68.83

100 5.76 36.83 53.64 312 69.81105 4.47 42.00 49.32 277 71.57107 3.94 41.90 49.42 277 71.57112 3.79 40.40 49.88 285 71.06117 3.09 36.69 50.71 255 58.98125 4.84 40.45 50.00 215 53.52130 4.73 38.75 51.54 226 53.52135 6.11 38.83 51.43 214 50.79140 5.75 42.50 48.84 124 32.53149 7.53 40.76 47.99 134 34.49155 10.28 43.66 47.85 104 28.09163 8.64 50.60 42.70 97 31.77169 5.20 49.22 42.40 151 49.00172 4.40 44.50 47.21 177 49.00175 5.69 39.75 50.53 216 52.72181 2.80 40.33 50.72 221 54.37185 2.31 32.66 56.70 270 54.00191 1.62 34.42 54.83 257 54.22205 3.22 41.15 47.72 211 55.00218 3.21 42.00 48.40 218 57.01227 4.79 40.82 49.51 217 54.79232 3.71 42.00 41.50 188 55.00239 6.30 35.29 55.00 191 48.14245 6.86 41.17 48.78 138 34.79251 7.41 40.08 49.44 250 63.04260 6.29 42.61 45.00 221 61.54264 6.82 45.12 46.00 122 34.94267 7.64 42.31 47.88 132 34.92272 7.58 43.66 46.69 202 55.69278 4.09 42.91 46.72 292 79.40

126

Table C-3 Characteristics of Feed and Digestate in Phase I Pilot ExperimentRunTime(days)

FeedTS(%

FM)

FeedVS(%

FM)

FeedVS/TS

DigestateTS

(% FM)

DigestateVS

(% FM)

Digestate

VS/TS

Digestate

TKN(%)

DigestateC/Nratio

1 - - - 18.62 14.15 0.76 1.59 26.556 - - - 18.68 13.45 0.72 1.30 30.81

15 15.80 13.13 0.83 12.06 7.79 0.65 1.71 20.9824 15.80 13.13 0.83 10.17 5.90 0.58 0.71 45.4129 15.80 13.13 0.83 11.48 7.07 0.62 0.82 41.7135 15.80 13.13 0.83 9.00 5.17 0.57 0.87 36.6841 15.80 13.13 0.83 8.84 5.26 0.60 0.89 37.1447 17.26 13.80 0.80 8.48 4.98 0.59 0.96 33.9553 17.26 13.80 0.80 7.88 4.46 0.57 0.90 34.9063 17.26 13.80 0.80 8.12 4.61 0.57 0.91 34.6870 20.75 16.47 0.79 7.81 4.69 0.60 0.89 37.4577 20.75 16.47 0.79 7.94 4.92 0.62 0.90 38.2283 20.75 16.47 0.79 7.80 4.87 0.62 0.92 37.7191 20.75 16.47 0.79 8.13 4.96 0.61 1.32 25.6798 22.09 18.65 0.84 8.18 5.17 0.63 1.80 19.52

105 25.11 19.91 0.79 8.07 4.87 0.60 2.01 16.69119 25.11 19.91 0.79 10.09 6.62 0.66 1.96 18.58127 25.11 19.91 0.79 11.52 7.50 0.65 2.17 16.66133 25.11 19.91 0.79 12.12 7.94 0.65 1.93 18.85141 25.11 19.91 0.79 16.87 10.88 0.65 1.88 19.06147 25.11 19.91 0.79 16.70 11.38 0.68 1.86 20.36151 22.83 18.64 0.82 18.47 12.49 0.68 ND ND158 22.83 18.64 0.82 17.40 12.18 0.70 ND ND165 22.83 18.64 0.82 13.10 9.10 0.70 ND ND173 22.83 18.64 0.82 11.61 8.09 0.70 ND ND180 22.83 18.64 0.82 11.63 7.80 0.67 ND ND185 22.83 18.64 0.82 11.37 7.50 0.66 ND ND193 22.83 18.64 0.82 11.39 7.64 0.67 ND ND200 22.83 18.64 0.82 11.77 7.66 0.65 ND ND207 22.83 18.64 0.82 11.95 7.83 0.66 ND ND215 22.83 18.64 0.82 11.03 7.32 0.66 ND ND224 22.83 18.64 0.82 12.16 7.69 0.63 1.74 20.18233 18.10 14.84 0.82 11.52 7.73 0.67 1.94 19.22240 18.10 14.84 0.82 12.57 8.91 0.71 1.99 19.79246 18.10 14.84 0.82 18.63 11.20 0.60 2.03 16.46253 18.10 14.84 0.82 9.62 6.35 0.66 2.68 13.69261 18.10 14.84 0.82 10.16 6.61 0.65 1.80 20.07264 19.22 15.15 0.79 16.17 9.71 0.60 2.02 16.52271 19.49 15.46 0.79 10.74 7.17 0.67 2.36 15.72280 19.49 15.46 0.79 17.73 8.85 0.50 1.84 15.07

%FM: Percentage of fresh matter, ND: Not determined.

