《固体废物处理与资源化》课程教学资源（文献资料）The anaerobic digestion process of biogas production from food waste - Prospects and constraints

Bioresource Technology Reports 8(2019)100310 Contents lists available at ScienceDirect REPORT Bioresource Technology Reports ELSEVIER journal homepage:www.journals.elsevier.com/bioresource-technology-reports The anaerobic digestion process of biogas production from food waste: Prospects and constraints Sagor Kumar Pramanik",Fatihah Binti Suja,Shahrom Md Zain",Biplob Kumar Pramanik. Department of Civil and Structural Engineering Faculty of Engineering,Universiti Kebangsaan Malaysia,43600 UKM Bangi,Selangor,Malaysia School of Engineering,RMIT University,Melbourne,VIC 3000,Australia ARTICLE INFO ABSTRACT Keywords: The unrestrained release of huge quantities of food waste (FW)has become a significant concern because it Anaerobic digestion causes intensive environmental pollution.However,FW is a proper substrate that can be treated by anaerobic Biogas digestion (AD)due to its excellent biodegradability and high-water content.Studies have demonstrated that the Co-digestion AD process of tumning FW into biogas is an effective solution for FW treatment.This manuscript reviews the Food waste characteristics of FW,the biological process and biochemical reaction involved in the AD process,various op- Pre-treatment erational parameters and classification of the AD process,and the co-digestion and pre-treatment of the AD process for biogas production.Both co-digestion and pre-treatment processes could improve the FW hydrolysis rate and methane production.However,further improvement of this technology is required to assess its eco- nomic feasibility.Challenges and future perspectives of biogas production from FW are also discussed to improve the performance of AD technology. 1.Introduction 157,154,74.7,51,and 44 kg per person in Australia,America,Japan, Germany,United Kingdom,India,and China,respectively. The quantity of food losses and waste has grown enormously in the Since FW creates harmful impacts at the environmental level,ap- past few years because of the rapid growth of the world economy and propriate management and treatment of FW have become the major population.It is estimated that approximately 33.3%of food produced objective of numerous countries across the world.One of the main globally for human consumption is lost or wasted through the food environmental impacts of FW is associated with the embedded carbon supply chain(i.e.,1.6 gigatons of food per year)which has a production from the earlier life cycle phases of food before it became waste. value of $750 billion (Ma and Liu,2019;Slorach et al.,2019).The Moreover,activities related to food production such as agriculture waste of food is a non-productive use of scarce resources (land,water (including land-use change),processing,manufacturing,transportation, and fertiliser)and leads to environmental degradation (Gokarn and storage,refrigeration,distribution and retail have an embedded GHG Kuthambalayan,2017;Slorach et al.,2019).Baroutian et al.(2018) effect (Papargyropoulou et al.,2014).The same study reported that reported that the total amount of FW produced by a single person is agriculture is associated with approximately 22%of all GHG emissions 160-295 kg/year all over the world.Food is wasted during the food compared to livestock production (about 18%).The final disposal of FW supply chain.The chain includes agricultural production,processing, in landfills has also given rise to major environmental pollutions.Clercq distribution,consumption and post-harvest handling stages(Gustavsson et al.(2017)showed that considerable amounts of GHG including et al,2011;Papargyropoulou et al.,2014).It can be noted that de- methane (CH4)and carbon dioxide(CO2)are produced when FW is veloped countries tend to have major losses (70 to 80%)associated with disposed of in landfills.They showed that the emission of GHG into the the retail and consumer stages,whereas food wastage is higher at the atmosphere contributes to global warming;methane is a potent GHG immediate post-harvest stages in developing countries (Gokarn and with a greenhouse effect that is 25 times more powerful than CO2. Kuthambalayan,2017).Papargyropoulou et al.(2014)revealed that the Slorach et al.(2019)reported that global food loss and waste generate quantity of food losses and waste not only fluctuated between devel- annually 6.7%of total anthropogenic GHG emissions.Papargyropoulou oped and developing countries but also varied in low-income countries et al.(2014)demonstrated another environmental effect associated having poor producers and customers.A recent report developed by with FW,which is the disturbance of the biogenic phases of phosphorus Magnet (2018)noted that annual FW generation reached 361,278, and nitrogen,applied as fertilisers in agriculture. Corresponding author. E-mail address:biplob.pramanik@rmit.edu.au (B.K.Pramanik). https:/doi.org/10.1016/.biteb.2019.100310 Received 9 July 2019;Received in revised form 19 August 2019;Accepted 20 August 2019 Available online 21 August 2019 2589-014X/@2019 Elsevier Ltd.All rights reserved

