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Journal of Environmental Management 130 (2013) 375e385

Contents lists avai

Journal of Environmental Management

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Review

Food waste and food processing waste for biohydrogen production:A review

Nazlina Haiza Mohd Yasin a, Tabassum Mumtaz b, Mohd Ali Hassan a,Nor’Aini Abd Rahman a,*

aDepartment of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, MalaysiabMicrobiology and Industrial Irradiation Division, Institute of Food and Radiation Biology, Bangladesh Atomic Energy Commission, Dhaka 1000, Bangladesh

a r t i c l e i n f o

Article history:Received 8 May 2013Received in revised form2 September 2013Accepted 4 September 2013Available online 10 October 2013

Keywords:BiohydrogenDark fermentationFood wasteFood processing wastePhysicochemical factors

* Corresponding author. Tel.: þ6 (0)3 89471945; faE-mail addresses: nazlinayasin@yahoo.com (N.H

hotmail.com (T. Mumtaz), alihas@upm.edu.myupm.edu.my (N. Abd Rahman).

0301-4797/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jenvman.2013.09.009

a b s t r a c t

Food waste and food processing wastes which are abundant in nature and rich in carbon content can beattractive renewable substrates for sustainable biohydrogen production due to wide economic prospectsin industries. Many studies utilizing common food wastes such as dining hall or restaurant waste andwastes generated from food processing industries have shown good percentages of hydrogen in gascomposition, production yield and rate. The carbon composition in food waste also plays a crucial rolein determining high biohydrogen yield. Physicochemical factors such as pre-treatment to seed culture,pH, temperature (mesophilic/thermophilic) and etc. are also important to ensure the dominance ofhydrogen-producing bacteria in dark fermentation. This review demonstrates the potential of food wasteand food processing waste for biohydrogen production and provides a brief overview of several physi-cochemical factors that affect biohydrogen production in dark fermentation. The economic viability ofbiohydrogen production from food waste is also discussed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogen is recognized as a clean, renewable and promisingfuture fuel. Production of hydrogen through a biological route uti-lizing waste biomass represents an important area of bio-energyproduction. Biological hydrogen production is a microbial conver-sion process, carried out by bacteria capable of synthesizinghydrogen producing enzymes, such as hydrogenase and nitroge-nase, in dark and photo-fermentation, respectively. Biologicalhydrogen production is classified into four categories, namely (1)biophotolysis of water using solar energy and algae/cyanobacteria,(2) photodecomposition of organic compounds using light energyand photosynthetic bacteria, (3) fermentative hydrogen evolutionto breakdown carbohydrate-rich substrates to hydrogen and otherproducts such as acids and alcohols using anaerobic bacteria and(4) hybrid systems combining dark and photofermentation eitherdirectly or in a series-type (Kothari et al., 2012).

Among all these processes, the dark fermentation route is themost feasible technology with commercial values as it does not

x: þ6 (0)3 89467590..M. Yasin), tabassum_baec@(M.A. Hassan), nor_aini@

All rights reserved.

require any external energy as well as light source and can be run atlow cost (Sreela et al., 2011a). Upon dark fermentative trans-formation, hydration of glucose molecule elucidates a concurrentgeneration of acetic acid and hydrogen in the ratio of 1:2 (Eq. (1)). Italso offers an excellent potential for practical application andintegration with emerging hydrogen and fuel cell technologies(Kim et al., 2011a).

C6H12O6þ2H2O/2CH3COOHþ 2CO2þ4H2 (1)

The majority of the research on biohydrogen production fromfood waste has been conducted under dark fermentation (Yasinet al., 2011, 2009; Kim et al., 2009, 2008a, 2004; Han and Shin,2004; Pan et al., 2008). Food waste, the major component inmunicipal solid waste accounted for 20e54% of the total waste inKorea (Kim et al., 2009, 2008b, 2008c), Japan (Komemoto et al.,2009), Thailand (Sreela et al., 2011b), China (Li et al., 2008), andMalaysia (Yasin et al., 2011; Latifah et al., 2009). It is a wastecomposed of raw and cooked food discarded before or during foodpreparation. It is high in volatile solids, moisture content andsalinity, thus it is the main source of odor, decay, vermin attraction,groundwater contamination and greenhouse gas emission (Kimet al., 2009, Kim and Shin, 2008; Lee and Chung, 2010).

Zhang et al. (2007) reported that food waste is the single-largestcomponent of the waste stream by weight in the United States. Asreported in ‘The Star’ in 10 June 2011, Malaysians throw away up to

Table 1Characteristics of food waste used in biohydrogen production studies.

References Parameters

pH Moisturecontent

Totalsolid

Volatilesolid

Total Kjeldalnitrogen

Total COD Totalcarbohydrate

C/N TotalVFA

Kim et al. (2008b) 11.80 nd 4.6b 4.4b 1.1c 44.2c nd nd 3.6c

Kim and Shin (2008) 4.60 nd 16.8b 16.1b 4.0c 162.2c 99.0c nd 13.5c

Li et al. (2008) nd 85.3b 14.7b 88.8b 2.0b nd nd 21.0 ndZhu et al. (2008) 4.70 nd 11.4c 10.5c 0.5c nd nd 9.0 1.1c

Lee et al. (2010) 4.35a nd 10b 9.5b 4.0c 150.0c 34.9c nd 5.5c

Jayalakshmi et al. (2009) 5.51 83.8b 16.2b 86.1b 2.3b nd nd 21.0 ndWang and Zhao (2009) 4.68 nd 17.6b nd 2.3b 211.8e nd nd ndChu et al. (2008) 4.90 nd 117.0c 108.0c 3.8d 142.0c 66.0c nd 5.4c

Shin et al. (2004) 5.80 nd 67.8c 63.7c 2.8d nd 25.5 18.3 ndIsmail et al. (2009) 6.25 72.0b 255.5c nd 0.2c nd nd nd ndWang et al. (2010) 4.50 nd 266.0c 256.0c 5.5c 346.0c 143.0c nd ndZhu et al. (2009) 4.70 nd 11.4c 10.5c 0.5c 19.3c nd nd ndTawfik et al. (2011) 6.50 nd 61c 56c nd 64c nd nd 2.7c

Valdez-Vazquez and Poggi-Varaldo (2009a, b) 7.20 nd nd 10.1c nd 110c 64c nd nd

nd not determined.a Average in range series.b % (w/w).c g/L.d % TS.e g O2/kg.

