Journal of Environmental Chemical Engineering 2 (2014) 1821–1830

Pre-treatment and anaerobic digestion of food waste for high ratemethane production – A review

Dhamodharan Krishna *, Ajay S. KalamdhadDepartment of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India

A R T I C L E I N F O

Article history:Received 20 February 2014Accepted 30 July 2014

Keywords:Food waste anaerobic digestionFood waste pre-treatmentAnaerobic reactorsBiogas from wastes

A B S T R A C T

Food waste with high decomposition potential can be successfully digested anaerobically for theproduction of biogas. As the fossil-fuel reserves decline anaerobic digestion can be a better alternative asa renewable energy source. The byproducts such as biogas with 50–60% methane content can beefficiently used for electricity production and the final digested sludge as a fertilizer. Even thoughanaerobic digestion is a proven technology, still there exist some technical difficulties (organic loadingrate, solid retention time, biogas composition, specific gas production) and scientific understandings(carbon to nitrogen ratio, volatile fatty acids production, pH variation, nutrient concentration) inoperating reactors for solid organic wastes. First the paper gives an overview of certain fundamentalaspects of anaerobic digestion considered important for the digestion of food waste and its biochemicalreactions. Then it describes food waste as the substrate for anaerobic digestion and its optimal conditionsfor the increased activity of biogas production. Finally it has been reviewed about the performance of thedifferent pre-treatment methods and anaerobic reactor configurations in the digestion of food waste forincreasing methane content in the biogas.

ã 2014 Elsevier Ltd. All rights reserved.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1821Anaerobic digestion process and biochemical reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1822Food waste as a substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823Optimum conditions required for anaerobes metabolic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824

Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824Carbon–nitrogen ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824Volatile fatty acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824Organic loading rate and solid retention time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824Nutrient concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825

Biochemical methane potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825Biogas composition and specific gas production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825

Pre-treatment processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825Thermal pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825Mechanical pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826Ultrasonic treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826Ozonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826

Abbreviations: AD, anaerobic digestion; FW, food waste; CH4, methane; CO2, carbon dioxide; MSW, municipal solid waste; TS, total solids; VS, volatile solids; COD,chemical oxygen demand; F/I, food to inoculum; C/N, carbon to nitrogen; HRT, hydraulic retention time; SRT, solids retention time; OLR, organic loading rate; UASB, upflowanaerobic sludge blanket; UASS, upflow anaerobic solid state reactor; SS-AD, solid state-AD; VFA, volatile fatty acids; H2, hydrogen; NH3, ammonia; H2S, hydrogen sulfide;BMP, biochemical methane potential; NaHCO , sodium bicarbonate; NaOH, sodium hydroxide; KOH, potassium hydroxide; Mg(OH) , magnesium hydroxide; Ca(OH) ,

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering

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calcium hydroxide.* Corresponding author. Tel.: +91 361 2582431; fax: +91 361 2582440.E-mail address: krishna.120187@gmail.com (D. Krishna).

http://dx.doi.org/10.1016/j.jece.2014.07.0242213-3437/ã 2014 Elsevier Ltd. All rights reserved.

1822 D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830

Alkali pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826Biological pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826Co-digestion of wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826

Anaerobic reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826Single and two-stage anaerobic reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1827Semi-dry AD process of organic solid waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828Upflow anaerobic solid state reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828Solid state anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828Hybrid bioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1828

Introduction

AD consists by a number of complex biochemical reactionscarried out by several types of microorganisms that require little orno oxygen to survive. During this process, a gas that is mainlycomposed of CH4 and CO2, also referred to as biogas, is produced.The amount of gas produced varies with the amount of organicwaste fed to the digester and temperature influences the rate ofdecomposition and gas production. The overall conversion processof complex organic matter into CH4 and CO2 can be divided intofour steps: hydrolysis, acidification, acetogenesis and methano-genesis. Biological processes like composting and AD haveadvantages because they are natural treatment processes overother technologies using microorganisms, which need less energyinput; and less harm to atmosphere over other technologies suchas incineration, pyrolysis, etc. AD has distinctive potential to actsimultaneously as waste treatment and resource recovery process.AD revealed an admirable life cycle analysis enactment paralleledto some other treatment technologies like composting orincineration as it can recover the energy. In addition, the residuesare stable and hence a compost potential for agriculture [1].

There is a major concern in reducing and recycling waste asmuch as possible during the treatment process with respect toboth energy and materials. AD is an old and currently developingtechnology that is considered promising for the future of gasproduction and waste reduction [2]. AD is one of the processesadopted for the treatment and recycling of MSW. Although, ADhas both benefits of waste treatment and energy generation, itincludes higher residence time and the inability of anaerobes tocompletely utilize every organics such as high lignin percentagewaste and highly insoluble organic polymers in MSW within theresidence time because of its lower hydrolysis rate [3]. Due to itscapability of reducing waste, high calorific value biogas produc-tion and pathogens free final product, AD of solid organic wastehas earned much attention [4]. AD earns many points than othertreatment process, with respect to global warming which plays amajor role in ecological balance [5,6]. AD is preferentially suitedfor high moisture content or semi-solid organic materials. ADtests performed with semi-solid and pasty proteins and lipidsproducts from slaughterhouses, pharmaceutical, food, beverageindustries, distilleries and municipal bio-wastes. Batch AD testwas conducted with the variety of substrates such as animal fat,flotation sludge, stomach and gut contents, blood, food leftoversshowed a biogas yield ranging from 0.3 to 1.36 L/g of VS added andfor the same substrates continuous AD test performed, HRTranging between 12 and 60 d according to the organic materialsadded, process temperature of 35 �C, was maintained to attainmaximum gas yield [7].

Roughly one third of all food produced for human consump-tion is wasted yearly totaling 1.3 billion tones, as reported byglobal food waste published in 2011 by the Food and AgricultureOrganization of the United Nation [8]. However this waste is

evenly distributed between developing and industrializednations with 40% of the FW in the developing nations occurringin the production and processing phases of consumption while inthe industrialized nations, 40% occurs at the retail and consumerlevels of consumption. When such amount of FW is digestedanaerobically it has the potential to generate 367 m3 of biogas perdry tonne at about 65% CH4 with an energy content of6.025 �10�9 T W h/m3, much amount of energy can be recovered.In addition, transportation of FW to landfills and greenhouse gasemissions from the landfill sites will be reduced by implementingAD. Thus AD of food waste needs few pre-treatments andoptimum conditions for better digestion and to recover maximumbiogas due to its readily available organics and highly complex oilcontents. The present paper is a review of various pre-treatmentstechniques, optimum conditions and types of anaerobic reactorsto enhance the bio-methane production from FW.

Anaerobic digestion process and biochemical reactions

When the anaerobic digester works properly, the conversion ofthe intermediate products (i.e., the products of the first three steps)is virtually complete, so that the concentrations of these are low atany time. In the hydrolysis process, macro molecules like proteins,polysaccharides and fats that compose the cellular mass of themicroorganisms are converted into smaller molecules that aresoluble in water: peptides, saccharides and fatty acids. Thehydrolysis or solubilization process is done by exo-enzymesexcreted by fermentative bacteria. Hydrolysis is a relatively slowprocess and generally it limits the rate of the overall AD process.Polymers are transformed into soluble monomers throughenzymatic hydrolysis.

n C6H10O5 þ n H2O !Hydrolysisn C6H12O6 (1)

The Reaction (1) is catalyzed by extracellular microbial enzymesknown as hydrolyses or lyses. Depending on the type of the reactionthey catalyze, these hydrolyses can be esterase, glycosidase orpeptidase. For example, lipases hydrolyze the ester bonds of lipids toproduce fatty acids and glycerol. Lyses, on the other side, catalyze thenon-hydrolytic removal of groups from substrates [9]. The majorclass of anaerobic bacteria degrading cellulose include Bacterioidessuccinogenes,Clostridium lochhadii, Clostridium cellobioporus, Rumi-nococcus flavefaciens, Ruminococcus albus, Butyrivibrio fibrosolvens,Clostridium thermocellum, Clostridium stercorarium and Micro-monospora bispora. The dungs of various animals such as cow,pig, etc. has been used as an inoculum in anaerobic digestion of foodwaste. The anaerobes present in the dungs belong to the digestivesystem of the species. The predominant bacteria found to degradethe hemicelluloses in the rumen are Bacterioides ruminicola.B. fibrisolvens, R. flavenfaciens, and R. albus[9]. The second step ofthe AD process is acidogenesis or acidification as given in Reaction(2), a process that results in the conversion of the hydrolyzed

D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830 1823

products into simple molecules with a low molecular weight, likeVFA (e.g., acetic-, propionic- and butyric acid), alcohols, aldehydesand gases like CO2, H2 and NH3. Acidification is affected by a verydiverse group of bacteria, the majority of which are strictlyanaerobic, i.e., the presence of oxidants like oxygen or nitrate istoxic. The presence of oxygen utilizing bacteria is important toremove all oxygen that might be introduced into the system. Theacidogenic bacteria are able to metabolize organic material down toa very low pH of around 4. Fig. 1 details about the anaerobicdigestion of organic materials.