Contents lists available at ScienceDirect Bioresource Technology Reports journal homepage: www.journals.elsevier.com/bioresource-technology-reports The anaerobic digestion process of biogas production from food waste: Prospects and constraints Sagor Kumar Pramanika , Fatihah Binti Sujaa , Shahrom Md Zaina , Biplob Kumar Pramanikb,⁎ a Department of Civil and Structural Engineering, Faculty of Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b School of Engineering, RMIT University, Melbourne, VIC 3000, Australia ARTICLE INFO Keywords: Anaerobic digestion Biogas Co-digestion Food waste Pre-treatment ABSTRACT The unrestrained release of huge quantities of food waste (FW) has become a significant concern because it causes intensive environmental pollution. However, FW is a proper substrate that can be treated by anaerobic digestion (AD) due to its excellent biodegradability and high-water content. Studies have demonstrated that the AD process of turning FW into biogas is an effective solution for FW treatment. This manuscript reviews the characteristics of FW, the biological process and biochemical reaction involved in the AD process, various op￾erational parameters and classification of the AD process, and the co-digestion and pre-treatment of the AD process for biogas production. Both co-digestion and pre-treatment processes could improve the FW hydrolysis rate and methane production. However, further improvement of this technology is required to assess its eco￾nomic feasibility. Challenges and future perspectives of biogas production from FW are also discussed to improve the performance of AD technology. 1. Introduction The quantity of food losses and waste has grown enormously in the past few years because of the rapid growth of the world economy and population. It is estimated that approximately 33.3% of food produced globally for human consumption is lost or wasted through the food supply chain (i.e., 1.6 gigatons of food per year) which has a production value of $750 billion (Ma and Liu, 2019; Slorach et al., 2019). The waste of food is a non-productive use of scarce resources (land, water and fertiliser) and leads to environmental degradation (Gokarn and Kuthambalayan, 2017; Slorach et al., 2019). Baroutian et al. (2018) reported that the total amount of FW produced by a single person is 160–295 kg/year all over the world. Food is wasted during the food supply chain. The chain includes agricultural production, processing, distribution, consumption and post-harvest handling stages (Gustavsson et al., 2011; Papargyropoulou et al., 2014). It can be noted that de￾veloped countries tend to have major losses (70 to 80%) associated with the retail and consumer stages, whereas food wastage is higher at the immediate post-harvest stages in developing countries (Gokarn and Kuthambalayan, 2017). Papargyropoulou et al. (2014) revealed that the quantity of food losses and waste not only fluctuated between devel￾oped and developing countries but also varied in low-income countries having poor producers and customers. A recent report developed by Magnet (2018) noted that annual FW generation reached 361, 278, 157, 154, 74.7, 51, and 44 kg per person in Australia, America, Japan, Germany, United Kingdom, India, and China, respectively. Since FW creates harmful impacts at the environmental level, ap￾propriate management and treatment of FW have become the major objective of numerous countries across the world. One of the main environmental impacts of FW is associated with the embedded carbon from the earlier life cycle phases of food before it became waste. Moreover, activities related to food production such as agriculture (including land-use change), processing, manufacturing, transportation, storage, refrigeration, distribution and retail have an embedded GHG effect (Papargyropoulou et al., 2014). The same study reported that agriculture is associated with approximately 22% of all GHG emissions compared to livestock production (about 18%). The final disposal of FW in landfills has also given rise to major environmental pollutions. Clercq et al. (2017) showed that considerable amounts of GHG including methane (CH4) and carbon dioxide (CO2) are produced when FW is disposed of in landfills. They showed that the emission of GHG into the atmosphere contributes to global warming; methane is a potent GHG with a greenhouse effect that is 25 times more powerful than CO2. Slorach et al. (2019) reported that global food loss and waste generate annually 6.7% of total anthropogenic GHG emissions. Papargyropoulou et al. (2014) demonstrated another environmental effect associated with FW, which is the disturbance of the biogenic phases of phosphorus and nitrogen, applied as fertilisers in agriculture. https://doi.org/10.1016/j.biteb.2019.100310 Received 9 July 2019; Received in revised form 19 August 2019; Accepted 20 August 2019 ⁎ Corresponding author. E-mail address: biplob.pramanik@rmit.edu.au (B.K. Pramanik). Bioresource Technology Reports 8 (2019) 100310 Available online 21 August 2019 2589-014X/ © 2019 Elsevier Ltd. All rights reserved. T

S.K.Pramanik,et al. Bioresource Technology Reports 8 (2019)100310 There are several old technologies for treating different types of methane yields.Bong et al.(2018)reported that fruit and vegetable wastes (e.g.,animal manure,household food waste,agricultural crop waste has low lipid content but relatively high cellulosic content, residue,and organic industrial waste)around the world.Historical whereas FW and kitchen waste have high lipid content because of the evidence indicates that,the anaerobic digestion process is one of the presence of animal fat and oil.Studies have reported that fruit and oldest technologies.Granado et al.(2017)reported that the concept of vegetable waste had a lipid content of 11.8%,whereas FW and kitchen anaerobic digestion has been introduced around 1870 through the de- waste were reported to have 33.22%and 21.6%of lipid content,re- velopment of the septic tank system.In 1939,the first anaerobic di- spectively (Wang et al,2014;Yong et al.,2015).Y.Li et al.(2017a) gestion plant was constructed in the USA for treating organic fraction of reported that FW rich in lipids could produce a higher amount of me- municipal solid waste,whereas several anaerobic digestion plants were thane compared to carbohydrates and proteins.However,high lipid constructed over the last few decades in Europe (Karthikeyan et al., content can cause system failure due to the formation of long-chain 2018).The development of anaerobic digestion technology has im fatty acids.This occurs when the mass transformation of soluble or- proved very rapidly after the energy crisis in the 1970s (Deepanraj ganics into bacteria decreases due to the destruction of the cellular et al.,2014).Currently,anaerobic digestion technology is being used membrane (Leung and Wang,2016).Y.Li et al.