N.H.M. Yasin et al. / Journal of Environmental Management 130 (2013) 375e385376

930 tons of unconsumed food (expired bread, eggs, old and rottenfruits) daily: which has doubled over the past three years (Aruna,2011). In Malaysia, 50% of the 31,000 tons of waste produced dailycomprised of organic kitchen waste from leftover food. Thus, foodwaste has gained interest as a potential feedstock for bioenergy.

In literature, the source of food wastes being employed forbiohydrogen studies were obtained mainly from dining hall orrestaurant waste (Yasin et al., 2011, 2009; Kim et al., 2009, 2008a,2004; Zhu et al., 2008). Food processing wastes such as tofu res-idue (Kim et al., 2011c), cheese whey (Davila-Vazquez et al., 2009),rice slurry (Fang et al., 2006) and apple pomace (Doi et al., 2010) etc.have also been tested for biohydrogen production. In addition, solidwastes such as wheat starch Argun et al., 2009), organic fraction ofmunicipal solid waste containing fruit and vegetable waste (Akutsuet al., 2009), jackfruit peel (Valdez-Vazquez et al., 2005) etc. canalso be considered as food processing waste.

Biohydrogen production from food waste containing carbohy-drates, fats, cellulose and hemicelluloses have different metabolicpathways which have not yet been studied in details(Vijayaraghavan et al., 2006). In general, biohydrogen productionresults from carbohydrate degradation through the acidogenesisand acetogenesis route and it is highly sensitive to certain envi-ronmental conditions such as pH, volatile fatty acids, temperature,hydrogen partial pressure, inoculum sources and food waste con-centrations (Ren et al., 2006).

With increasing energy demand worldwide, utilizing renewableresources such as food wastes and food processing wastes forbiohydrogen production can be a novel and promising approach forsubstituting fossil fuels while at the same time solving the wastedisposal problem. This review gives an overview of the researchconducted on biohydrogen production from food waste and foodprocessing wastes by dark fermentation. Several physico-chemicalfactors/parameters that need to be considered for biohydrogenproduction such as chemical nature of the waste, pH, volatile fattyacids, hydrogen partial pressure, inoculum sources, and pre-treatment to mixed culture etc. are also discussed in this review.

2. Biohydrogen from food waste

As shown in Table 1, food waste, in general, has good potential forenergy production through anaerobic degradation due to its

characteristics such as moisture content (72e85.2%), high substrateconcentration (COD: 19.3e346 g/L; Carbohydrate: 25.5e143 g/L) andhigh carbon to nitrogen (C/N) ratio (9e21) (Hwang et al., 2011;Elbeshbishy et al., 2011a; Jayalakshmi et al., 2009). The physico-chemical characteristics of food waste are very important indesigning and operation of an anaerobic digestion system for bio-hydrogen production. Pre-treatment of food waste, temperature, pHand low hydrogen partial pressure etc. are regarded as importantparameters in influencing biohydrogen production and yield (Kimet al., 2009). In addition, other characteristics such as moisture con-tent, volatile solid composition, nutrient content, particle size andbiodegradability of food waste are also important to achieve highbiohydrogen yield (Zhang et al., 2007). Table 1 shows the character-istics of food waste being used in biohydrogen production studies.Different characteristics were observed for each study indicating thatthe composition of this feedstock differed depending on the type ofwaste generated in different countries and cafeterias and also on thetype of actual food waste being used by different researchers.

In order to have optimum concentration of food components,water is usually added to homogenize food waste thereby stimu-lating the degradation rate for biohydrogen production (Chu et al.,2008; Shin et al., 2004). Ismail et al. (2009) found the optimumbiohydrogen production to be at controlled COD concentration of200 g/L food waste while Han and Shin (2004) controlled thedilution rate of food waste to achieve high biohydrogen yield. Highcarbon to nitrogen (C/N) ratios resulted in better biohydrogenproduction even though nitrogen is also an important source ofnutrient to be added in anaerobic fermentation (Mohan et al.,2009). Kim et al. (2010) found that C/N ratios of over 20 resultedin the decline of biohydrogen production. Therefore, based on ourliterature search, C/N ratios of food waste should fall between 20and 21 for optimum yield.

Biohydrogen production from food waste has been carried outwidely by many researchers using mixed cultures from anaerobicsludge, manure and compost in batch, repeated batch, semi-continuous and continuous modes (Table 2). This implied that thepresence of indigenous microorganisms and high carbon content infood waste made it suitable as a feedstock for biohydrogen pro-duction under non-sterile conditions. So far, pure culture inoculumhas not been employed by any researcher for the production ofbiohydrogen from food waste. However, Jo et al. (2007) managed to

Table 2Comparison of the biohydrogen yield on different types of food wastes.

Reference Substrate Inoculum Reactor type Pre-treatment to inoculum Temperature(�C)

pH Biohydrogenyield

Lee and Chung (2010) Food waste Recycled sludge UASB na 30 5.5 1.82a

Ismail et al. (2009) Food waste POME sludge Batch Heat: 80 �C, 20 min 55.7 m7.5 120b

Tawfik et al. (2011) Municipal food waste Anaerobic sludge fromseptic tank treatingblack wastewater

ABR Heat: 90 �C, 20 min 26 5.0e6.0 4.9c

Lee et al. (2010) Food waste Anaerobic sludgefrom methane plant

Semi-continuous Heat: 90 �C. 30 min 55 5.4e5.7 2.5c

Kim et al. (2011c) Sonicated food waste Sewage sludge SBHR na 30 5e6 2.1c

Kim et al. (2009) Food waste (Heat treated: 90 �C, 20 min No inoculum Repeated batch na 35 >5.0 2.05c

Shin et al. (2004) Food waste Mesophilicacidogenic culture

Batch na 35 m4.5 0.1c

Thermophilicacidogenic culture

Batch 55 0.9c

Kim et al. (2011b) Food waste Sewage sludge Batch Heat: 90 �C, 20 min 35 m8.0 2.11d