127

Appendix D

Data of Phase II Pilot Experiment

128

Table D-1 Operational Parameters of Anaerobic Digestion During Phase II PilotExperimentRun Time

(day)pH VFA

(mg/L)Alkalinity

(mg/L)VFA/Alk ratio TAN

(mg/L)Free Ammonia

(mg/L)11 7.14 15300 13300 1.15 1540 7619 7.16 17775 17000 1.05 1652 8525 7.20 19000 18000 1.06 1785 10132 7.43 17050 17450 0.98 1660 15339 8.10 11100 17700 0.63 1442 46447 8.23 6300 19500 0.32 1491 58254 7.82 5700 15450 0.37 1862 37161 7.91 5325 13750 0.39 2016 47368 7.76 4700 11750 0.40 1890 33775 7.71 4975 11600 0.43 1880 30581 7.70 5100 11550 0.44 2058 32789 7.70 5000 11000 0.45 2065 32896 7.56 5350 10250 0.52 1897 228

103 7.85 4400 10500 0.42 1974 416112 7.75 3575 10500 0.34 2198 384119 7.54 4450 9400 0.47 2107 244127 7.82 6450 11400 0.57 2100 419135 7.75 5900 10750 0.55 2114 370141 7.55 6525 8800 0.74 2030 240146 7.23 5700 8500 0.67 2030 122154 7.19 7350 9650 0.76 3066 169160 7.00 7500 9500 0.79 2100 76

Table D-2 Performance Parameters of Anaerobic Digestion During Phase II PilotExperimentRun Time

(days)GPR

(L/Lreactor vol./d)% CO2 % CH4 Methane Yield

(L CH4/kg VS)VS Removal

(%)55 4.54 41.69 48.79 317 82.1663 5.30 41.60 48.02 308 80.4674 5.65 40.32 48.58 331 83.9684 5.53 40.89 48.42 309 79.3289 4.73 40.83 49.18 321 81.49

102 5.70 39.98 49.37 353 88.06114 5.75 38.73 50.64 361 86.67119 6.77 42.00 48.00 310 81.56126 5.64 41.90 47.69 305 80.43135 6.64 36.90 47.07 326 79.59145 6.07 43.15 46.82 283 77.13146 6.04 42.76 47.28 161 43.50159 7.79 43.53 46.76 278 74.86

129

Table D-3 Characteristics of Feed and Digestate in Phase II Pilot ExperimentRunTime(days)

FeedTS(%

FM)

FeedVS(%

FM)

FeedVS/TS

DigestateTS

(% FM)

DigestateVS

(% FM)

DigestateVS/TS

11 - - - 5.68 3.40 0.6019 - - - 11.91 7.27 0.6125 - - - 8.22 4.86 0.5932 - - - 7.42 4.76 0.6439 - - - 8.28 5.05 0.6147 - - - 9.77 5.73 0.5951 16.07 13.97 0.87 5.21 2.97 0.5758 15.89 13.70 0.86 5.83 3.32 0.5765 14.66 12.61 0.86 5.17 2.91 0.5671 16.38 14.39 0.88 5.17 2.91 0.5678 15.08 13.17 0.87 5.06 2.87 0.5779 13.44 11.38 0.85 5.64 3.32 0.5986 13.88 11.93 0.86 5.63 3.34 0.5993 13.88 11.93 0.86 5.20 2.92 0.56

100 17.85 15.28 0.86 4.82 2.66 0.55108 17.85 15.28 0.86 4.38 2.35 0.54110 17.85 15.28 0.86 5.09 2.83 0.56116 17.85 15.28 0.86 6.80 3.74 0.55124 17.85 15.28 0.86 7.06 3.95 0.56132 17.85 15.28 0.86 6.64 3.64 0.55139 16.83 14.45 0.86 6.86 3.99 0.58146 17.59 15.18 0.86 6.43 3.57 0.55150 17.59 15.18 0.86 4.99 3.05 0.61160 16.26 14.01 0.86 4.99 3.05 0.61

%FM: Percentage of fresh matter

130

Appendix E

Methodology for Calculation of Energy Balance

131

Methodology for Energy Balance Calculation of Inclined Reactor

The energy balance of inclined reactor is based on its energy consumption and energyproduction. Energy is required for shredding the waste (ES), waste feeding and withdrawal(EFW), re-circulation of reactor content (ER) and heating (EH) which includes energy forheating the substrate (EHS) as well as energy for maintaining constant thermophilictemperature (EMT). The production of energy (EP) is only from the methane produced.Method for calculation of each type of these energies has been given in the followingsection.