(2017a)stated that FW not only for the treatment of organic wastes but also for the treatment rich in carbohydrate content will affect the carbon and nitrogen ratio of wastewater.For installing large-scale biogas plants,Germany and (C/N),and thus,nutrient restrictions and quick acidification could Switzerland are the pioneer countries in the global biogas industry occur due to increased organic matter (Karthikeyan et al.,2018).The United States has about 2100 current The total solids and volatile solids of each type of FW fall in the operational anaerobic digestion plants,whereas Asia has the largest ranges of 10.7%-41%and 10%-34.4%,respectively (Table 1),in- number of small-scale household anaerobic digesters that are used in dicating that water accounts for 60%-90%in fruit and vegetable waste, rural areas for lighting and cooking (Vasco-correa et al.,2018).They kitchen waste and FW.FW is considered to be a readily biodegradable also pointed out that socio-economic hurdle,existing infrastructure, organic substrate because of its large quantity of moisture content policymakers,technology availability and consistency are the main (Zhang et al.,2014).The characteristics of FW also define the relative differences in the development of anaerobic digestion plants around the quantities of organic carbon and nitrogen present in the FW.The C/N world. ratio for each type of FW was found to vary in the range of 12.7-28.84, AD is an excellent alternative for FW treatment compared to waste and displayed an acidic pH of 4.1-6.5 (Table 1).The methane pro- treatment,energy supply,and environmental protection (Leung and duction of every category of FW fall in the range of 346-551.4 mL/ Wang,2016).There are numerous benefits related to the AD process g VSadded (Table 1),which is higher compared to cow manure (233 mL/ such as decreased GHG emission,digestate for application in agronomy, g VSadded),grass silage (306 mL/g VSadded),and oat straw (203 mL/ small footprint production,and the generation of high-quality renew- g VSadded)(Huttunen and Rintala,2007). able fuel (Ariunbaatar,2014).However,the drawbacks of the AD process such as relatively high capital costs,long retention time,and 3.Biological process and biochemical reaction involved in the AD the required control of certain key parameters (e.g.,pH,temperature, process feed rate,alkalinity)prevents it from being widely implemented (Ariunbaatar,2014).Herein,the objective of this paper is to review the Anaerobic digestion is a biological process,which breaks down characteristics of FW,the biological process and biochemical reaction complex organic matter into simpler chemicals components in the ab- involved in the AD process,various operational parameters and clas. sence of oxygen.During this process,a gas that is mainly composed of sification of the AD process,and the co-digestion and pre-treatment of CH4 and CO2,also referred to as biogas,is produced as the end products the AD process for biogas production.This paper also discusses the under ideal conditions.A minor quantity of hydrogen sulphide (H2S). challenges and future perspectives of enhancing biogas production from ammonia (NH3),and other gases are also present when biogas is pro- FW and maintaining the AD process effectively. duced in the AD plant (Monnet,2003).The AD process can be divided into four stages i.e.hydrolysis,acidogenesis,acetogenesis,and metha- 2.Characteristics of FW nogenesis,as it is a multi-step biochemical process (Zhang et al.,2014). In the AD procedure,various kinds of bacteria degrade the organic FW is characterised by complex components and organic material. substance continuously in a multi-step method and via parallel reac- There are several types of FW such as fruit and vegetable waste, tions.Microorganisms play an essential role in the AD process,and the household and restaurant FW,brewery waste,and dairy waste (Xu bacterial groups are dissimilar among the phases of hydrolysis,acid. et aL,2018).Studies have found that the composition of FW varies ification,and methane production (P.Wang et al.,2018).Li et al. based on geographical changes,seasonal changes,cooking procedures, (2015)investigated the relationship between microbial community and consumption patterns (Meng et al.,2015;Xu et al.,2018).They structure and process stability and compared the microbial community reported that FW consists of various organic components such as pro- structure in both stable and deteriorative phase using 454-pyr- teins,carbohydrate polymers (starch,cellulose,hemicelluloses,and osequencing.They reported that bacteria are responsible for the de- lignin),lipids,and organic acids.Fisgativa et al.(2016)studied 102 gradation of FW to intermediate metabolites which can be later used by different FW samples and reported that the characteristic of FW dis- methanogens.They concluded that acid-producing bacteria such as played high coefficient of variance (CV).They indicated that the var- Acholeplasma and Actinomyces increased dramatically at deteriorative iations of 24%of the studied characteristics were described by the phase compared with stable stage,which may be the failure indicator in geographical change,seasonal change and the type of collection source. anaerobic digester treating FW.In hydrolysis,insoluble complex poly- They observed that FW has an average pH of 5.1(CV 13.9%),carbon mers comprising carbohydrates,proteins,lipids,and other organics are and nitrogen ratio of 18.5%(CV 31.8%),36%of carbohydrates (CV converted into smaller soluble molecules.It can be noted that hydro- 57.2%),26%of protein (CV 62.2%),15%of fats (CV 52.0%),and lysis is a comparatively slow stage and therefore can limit the rate of the biomethane potential of 460.0 NL CH4/kg VS(CV 19%) entire AD process,especially when FW is used as the feedstock(Kothari As mentioned in Table 1,the percentage range of degradable car- et al.,2014;Leung and Wang,2016;Ostrem,2004;A.Zhang et al. bohydrates,proteins,and lipids is (5.7%-53%),(2.3%-28.4%),and 2015;Zhang et al.,2014).In acidogenesis,monomers and dissolved (1.3%-30.3%),respectively.Meng et al.