The monomers results of first reaction become substrates forthe microorganisms in the second stage where they are convertedinto organic acids by a group of bacteria. In the third step,acetogenesis, the products of the acidification are converted intoacetic acids, hydrogen, and CO2 by acetogenic bacteria. The firstthree steps of AD are often grouped together as acid fermentation.It is important to note that in the acid fermentation, no organicmaterial is removed from the liquid phase: it is transformed into aform suitable as substrate for the subsequent process ofmethanogenesis.

nC6H12O6 !Acid form bacteria3nCH3COOH (2)

In the final step of the AD process as in Reaction (3), theproducts of the acid fermentation (mainly acetic acid) areconverted into CO2 and CH4 by acetoclastic methanogens. Onlythen will organic material be removed, as the produced methanegas will largely desorb from the liquid phase. In each of the foursequential steps, the catabolic reactions described above developtogether with anabolic activity. The free energy exhausted from thereactions is partially utilized for synthesis of the anaerobespopulations. Therefore, a large fraction of the digested organicmatter is converted into biogas. These organic acids primarilyacetic acid form the substrate for the third-stage

Fig. 1. Anaerobic digestion processes for recovery of biogas.

CH3COOH !Methane forming bacteriaCH4 þ CO2 (3)

In the third step, CH4 can be generated in two routes, fermentingacetic acid to CH4 and CO2 by acetoclastic methanogens and usingCO2 as a source of carbon and hydrogen as a reducing agent byhydrogenotrophic methanogens or format generated by otherbacterial species as given in Reactions (4–6) [10]. The mostcommonly found methanogens genera, in the biogas reactors areMethanobacterium, Methanothermobacter (formerly Methanobacte-rium), Methanobrevibacter, Methanosarcina, and Methanosaeta(formerly Methanotrix) [11,12].

CO2 þ 4H2 !ReductionCH4 þ 3H2O (4)

Similarly CO2 can be hydrolyzed to carbonic acid and to methane

CO2 þ H2O !HydrolysisH3CO3 (5)

4H2 þ H2CO3 !ReductionCH4 þ 3H2O (6)

The CO2 in the biogas are undesirable. They are removed foroptimum performance of biogas as fuel. CO2 is removed bypassing the gas into lime water which turns milky due toformation of calcium carbonate. H2S is also another undesirable inthe biogas due to presence of sulphate in the substrate. But thepercentage of H2S present in biogas varies according to sulphatepercentage.

Food waste as a substrate

The degradability rate of the substrate greatly depends on thecomposition of the input material considered for the digestion. It isalso very difficult to estimate or measure the percentage ofcarbohydrates, lipids, and proteins in a heterogeneous substratesuch as FW due to the dynamic and sensitive nature of thebiological process to the input composition. There are severaltechniques available to estimate the BMP of the complex substratesuch as FW including ultimate analysis (carbon, nitrogen andhydrogen), molecular formula (if known), computer simulation,and a literature review of experimentally determined biogas yields[7,8,13,14]. As waste analysis is one of the most important steps inthe AD process, knowing the general composition of the inputmaterial to the system is essential for calculating the amount andcomposition of the biogas produced as well as the amount ofenergy contained in the biogas.

Cho et al. [13] did a laboratory experiment on BMP and solidstate AD of korean food wastes. The study concluded that, themethane yield and anaerobic biodegradability of Korean MFW arehigh: 472 mL CH4/g VS added and 86%, respectively and to performsolid-state anaerobic digestion of Korean food wastes discharged at15–30% TS successfully, the VFAs produced rapidly at the initialstage of fermentation need to be controlled using a two-phasedigestion method.

Kim et al. [14] evaluated the effect of temperature and hydraulicretention time on anaerobic digestion of food waste in three stagereactor in which Clostridium species were isolated and again wasinoculated to find the better performance. The results showed thatthe 12 d HRT in thermophilic condition produced maximummethane yield (223 L CH4/kg soluble coddegraded) and provides abest way for the anaerobic digestion of food waste.

Liu et al. [11] stated the effect of F/I ratio on biogas yields of foodwaste, green waste and their mixture in thermophilic andmesophilic conditions. The results showed a negative relationbetween biogas yield and the F/I ratios in the range of 1.6–5.0. At50 �C and an F/I of 1.6, the biogas yields of 778, 631 and 716 mL/g VScould be obtained, respectively, for food waste, green waste andtheir mixture after 25 d of digestion. The author concluded that the

1824 D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830

biogas yield was much better in thermophilic conditions compareto mesophilic conditions.

Optimum conditions required for anaerobes metabolic activity

To maintain anaerobes with a high metabolic activity, it isnecessary to have controlled environmental conditions. Themethanogens are very sensible towards adverse environmentalconditions so it is very important to maintain optimal conditionsfor these microbes. The rate of biogas production depends: thenature of the substrate, TS, temperature, pH, toxicity, mixing,nutrients, slurry concentration, retention time, digester type,carbon to nitrogen ratio, alkalinity, initial feeding, total VS, COD,etc. Russell [15] has stated that the concentration of water-solublesubstances such as sugar, amino acid, proteins and mineralsdecrease with age of plants. This is because that non-water solublesubstances like lignin, cellulose, hemi cellulose and polyamidesincrease in content with the age of the plant. Therefore thevegetative matter from younger plants produces more biogascompared to those from the older plants. For waste products fromanimals, the type and age of animal, its feeding and livingconditions, the age and storage of the waste product are factorsaffecting the quality and quantity of the gas produced. Generallyfinely ground waste products is said to produce more biogas due tolarge surface area of contact with microbes.

Temperature

Anaerobic process is so sensible to temperature; change ofacetic acid to methane depends mostly on temperature butconversion to acetic acid will not affect much by slight temperaturevariations. Speece [16] has reported that the environmentaltemperature has a major influence on the anaerobic microbialsystems, which affects the metabolic rate, ionization equilibria,substrate solubility and fats and bioavailability of iron. Highertemperature affects the activity of hydrogenotropic methanogensin the anaerobic process and enriches hydrogen producing bacteriaand spore forming bacteria [17]. The optimum temperature foranaerobic digestion is represented in Table 1.

pH

The pH of the reactor affects the AD process and efficiency of thedigestion process. Methanogens work effectively between pHrange of 6.5–8.2, with an optimum pH 7.0 [22,23]. Although it hasbeen proven that the optimal range of pH for obtaining maximalbiogas yield in AD is 6.5–7.5, Liu and Fang [24] has reported that pHrange is relatively wide in plants and the optimal value of pH varieswith substrate and digestion technique. The pH varies because ofmany parameters such as VFA, bicarbonate concentration, andalkalinity of the system and also by fraction of CO2 produced duringthe process. To maintain constant pH value it is essential to controlthe relationship between the VFA and bicarbonate concentrations[25]. For controlling pH, NaHCO3 and NaOH were added in thecontinuous reactor during startup period [26].

Table 1Optimum conditions required for anaerobes metabolic activity.

Parameters Optimum conditions

Temperature Mesophilic range (35–40 �CThermophilic range (50–65

pH 6.3–7.8

Carbon–nitrogen ratio 25–30

Volatile fatty acid 2000–3000 mg/L

Organic loading rate (OLR) and nutrient concentration Varies according to the sub

Carbon–nitrogen ratio

Nitrogen is the major nutrient for the growth of microbes. Theamount of nitrogen uptake by the aerobic and anaerobic microbesvaries according to their nature. The review stated that bacteria inanaerobic digestion use the carbon 25–35 times higher thannitrogen. For better digestion, the ratio of the carbon to nitrogenshould be about 25–30:1 in the substrate. Nitrogen is one of themajor limiting nutrient in the treatment of MSW, due to that othersources such as manure, clean sewage-sludge (bio solids), and ureacan be used as a supplementarysource. If the nitrogen content is low,microbial populations remain less and it might take longer durationto digest the available carbon. Excess nitrogen, may also causeproblem of ammonia formation which affects the anaerobic process.For solid wastes with a high C/N ratio, the ammonia inhibition effectcan be controlled by dilution with water; it decreases theconcentration of ammonia toxicity. The concentrations of carbonand nitrogen indicate the performance of the anaerobic digestionprocess. Whereas carbon acts as the energy source of the micro-organisms and nitrogen plays a role in enhancing microbialpopulation. Nitrogen (nitrate) can be utilized in two ways by themicrobes in anaerobic digestion such as assimilatory (nitrate usedas electron acceptor converted to nitrogen gas also called asdenitrification) and dissimilatory (nitrate converted to ammoniaalso called as ammonification). More nitrate addition leads toammonification and less leads to nitrogen deficiency. By this waynitrogen controls the microbial population.