Hallenback and Ghosh (2009) Food waste No inoculum Batch na 50 m8.0 1.79d

Kim et al. (2008b) Food waste Sewage sludge ASBR Heat: 90 �C, 10 min 35 >5.3 1.12d

Kim and Shin (2008) Food waste Sewage sludge Continuous Alkali: pH 12.5, 1 d 35 5.3 0.87d

Kim et al. (2010) Food waste(Alkali treated with 6 N KOH, pH 12.5, 1d)

Sewage sludge ASBR Heat: 90 �C, 15 min 35 5.3 0.9d

Li et al. (2008) Co-digestion of food waste andsewage sludge (Heat treated at 80 �C, 15 min)

Aged refuse landfill Batch Attached growth toactivated carbon

36 4.83e5.83 187.46e

Zong et al. (2009) Food waste Sewage sludge Batch Chemical: Linearalkylbenzene sulfonate

35 5.6 154.8e

Pan et al. (2008) Food waste Anaerobic sludge Batch na 35 m7.5 39e

50 57e

Valdez-vazquez et al. (2009) 60% food waste and40% paper

Solid substrateanaerobic digester

Semi-continuous na 35 5.5 165e

55 6.4 360e

Valdez-Vazquez andPoggi-Varaldo, (2009b)

60% food waste and40% paper

Solid substrateanaerobic digester

Repeated batch na 55 5.51e6.31 54.8e

Chu et al. (2008) Food waste (grain,vegetables, meats and fish)

Sewage sludge Continuous Adapted in organicwaste for 2 years

55 5.5 205f

Zhu et al. (2008) Co-digestion of municipalfood waste(grains, vegetables, meats)and sewage sludge

Digested sludge growon sucrose medium

Batch nd 35 m7.0 112f

Sreela et al. (2011b) Co-digestion of foodwaste and sludge

Anaerobic sludge Batch Heat: 105 �C, 3 h 30 nd 102.63f

Elbeshbishy et al. (2011b) Sonicated food waste No inoculum Batch na 37 m5.5 97f

Jayalakshmi et al. (2009) Kitchen waste Digested slurry Continuous(Inclined plugflow reactor)

Heat: 100 �C, 30 min nd 5.6 72f

Danko et al. (2008) Food waste (pork lard, cabbage,chicken breast, potato flakes)

Suspended sludge Batch Chemical:Bromoethane-sulfonate (BES)

37 m6.5 154.8g

Li et al. (2011) Kitchen waste I-CSTR system I-CSTR na 35 5.5 96g

Han and Shin (2004) Food waste (grains, vegetables, meats) Sewage sludge Continuous Heat: Boiled for 15 min 35 m6.5 0.33h

Wang et al. (2010) Kitchen waste (rice, noodles, cake) Mixed sludge I-CSTR na 55 4.4 2.1i

Lee et al. (2008) Vegetable kitchen waste Kitchen waste compost Batch No treatment 55 7.0 0.57i

Yasin et al. (2009) Food waste (rice, fish and vegetable) POME sludge Batch Heat: 80 �C, 30 min 55 m7.0 83j

Yasin et al. (2011) Food waste (rice, fish and vegetable) POME sludge Batch Heat: 80 �C, 30 min 55 5.5 79j

Zhu et al. (2011) Co-digestion of municipal food waste(grains, vegetables, meats) andsewage sludge

Sewage sludge Semi-continuous Heat: 100 �C, 10 min with periodictreatment every 2e5 weeks

35 m7.0 0.93k

Eroglu et al. (2006) Food waste Cattle dung compost Batch Heat: Boiled for 15 min 37 m6.8 671l

Munoz-Paez et al. (2012) 60% food waste and 40% paper Anaerobic digester sludge Batch nd 37 6.3 nd

(continued on next page)

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Table

2(con

tinu

ed)

Referen

ceSu

bstrate

Inoc

ulum

Rea

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type

Pre-trea

tmen

tto

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ulum

Temperature

(�C)

pH

Biohyd

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nyield

Kim

etal.(20

04)

Co-digestion

offood

waste

and

sewag

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Sewag

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Batch

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t:90

� C.1

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nd

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nd

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nd:not

determined

.PO

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Palm

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effluen

t.ABR:Anae

robicba

fflereactor.

ASB

R:Anae

robicsludge

batchreactor.

CST

R:Con

tinuou

sstirredtankreactor.

I-CST

R:Interm

ittentco

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SBHR:So

nicated

biolog

ical

hyd

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ydrate.

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minitialpH.

N.H.M. Yasin et al. / Journal of Environmental Management 130 (2013) 375e385378

isolate a single strain of Clostridium tyrobutyricum JM1 from thebiohydrogen fermentation of food waste.

Some of the authors co-digested food waste with sludge (Kimet al., 2004; Zhu et al., 2008) and aged landfill refuse (Mohanet al., 2009) in order to balance the carbon to nitrogen ratio andto increase proteinaceous content and improve biohydrogen pro-duction by microorganisms. Kim and Lee (2010) suggested thatanaerobically digested sludge could be used as additional nutrientsfor the growth and metabolic activity of hydrogen-producingbacteria.

A few studies were conducted using non-sterile food wastewithout inoculum addition (Kim et al., 2011d, 2009). It was foundthat heat-treated food waste could produce high biohydrogen yieldwithout any problems related to hydrogen-consuming bacteriawhen compared to untreated food waste. Similarly, sonication offood waste with heat and without inoculum was applied byElbeshbishy et al. (2011a, 2011b) for biohydrogen production. Thisresult showed that pre-treatment of food waste could enhancebiohydrogenproduction efficiency and therefore can be regarded asan important parameter influencing biohydrogen production.