(2015)and Xu et al.(2018) compounds such as sugars,amino acids,and fatty acids resulting from indicated that carbohydrates and proteins have a higher hydrolysis rate hydrolysis are converted into simple molecules with a small molecular due to its rapid degradability compared to lipid.Thus,quickly de- weight such as volatile fatty acids(ie.,propionic,butyric,acetic acid), gradable carbohydrates and lipid-rich food wastes can produce high alcohols,and different kinds of gases (CO2,H2,and NH3)(A.Zhang

There are several old technologies for treating different types of wastes (e.g., animal manure, household food waste, agricultural crop residue, and organic industrial waste) around the world. Historical evidence indicates that, the anaerobic digestion process is one of the oldest technologies. Granado et al. (2017) reported that the concept of anaerobic digestion has been introduced around 1870 through the de￾velopment of the septic tank system. In 1939, the first anaerobic di￾gestion plant was constructed in the USA for treating organic fraction of municipal solid waste, whereas several anaerobic digestion plants were constructed over the last few decades in Europe (Karthikeyan et al., 2018). The development of anaerobic digestion technology has im￾proved very rapidly after the energy crisis in the 1970s (Deepanraj et al., 2014). Currently, anaerobic digestion technology is being used not only for the treatment of organic wastes but also for the treatment of wastewater. For installing large-scale biogas plants, Germany and Switzerland are the pioneer countries in the global biogas industry (Karthikeyan et al., 2018). The United States has about 2100 current operational anaerobic digestion plants, whereas Asia has the largest number of small-scale household anaerobic digesters that are used in rural areas for lighting and cooking (Vasco-correa et al., 2018). They also pointed out that socio-economic hurdle, existing infrastructure, policymakers, technology availability and consistency are the main differences in the development of anaerobic digestion plants around the world. AD is an excellent alternative for FW treatment compared to waste treatment, energy supply, and environmental protection (Leung and Wang, 2016). There are numerous benefits related to the AD process such as decreased GHG emission, digestate for application in agronomy, small footprint production, and the generation of high-quality renew￾able fuel (Ariunbaatar, 2014). However, the drawbacks of the AD process such as relatively high capital costs, long retention time, and the required control of certain key parameters (e.g., pH, temperature, feed rate, alkalinity) prevents it from being widely implemented (Ariunbaatar, 2014). Herein, the objective of this paper is to review the characteristics of FW, the biological process and biochemical reaction involved in the AD process, various operational parameters and clas￾sification of the AD process, and the co-digestion and pre-treatment of the AD process for biogas production. This paper also discusses the challenges and future perspectives of enhancing biogas production from FW and maintaining the AD process effectively. 2. Characteristics of FW FW is characterised by complex components and organic material. There are several types of FW such as fruit and vegetable waste, household and restaurant FW, brewery waste, and dairy waste (Xu et al., 2018). Studies have found that the composition of FW varies based on geographical changes, seasonal changes, cooking procedures, and consumption patterns (Meng et al., 2015; Xu et al., 2018). They reported that FW consists of various organic components such as pro￾teins, carbohydrate polymers (starch, cellulose, hemicelluloses, and lignin), lipids, and organic acids. Fisgativa et al. (2016) studied 102 different FW samples and reported that the characteristic of FW dis￾played high coefficient of variance (CV). They indicated that the var￾iations of 24% of the studied characteristics were described by the geographical change, seasonal change and the type of collection source. They observed that FW has an average pH of 5.1 (CV 13.9%), carbon and nitrogen ratio of 18.5% (CV 31.8%), 36% of carbohydrates (CV 57.2%), 26% of protein (CV 62.2%), 15% of fats (CV 52.0%), and biomethane potential of 460.0 NL CH4/kg VS (CV 19%). As mentioned in Table 1, the percentage range of degradable car￾bohydrates, proteins, and lipids is (5.7%–53%), (2.3%–28.4%), and (1.3%–30.3%), respectively. Meng et al. (2015) and Xu et al. (2018) indicated that carbohydrates and proteins have a higher hydrolysis rate due to its rapid degradability compared to lipid. Thus, quickly de￾gradable carbohydrates and lipid-rich food wastes can produce high methane yields. Bong et al. (2018) reported that fruit and vegetable waste has low lipid content but relatively high cellulosic content, whereas FW and kitchen waste have high lipid content because of the presence of animal fat and oil. Studies have reported that fruit and vegetable waste had a lipid content of 11.8%, whereas FW and kitchen waste were reported to have 33.22% and 21.6% of lipid content, re￾spectively (Wang et al., 2014; Yong et al., 2015). Y. Li et al. (2017a) reported that FW rich in lipids could produce a higher amount of me￾thane compared to carbohydrates and proteins. However, high lipid content can cause system failure due to the formation of long-chain fatty acids. This occurs when the