Volatile fatty acid

The different concentrations of VFA (equivalent to acetic acid) inan anaerobic batch system were shown to have a derivative effecton each phase of the hydrolysis, acidogenesis and biogasproduction related to the AD process. Apart from pH, VFA of thereactor inhibits the cellulolytic activity at concentrations �2 g/L.Therefore, the rate of cellulose hydrolysis and the glucosefermentation also gets affected above 4 g/L thereby havinginhibitory effect on the production of biogas. In addition, CH4 toCO2 ratio gives much difference above 6 g/L VFA if cellulose andglucose is used as major source [27].

In the co-digestion of cellulose waste material with paper asprimary substrate, biogas production was reduced more than a halfdue to 1 g/L initial VFA, showing inhibition of the hydrolysisprocess. In case of glucose as primary substrate biogas productionwas more than halved when VFA was above 8 g/L which indicatedthat the digestion process was less sensitive to inhibition by VFA[28]. The accumulation of VFA in specific place results in derangedmicrobial consortia, which leads to failure of the anaerobicdigestion process operation [29,30].

Organic loading rate and solid retention time

The relative residence time needed for full degradation and thelimit of anaerobic microbes to degrade the substrates can becarried out by BMP assay [31]. The investigation on the effects of

References

)�C)

van Haandel and Lettinga (1994) [19]; Arsova (2010) [18]

Xiaojiao et al. (2012) [20]Ghosh and Pohland (1974) [21]Eastman and Ferguson (1981) [22]

strate and inoculum –

D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830 1825

stepwise increase in OLR and SRT on integrated two-stage processstated that at steady state, the optimal OLR and SRT were found tobe 22.65 kg VS/m3d (160 h) for hydrogen fermentation reactor and4.61 (26.67 d) for methane fermentation reactor, respectively [32].Stable operation of single stage wet AD of FW at an OLR of 9.2 kg VS(15.0 kg COD)/m3/d was achieved with a high VS reduction (91.8%)and high methane yield (455 mL g/VS/d). Norio et al. [32] statedthat the cell density increased in the periods without organicloading, and reached to 10.9�1010 cells m/L after 187 d, which wasaround 15 times higher than that of the seed sludge.

Nutrient concentration

Trace inorganic elements such as iron, nickel, cobalt, and zinchave to be in desired concentrations to initiate the digestionprocess. Wastes are different in their nutrients concentration, sofor the proper function of AD an appropriate amount of inorganicelements have to be added. Finding the nutrient requirements canbe tough and it depends on characteristics of the waste, nutrientsavailability, the reactor design and other parameters [33].

Biochemical methane potential

BMP is the important and valuable assay for the interpretationof AD. In recent years, BMP tests have increased which is evidentfrom the broad band of research carried out with BMP assays [34].The BMP assay results of Korean mixed FW digestion incomparison to other individual FW like boiled rice, fresh cabbageand cooked meat digestion with cellulose as control has shownhigher methane production (472 mL/g VS) with 86% reduction oftotal VS [13]. From the analysis of FW collected in the city of SanFrancisco, California it was stated that the nutrient contents werewell balanced for the growth of anaerobic microorganisms for theproduction of methane and the methane yields of 348 and435 mL/g VS after 10 and 28 d digestion [36].

The AD of fruits and vegetable solid waste produced methanewith a minimum of 0.3 L/g VS added in every sample which showsthe commercial value in AD [36]. In the AD of Bermuda grass, theconversion of cellulose to simpler organic was higher thanhemicellulose by the supply of external nitrogen both inmesophilic and thermophilic conditions and it requires leastamount of enzymes and energy [37]. From the co-digestion of FWwith piggery wastewater suggested the addition of trace elementsincreased the biogas production in FW AD and results a highmethane yield of 0.396 m3/kg VS added and 75.6% VS destruction[25]. BMP experiment was conducted with four different foodwaste (meatball, chicken, cranberry and ice cream processingwastes) for co-digestion with flushed dairy manure at a ratio of3.2% food waste and 96.8% manure (by volume), which equated to14.7% (ice-cream) to 80.7% (chicken) of the VS being attributed tothe food waste to find the suitability for anaerobic digestion. Allexperiments showed increase in methane production from 67.0%(ice cream waste) to 2940% (chicken processing waste) comparedto digesting manure as single substrate. It proves high methanepotential for food waste additions compared to low methaneproduction potential of the flushed dairy manure [38].

Biogas composition and specific gas production

Chea etal. [39] reportedthat the upflowanaerobicfiltersattachedwith solid feed anaerobic produced 2 m3 of biogas per m3 of reactorvolume per day, whereas the conventional biogas digester produced0.1 m3–0.2 m3/m3 volume of digester volume per day. Ghangrekaret al. [40] investigated the AD of organic fractions of MSW in twophase reactor with continuous operation. During the reactor’s

startup period, the process was stable and therewas no occurrence ofinhibition as methane composition leveled off at 66% with higherrate of biogas production and the reactorwasfed in continuousmodeand methane content of the biogas reduced to 30–40%. Carrere et al.[41] evaluated the effectof different biogas production rates on UASBreactor performance and on the characteristics of the sludgedeveloped and observed that biogas yield higher than 0.7 m3/m3dwas sufficient to carry out natural mixing inside the reactor.However, very high biogas yield, greater than 2.3 m3/m3d wasobserved to be unfavorable for determining the requisite sludge ageand necessary strength of granules.

Pre-treatment processes

AD is now practiced widely for the volume reduction of sludgeand energy recovery. Several advantages such as low energyrequirement, low sludge production, low nutrient requirementsare advantages of AD. Also AD does not disturb the stabilizedheavy metals in the sludge and hence can be safely disposed intoland fill sites.

Different pre-treatments can be utilized for solid waste such asmechanical (ultrasound, high pressure and lysis), thermal, chemi-cal (ozonation, alkali) and biological pre-treatments [42,43]. Thesludge pre-treatment is possible to reduce the necessary retentiontime of sludge for digestion, the final quantity of the sludge andalso enhances biogas production [2,31].

The effects on biodegradability of substrate vary depending onits characteristics for example ligno-cellulose substrates such asagricultural waste, Paper and pulp waste requires more time for itshydrolysis which affects biodegradability however in case of easilyavailable organics such as food waste, glucose takes less time todegrade in compare. Pre-treatment effects for waste from foodindustry are mainly correlated with the organic wastes ofslaughterhouse because of its organic composition [5,44–47] orit is also correlated with waste from the dairy industry [48,49].Some of the pre-treatments have been in application are thermaland chemical, followed by ultrasonic and microwave pre-treat-ments. Particle-size reduction of waste from the food industry isinduced by chemical and ultrasonic pre-treatments, whilesolubilization results from all pre-treatment types applied.

Effects from the pre-treatment of waste from the food industryare varying and highly dependent on both the pre-treatmentmechanism and the substrate composition. Thermal pre-treatment(70 and 133 �C) and chemical (alkali) pre-treatments for slaughter-house waste was not found effective, because of its higherbiodegradable nature [5,45]. In contrast, Salminen et al. [46] founda significant increase in soluble COD from chemical (acid and alkali)as well as ultrasonic and low temperature thermal (70 �C) pre-treatment of different substrates from the meat processing industry.

The solubilizations due to pretreatment leads enhancedbiodegradability in few cases, but in some cases combinations ofpre-treatment and substrate the biodegradability gets decreaseddue to inhibitory intermediate formation [46]. Formation of toxiccompounds was also reported from the high temperature thermalpre-treatment (133 �C, >3 bar) [44].

Thermal pre-treatment

Bougrier and co-workers [50] reported the impacts of thermalpre-treatment on the AD of waste activated sludge. It was foundthat thermal treatment at 190 �C gave better degradation then at135 �C in terms of total COD, lipids, carbohydrates and proteinremovals and methane production. However, treatment at 190 �Cproduced refractory soluble COD. In every case, with or withoutpre-treatments, lipids degradation yield (67% without pre-treatment and 84% with 190 �C treatments) was higher than

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carbohydrates (56% without pre-treatment and 82% with 190 �Ctreatments) and proteins (35% without pre-treatment and 46%with 190 �C treatments) degradation yields. Methane productionwas seen to increase by 25% after the 190 �C treatment.