Some of the authors simulated food waste by adding differentratios of carbohydrate, protein, lipids and cellulose in order tomaintain the same composition and degradation rates as comparedto the real restaurant waste. Danko et al. (2008) simulated lipid,cellulose, protein and carbohydrate in the ratio of 1:1:3.5:20 bymixing pork lard, cabbage, chicken breast and potato flakes,respectively. They found that this ratio could simulate actualrestaurant waste and resulted in higher hydrogen production po-tential when mixed with granular sludge. Yasin et al. (2009, 2011)simulated the 2:1:1 ratio of carbohydrate, protein and fiber bymixing rice, fish and vegetable, respectively to simulate the actualrestaurant waste. Some authors mixed grains, vegetables, meat andfish to mimic the real composition of municipal food waste (Hanand Shin, 2004; Chu et al., 2008; Zhu et al., 2011, 2008; Sreelaet al., 2011a).

3. Biohydrogen from food processing waste

Food processing industries e.g., tofu-, cereal-, cheese- andpotato-processing industries generate huge amount of wastewaterwith high concentrations of sugars and starch (Van-Ginkel et al.,2005). Utilization of these wastes for biohydrogen productionbenefits food processing industries since they can use the energy asa source of electricity. Table 3 lists the types of food processingwaste from industrial effluent being used as feedstock for bio-hydrogen production. Apple pomace, pineapple waste, mixed fruitpeels, and fully ripened fruits such as apples, pears and grapeswhich are high in sugar content but do not have market value, areattractive substrates for the production of biohydrogen (Doi et al.,2010; Vijayaraghavan et al., 2007; Feng et al., 2010; Hwang et al.,2011).

Using fully ripened fruits and tofu-processing waste, Hwanget al. (2011) and Kim and Lee (2010) obtained hydrogen yields of2.2 and 2.3 mol H2 mol�1 glucose, respectively. However, tofu-processing waste required pre-treatment such as heat- and acid-treatment of the substrate to increase the soluble carbohydratecontent in the feedstock (Kim and Lee, 2010; Kim et al., 2011a,2011b). The use of cheese whey, rich in protein, also resulted inhigh biohydrogen yields of 122 mL H2 L�1 media d�1 and2.8 mol H2 mol�1 hexose from the experiments conducted byCastello et al. (2009) and Davila-Vazquez et al. (2009), respec-tively. On the other hand, only 0.79 mol H2 mol�1 glucose wasobtained using cereal wastewater by Oh and Logan (2005) asshown in Table 3. Mixed microbial culture was used as the seedculture in all the studies. Based on our literature search, only

Table 3Comparison on the biohydrogen yield from different food processing wastes.

References Substrate Inoculum Reactor type Pre-treatment to inoculum Temperature(�C)

pH Hydrogenyield

Kim and Lee (2010) Tofu processing waste (Heattreated: 110 �C, 30 min)

Anaerobic digester sludge CSTR na 60 5.5 2.3a

Hwang et al. (2011) Fully ripened fruits(apple, pear, grapes)

Sewage sludge Batch Heat: Boiled for 30 min 35 5.1e6.1 2.2a

Kargi et al. (2012) Cheese whey powder(Heat treated: 121 �C, 15 min)

Sewage sludge Batch nd 55 l7.0 1.03a

Argun and Kargi (2009) Waste ground wheat solution Anaerobic sludge Batch Heat: repeated boilingevery 5 h

37 l7.0 1.0a

Oh et al. (2005) Cereal wastewater Dewatered sewage sludge Batch Heat: 140 �C, 2 h 24 6.0 0.79a

Venetsaneas et al. (2009) Cheese whey ND Continuous na 35 5.2 0.78a

Fang et al. (2006) Rice slurry Anaerobic digester sludge Batch Heat: 100 �C, 30 min 37 4.5 346b

Davila-vazquez et al. (2009) Cheese whey Anaerobic granular sludge CSTR Heat: Boiled for 40 min 37 5.9 2.8c

Doi et al. (2010) Apple pomace waste Rice rhizosphere microflora Batch na 35 l6.0 2.3c

Doi et al. (2009) Wasted bread Rice rhizosphere Continuous nd 35 5.70e5.75 1.3c

Kim et al. (2011c) Tofu residue (Acid treatedwith 1% HCl)

Sewage sludge Batch Heat: 90 �C, 15 min 35 l7.0 1.48d

Kim et al. (2011a) Tofu processing waste(Acid treated with 0.5%HCl for 5 min)

Sewage sludge CSTR Heat: 90 �C, 15 min 60 5.5 1.20d

MBR 1.87d

Karlsson et al. (2008) Food industry residueand manure

Anaerobic digester sludge CSTR na 55 End point: 6.3e6.8 16.5e

Tenca et al. (2011) Co-digestion of swinemanure with fruit andvegetable waste

10 L reactor digesting glucoseand fruit waste

Semi-continuous na 55 5.45 (not controlled) 126f

Ferchichi et al. (2005) Crude cheese whey Clostridiumsaccharoper-butylacetonicum

Batch nd 30 k6.0 0.028g

Wang et al. (2006) Pineapple waste Municipal sewage sludge Batch Acid: nd 37 7.5 5920h

Azbar et al. (2009) Cheese whey wastewater Anaerobic digester sludge CSTR Heat: 85 �C, 45 min 55 5.5 22h

Castello et al. (2009) Cheese whey Acidogenic lab scale reactor UASB No treatment 30 nd 122i

Yang et al. (2007) Cheese processing wastewater Sewage sludge CSTR nd 35e38 4.0e5.0 2.3j

Van-Ginkel et al. (2005) Potato processing waste Soil from tomato plant Batch Heat: 100 �C, 2 h ND 6.4 1.0k

Vijayaraghavan et al. (2007) Mixed fruit peel Cow manure Continuous Acid: pH 5.0 followed byHeat: 105 �C. 1 h

nd 5.4e5.7 nd

na: not available.nd: not determined.CSTR e Continuous stirred tank reactor.MBR e Membrane bioreactor.UASB e Upflow anaerobic sludge blanket.

a mol H2 mol�1 glucose.b mL H2 g�1 carbohydrate.c mol H2 mol�1 hexose.d mol H2 mol�1 hexoseadded.e mL H2 g�1 VS.f mL H2 g�1 VS added.g L h�1.h mmol H2 g�1 COD.i mmol H2 L�1 media d�1.j mM g�1 COD.k mL H2 mL�1 media.l initial pH.