mass transformation of soluble or￾ganics into bacteria decreases due to the destruction of the cellular membrane (Leung and Wang, 2016). Y. Li et al. (2017a) stated that FW rich in carbohydrate content will affect the carbon and nitrogen ratio (C/N), and thus, nutrient restrictions and quick acidification could occur due to increased organic matter. The total solids and volatile solids of each type of FW fall in the ranges of 10.7%–41% and 10%–34.4%, respectively (Table 1), in￾dicating that water accounts for 60%–90% in fruit and vegetable waste, kitchen waste and FW. FW is considered to be a readily biodegradable organic substrate because of its large quantity of moisture content (Zhang et al., 2014). The characteristics of FW also define the relative quantities of organic carbon and nitrogen present in the FW. The C/N ratio for each type of FW was found to vary in the range of 12.7–28.84, and displayed an acidic pH of 4.1–6.5 (Table 1). The methane pro￾duction of every category of FW fall in the range of 346–551.4 mL/ g VSadded (Table 1), which is higher compared to cow manure (233 mL/ g VSadded), grass silage (306 mL/g VSadded), and oat straw (203 mL/ g VSadded) (Huttunen and Rintala, 2007). 3. Biological process and biochemical reaction involved in the AD process Anaerobic digestion is a biological process, which breaks down complex organic matter into simpler chemicals components in the ab￾sence of oxygen. During this process, a gas that is mainly composed of CH4 and CO2, also referred to as biogas, is produced as the end products under ideal conditions. A minor quantity of hydrogen sulphide (H2S), ammonia (NH3), and other gases are also present when biogas is pro￾duced in the AD plant (Monnet, 2003). The AD process can be divided into four stages i.e. hydrolysis, acidogenesis, acetogenesis, and metha￾nogenesis, as it is a multi-step biochemical process (Zhang et al., 2014). In the AD procedure, various kinds of bacteria degrade the organic substance continuously in a multi-step method and via parallel reac￾tions. Microorganisms play an essential role in the AD process, and the bacterial groups are dissimilar among the phases of hydrolysis, acid￾ification, and methane production (P. Wang et al., 2018). Li et al. (2015) investigated the relationship between microbial community structure and process stability and compared the microbial community structure in both stable and deteriorative phase using 454-pyr￾osequencing. They reported that bacteria are responsible for the de￾gradation of FW to intermediate metabolites which can be later used by methanogens. They concluded that acid-producing bacteria such as Acholeplasma and Actinomyces increased dramatically at deteriorative phase compared with stable stage, which may be the failure indicator in anaerobic digester treating FW. In hydrolysis, insoluble complex poly￾mers comprising carbohydrates, proteins, lipids, and other organics are converted into smaller soluble molecules. It can be noted that hydro￾lysis is a comparatively slow stage and therefore can limit the rate of the entire AD process, especially when FW is used as the feedstock (Kothari et al., 2014; Leung and Wang, 2016; Ostrem, 2004; A. Zhang et al., 2015; Zhang et al., 2014). In acidogenesis, monomers and dissolved compounds such as sugars, amino acids, and fatty acids resulting from hydrolysis are converted into simple molecules with a small molecular weight such as volatile fatty acids (i.e., propionic, butyric, acetic acid), alcohols, and different kinds of gases (CO2, H2, and NH3) (A. Zhang S.K. Pramanik, et al. Bioresource Technology Reports 8 (2019) 100310 2

S.K.Pramanik,et al. Bioresource Technology Reports 8(2019)100310 Table 1 Characteristics and methane potential of some food waste reported in literatures. Source TS(%)VS(%)VS/TS (% C/N ratio pH Carbohydrates(%)Proteins (%Lipids(%) Methane yield (mL/g VS) Reference KW 24.9 23.1 92.8 18.24 49 17.3 23 501 J.Jiang et al.(2018) FVW 13.8 12.88 93.4 4.5 7.74 3.28 2.87 516 Edwiges et al (2018) FW 20 19.26 96.3 15.5 47.6 24.1 28.3 548.1 Li et al.(2018) 19.2 18.45 96.1 12.7 41.3 28.4 30.3 541.1 20.9 20.02 95.8 14 38.4 26.3 25.3 545.3 FW 10.86 10.22 94 15.18 4.16 5.71 2.29 1.31 460 Xiao et al (2019) 25.94 24.59 94.7 17.5 48 15.1 10.6 346.2 Shi et al.(2018) w 10.69 10.06 4.18 5.69 2.29 1.30 477-459 Xiao et al.(2018) 24.3 22.5 16 23.11 5.02 3.38 386.7-551.4 Liu et al.(2017) FW 19.1 18.53 97 17.7 10.8 4 37 536.19 Y.Li et al.(2017a) 17.2 16.72 97.2 13.4 1 2.9 441.23 20.5 19.78 96.5 17.8 3.6 5.3 531.30 19.7 19.05 96.7 16.8 10.1 53737 19.6 18.91 96.5 15.4 9.3 533.01 20.0 19.26 96.3 15.5 544.95 AFW 41 34.44 4 经 25 Naroznova et aL (2016) VFW 24 22.32 93 53 5 14 425 19.1 17.80 93.2 14.41 4.5 11.8 2.5 3.5 372.1 Li et al.(2016a) W 23.2 21.7 93.5 4.4 13.7 2.9 6.5 425.2 W.Zhang et al.(2015) 20.05 19.21 95.81 28.4 33.22 14.03 25.25 381 Yong et al.(2015) FW 29.4 28.01 95.3 14.2 4.1 18.1 19 529 Browne and Murphy (2013) FW 18.1 17.1 94 13.2 6.5 11.2 3.3 2.3 479.5 Zhang et al.(2011) Note:FVW:fruit and vegetable waste,KW:kitchen waste,FW:food waste,AFW:animal food waste,VFW:vegetable food waste. et al.,2015).In acetogenesis,acetogenic bacteria use volatile fatty acids utilisation rates,and bacterial development (Khalid et al.,2011).The for their growth and the growth of these bacteria is slow with a dou same study reported that cell energy fatigue and intracellular substance bling time of 1.5 to 4 days (Kothari et al.,2014).The concentration of leakage could occur due to this lower temperature.They also indicated products created in this phase differs according to the type of bacteria that AD operating under mesophilic conditions offers higher stability as well as culture circumstances such as temperature and pH(Ostrem, and requires lower energy cost compared to the thermophilic condition. 