Thermal pre-treatment was initially used for sludge dewater-ability and allows the degradation of gel structure [51]. It also leadsto the solubilization of complex sludge particulates and enhancesAD [52–54]. Hydrolysis which is the rate limiting step can beenhanced and the retention time for AD could be reduced bythermal pre-treatment. An optimum temperature of 160–180 �Cand treatment duration of 30–60 min was reported for sewagesludge [55–57]. Pre-treatment at moderate temperature of 70 �Chas also been reported but it takes a longer pre-treatment time[58,59]. The enhancement in biogas production by thermal pre-treatment is related to COD solubilization [50]. Salminen andRintala [59] reviewed many pre-treatment techniques to increasemethane production from feather by combined thermal andenzymatic treatments and reported an increase in methane yield of37–51% whereas thermal (70–120 �C), chemical (NaOH: 2–10 g/L,2–24 h) and enzymatic treatments were not much efficient inyielding methane which increased only in the range of 5–32%.

Mechanical pre-treatment

Kouichi et al. [10] has reported that AD batch experimentsrevealed that particle size has been reduced by bead milling at1000 rpm increased the methane yield by 28% in comparison withother disposer treatment. FW depicted the mean particle size ofsubstrates ground decreased from 0.843 to 0.391 mm by the resultsof bead mill pre-treatment for particle size reduction andsolubilization increased approximately 40% of the total COD bybead milling. In order to increase the solubilization of organicmatter and reduce the size of the fleshings various pre-treatmentmethods were reported to increase the biogas recovery and toreduce retention time in the reactor. Mincing the fleshings withcutter and feeding it into conventional anaerobic digester is one ofthe studies reported by Shanmugam and Horan [4]. Pre-treatmentof the substrate by mechanical disintegration (size reduction ofparticles) had positive effects on the anaerobic biodegradability ofthe substrate. The obvious reason is the increase of the availablespecific surface to the medium [45].

Ultrasonic treatment

Ultrasonic treatment works similar to mechanical pretreatmentwhich disrupts the cell structure and flocs. Two mechanisms linkedwith ultrasonic treatment; cavitation, which happens at lowfrequencies, and chemical reactions due to the release ofOH�, HO2

�, H� radicals at high frequencies. In sludge treatment,low frequencies between 20–40 kHz were most effective [50].Ultrasound pre-treatment of sewage sludge at 31 kHz for 64 s hasreduced the retention time of AD from 22 to 8 d and a VS removal of44% as reported by Tiehm et al. [55]. This method has beenconsidered for sludge pre-treatment but the energy needed is high.Comparative study of the effect of ultrasonication on the anaerobicbiodegradability of food waste in single and two-stage systems wasconducted by Elsayed and George [60]. The results showed theultrasonication in first stage hydrogen reactor followed bymethane reactor yields more hydrogen (4.8 L H2/Lreactor) andmethane production (3.2 L CH4/Lreactor) [60].

Ozonation

Ozonation can be carried out as a pre-treatment for sludgeprior to AD [61]. Weemaes and Verstraete [62] reportedOzonation pre-treatment for mixed sludge which led to an

increase of CH4 production from 110 to 220 mL/g. CODin with aretention time of 30 d at a temperature of 33 �C. Ozonationpre-treatment of sewage sludge led to an increase of methaneproduction from 82 to 173 mL/g. CODin with a retention time of30 d [61]. Ozonation pre-treatment was not carried out for the FWdue to readily degradable organic waste and it’s not necessary incase of FW.

Alkali pre-treatment

Alkali treatment is one of the best practice for complex mattersolubilization, with the efficiency in order NaOH > KOH > Mg(OH)2and Ca(OH)2 [63]. But too high concentrations of Na+ or K+ maycause subsequent inhibition of AD [64]. Alkali pre-treatment hasalso been performed with combinations of thermal or ultrasonicpre-treatment. The studies by Yiying et al. [65] report thecombination of alkali and ultrasonic pre-treatment for wasteactivated sludge. The solubilization of COD levels was higher thanthose with ultrasonic or alkaline pre-treatment alone.

Biological pre-treatment

Hee and Song [66] investigated the effect of enzymaticsolubilization of FW and methane production in UASB reactor.The optimum conditions of FW hydrolysis were enzyme mixtureratio of 1:2:1 with carbohydrase:protease:lipase, respectively, 0.2%(w/w FW) of mixture dose, and 10 h hydrolysis reactions. Upto 95%of soluble COD removal efficiency with an observed methane gasyield of 350 mL-CH4/g-soluble COD and 9.1 g-soluble COD/L/d ofOLR. Raynal et al. [67] have evaluated that hydrolysis rate from theCOD removal in the anaerobic digester per unit time and expressedin grams COD/L d. However, few solid wastes have a lowbiodegradability in spite of the high COD content and, so furtherstudies needs to look in the enhancement of biomethenizationprocess of such wastes [64].

Co-digestion of wastes

For the co-digestion of a mixture of 70% manure, 20% FW and10% sewage sludge (TS concentration around 4%) at 36 �C, for anOLR of 1.2 g VS/L d the maximum value obtained was 603 L CH4/kgVS feed [68]. Different pre-treatment methods have beendiscussed above in order to enhance its biodegradability.Co-digestion is another concept employed in AD in which thesludge is mixed with another waste substrate with high organiccontent and requires disposal [69]. Xing et al. [42] reported thatco-treatment of sludge with other organic substrate can be doneto enhance the AD.

The co-digestion of fruit and vegetable waste and wasteactivated sludge in acidogenic continuous stirred tank reactors andmethanogenic inclined tubular digesters operated at 30 �C wereused. Optimized AD was reached in a two-stage system at an OLR of5.7 kg VS/m3/d, overall HRT of 13 d (3 d acidogenic HRT, 10 dmethanogenic HRT), with 40% VS reduction and a system biogasyield of 0.37 m3/kg VS added [70].

Anaerobic reactors

In past two decades much efforts has been committed towardsthe performance of AD especially in organic solid waste treatment.In the treatment of solid waste problem was confronted inhydrolysis process particularly in complex polymeric substances[9,23,35]. The applied solid content in association with thesubstrate loading rate is critical to the cost, performance andstability of anaerobic solid waste digesters [6,71]. In continuous AD

D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830 1827

of slaughterhouse or co-digestion of slaughterhouse waste withmanure, fruit and vegetable waste the specific CH4 yield was foundto be 351–381 mL CH4/g VS d like 270–350 mL CH4/g total VS inbatch. The CH4 production at 10% solids was less in comparison to5% solids, it is due to excess substrate or less buffering capacity[70]. The researchers used many reactors such as single and twostage reactor, upflow anaerobic solid state reactor, semi dryAnaerobic digester, solid state anaerobic digester and hybridreactors for the treatment of organic waste which is well suitablefor food waste treatment has been discussed below.

Single and two-stage anaerobic reactors

In single stage reactor, all the four process hydrolysis, acido-genesis, acetogenesis and methanogenesis has taken place in thesame reactor, i.e., conversion of polymeric organic compounds toCH4, H2S, NH3 and CO2 are observed to occur in the same reactor. Butin two-stage reactor, the Hydrolysis and acidogenesis take place ininitial reactor and the utilization of those acids by methanogenesis isanticipated to take place in the final reactor. The separation ofdigesters as hydrolyzers and methanizers changes the processdynamics of thedigesters byallowingtheindividual bacterialspeciesseparately to perform the hydrolysis and methanogenesis. It hasbeen found that the performance of two-stage anaerobic digesters isproviding more efficient than the single stage digesters [36,47,48,71–75]. Instead of that many other researchers Borja et al. [52], Onget al. [76], Vavilin et al. [77], Veeken et al. [78] and Viturtia-Mtz et al.[79] reported that single stage are much better than two stagedigesters particularly for proteinaceous waste. The amount NH3

produced need to be sufficient for neutralizing the accumulated VFAand then it forms ammonium acetate or ammonium bicarbonatemaintain favorable pH. More ammonia production must affect theprocess of methanogens by neutralizing all the VFA and affects CH4

content in biogas.Trouque and Forster [80] has reported that VS reduction in

3 dual digestion configurations were similar, but it was moreeffective than single stage and two-stage with VS reduction of 60%,however with single stage it was low. The production of methaneby hydrogenotropic methanogen using hydrogen produced duringhydrolysis process, this process was disturbed in two stagereactors, i.e., syntrophic relation gets affected between hydrogenformers and consumers which leads to reactor performancereduction [81]. Costello et al. [82,83] observed that the two-stagereactors maintains pH for proper functioning of both hydrolyticacidogenic and methanogenic bacteria at pH 6 and pH 7respectively. Kim et al. [84] explained that two phase reactorhas much better performance than the single phase reactorbecause it would not affect the slow growing methanogens bychanges in environmental conditions. The authors have denotedthat methane forming process was disturbed by lower tempera-ture which might be due to the unbalanced reaction rate betweenacetogens and methanogens and confirmed two-phase was more

Table 2Anaerobic digestion of food waste and its reactor configurations.