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Fig. 1. The effect of pre-treatment, pH and temperature on biohydrogen production by mixed culture: (a) Mixed culture, (b) Mixed culture after pre-treatment, (c) Mixed culturewithout pre-treatment (d) Mixed culture at pH 5.0e6.0, (e) Mixed culture at pH 7.0e8.0 and (f) Mixed culture without pre-treatment at mesophilic temperature.

N.H.M. Yasin et al. / Journal of Environmental Management 130 (2013) 375e385380

Ferchichi et al. (2005) using a single culture of Clostridium sac-charoperbutylacetonicum for biohydrogen production from crudecheese whey.

4. Components of food waste affecting biohydrogenproduction

Food waste and food processing wastes are potential feedstocksfor biohydrogen production. However, the effects of the variation ofcarbohydrate, fat, protein, cellulose and hemicellulose contents inthe mixture of food waste are not clearly understood bybiochemical means. For example the highest biohydrogen yield of2.79 mmol H2 L�1 d�1 was obtained by Fang et al. (2006) from riceslurry which is rich in carbohydrate content while Lee et al. (2008)achieved the highest biohydrogen yield of 18.4 mmol H2 L�1 d�1

from vegetable kitchen waste which is rich in cellulose content.Carbohydrate has been reported to be the most suitable feedstockfor biohydrogen production although other components in foodwaste such as fat, protein and cellulose can also be used as substrate(Lay et al., 2003; Lee et al., 2010). Theoretically, hydrogen is difficultto be produced from lipid and protein degradation (Lay et al., 2003).

The limiting factor for biohydrogen production from differentcomponents of food waste is the hydrolysis rate. Fermentativebacteria hydrolyze and ferment carbohydrates, protein and lipids tovolatile fatty acids which are then further hydrolyzed into acetate,carbon dioxide and hydrogen by acetogenic bacteria. Fermentativebacteria such as Clostridium sp., Enterobacter sp., Thermoanaer-obacter sp. and other spore forming bacteria involved in bio-hydrogen production contain hydrogenase enzyme by whichhydrogen as well as energy in the form of ATP are produced duringthe degradation process.

4.1. Carbohydrate

Carbohydrate rich substrates such as rice slurry (Fang et al.,2006), food waste rich in carbohydrate content (Kim et al., 2011,2009, 2008a, 2008b; Wang et al., 2010), wheat (Argun et al.,

2009), cheese, tofu and potato processing waste (Kim et al.,2011c; Van-Ginkel et al., 2005; Yang et al., 2007) etc. have beenshown to be suitable substrates for biohydrogen production(Antonopoulou et al., 2011). During carbohydrate hydrolysis, hy-drolytic bacteria produce simple sugars such as sucrose, glucose,xylose or hexose (Kapdan and Kargi, 2006). The product of carbo-hydrate hydrolysis mainly depends on the microorganisms presentin the culture broth during biohydrogen production (Antonopoulouet al., 2011). For example stochiometrically, Clostridium sp. canproduce 2 mol of hydrogen with 1 mol of n-butyrate or 4 mol ofhydrogen with 2 mol of acetate from 1 mol of hexose. Meanwhile,no hydrogen gas will be produced if the end products are lactateand propionate (Wang and Chang, 2008; Kim et al., 2008a).

The rate of carbohydrate hydrolysis is faster compared to lipidand protein. Lay et al. (2003) found that biohydrogen productionfrom carbohydrate-rich substrate is twenty times greater whencompared to lipid and protein-rich substrates. Wang and Chang(2008) reported that cassava starch could produce 4 L H2 L�1 h�1

with 80e93% substrate conversion in a continuous system. Kim andLee (2010) conducted various pre-treatments such as ultra-sonication, homogenization, pH and heat treatment to tofu-processing wastewater in order to enhance carbohydrate solubili-zation for better biohydrogen production efficiency. Sagnak et al.(2011) applied both acid and heat treatments to obtain mono-meric sugar for biohydrogen production. Wang et al. (2010) addedamylases to a kitchen-waste-fed in hydrogen fermenter to increasethe efficiency of starch hydrolysis.

4.2. Fats (Triglycerides)

Oils and dairy products are the sources of lipids in food wasteand food processing waste (Cirne et al., 2007). In anaerobic hy-drolysis by microorganisms, bacteria secrete lipase for lipid hy-drolysis Campbell and Farrell, 2009; Cirne et al., 2007). Thehydrolysis of triacylglycerides resulted in free fatty acid chains andglycerol. During the b-oxidation pathway, glycerol and free fattyacids could be further hydrolyzed to acetyl-CoA and acetate

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resulting in hydrogen evolution from NADH oxidation (Lay et al.,2003; Boyer, 2006). The oxidation state is different depending onthe number of double bonds of the side chains of the fatty acids(Karlsson et al., 2008).

The presence of lipids and fats in an anaerobic fermenterresulted in flotation, clogging and mass transfer problems. Hightemperature is suggested to counteract clogging problems duringanaerobic degradation of lipid (Lee et al., 2010). The process ofbiohydrogen production from lipid hydrolysis would be slowerwhen compared to carbohydrate hydrolysis (Vidal et al., 2000). Thiswas due to the ability of hydrogenotrophic methanogens toconsume hydrogen-producing bacteria (Cirne et al., 2007). Hence itcan be said that lipids are not suitable to be used as the sole sub-strate for biohydrogen production (Vidal et al., 2000; Cirne et al.,2007; Lee et al., 2010). The accumulation of biohydrogen fromlipid was also accompanied by volatile fatty acid productionresulting in pH decrease (Lay et al., 2003). Thus, controlling tem-perature and pH are crucial in order to optimize biohydrogenproduction from lipid-rich waste.

4.3. Protein

Proteins are polypeptides formed by joining covalently linkedamino acids (peptide bonds). Foodwaste and food processing wastefrom cheese, whey, casein, fish, meat, chicken and eggs containsignificant amounts of protein (Lay et al., 2003). During anaerobicdegradation of proteins, hydrogen-producing bacteria such asClostridium sp. hydrolyze proteins into polypeptides and aminoacids by secreting protease enzyme. The hydrolysis is carried out byproteases excreted by microorganisms (Vidal et al., 2000; Jokelaand Rintala, 2003). Then, the amino acids are further brokendown into volatile fatty acids, carbon dioxide, hydrogen, ammoniaand reduced sulphur (Lay et al., 2003; Jokela and Rintala, 2003).