The net energy production NEP (MJ) is the difference between the energy produced andconsumed by the process that is shown in Figure 1 and given by the following equation:

NEP = EP - ES – EFW – ER – EH

Figure E-1 Energy inputs and output of inclined anaerobic digester

1.1 Energy Production

The energy production EP (MJ) equivalent to methane and hydrogen content of theproduced biogas can be calculated by the equation below.

EP = (MP X L.H.V. of CH4) + (HP X L.H.V. of H2)

whereMP = methane production (L CH4)L.H.V. of CH4= 0.03618 MJ/L CH4 (Ruggeri et al., 2010)HP = Hydrogen production (L H2)L.H.V. of H2 = 0.0108 MJ/L H2 (Ruggeri et al., 2010)

1.2 Energy Consumption

Energy consumption has been described under sub-categories as follows:

1.2.1 Energy required for shredding (ES)

The assumption here is that about 100 kg of waste is shredded by the use of 0.5 L gasoline.

FeedingWithdrawalRecirculation

ShreddingHeating

Biogas

131

Methodology for Energy Balance Calculation of Inclined Reactor

The energy balance of inclined reactor is based on its energy consumption and energyproduction. Energy is required for shredding the waste (ES), waste feeding and withdrawal(EFW), re-circulation of reactor content (ER) and heating (EH) which includes energy forheating the substrate (EHS) as well as energy for maintaining constant thermophilictemperature (EMT). The production of energy (EP) is only from the methane produced.Method for calculation of each type of these energies has been given in the followingsection.

The net energy production NEP (MJ) is the difference between the energy produced andconsumed by the process that is shown in Figure 1 and given by the following equation:

NEP = EP - ES – EFW – ER – EH

Figure E-1 Energy inputs and output of inclined anaerobic digester

1.1 Energy Production

The energy production EP (MJ) equivalent to methane and hydrogen content of theproduced biogas can be calculated by the equation below.

EP = (MP X L.H.V. of CH4) + (HP X L.H.V. of H2)

whereMP = methane production (L CH4)L.H.V. of CH4= 0.03618 MJ/L CH4 (Ruggeri et al., 2010)HP = Hydrogen production (L H2)L.H.V. of H2 = 0.0108 MJ/L H2 (Ruggeri et al., 2010)

1.2 Energy Consumption

Energy consumption has been described under sub-categories as follows:

1.2.1 Energy required for shredding (ES)

The assumption here is that about 100 kg of waste is shredded by the use of 0.5 L gasoline.

FeedingWithdrawalRecirculation

ShreddingHeating

Biogas

131

Methodology for Energy Balance Calculation of Inclined Reactor

The energy balance of inclined reactor is based on its energy consumption and energyproduction. Energy is required for shredding the waste (ES), waste feeding and withdrawal(EFW), re-circulation of reactor content (ER) and heating (EH) which includes energy forheating the substrate (EHS) as well as energy for maintaining constant thermophilictemperature (EMT). The production of energy (EP) is only from the methane produced.Method for calculation of each type of these energies has been given in the followingsection.

The net energy production NEP (MJ) is the difference between the energy produced andconsumed by the process that is shown in Figure 1 and given by the following equation:

NEP = EP - ES – EFW – ER – EH

Figure E-1 Energy inputs and output of inclined anaerobic digester

1.1 Energy Production

The energy production EP (MJ) equivalent to methane and hydrogen content of theproduced biogas can be calculated by the equation below.

EP = (MP X L.H.V. of CH4) + (HP X L.H.V. of H2)

whereMP = methane production (L CH4)L.H.V. of CH4= 0.03618 MJ/L CH4 (Ruggeri et al., 2010)HP = Hydrogen production (L H2)L.H.V. of H2 = 0.0108 MJ/L H2 (Ruggeri et al., 2010)

1.2 Energy Consumption

Energy consumption has been described under sub-categories as follows:

1.2.1 Energy required for shredding (ES)

The assumption here is that about 100 kg of waste is shredded by the use of 0.5 L gasoline.

FeedingWithdrawalRecirculation

ShreddingHeating

Biogas

132

ES (MJ) = 0.5 L (Petrol) x weight of waste (kg) x 34.8111100 kg (waste)

where

34.8111 MJ = 1L Petrol

1.2.2 Energy required for feeding and withdrawal (EFW)

EFW (MJ) = P x T x 3.6whereP = Motor power (kW)T = Duration of operation (h)3.6 MJ = 1 kWh

1.2.3 Energy required for recirculation (ER)

It is calculated using the same formula as EFW.