Sl. no. Waste type Reactor configuration Biogas yield

1 Food waste Batch solid state AD 472 mL CH4/2 Food waste Three stage anaerobic reactor 223 mL CH4/

coddegraded3 Food and green waste Batch reactor 716 mL/g VS

4 Dairy manure and foodwaste

Hybrid anaerobic solid–liquidbioreactor

302 mL/g VSmaterials

5 Food waste Single stage wet AD 455 mL/g VS

6 Food waste Leached bed reactor –

beneficial than the single-phase reactor; it is also proved in VSreduction efficiency.

Held et al. [85] stated that segregation of organic waste in twocommon fractions and two stage anaerobic process of liquidportion in an upflow anaerobic filter is a betting technology topresent technology. It was expressed that, CH4 content was higherin the second stage than the first stage Ince et al. [86] and Liu andFang [24] who reported that the single stage system requires 37 dHRT whereas the two-stage requires 30 d HRT for maximum CODremoval (80%). Mesophilic condition is recommended if two-phasesystem is used for proteinous waste like fish and meat and reportedthat NH3 toxicity was observed to be the rate-limiting step even atlow HRT continuous stirred tank reactor both in mesophilic andthermophilic condition [87]. Hansen et al. [88] reported that singlephase system is more advantageous than the two phase system asdenitrification and methanogenesis in a single reactor would beoptimum for the neutralizing capacity of VFA and further itprevents from the drop in pH and VFA toxicity.

Two-phase system can be considered to be more advantageousover the single stage under different climatic conditions, i.e., lowsummer and high winter. Furthermore, it can be stated that thehydrolysis of protein is the rate-limiting step for methogenesis. Thehydrolysis of lipids and carbohydrates increased with increase inHRT, whereas protein hydrolysis occurred only in methanogeniccondition and further revealed that hydrolysis was the rate limitingstep for conversion of carbohydrates, while both hydrolysis andacidification were the rate limiting step for conversion of protein,hence two phase system was suggested for the protein enrichedwastes Miron et al. [89]. Braun et al. [69] have studied thesuitability of two-phase system for fruit and vegetable waste andthe acetogenic and methanogenic reactor was expected to operatesuccessfully. The researchers have further reported that the stabletwo-phase system was achieved for vegetable and fruit waste withHRT of 3 d for acetogenic reactor and 10 d for methanogenicdigestion, which is again supporting the hydrolyzer to methanizer.

Yu et al. [90] reported that the efficient degradation of organicmatter is dependent on coordinate metabolism of acid forming andCH4 forming bacteria. It was emphasized that separating theoptimum condition for each bacterial group can increase theanaerobic process stability and overall degradation rate and hencethe two-phase system was considered more efficient than single-phase system. Costello et al. [82] has reported that themethanogenic phase outlet gives better quality in two phasesthan in the single phase AD for synthetic waste, which was alsosupported by Nozhevnikova et al. [91]. Shimizu et al. [92] that theoverall gas production rate of two phase process was four timesgreater than single phase process.

Kaul and Nandy [93] have reported that CH4 production ratefrom two-phase digesters was 7 times higher than that of theconventional single stage digester. Parkin and Owen [94] statedthat the phase separation of digester would only be feasible for thesubstrate where hydrolysis step is clearly the overall rate-limiting

Conditions maintained References

g VS added Mesophilic condition Cho and Park (1995) [13]g soluble Thermophilic conditions and varies

HRTKim et al. (2006) [14]

of mixture Thermophilic condition Guangqing et al. (2009)[11]

of fine Mesophilic condition Mashad and Zhang (2010)[98]

Mesophilic condition and 10.5 kgVS OLR

Norio et al. (2012) [32]

Mesophilic condition Xu et al. (2012) [99]

1828 D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830

step which was also confirmed by Miron et al. [89] who stated thathydrolysis of lipids and carbohydrates increased with increasingSRT, whereas protein hydrolysis only occurred under methano-genic conditions. Under methanogenic conditions, hydrolysis wasthe rate-limiting step in the whole digestion process. Tembhurkarand Mhaisalkar [95] experimented biomethanation of kitchenwaste suitably adopting two phase anaerobic treatment usinganaerobic fixed film fixed bed reactor. The acidogenesis is process(first phase) suitably incorporated a new approach wherein thekitchen waste was appropriately kept in submerged conditions inthe acidogenesis reactor to obtain leachates.

Semi-dry AD process of organic solid waste

Based on semi-dry (20% dry matter) anaerobic process, two full-scale industrial plants have reported to be installed in Italy underthermophilic condition. From the experiments conducted, it isreported that the plant is able to produce nearly 2.500 m3/d ofbiogas and 39 tons/d of digested sludge (10% dry matter) from500 tons of MSW. The dewatered digested sludge are then co-composted with parts of the fresh organic fraction to produce richcomposts (55% dry matter) [22,44].

Upflow anaerobic solid state reactor

The experimental results of the UASS concept equipped withliquid recirculation proved its feasibility in the digestion of solidwaste to methane gas. It had been shown that the UASS reactor’shydrolytic and methanogenic performance is amongst the highestreported for the digestion of solid biomass. When connected to ananaerobic filter, the methane production of the UASS reactor can bestabilized at OLRs as high as 17 g VS/L/d [96] and potentialdrawback of this was the restricted use for colloidal substancessuch as starch as they can lead to compaction and clogging of thesolid state bed.

Solid state anaerobic digestion

SS-AD has been more advantageous over liquid AD in variousaspects including smaller reactor volume, lower energy require-ments for heating, minimal material handling, and lower totalparasitic energy loss. Biogas production from SS-AD is more or lessequal to liquid AD. SS-AD generally occurs at solid concentrationshigher than 15%. In contrast, liquid AD handles feedstocks with solidconcentrations between 0.5% and 15%. Animal manure, sewagesludge, and food waste are generally treated by liquid AD, whileOrganic fraction of MSW and lignocellulosic biomass such as cropresidues and energy crops can be processed through SS-AD [32].

Hybrid bioreactors

Several different designs of hybrid reactors have been proposed.The majority of the laboratory and full scale examples of hybridreactors have been realized following a simpler design, where thefilter is located in the upper part of the reactor without any gas,solid, and liquid separation device. Studies have been undertakenon AD of the solid fraction of kitchen waste using this type ofreactor [97]. The anaerobic digestion of food waste done bydifferent researchers was given in Table 2.

Conclusion

The review suggests that AD is superior in treating the FW withits high energy recovery and controlled environmental issues. Themajor constraints associated with AD of FW for the production ofbiogas with high methane content have been reviewed. Based on

the outcome of laboratory studies by researchers on BMP, optimalconditions, pre-treatment methodologies and various reactoroperations for increased biogas production, the processes variesaccording to the nature of the substrate and active inoculumaddition. Finally the authors conclude that AD of FW is good for theconversion of waste to valuable biogas, still research need to befocussed on reactor design and degradation of oil content in the FWto increase the percentage of methane content in biogas.

Acknowledgement

The authors gratefully acknowledge the financial support of theMinistry of Drinking water and Sanitation, Government of India.

References

[1] L. Mata-Alvarez, Biomethanization of Organic Fraction of Municipal SolidWaste, IWA Publishing, London, 2003 ISBN: 1-900222-14-0.

[2] A. Valo, H. Carrere, J.P. Delgenes, Thermal, chemical and thermo-chemicalpretreatment of waste activated sludge for anaerobic digestion, J. Chem.Technol. Biotechnol. 79 (2004) 1197–1203, doi:http://dx.doi.org/10.1002/jctb.1106.

[3] P.K. Chandran, R. Saravanan, S. Renganarayanan, Pretreatment processes forenhanced biomethanation of MSW, Pollut. Res. 25 (1) (2006) 139–146.

[4] P. Shanmugam, N.J. Horan, Optimising the biogas production from leatherfleshing waste by co-digestion with MSW, Bioresour. Technol. 100 (2009)4117–4120, doi:http://dx.doi.org/10.1016/j.biortech.2009.03.052. 19395254.

[5] J.M. Baldasano, C. Soriano, Emission of geenhouse gases from anaerobicdigestion processes, comparison with other MSW treatments. Abstract of theSecond International Symposium on AD of Solid Wastes. Barcelona, 2 (92)274–77 (1999).