The rate of protein degradation is slower than carbohydrate andlipid degradation (Vidal et al., 2000). Sometimes it is found thatprotein degradation is incomplete. In order to increase the ten-dency of protein to degrade, Kargi et al. (2012) applied heat-treatment to cheese whey powder to precipitate the proteinbefore it was further hydrolyzed for biohydrogen production(Table 3). Thus, protein alone is not a good substrate for bio-hydrogen production but it can be added to the substrate to in-crease nutritive value for hydrogen-producing bacteria (Xioa et al.,2010; Siddiqui et al., 2011).

4.4. Cellulose and lignocellulose

Cellulose and lignocelluloses are the most abundant bio-polymers from plants including fruits and vegetables that containdifferent kinds of sugars and can be regarded as a beneficialfeedstock for biohydrogen production (Lalaurette et al., 2009).Biohydrogen production from food waste and food processingwaste containing large amounts of cellulose such as jackfruit peels(Vijayaraghavan et al., 2006), apple waste (Lay et al., 1999; Hwanget al., 2011), pineapple waste (Wang et al., 2006), food wastecontaining kimchi (Jo et al., 2007), and vegetable kitchen waste(Lee et al., 2008; Hwang et al., 2011) results in interesting andvaried biohydrogen yields. However, it was reported that cellulosewas hardly degraded by biological treatments due to its crystallineand rigid structure (Lee et al., 2008). In order to avoid this prob-lem, some studies applied physical treatments such as steam-explosion or chemical treatments such as acid or alkaline treat-ment to disrupt the rigid structure of cellulosic and lignocellulosicmaterials and to saccharify the sugars for the production of bio-hydrogen by cellulolytic bacteria in anaerobic conditions (Levinet al., 2009).

5. Physicochemical factors affecting biohydrogen productionfrom food waste

Controlled environmental conditions during biohydrogen pro-duction are very important to limit hydrogen-consuming bacteriain the biohydrogen fermenter. When environmental conditions arefavorable for hydrogen-producing bacteria to grow, hydrogen-consuming bacteria such as solvent producing bacteria andmethanogens could be suppressed (Kim et al., 2008a). Thefollowing sections detail the effects of pre-treatment, pH, volatilefatty acids, substrate concentration and temperature on the pro-duction of biohydrogen from food waste and food processing wastein dark fermentation.

5.1. Pre-treatment

The elimination of hydrogen-consuming bacteria is important inbiohydrogen production to favor the growth of hydrogen-producing bacteria. Hydrogen-producing bacteria are stable inharsh environments such as heat, chemical or pH shock, resultingin the germination of spores (Valdez-Vazquez and Poggi-Varaldo,2009a). Fig. 1 shows the changes of microbial community inmixed culture. Themixed culture contains indigenous bacteria suchas hydrogen-producing bacteria encapsulated with spores,methane-producing bacteria and acid-producing bacteria (Fig. 1a).Therefore, mixed culture containing indigenous microorganismsneed to be pre-treated by heat, chemical or pH shock to promotegermination of hydrogen-producing bacteria and elimination ofhydrogen-consuming bacteria (Kim and Shin, 2008).

Fig. 1b shows that after pretreatment, the hydrogen-producingbacteria germinate from the spores and the number of methane-and acid-producing bacteria reduces. The dominance of hydrogen-producing bacteria such as Clostridium sp., and Caloramator aus-tralicus after heat treatment has been reported by several authorsfor biohydrogen production study using food waste as a substrate(Yasin et al., 2011; Han and Shin, 2004; Vijayaraghavan et al., 2006).However, if no pretreatment was applied to the mixed culture, thenumber of methane- and acid-producing bacteria increase andhydrogen-producing bacteria remain encapsulated by the spores(Fig. 1c,f).

As shown in Tables 2 and 3, in most cases, heat-treatment of themixed culture by boiling or autoclavingmethod has been employedfor treating food waste and food-processing waste for biohydrogenproduction. The heat treatment process was chosen by most of theresearchers because it is simple, cheap and does not require a longperiod of time compared to alkali, acid or other chemical treat-ments. The temperature and treatment time vary from 80 to 140 �Cand 15 min to 3 h, respectively (Tables 2 and 3). In contrast,treatment by alkali, acid or chemical shock requires at least a dayand the use of chemicals are more expensive and time consumingwhen compared to heat treatment (Kim and Shin, 2008; Dankoet al., 2008).

Argun and Kargi (2009) repeated the heat treatment process forfive times to ensure the efficiency of hydrogen-producing bacteriato produce biohydrogen. They also suggested prolonging thetreatment time to eliminate hydrogen-consuming bacteria such asmethanogens and homoacetogens. Zhu et al. (2011) also appliedheat treatment every two to five weeks in semi-continuous systemto ensure that the system was not disrupted by hydrogen-consuming bacteria. Vijayaraghavan et al. (2006, 2007) appliedacid treatment followed by not less than three period of heattreatment before isolation of microbes in cow dung for bio-hydrogen production from jackfruit peel.

Kim et al. (2011d, 2009) applied heat treatment at 90 �C for20 min to germinate hydrogen-producing bacteria in food waste.

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They found that untreated food waste resulted in organic acidproduction with low biohydrogen yield. Kim et al. (2010) appliedalkali treatment using potassium hydroxide at pH 12.5 for one dayto enhance biohydrogen production even though heat treatedsewage sludge was used as a seed culture. Elbeshbishy et al. (2011a,2011b) sonicated food waste at 70 �C for 30 min to enhance bio-hydrogen production.