1.2.4 Energy required for heating (EH)

a. Energy required for heating the substrate (EHS)

This is the energy required to heat influent substrate. The energy consumed for the heatingof substrate for each organic loading rate has been measured. The total heat energy (EH,MJ) required to raise the temperature of waste from its ambient value (To i.e. 28 °C,average ambient temperature of Pathumthani province) to the reactor’s temperature Ti (50°C) can be calculated by equation below.

EH (MJ) = M x Cp x (Ti - To)1000

whereM = Weight of waste in killograms

Cp = Specific heat of feed (kJ/kg.°C)1000 = Factor to change kJ to MJThe influent had TS and moisture content of about 15% and 85% respectively. Therefore,for calculation of EH, Cp of water (i.e. 4.186 kJ/kg.°C) has been used for 85% moistureand Cp of peat (i.e. 1.88 kJ/kg.°C) has been used for 15% solids. The reason for using Cpof peat is that the solids in my reactor feed have 80% volatile solids or organic matter andare similar in nature to peat which is the soil formed of partially decayed plants.

b. Energy for maintaining thermophilic temperature (EMT)

This is energy required to maintain the reactor temperature more than ambient temperature(i.e. at 50 °C). Since the temperature of the reactor was kept constant, therefore EMT isequal to the heat energy lost through the walls of the reactor. Heat transfer rate or heat loss(Q in Joule/second) from the cylindrical surface of the reactor (a composite wall consistingof three layers namely stainless steel, cotton and Al/PE foam insulation) was calculatedusing the following formula (Cengel, 2003 p.149).

133

Heat transfer rate or heat loss (Q) from the top and bottom bases of the reactor wascalculated using the following formula (Cengel, 2003 p.134).

where

Ti = Temperature inside the reactor (°C)T0 = Temperature outside the reactor (°C)r1 = Radius of cylinder w.r.t first layer (stainless steel) of composite wallr2 = Radius of cylinder w.r.t second layer (cotton) of composite wallr3 = Radius of cylinder w.r.t third layer (Al/PE foam insulation) of composite wallr4 = Radius of cylinder w.r.t outer surface of third layer of composite wallL = Length of cylinderK1, K2 and K3 = thermal conductivities of layers 1, 2 and 3hi = Heat transfer coefficient from inside reactor to first layerho = Heat transfer coefficient from last layer to airL1, L2 and L3 are thicknesses of layers 1, 2 and 3A = Surface area of the base of cylinder

=1/hiA + L1/K1A + L2/K2A + L3/K3A + 1/h0 A

Ti - T0Q =

1/hi(2πr1L) + ln (r2/r1)/K1(2πL) + ln (r3/r2)/K2(2πL) + ln (r4/r3)/K3(2πL) + 1/h0(2π4L)

Q =

Ti - T0

134

Appendix F

Sample Calculation of GHG Emission Potential

135

Calculation of GHG Emission Potential of Digestate

The formula for methane emission potential of digestate is given as:

1000)1())12/16(((

/4

OXRFMCFDOCDOCTSM

digestatekgCHgemissionMethane

F

and

GHG emission (gCO2-eq/kg digstate) = Methane emission (g CH4/kg digestate) x 25

whereM = Mass of digestate = 1 kgTS = Total solid content of digestate = 0.1045DOC = Carbon content of digestate in TS = 0.356DOCF = Fraction of DOC dissimilated. The model is described as 0.014T+0.28, at

35C, the value is computed as 0.77.MCF = Methane correction factor for open dumpsite of < 5 m depth = 0.4F = Fraction of methane in landfill gas = 0.5R = Recovered methane (kg) = 0OX = Oxidation factor = 0

Thus,

1000)12/16(5.04.077.0356.01045.01

/

digestatekgmethaneofgemissionMethane

= 7.6 g CH4/kg digestate

GHG emission of digestate = 7.6 (g CH4/kg digestate) x 25 = 190 gCO2-eq/kg digstate

Since, 0.7335 kg digestate was produced by digester for 1 kg waste fed, therefore, theGHG emission potential of digestate per unit weight of waste fed will be:

GHG emission potential of digestate obtained from 1 kg waste fed = 190 x (0.7335/1)= 139 gCO2-eq/kg waste

The weights of material decreased at every step of its management, which were used forGHG emission potential calculation and are given as follows:

Waste fed 1 kgDigestate 0.7335 kgStored digestate 0.7335 kgDewatered-stored digestate 0.153 kgStored-cured digestate 0.123 kg