2004).Methanogenesis is the final step in the anaerobic digester,where AD operating under thermophilic condition provides several benefits the methanogens use acetic acid,hydrogen,and carbon dioxide to such as the higher growth of methanogenic bacteria at a higher tem- produce methane gas.Most methanogen bacteria require an optimum perature,reduced retention time,destruction of pathogens,enhanced pH range between 6.5 and 7.5 (Leung and Wang,2016).The four steps digestibility and better degradability of solid substrates,besides dif- of anaerobic biodegradation process such as hydrolysis,acidogenesis, ferentiating liquid and solid portions (Dobre et al,2014;A.Zhang acetogenesis,and methanogenesis,and the bacteria involved in each et al.,2015).However,the drawbacks of the thermophilic condition stage of the AD process,are schematically displayed in Fig.1. must be considered.There are several demerits of the thermophilic condition such as a greater amount of disproportion and higher energy 4.Parameters affecting the AD process of biogas production requirement because of the associated high temperature (A.Zhang etal.,2015) Different categories of bacteria are engaged in the AD process.The bacteria need an environment that has reached equilibrium to produce 4.2.pH and VFA biogas from FW.Leung and Wang(2016)reported that a stable process could be achieved if the bacteria stayed in an ideal condition.Fluc pH is the most significant parameter that affects the performance tuations in environmental conditions could interrupt the micro- and stability of an anaerobic digester.Microorganisms are sensitive to organism's equilibrium,which could perhaps impede or even close pH.This is because every group of bacteria needs a different pH range down the AD process(Ostrem,2004).Therefore,operational conditions for their growth (Appels et al.,2008).The ideal pH range for hydrolysis. (such as temperature,pH and VFA,the ratio of carbon and nitrogen, acetogenesis,and methanogenesis is almost 6.0,6.0-7.0,and 6.5-7.5 retention time,and organic loading rate)in the AD process need to be respectively (Leung and Wang,2016).Gerardi (2003)reported that the continuously observed and maintained within optimum ranges.The pH required for acid-forming bacteria and methane-forming bacteria effect and optimised range of these parameters on biogas production is>5.0 and 6.2,respectively,for acceptable enzymatic activity.Me- are explained in the following section. thanogenic bacteria display better performance in a pH range of 6.8-7.2 (Yadvika et al.,2004).CH4 production was found to be 75%more ef- 4.1.Temperature ficient with a pH of>5.0 (Yadvika et al.,2004).Krishna and Kalamdhad (2014)indicated that other main factors contribute to the Methane production is highly influenced by temperature,which fluctuation of pH such as alkalinity,volatile fatty acid (VFA),the affects bacterial performance within an anaerobic digester.Both me- quantity of CO2 production,and the concentration of bicarbonate thanogenic and volatile acid-forming microorganisms are affected by (HCO)during the AD process.They reported that the relationship temperature.Changes in temperature extensively influence the perfor between VFA and HCO3 concentrations should be controlled,as it helps mance of methanogenic microorganisms compared to the operating in adjusting the optimum pH during the AD process.For a stable and temperature (Gerardi,2003).The AD process can take place at various well-buffered digestion process,it is important to maintain at least temperatures,which are normally classified into three types,i.e.psy. 1.4:1 (molar ratio)of HCO3/VFA or a buffering capacity of chrophilic,mesophilic,and thermophilic temperatures.The mesophilic 70 meg CaCO3/L (Appels et al.,2008).Volatile fatty acids are short- and thermophilic temperature ranges are between 20 and 40C(usually chain fatty acids (acetic acid,propionic acid,butyric acid,and valeric 35'C)and 50 and 65'C(typically 45'C),respectively (Kothari et al., acid),which are the primary intermediate products produced from the 2014).A lower temperature reduces methane production,FW AD of FW (Zhang et al.,2014).Xu et al.(2014)and Shi et al.(2018)

et al., 2015). In acetogenesis, acetogenic bacteria use volatile fatty acids for their growth and the growth of these bacteria is slow with a dou￾bling time of 1.5 to 4 days (Kothari et al., 2014). The concentration of products created in this phase differs according to the type of bacteria as well as culture circumstances such as temperature and pH (Ostrem, 2004). Methanogenesis is the final step in the anaerobic digester, where the methanogens use acetic acid, hydrogen, and carbon dioxide to produce methane gas. Most methanogen bacteria require an optimum pH range between 6.5 and 7.5 (Leung and Wang, 2016). The four steps of anaerobic biodegradation process such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis, and the bacteria involved in each stage of the AD process, are schematically displayed in Fig. 1. 4. Parameters affecting the AD process of biogas production Different categories of bacteria are engaged in the AD process. The bacteria need an environment that has reached equilibrium to produce biogas from FW. Leung and Wang (2016) reported that a stable process could be achieved if the bacteria stayed in an ideal condition. Fluc￾tuations in environmental conditions could interrupt the micro￾organism's equilibrium, which could perhaps impede or even close down the AD process (Ostrem, 2004). Therefore, operational conditions (such as temperature, pH and VFA, the ratio of carbon and nitrogen, retention time, and organic loading rate) in the AD process need to be continuously observed and maintained within optimum ranges. The effect and optimised range of these parameters on biogas production are explained in the following section. 