5.2. pH

pH plays a crucial role in influencing biohydrogen productionfrom food waste and food processing waste in terms of enzymeactivity, biohydrogen generation rate and metabolic pathways (Chuet al., 2008; Zhu et al., 2009). pH is very important in order to limithydrogen-consuming bacteria especially methanogens which areactive in the narrow pH range from 6.3 to 7.8 (Pan et al., 2008).Fig. 1e shows that methane and acid-producing bacteria aredominant in fermentation even though the inoculum was pre-treated at pH 6.3 to 7.8. The number of hydrogen-producing bac-teria becomes very low if the temperature is not suitable for thesebacteria to grow. Most of the reports showed that the suitable pHfor hydrogen-producing bacteria is in the range of 5.0e6.0 asindicated in Fig. 1d and Table 2.

Table 2 shows that biohydrogen production has been con-ducted mostly at initial pH 7.0 (Yasin et al., 2009; Kim et al., 2011c;Argun et al., 2009; Zhu et al., 2008, 2011; Feng et al., 2010; Argunand Kargi, 2009). The hydrogen-producing bacteria might besuppressed if too low pH was applied initially and could affectsubstrate utilization (Kim et al., 2011d). Our study also proved thatbiohydrogen production at initial pH 7.0 resulted in higher bio-hydrogen yield when compared to biohydrogen production atcontrolled pH 5.5 throughout the study Yasin et al (2011). Low pHat the initial stage of biohydrogen production resulted in sup-pression of hydrogen-producing bacteria for further degradationof the substrate. The addition of acid and alkali during hydrolysisprocess resulted in excess chemical addition and unstable bacte-rial adaptation in the culture broth although there were manystudies which suggested the suitable pH of biohydrogen produc-tion to be controlled at pH 5.0e6.0. Therefore, neutral initial pHresulted in high biohydrogen yield due to bacterial adaptation tothe hydrolysis stages in the anaerobic degradation pathway (Renet al., 2006). Kim et al. (2011b, 2011c, 2011e) adjusted the initialpH at 7.0e8.0 and allowed the pH to drop and then maintained at5.5e6.0.

However, no biohydrogen production was reported at pH lowerthan 4.0 or above 8.0. Very acidic pH (lower than pH 4.0) leads tocell maintenance disruption in which energy (ATP) will be used tomaintain the neutrality instead of producing biohydrogen (Zonget al., 2009; Hallenbeck and Ghosh, 2009). Due to the lack of ATPfunction, the activities of enzymes such as hydrogenase and iron-containing enzyme will be inhibited in hydrogen-producing bac-teria (Lay et al., 2003). Kim et al. (2011d) reported that initial pH 5.0slowed down hydrogen-producing bacteria germination and low-ered hydrogenase enzyme production. They also showed that thehigher the initial pH, the longer the lag period for biohydrogenevolution without affecting hydrogenase activity and butyrateproduction. Very alkaline pH (pH more than 8.5) resulted in a longlag phase of biohydrogen production due to bacterial adaptationtime to the alkaline culture broth to produce organic acids as wellas biohydrogen.

In addition, initial pH studies are usually applicable in the batchfermentation system as indicated in Tables 2 and 3. This is due tothe rapid decrease of pH resulting in organic acids productionduring anaerobic degradation. The short period in batch fermen-tation could result in high biohydrogen production and yield. On

the other hand, biohydrogen production in continuous system re-quires controlled pH to maintain the activity of hydrogen-producing bacteria. Addition of buffer and alkali such as sodiumhydroxide, potassium hydroxide and calcium carbonate arerequired to improve biohydrogen production (Pan et al., 2008; Yanget al., 2007; Lee et al., 2008; Zhu et al., 2008). Livestock manure, forexample, can provide high buffering capacity and wide variety ofnutrients and nitrogen necessary for optimal bacterial growth(Maranon et al., 2012).

5.3. Temperature

Temperature is one of the most important parameters influ-encing biohydrogen production from food waste and food pro-cessing waste. Mesophilic temperature is cost effective and easyto be controlled at industrial scale. In most cases, biohydrogenproduction has been conducted under mesophilic temperature asindicated in Tables 2 and 3 Biohydrogen production in mesophiliccondition (temperature range from 30 to 37 �C) use lower energysince ambient temperature can be used to convert waste tobiohydrogen directly. However, thermophilic temperature resul-ted in higher biohydrogen yield (Tables 2 and 3). For instances,Venetsaneas et al. (2009) and Kargi et al. (2012) reported that theyield of biohydrogen from cheese whey was 0.78 and1.03 mol H2 mol�1 glucose at mesophilic temperature (35 �C) andthermophilic temperature (55 �C), respectively.

Kim et al. (2011e) demonstrated that the poor performance ofbiohydrogen production from food waste obtained in mesophilictemperature at 35 �C was due to lactic acid production thatinhibited the growth of hydrogen-producing bacteria. Theyproved that biohydrogen could be effectively produced at ther-mophilic temperature from food waste naturally even though nopre-treated inoculum was used as a seed culture due to the sup-pression of lactic acid bacteria. Chu et al. (2008) also conductedexperiments for biohydrogen production at 55 �C while 35 �C waseffective for methane production. Heat-treatment or the use ofthermophilic temperature during biohydrogen production resul-ted in germination of hydrogen-producing bacteria and the sup-pression of acid-producing bacteria (Kim et al., 2011c). Thus,biohydrogen production from food waste and food-processingwaste was suggested to be effective at thermophilic temperaturebetween 50 and 60 �C.

5.4. Volatile fatty acids

During anaerobic degradation, intermediate by-products suchas acetic acid, butyric acid, lactic acid and propionic acid are pro-duced (Ren et al., 2006). The production of hydrogen is accompa-nied by the production of acetic acid and butyric acid. In contrast,no hydrogen could be produced if the by-products are lactic acidand propionic acid (Kim et al., 2008a).