[6] D. Riggle, Acceptance improves for large-scale anaerobic digestion, Biocycle 39(6) (1998) 51–55.

[7] R. Braun, E. Brachtl, M. Grasmug, Co-digestion of proteinaceous industrialwaste, Appl. Biochem. Biotechnol. 109 (2003) 139–153, doi:http://dx.doi.org/10.1385/ABAB:109:1-3:139.

[8] Food and Agriculture Organization of the United Nation (FAOUN). http://www.fao.org/news/story/en/item/74192/icode.

[9] T. Noike, G. Endo, J.E. Chang, J. Yaguchi, J. Matsumoto, Characteristics ofcarbohydrate degradation and the rate-limiting step in anaerobic digestion,Biotechnol. Bioeng. 28 (1985) 1482–1489, doi:http://dx.doi.org/10.1002/bit.260271013. 18553595.

[10] K. Izumi, Y. Okishio, N. Nagao, C. Niwa, S. Yamamoto, T. Toda, Effects of particlesize on anaerobic digestion of food waste, Int. Biodeter. Biodeg. 64 (2010)601–608, doi:http://dx.doi.org/10.1016/j.ibiod.2010.06.013.

[11] G. Liu, R. Zhang, H.M. El-Mashad, R. Dong, Effect of feed to inoculum ratios onbiogas yields of food and green wastes, Bioresour. Technol. 100 (21) (2009)5103–5108, doi:http://dx.doi.org/10.1016/j.biortech.2009.03.081. 19570675.

[12] S. Connaughton, G. Collins, V. O’Flaherty, Development of microbialcommunity structure and activity in a high-rate anaerobic bioreactor at181 �C, Water Res. 40 (2006) 1009–1017, doi:http://dx.doi.org/10.1016/j.watres.2005.12.026. 16488461.

[13] K.C. Jae, C.P. Soon, H.N. Chang, Biochemical methane potential and solid stateanaerobic digestion of Korean food wastes, Bioresour. Technol. 2 (1995)245–253.

[14] J.K. Kim, B.R. Oh, Y.N. Chun, S.W. Kim, Effects of temperature and hydraulicretention time on anaerobic digestion of food waste, J. Biosci. Bioeng. 102 (4)(2006) 328–332, doi:http://dx.doi.org/10.1263/jbb.102.328. 17116580.

[15] R.E. Anderson, Biological Path to Self-Reliance, Van Nostrand ReinholdCompany, New York, 1979, pp. 219–307.

[16] R.E. Speece, Anaerobic Biotechnology for Industrial Wastewaters, ArchaePress, Nashville, TE, USA, 1996, pp. 394–400.

[17] F. Cecchi, P. Pawan, J. Mata Alvarez, A. Bassetti, A. Farneti, Analysis of theThermophilic Semi Dry Anaerobic Fermentation of the Organic Fraction of theMSW Sorted by Plant, in: E. Berlin (Ed.), 1, Recycling International, Berlin, 1989,pp. 241–247.

[18] L. Arsova, Anaerobic Digestion of Food Waste: Current Status, Problems and anAlternative Product, Dissertation, Columbia University, Berlin, 2010.

[19] A.C. Van Haandel, G. Lettinga, Anaerobic Sewage Treatment - A Practical Guidefor Regions with a Hot Climate, Wiley, New York, 1994.

[20] X. Wang, G. Yang, Y. Feng, G. Ren, X. Han, Optimizing feeding composition andcarbon–nitrogen ratios for improved methane yield during anaerobic co-digestion of dairy, chicken manure and wheat straw, Bioresour. Technol. 120(2012) 78–83, doi:http://dx.doi.org/10.1016/j.biortech.2012.06.058.22784956.

[21] S. Ghosh, F.G. Pohland, Kinetics of substrate assimilation and productformation in anaerobic digestion, J. Water Pollut. Control Fed. 46 (4) (1974)748–759.

[22] J.A. Eastman, J.F. Ferguson, Solubilization of particulate organic carbon duringthe acid phase of AD, Journal of the Water Pollution Control Federation 53(1981) 352–366.

D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830 1829

[23] P.A. Sosnowski, S. Wieczorek, S. Ledakowicz, Anaerobic co-digestion of sewagesludge and organic fraction of MSWs, Adv. Environ. Res. 7 (2002) 609–616.

[24] C.F. Liu, Prediction of methane yield at optimum pH for anaerobic digestion oforganic fraction of municipal solid waste, Bioresour. Technol. 99 (2008)882–888, doi:http://dx.doi.org/10.1016/j.biortech.2007.01.013. 17369040.

[25] L. Zhang, Y.W. Lee, D. Jahng, Anaerobic co-digestion of food waste and piggerywastewater: focusing on the role of trace elements, Bioresour. Technol. 102(2011) 5048–5059, doi:http://dx.doi.org/10.1016/j.biortech.2011.01.082.21349706.

[26] R. Goel, T. Tokutomi, H. Yasui, Anaerobic digestion of excess activated sludgewith ozone pretreatment, Water Sci. Technol. 47 (2003) 207–214. 12926690.

[27] I. Siegert, C. Banks, The effect of VFA additions on the anaerobic digestion ofcellulose and glucose in batch reactors, Process Biochem. 40 (2005)3412–3418, doi:http://dx.doi.org/10.1016/j.procbio.2005.01.025.

[28] S.J. Komisar, C.G. Weinert, R.F. Hickey, S. Veltman, M.S. Switzenbaum,Complete on-site treatment of aircraft deicing wastewater using sequencinganaerobic fluidized bed-slow sand filter system. Proc. Water EnvironmentFederation 71st Annual Tech. Conf, USA, 1998.

[29] K. Vijayaraghavan, V. Sudharsan Varma, S.P. Kamala Nalini, Hydrogengeneration from biological solid waste of milk processing effluent treatmentplan, Int. J. Curr. Trends Res. 1 (2012) 17–23.

[30] R.A. Labatut, L.T. Angenent, N.R. Scott, Biochemical methane potential andbiodegradability of complex organic substrates, Bioresour. Technol. 102 (2011)2255–2264, doi:http://dx.doi.org/10.1016/j.biortech.2010.10.035. 21050752.

[31] Y. Li, S.Y. Park, J. Zhu, Solid-state anaerobic digestion for methane productionfrom organic waste, Renew. Sustain. Energy Rev.15 (2011) 821–826, doi:http://dx.doi.org/10.1016/j.rser.2010.07.042.

[32] N. Nagao, N. Tajima, M. Kawai, C. Niwa, N. Kurosawa, T. Matsuyama, F.M. Yusoff,T. Toda, Maximum organic loading rate for the single-stage wet anaerobicdigestion of food waste, Bioresour. Technol. 118 (2012) 210–218, doi:http://dx.doi.org/10.1016/j.biortech.2012.05.045. 22705526.

[33] F. Raposo, V. Fernández-Cegrí, M.A. De La Rubia, R. Borja, F. Béline, C. Cavinato,G. Demirer, B. Fernández, M. Fernández-Polanco, J.C. Frigon, R. Ganesh,P. Kaparaju, J. Koubova, R. Méndez, G. Menin, A. Peene, P. Scherer, M. Torrijos,H. Uellendahl, I. Wierinck, V. de Wilde, Biochemical methane potential (BMP)of solid organic substrates: evaluation of anaerobic biodegradability usingdata from an international interlaboratory study, J. Chem. Technol. Biotechnol.86 (8) (2011) 1088–1098, doi:http://dx.doi.org/10.1002/jctb.2622.

[34] R. Zhang, H.M. El-Mashad, K. Hartman, F. Wang, G. Liu, C. Choate, P. Gamble,Characterization of food waste as feedstock for anaerobic digestion, Bioresour.Technol. 98 (2007) 929–935, doi:http://dx.doi.org/10.1016/j.bio-rtech.2006.02.039. 16635571.

[35] V.N. Gunaseelan, Biochemical methane potential of fruits and vegetable solidwaste feedstocks, Biomass Bioenergy 26 (2004) 389–399, doi:http://dx.doi.org/10.1016/j.biombioe.2003.08.006.

[36] S. Ghosh, M.P. Henry, Hemicellulose conversion by anaerobic digestion,Biomass 6 (1985) 257–269, doi:http://dx.doi.org/10.1016/0144-4565(85)90052-6.

[37] L.M. Sol, L. Stephanie, Characterizing food waste substrates for co-digestionthrough biochemical methane potential (BMP) experiments, Waste Manag. 3(2013) 2664–2669.

[38] P. Sankar Ganesh, R. Sanjeevi, S. Gajalakshmi, E.V. Ramasamy, S.A. Abbasi,Recovery of methane-rich gas from solid feed anaerobic digestion of ipomoea(Ipomoea carnea), Bioresour. Technol. 99 (2008) 812–818, doi:http://dx.doi.org/10.1016/j.biortech.2007.01.024. 17368892.