4.1. Temperature Methane production is highly influenced by temperature, which affects bacterial performance within an anaerobic digester. Both me￾thanogenic and volatile acid-forming microorganisms are affected by temperature. Changes in temperature extensively influence the perfor￾mance of methanogenic microorganisms compared to the operating temperature (Gerardi, 2003). The AD process can take place at various temperatures, which are normally classified into three types, i.e. psy￾chrophilic, mesophilic, and thermophilic temperatures. The mesophilic and thermophilic temperature ranges are between 20 and 40 °C (usually 35 °C) and 50 and 65 °C (typically 45 °C), respectively (Kothari et al., 2014). A lower temperature reduces methane production, FW utilisation rates, and bacterial development (Khalid et al., 2011). The same study reported that cell energy fatigue and intracellular substance leakage could occur due to this lower temperature. They also indicated that AD operating under mesophilic conditions offers higher stability and requires lower energy cost compared to the thermophilic condition. AD operating under thermophilic condition provides several benefits such as the higher growth of methanogenic bacteria at a higher tem￾perature, reduced retention time, destruction of pathogens, enhanced digestibility and better degradability of solid substrates, besides dif￾ferentiating liquid and solid portions (Dobre et al., 2014; A. Zhang et al., 2015). However, the drawbacks of the thermophilic condition must be considered. There are several demerits of the thermophilic condition such as a greater amount of disproportion and higher energy requirement because of the associated high temperature (A. Zhang et al., 2015). 4.2. pH and VFA pH is the most significant parameter that affects the performance and stability of an anaerobic digester. Microorganisms are sensitive to pH. This is because every group of bacteria needs a different pH range for their growth (Appels et al., 2008). The ideal pH range for hydrolysis, acetogenesis, and methanogenesis is almost 6.0, 6.0–7.0, and 6.5–7.5, respectively (Leung and Wang, 2016). Gerardi (2003) reported that the pH required for acid-forming bacteria and methane-forming bacteria is > 5.0 and 6.2, respectively, for acceptable enzymatic activity. Me￾thanogenic bacteria display better performance in a pH range of 6.8–7.2 (Yadvika et al., 2004). CH4 production was found to be 75% more ef- ficient with a pH of > 5.0 (Yadvika et al., 2004). Krishna and Kalamdhad (2014) indicated that other main factors contribute to the fluctuation of pH such as alkalinity, volatile fatty acid (VFA), the quantity of CO2 production, and the concentration of bicarbonate (HCO3) during the AD process. They reported that the relationship between VFA and HCO3 concentrations should be controlled, as it helps in adjusting the optimum pH during the AD process. For a stable and well-buffered digestion process, it is important to maintain at least 1.4:1 (molar ratio) of HCO3/VFA or a buffering capacity of 70 meq CaCO3/L (Appels et al., 2008). Volatile fatty acids are short￾chain fatty acids (acetic acid, propionic acid, butyric acid, and valeric acid), which are the primary intermediate products produced from the AD of FW (Zhang et al., 2014). Xu et al. (2014) and Shi et al. (2018) Table 1 Characteristics and methane potential of some food waste reported in literatures. Source TS (%) VS (%) VS/TS (%) C/N ratio pH Carbohydrates (%) Proteins (%) Lipids (%) Methane yield (mL/g VS) Reference KW 24.9 23.1 92.8 18.24 – 49 17.3 23 501 J. Jiang et al. (2018) FVW 13.8 12.88 93.4 4.5 7.74 3.28 2.87 516 Edwiges et al. (2018) FW 20 19.26 96.3 15.5 – 47.6 24.1 28.3 548.1 Li et al. (2018) 19.2 18.45 96.1 12.7 – 41.3 28.4 30.3 541.1 20.9 20.02 95.8 14 – 38.4 26.3 25.3 545.3 FW 10.86 10.22 94 15.18 4.16 5.71 2.29 1.31 460 Xiao et al. (2019) FW 25.94 24.59 94.7 17.5 – 48 15.1 10.6 346.2 Shi et al. (2018) FW 10.69 10.06 94 – 4.18 5.69 2.29 1.30 477–459 Xiao et al. (2018) FW 24.3 22.5 92.6 23.11 5.02 – 3.38 – 386.7–551.4 Liu et al. (2017) FW 19.1 18.53 97 17.7 – 10.8 4 3.7 536.19 Y. Li et al. (2017a) 17.2 16.72 97.2 13.4 – 9.4 4.5 2.9 441.23 20.5 19.78 96.5 17.8 – 11 3.6 5.3 531.30 19.7 19.05 96.7 16.8 – 10.1 4.3 4.7 537.37 19.6 18.91 96.5 15.4 – 9.3 4.6 5 533.01 20.0 19.26 96.3 15.5 9.2 4.6 5.5 544.95 AFW 41 34.44 84 – – 52 12 25 – Naroznova et al. (2016) VFW 24 22.32 93 – – 53 5 14 425 KW 19.1 17.80 93.2 14.41 4.5 11.8 2.5 3.5 372.1 Li et al. (2016a) FW 23.2 21.7 93.5 4.4 13.7 2.9 6.5 425.2 W. Zhang et al. (2015) FW 20.05 19.21 95.81 28.4 – 33.22 14.03 25.25 381 Yong et al. (2015) FW 29.4 28.01 95.3 14.2 4.1 – 18.1 19 529 Browne and Murphy (2013) FW 18.1 17.1 94 13.2 6.5 11.2 3.3 2.3 479.5 Zhang et al. (2011) Note: FVW: fruit and vegetable waste, KW: kitchen waste, FW: food waste, AFW: animal food waste, VFW: vegetable food waste. S.K. Pramanik, et al. Bioresource Technology Reports 8 (2019) 100310 3

S.K.Pramanik,et al. Bioresource Technology Reports 8(2019)100310 Food Waste Complex polymers PH-S.5-60 Carbohydrates Proteins Lipids Clostridium,Acctivibrio, Clostridium,Proteus Clostridium. Fermentative Staphylococcus,Bacteroides Peptococcus,Vibrio. Micrococcus, Bacteroides,Bacillus Staphylococcus bacteria PH-6.0-7.0 Acidogenesis Monomers Sugars Amino acids LCFA Clostridium. Lactobacillus,Escherichia.Staphylococcus. Acidogenic Eubacterium limosum. Micrococcus,Bacillus,Sarcina,Veillonella, Streptococcus Pscudomonas.Desulfovibrio,Desulfuromonas. bacteria Desulfobacter,Selenomonas,Streptococcus VFAs and Clostridium,Syntrophobacter PH-6.0-7.0 Acetogenesis alcohols Syntrophomonas Acetogenic Clostridium,Syntrophobacter,Syntrophomonas bacteria Acetate oxidizing bacteria Acetate H2,C02 Homoacetogenic bacteria Methanobacterium,Methanocalculus Methanosarcina,Methanospirillum, Methanogenic PH-6.57.5 Methanogenesis Methanobrevibacter,Methanoplanus Methanosacta Methanoregula,Methanococcus, archaea Methanoculleus Acetotrophic CH4,CO2 Hydrogenotrophic methanogens methanogens Fig.1.Four biological steps and the respective bacterial groups engaged in every phase of the AD process.Information collected from Deepanraj et al.