Food waste degradation at ambient temperature is governed bylactic acid bacteria, the main cause of hydrogen fermentation fail-ure (Kim et al., 2009; Jo et al., 2007). Pre-treatments such as heat,chemical or pH shock to food waste or to seed culture are thereforeapplied to inhibit the effects of lactic acid bacteria during fermen-tation and to inhibit the activity of other hydrogen-consumingbacteria (Kim et al., 2009; Valdez-Vazquez et al., 2005). In harshenvironments, for example, during pre-treatment, spores ofhydrogen-producing-bacteria germinate while hydrogen-consuming bacteria could not survive and thus can be suppressed(Valdez-Vazquez et al., 2009). The controlled environmental con-ditions resulted in butyrate being the main metabolite that sup-presses lactic acid and propionic acid production in food waste andfood processing waste fermentation. When butyrate is the end-

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product, a theoretical maximum of 2 mol H2 per mole of glucose isobtained:

C6H12O6þ2H2O/CH2CH2CH2COOH þ 2H2 þ 2CO2 (2)

6. Future prospects and challenges of biohydrogenproduction from food waste

Utilization of food waste to produce bioenergy is gaining mo-mentum owing to the looming shortage of foodworldwide. Again, alarge amount of food goes to waste everyday across the world.According to UN Food and Agriculture Organization (FAO), 1.3billion tonnes of food is wasted every year. This underutilizedresource could be a potential energy source provided thateconomically viable technologies are available. Food waste, whichcomprises mainly of starch, protein, and fat with a small fraction ofcellulose and hemi-cellulose, constitutes a possible source forbioenergy production (Zong et al., 2009).

Recently, hybrid fermentation technology combining darkfermentation and photo-fermentation process has been proposedfor biohydrogen production. In hybrid fermentation, the anaerobicfermentation of carbohydrates produces intermediates (low mo-lecular weight organic acids) which can be converted into bio-hydrogen by photosynthetic bacteria Hallenbeck and Ghosh, 2009).Eroglu et al. (2006) observed three fold increments in biohydrogenproduction from olive mill wastewater compared to photo-fermentation alone when hybrid fermentation technology wasapplied. Similarly, two stage fermentation which is hydrogenfermentation followed bymethane production have been proposedby many researchers (Han and Shin, 2004; Demirel et al., 2010).Through this hydromethane production strategy, food waste basedsubstrate could be wisely utilized thus reducing waste stream inthe landfill. Using a two-step process and sucrose as substrate, Taoet al. (2007) obtained total hydrogen yield of 6.63 mol H2 mol�1

sucrose compared to 3.67 in dark fermentation. Thus, hybrid and/ortwo stage fermentation can be suggested to improve conventionalbiohydrogen production in both dark and fotosynthetic fermenta-tion. Other by-products such as acetone, butanol and ethanol (ABE)normally produced during anaerobic fermentation of food wastecould also be harnessed for liquid biofuel production.

Hydrogen produces through a range of renewable primary en-ergy sources such as wind, waste biomass and solar energy, is idealfor gradually replacing fossil fuels. Despite of the fact that, intensiveefforts have been given to generate hydrogen from food wastes orany other kind of biowastes, in general, there is a long way to go togenerate hydrogen in a cost-effective manner. As of now, com-mercial success has not been achieved to make these technologieseconomically feasible. Several studies have indicated that the costof biohydrogenproduction largely depends on the cost of feedstock.The choice of food waste based material becomes a good ideaconsidering the substrate cost. However, the cost of collection andtransportation are directly proportional to the cost of feedstock andpose a significant challenge to their use in energy production. Be-sides, appropriate hydrogen storage, transport infrastructure andutilization system need to be developed (Balat and Kirtay, 2010). Ina recent review on the prospects and challenges of hydrogen pro-duction from renewable resources, Abassi and Abbasi (2011)pointed out that the production of ‘renewable’ hydrogen (forexample from wind-electrolysis and biomass routes) is prohibi-tively costly. The cost analysis results showed that several systemimprovements are necessary to bring down the cost of hydrogenproduction to be competitive with gasoline. On the other hand,based on simulated results, Li et al., (2012) showed the total annualrevenue of beverage wastewater should be 2,658,000 USD/yr using

the Aspen simulation while 81,000 USD/yr based on local priceevaluation, indicating the feasibility of commercializing bio-hydrogen energy (Li et al., 2012).

A survey was carried out to predict the development of thebiohydrogen sector in the globe. Simulation results revealed thatChinawill have the largest biohydrogenmarket, followed by the US,Japan and India. It has been suggested that investment in bio-hydrogen technologies should have priority over investment inhydrogen infrastructure to maximize the biohydrogen output ofthese countries (Lee and Chiu, 2012).

Additional research is required to improve the efficiency ofbiohydrogen production from food waste through the anaerobicfermentation processes to be competitive with fossil fuel technol-ogies. Extent of biodiversity of the hydrogen-producing microbialcommunity is yet to be discovered. Utilization of these unexploredbiodiversity is a good strategy for harnessing microbial resourcesand their gene resources to further improve the yield and pro-duction rates of biohydrogen production (Show et al., 2012). It istherefore hoped that, in the near future, with scientific and engi-neering breakthroughs, the limitations for biowaste to biohydrogengeneration route can be solved and viewed as a key and econom-ically viable component to a renewable energy based economy.

7. Conclusion

It has been reported that the waste utilization is the mosteconomical process for renewable energy production (biogas andhydrogen/biohydrogen) and to clean the environment (Kothariet al., 2010). This review has revealed that biohydrogen could beefficiently produced from renewable waste feedstock like foodwaste and food processing waste. These unused resources are beingcontinuously generated and can be made available to producebiohydrogen whenever necessary.

Compared to the lipid, protein and cellulose components, thecarbohydrate fraction in food waste plays an important role in thehydrolysis step during anaerobic degradation. Biohydrogen pro-duction is also influenced by several environmental factors such aspH, temperature, pre-treatment using seed culture and lowhydrogen partial pressure. Maintaining initial culture pH at 7.0 inbatch fermentation and at pH 5.5 in continuous fermentation hadbeen shown to enhance the activity of the hydrogenase enzymeYasin et al (2011). The other controlled environmental factors suchas the use of high temperature at 50e60 �C and heat shock to theseed culture could suppress hydrogen-consuming bacteria andenhance the growth of hydrogen-producing bacteria. Thus, it issuggested that enhanced biohydrogen production from food wasteand food processing waste can be achieved using controlled con-ditions as elaborated in this review. On another perspective, darkfermentation of food waste for biohydrogen production has thepotential to create an impact on the global energy market for theproduction of energy from a cheap and renewable carbon source.

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