[39] C. Eliyan, A. Radha, J.P. Jeanger, V. Chettiyappan, Anaerobic digestion of MSW inthermophilic continuous operation. International Conference on SustainableSolid Waste Management, 2009, 377–384.

[40] M.M. Ghangrekar, S.R. Asolekar, S.G. Joshi, Characteristics of sludge developedunder different loading conditions during UASB reactor start-up andgranulation, Water Res. 39 (2005) 1123–1133, doi:http://dx.doi.org/10.1016/j.watres.2004.12.018. 15766967.

[41] H. Carrèrea, Y. Rafrafi, A. Battimelli, M. Torrijos, J.P. Delgenes, G. Ruysschaert,Methane potential of activated sludge and fatty residue: Impact ofCo-digestion and alkaline pretreatments, Open Environ. Eng. J. 3 (2010)71–76, doi:http://dx.doi.org/10.2174/1874829501003010071.

[42] X. Wang, Y. Zhao, A bench scale study of fermentative hydrogen and methaneproduction from food waste in integrated two-stage process, Int. J. HydrogenEnergy 34 (2009) 245–254, doi:http://dx.doi.org/10.1016/j.ijhy-dene.2008.09.100.

[43] C. Cozzolino, A. Bassetti, P. Rondelli, Industrial application of semi dryanaerobic digestion process of organic solid waste. The InternationalSymposium on Anaerobic Digestion of Solid Waste, Venice, April 14–17, 1992.

[44] A. Hejnfelt, I. Angelidaki, Anaerobic digestion of slaughterhouse by-products,Biomass Bioenergy 33 (8) (2009) 1046–1054.

[45] S. Luste, S. Luostarinen, M. Sillanpää, Effect of pre-treatments on hydrolysisand methane production potentials of by-products from meat-processingindustry, J. Hazard. Mater. 164 (1) (2009) 247–255, doi:http://dx.doi.org/10.1016/j.jhazmat.2008.08.002. 18805637.

[46] E. Salminen, J. Einola, J. Rintala, The methane production of poultryslaughtering residues and effects of pre-treatments on methane productionof poultry feather, Environ. Technol. 24 (9) (2003) 1079–1086, doi:http://dx.doi.org/10.1080/09593330309385648. 14599141.

[47] A. Battimelli, H. Carrère, J.P. Delgenès, Saponification of fatty slaughterhousewastes for enhancing anaerobic biodegradability, Bioresour. Technol. 100 (15)

(2009) 3695–3700, doi:http://dx.doi.org/10.1016/j.biortech.2008.12.029.19200718.

[48] L. Palmowski, L. Simons, R. Brooks, Ultrasonic treatment to improve anaerobicdigestibility of dairy waste streams, Water Sci. Technol. 53 (2006) 281–288.16784213.

[49] C. Bougrier, J.P. Delgenès, H. Carrère, Impacts of thermal pre-treatments on thesemi-continuous anaerobic digestion of waste activated sludge, Biochem. Eng.J. 34 (2007) 20–27, doi:http://dx.doi.org/10.1016/j.bej.2006.11.013.

[50] H. Carrère, C. Bougrier, D. Castets, J.P. Delgenès, Impact of initial biodegrad-ability on sludge anaerobic digestion enhancement by thermal pretreatment,J. Environ. Sci. Health 43 (2008) 1551–1555, doi:http://dx.doi.org/10.1080/10934520802293735.

[51] S.K. Bhattacharya, R.L. Madura, D.A. Walling, J.B. Farrell, Volatile solidsreduction in two-phase and conventional anaerobic sludge digestion, WaterRes. 30 (5) (1996) 1041–1048, doi:http://dx.doi.org/10.1016/0043-1354(95)00252-9.

[52] R. Borja, B. Rincón, F. Raposo, J. Alba, A. Martin, Kinetics of mesophilicanaerobic digestion of the two-phase olive mill solid waste, Biochem.Eng. J. 15 (2003) 139–145, doi:http://dx.doi.org/10.1016/S1369-703X(02)00194-8.

[53] R.T. Haug, D.C. Stuckey, J.M. Gossett, P.L. Mac Carty, Effect of thermalpretreatment on digestibility and dewaterability of organic sludges, J. WaterPollu. Control Fed. 23 (1978) 73–85.

[54] S. Tanaka, T. Kobayashi, K.I. Kamiyama, M. Lolita, Effects of thermochemicalpretreatment on the anaerobic digestion of waste activated sludge, Water Sci.Technol. 35 (1997) 209–215, doi:http://dx.doi.org/10.1016/S0273-1223(97)00169-8.

[55] A. Tiehm, K. Nickel, U. Neis, The use of ultrasound to accelerate the anaerobicdigestion of sewage sludge, Water Sci. Technol. 36 (1997) 121–128, doi:http://dx.doi.org/10.1016/S0273-1223(97)00676-8.

[56] A.C. Van Haandel, G. Lettinga, Anaerobic Sewage Treatment -a Practical Guidefor Regions with a Hot Climate, Wiley, New York, 1994.

[57] I. Ferrer, S. Ponsá, F. Vázquez, X. Font, Increasing biogas production by thermal(70 �C) sludge pre-treatment prior to thermophilic anaerobic digestion,Biochem. Eng. J. 42 (2008) 186–192, doi:http://dx.doi.org/10.1016/j.bej.2008.06.020.

[58] H.N. Gavala, U. Yenal, I.V. Skiadas, P. Westermann, B.K. Ahring, Mesophilic andthermophilic anaerobic digestion of primary and secondary sludge. Effect ofpre-treatment at elevated temperature, Water Res. 3 (2003) 4561–4572, doi:http://dx.doi.org/10.1016/S0043-1354(03)00401-9. 14568041.

[59] E.A. Salminen, J.A. Rintala, Semi-continuous anaerobic digestion of solidpoultry slaughterhouse waste: Effect of hydraulic retention time and loading,Water Res. 36 (2002) 3175–3182, doi:http://dx.doi.org/10.1016/S0043-1354(02)00010-6. 12188113.

[60] E. Elbeshbishy, G. Nakhla, Comparative study of the effect of ultrasonication onthe anaerobic biodegradability of food waste in single and two-stage systems,Bioresour. Technol. 102 (2011) 6449–6457, doi:http://dx.doi.org/10.1016/j.biortech.2011.03.082. 21498070.

[61] I.T. Yeom, K.R. Lee, K.H. Ahn, S.H. Lee, Effects of ozone treatment on thebiodegradability of sludge from municipal wastewater treatment plants,Water Sci. Technol. 46 (2002) 421–425. 12361042.

[62] M. Weemaes, M. Verstraete, Evaluation of current wet sludge disintegrationtechniques, J. Chem. Technol. Biotechnol. 73 (1998) 83–92.

[63] J. Kim, C. Park, T.H. Kim, M. Lee, S. Kim, S.W. Kim, J. Lee, Effects of variouspretreatments for enhanced anaerobic digestion with waste activated sludge,J. Biosci. Bioeng. 95 (2003) 271–275, doi:http://dx.doi.org/10.1016/S1389-1723(03)80028-2. 16233404.

[64] L. Neves, R. Ribeiro, R. Oliveira, M.M. Alves, Enhancement of methaneproduction from barley waste, Biomass Bioenergy 30 (2006) 599–603, doi:http://dx.doi.org/10.1016/j.biombioe.2005.12.003.

[65] J. Yiying, L. Huan, M.R. Bux, W. Zhiyu, N. Yongfeng, Combined alkaline andultrasonic pretreatment of sludge before aerobic digestion, J. Environ. Sci. 21(2008) 279–284.

[66] H.C. Moon, I.S. Song, Enzymatic hydrolysis of food waste and methaneproduction using UASB bioreactor, Int. J.Green Energy 8 (3) (2011) 361–371,doi:http://dx.doi.org/10.1080/15435075.2011.557845.

[67] J. Raynal, J.P. Delgenès, R. Moletta, Two-phase anaerobic digestion of solidwastes by a multiple liquefaction reactors process, Bioresour. Technol. 65(1998) 97–103, doi:http://dx.doi.org/10.1016/S0960-8524(98)00009-1.

[68] E. Maranon, L. Castrillon, G. Quiroga, Y. Fernandez-Nava, L. Gomez, M.M.Garcia, Co-digestion of cattle manure with food waste and sludge to increasebiogas production, Waste Manag. 32 (10) (2012) 1821–1825.