(2014), Gonzalez-Fernandez et al.(2015),Kothari et al.(2014),Leung and Wang (2016),and P.Wang et al.(2018). reported that methane production was inhibited completely when the for an effective AD process (Kothari et al.,2014).Zhang et al.(2014) concentrations of VFA fell in the range of 5800 to 6900 mg/L. reported that the C/N ratio greatly influences the stability of the AD process.This is because the optimal C/N ratio not only helps to main- 4.3.Carbon and nitrogen ratio tain a suitable environment,but it also helps to control proper nutrient balance through the development of microorganisms.The microbial The C/N ratio represents the relationship between the quantity of population could increase gradually if the quantity of nitrogen is low in carbon and nitrogen present in FW.An optimum C/N ratio is required the FW,and thus,more time will be required to decompose the existing

reported that methane production was inhibited completely when the concentrations of VFA fell in the range of 5800 to 6900 mg/L. 4.3. Carbon and nitrogen ratio The C/N ratio represents the relationship between the quantity of carbon and nitrogen present in FW. An optimum C/N ratio is required for an effective AD process (Kothari et al., 2014). Zhang et al. (2014) reported that the C/N ratio greatly influences the stability of the AD process. This is because the optimal C/N ratio not only helps to main￾tain a suitable environment, but it also helps to control proper nutrient balance through the development of microorganisms. The microbial population could increase gradually if the quantity of nitrogen is low in the FW, and thus, more time will be required to decompose the existing Food Waste VFAs and alcohols Carbohydrates Lipids Proteins Sugars Amino acids LCFA Acetate H2, CO2 CH4, CO2 Clostridium, Proteus, Peptococcus, Vibrio, Bacteroides, Bacillus Clostridium, Acetivibrio, Staphylococcus, Bacteroides Clostridium, Micrococcus, Staphylococcus Lactobacillus, Escherichia, Staphylococcus, Micrococcus, Bacillus, Sarcina, Veillonella, Pseudomonas, Desulfovibrio, Desulfuromonas, Desulfobacter, Selenomonas, Streptococcus Clostridium, Eubacterium limosum, Streptococcus Clostridium, Syntrophobacter, Syntrophomonas Clostridium, Syntrophobacter, Syntrophomonas Methanosarcina, Methanospirillum, Methanosaeta Methanobacterium, Methanocalculus, Methanobrevibacter, Methanoplanus, Methanoregula, Methanococcus, Methanoculleus Fermentative bacteria Acidogenic bacteria Acetogenic bacteria Methanogenic archaea pH= 5.5-6.0 Hydrolysis Acidogenesis pH= 6.0-7.0 Methanogenesis pH= 6.5-7.5 pH= 6.0-7.0 Acetogenesis Complex polymers Monomers Acetate oxidizing bacteria Homoacetogenic bacteria Acetotrophic methanogens Hydrogenotrophic methanogens Fig. 1. Four biological steps and the respective bacterial groups engaged in every phase of the AD process. Information collected from Deepanraj et al. (2014), Gonzalez-Fernandez et al. (2015), Kothari et al. (2014), Leung and Wang (2016), and P. Wang et al. (2018). S.K. Pramanik, et al. Bioresource Technology Reports 8 (2019) 100310 4

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Table 2 Comparison of different anaerobic digestion process for food waste. Process Reactor volume and type Inoculum Operating condition Methane yield (mL/ g VS) VS removal (%) Reference Single-stage 2 L semi-continuous CSTR, mesophilica Anaerobic sludge OLR = 5 g VS/L day, HRT = 20 days, pH = 7.7 494 74.7 Jo et al. (2018) Two-stage 0.5 L and 2 L semi-continuous CSTR, mesophilic OLR = 4 g VS/L day, HRT = 25 days, pH = 7.5 511 78.9 Single-stage 230 L CSTR, thermophilicb Anaerobic sludge OLR = 3.5 kg VS/m3 day, HRT = 20 days 450 93.6 Micolucci et al. (2018) Two-stage 200 and 760 L CSTR, thermophilic 550 96.2 Single-stage 4 L semi-continuous CSTR, thermophilic Mesophilic anaerobic sludge HRT = 30 days, pH = 8.31, TVFA = 0.87 g HAc/L 477 83.22 Xiao et al. (2018) Two-stage 2 L and 8 L semi-continuous CSTR, thermophilic HRT = 30 days, pH = 3.83 and 8.17, TVFA = 1.64 and 1.08 g HAc/L 459 82.02 Single-stage 1 and 20 L semi-continuous, mesophilic Anaerobic seed sludge OLR = 1.6–10 g VS/L, pH = 5.1–7.8 199 30.1 Zhang et al. (2017) Two-stage 249 44.2 Three-stage 307 83.5 Single-stage 5 m3 reactor, mesophilic Anaerobic sludge OLR = 3.79 kg VS/m3/day, pH = 7.32 380 96 Grimberg et al. (2015) Two-stage OLR = 0.78 kg VS/m3/day, pH = 5.2, 8.4 446 93 Single-stage 6 L CSTR, mesophilic Mesophilic seed sludge OLR = 2.4 g VS/L/day, pH = 7.77, HRT = 30 days 440 74.1 Wu et al. (2015) Temperature-phase two￾stage 1.5 L thermophilic CSTR, 6 L mesophilic CSTR OLR = 14.2 and 2.6 g VS/L/day, pH = 5.36 and 7.59, HRT = 3 and 12 days 440 80.1 Mesophilic 3 L continuous CSTR Anaerobic seed sludge OLR = 7.75 g VS/L/day, HRT = 10 days, pH = 7.64 350 – Q. Li et al. (2017) Thermophilic OLR = 5.19 g VS/L/day, HRT = 15 days, pH = 7.86. 407 Mesophilic 500 mL laboratory-scale bottle (semi￾continuous) Mesophilic sewage sludge OLR = 1.5 g VS/L/day, HRT = 20 days 371 94.7 Liu et al. (2017) Thermophilic Thermophilic sewage sludge OLR = 2.5 g VS/L/day, HRT = 20 days 541 93 Batch 75 L upflow anaerobic reactor, mesophilic Seed sludge OLR = 6.1 kg COD/m3/day, HRT = 30 days, pH = 7.5 266a 80.9b Park et al. (2018) Continuous OLR = 7.9 kg COD/m3/day, HRT = 13 days, pH = 7.5 326a 71.2b Batch 1 L glass digesters, mesophilic Anaerobic activated sludge OLR = 8 g VS/L, pH = 7.5 388 – Zhang et al. (2013) Continuous OLR = 10 g VS/L, pH = 7.1–7.7 317 Wet 6.0 L continuous reactor, mesophilic Mesophilic seed sludge OLR = 2.35 kg VS/m3/day, SRT = 20 days, pH = 7.39 370 80.1 Yi et al. (2014) Dry OLR = 9.41 kg VS/m3/day, SRT = 20 days, pH = 7.82 480 85.6 a L/kg COD. b COD removal. S.K. Pramanik, et al. Bioresource Technology Reports 8 (2019) 100310 5