[69] R. Braun, E. Brachtl, M. Grasmug, Co-digestion of proteinaceous industrialwaste, Appl. Biochem. Biotechnol. 109 (2003) 139–153, doi:http://dx.doi.org/10.1385/ABAB:109:1-3:139.

[70] R.M. Dinsdale, G.C. Premier, F.R. Hawkes, D.L. Hawkes, Two-stage anaerobicco-digestion of waste activated sludge and fruit or vegetable waste usinginclined tubular digesters, Bioresour. Technol. 72 (2000) 159–168, doi:http://dx.doi.org/10.1016/S0960-8524(99)00105-4.

[71] G. Lissens, P. Vandevivere, L. De Baere, E.M. Biey, W. Verstrae, Solid wastedigesters: process performance and practice for MSW digestion, Water Sci.Technol. 44 (8) (2001) 91–102. 11730142.

[72] J.M. Mata-Alvarez, A dynamic simulation of a two-phase anaerobic digestionsystem for solid wastes, Biotechnol. Bioeng. 30 (1987) 844–851, doi:http://dx.doi.org/10.1002/bit.260300706. 18581519.

1830 D. Krishna, A.S. Kalamdhad / Journal of Environmental Chemical Engineering 2 (2014) 1821–1830

[73] T. Kuba, H. Furumai, T. Kusuda, A kinetic study on methanogenesis by attachedbiomass in a fluidized bed, Water Res. 24 (11) (1990) 1365–1372, doi:http://dx.doi.org/10.1016/0043-1354(90)90155-Y.

[74] A.H. Mouneimne, H. Carrère, N. Bernet, J.P. Delgenès, Effect of saponification onthe anaerobic digestion of solid fatty residues, Bioresour. Technol. 90 (2003)89–94, doi:http://dx.doi.org/10.1016/S0960-8524(03)00091-9. 12835063.

[75] V. Sonakya, N. Raizada, V.C. Kalia, Microbial and enzymatic improvement ofanaerobic digestion of waste biomass, Biotechnol. Lett. 23 (2001) 1463–1466,doi:http://dx.doi.org/10.1023/A:1011664912970.

[76] H.K. Ong, P.F. Greenfield, P.C. Pullammanappallil, An operational strategy forimproved biomethanation of cattle-manure slurry in an unmixed, single-stagedigester, Bioresour. Technol. 73 (2000) 87–89, doi:http://dx.doi.org/10.1016/S0960-8524(99)00139-X.

[77] V.A. Vavilin, L.Y. Lokshina, Modeling of volatile fatty acids degradation kineticsand evaluation of microorganism activity, Bioresour. Technol. 57 (1996) 69–80,doi:http://dx.doi.org/10.1016/0960-8524(96)00052-1.

[78] A. Veeken, S.H. Kalyuzhnyi, H. Scharff, Effect of pH and VFA on hydrolysis oforganic solid waste, J. Environ. Eng. 126 (12) (2000) 1076–1081, doi:http://dx.doi.org/10.1061/(ASCE)0733-9372(2000)126:12(1076).

[79] A.Mtz-Viturtia,J.M.Alvarez,F.Cecchi,Two-phasecontinuousanaerobicdigestionof fruit and vegetable wastes, Resour. Conserv. Recy. 13 (1995) 257–267.

[80] S.L. Lafitte-Trouqué, C.F. Forster, Dual anaerobic co-digestion of sewage sludgeand confectionery waste, Bioresour. Technol. 71 (2000) 77–82, doi:http://dx.doi.org/10.1016/S0960-8524(99)00043-7.

[81] S.R. Harper, F.G. Pohland, Recent developments in hydrogen managementduring anaerobic biological wastewater treatment, Biotechnol. Bioeng. 28(1986) 585–602, doi:http://dx.doi.org/10.1002/bit.260280416. 18555364.

[82] D.J. Costello, P.F. Greenfield, P.L. Lee, Dynamic modelling of a single-stage high-rate anaerobic reactor—I. Model derivation, Water Res. 25 (7) (1991) 847–858,doi:http://dx.doi.org/10.1016/0043-1354(91)90166-N.

[83] D.J. Costello, P.F. Greenfield, P.L. Lee, Dynamic modelling of a single-stage high-rate anaerobic reactor—II. Model verification, Water Res. 25 (7) (1991)859–871, doi:http://dx.doi.org/10.1016/0043-1354(91)90167-O.

[84] M. Kim, Y.H. Ahn, R.E. Speece, Comparative process stability and efficiency ofanaerobic digestion; mesophilic vs. thermophilic, Water Res. 36 (2002)4369–4385, doi:http://dx.doi.org/10.1016/S0043-1354(02)00147-1. 12420941.

[85] C. Held, M. Wellacher, K.H. Robra, G.M. Gübitz, Two-stage anaerobicfermentation of organic waste in CSTR and UFAF-reactors, Bioresour. Technol.81 (2002) 19–24, doi:http://dx.doi.org/10.1016/S0960-8524(01)00108-0.11708753.

[86] O. Ince, G.K. Anderson, B. Kasapgil, Control of organic loading rate using thespecific methanogenic activity test during start-up of anaerobic digestion

systems, Water Res. 29 (1) (1995) 349–355, doi:http://dx.doi.org/10.1016/0043-1354(94)E0102-C.

[87] L. Guerrero, F. Omil, R. Méndez, J.M. Lema, Anaerobic hydrolysis andacidogenisis of wastewaters from food industries with high content oforganic solids and protein, Water Res. 33 (15) (1999) 3281–3290, doi:http://dx.doi.org/10.1016/S0043-1354(99)00041-X.

[88] K.H. Hansen, I. Angelidaki, B.K. Ahring, Anaerobic digestion of swine manures:inhibition by ammonia, Water Res. 32 (1998) 5–12, doi:http://dx.doi.org/10.1016/S0043-1354(97)00201-7.

[89] Y. Miron, G. Zeeman, J.B. Lier, G. Lettinga, The role of sludge retention time inthe hydrolysis and acidification of lipids, carbohydrates and proteins duringdigestion of primary sludge in CSTR system, Water Res. 34 (5) (2000)1705–1713, doi:http://dx.doi.org/10.1016/S0043-1354(99)00280-8.

[90] H.W. Yu, Z. Samani, A.M. Hanson, G. Smith, Energy recovery from grass usingtwo-phase anaerobic digestion, Waste Manag. (New York) 22 (2002) 1–5, doi:http://dx.doi.org/10.1016/S0956-053X(00)00121-5. 11942700.

[91] A.N. Nozhevnikova, O.R. Kotsyurbenko, S.N. Parshina, Anaerobic manuretreatment under extreme temperature conditions, Water Sci. Technol. 40(1999) 215–221, doi:http://dx.doi.org/10.1016/S0273-1223(99)00387-X.

[92] T. Shimizu, K. Kudo, Y. Nasu, Anaerobic waste-activated sludge digestion- abioconversion mechanism and kinetic model, Biotechnol. Bioeng. 41 (1993)1082–1091, doi:http://dx.doi.org/10.1002/bit.260411111.

[93] S.N. Kaul, T. Nandy, Biogas recovery from industrial wastewaters, J. IndianAssoc. Environ. Manag. 24 (1992) 113–189.

[94] F.G. Parkin, W.F. Owen, Fundamentals of anaerobic digestion of waste watersludge’s, ASCE, J. Environ. Eng. 112 (5) (1986) 870–919.

[95] A.R. Tembhurkar, V.A. Mhaisalkar, Studies on hydrolysis and acidogenesis ofkitchen waste in two phase anaerobic digestion, J. Inst. Public Health Eng. 2(2008) 10–18.

[96] J. Mumme, B. Linke, R. Tölle, Novel upflow anaerobic solid-state (UASS) reactor,Bioresour. Technol. 101 (2010) 592–599, doi:http://dx.doi.org/10.1016/j.bio-rtech.2009.08.073. 19748268.

[97] X.H. Lou, W.J. Yuan, T.J. Hwa, A hybrid anaerobic solid–liquid bioreactor forfood waste digestion, Biotechnol. Lett. 24 (2002) 757–761, doi:http://dx.doi.org/10.1023/A:1015517617907.

[98] H.M. El-Mashad, R. Zhang, Biogas production from co-digestion of dairymanure and food waste, Bioresour. Technol. 101 (2010) 4021–4028, doi:http://dx.doi.org/10.1016/j.biortech.2010.01.027. 20137909.

[99] S.Y. Xu, O.P. Karthikeyan, A. Selvam, J.W.C. Wong, Effect of inoculum tosubstrate ratio on the hydrolysis and acidification of food waste in leach bedreactor, Bioresour. Technol.126 (2012) 425–430, doi:http://dx.doi.org/10.1016/j.biortech.2011.12.059. 22227144.