The anaerobic digestion of solid organic waste

Azeem Khalid a,⇑, Muhammad Arshad b, Muzammil Anjuma, Tariq Mahmood a, Lorna Dawson c

aDepartment of Environmental Sciences, PMAS Arid Agriculture University, Rawalpindi-46300, Pakistanb Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad-38040, Pakistanc The James Hutton Institute, Craigiebuckler Aberdeen AB15 8QH, Scotland, UK

a r t i c l e i n f o

Article history:Received 17 May 2010Accepted 30 March 2011Available online 6 May 2011

a b s t r a c t

The accumulation of solid organic waste is thought to be reaching critical levels in almost all regions ofthe world. These organic wastes require to be managed in a sustainable way to avoid depletion of naturalresources, minimize risk to human health, reduce environmental burdens and maintain an overall bal-ance in the ecosystem. A number of methods are currently applied to the treatment and managementof solid organic waste. This review focuses on the process of anaerobic digestion which is consideredto be one of the most viable options for recycling the organic fraction of solid waste. This manuscript pro-vides a broad overview of the digestibility and energy production (biogas) yield of a range of substratesand the digester configurations that achieve these yields. The involvement of a diverse array of microor-ganisms and effects of co-substrates and environmental factors on the efficiency of the process hasbeen comprehensively addressed. The recent literature indicates that anaerobic digestion could be anappealing option for converting raw solid organic wastes into useful products such as biogas and otherenergy-rich compounds, which may play a critical role in meeting the world’s ever-increasing energyrequirements in the future.

� 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17372. Anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17383. Anaerobic co-digestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17384. Anaerobic bioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17395. Biogas yield from anaerobic digestion of solid organic waste. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17396. Role of microorganisms in anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17417. Factors affecting anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1741

7.1. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17417.2. pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17417.3. Moisture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17417.4. Substrate/carbon source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17427.5. Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17427.6. C/N Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1742

8. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1742References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1742

1. Introduction

Solid organic waste removal has become an ecological problem,brought to light as a result of an increase in public health concerns

and environmental awareness. The average solid waste generationrate in 23 developing countries is 0.77 kg/person/day (Troschinetzand Mihelcic, 2009) and is increasing. At present, worldwide muni-cipal solid waste generation is about two billion tons per year,which is predicted to increase to 3 billion tons by 2025 (Charleset al., 2009). The production of fruit and vegetable waste is alsovery high and becoming a source of concern in municipal landfillsbecause of its high biodegradability (Bouallagui et al., 2005).

⇑ Corresponding author. Tel.: +92 51 9062221, 4420827; fax: +92 51 9290160.E-mail addresses: azeemuaf@yahoo.com, azeem@uaar.edu.pk (A. Khalid).

Reproduced from Waste Management, 31: 1737-1744 (2011).

Azeem Khalid: Participant of the 30th UM, 2002-2003, & the 2nd UO, 2012-2013.

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Recently, the organic fraction of solid waste has been recog-nized as a valuable resource that can be converted into usefulproducts via microbially mediated transformations (Yu and Huang,2009; Lesteur et al., 2010). There are various methods available forthe treatment of organic waste but anaerobic digestion appears tobe a promising approach (Lee et al., 2009c). Anaerobic digestion in-volves a series of metabolic reactions such as hydrolysis, acidogen-sis and methanogensis (Themelis and Ulloa, 2007). Anaerobicdigestion of organic waste in landfills releases the gases methaneand carbon dioxide that escape into the atmosphere and pollutethe environment (Zhu et al., 2009). Under controlled conditionsthe same process has the potential to provide useful products suchas biofuel and organic amendment (soil conditioner) and the treat-ment system does not require an oxygen supply (Chanakya et al.,2007; Guermoud et al., 2009). Further, methane and hydrogen aspotential fuels are considered comparatively cleaner than fossilfuel. In addition, this has the benefit of not depending on fossil fuelfor energy consumption (Jingura and Matengaifa, 2009). Thus,anaerobic digestion represents an opportunity to decrease envi-ronmental pollution and at the same time, providing biogas and or-ganic fertilizer or carrier material for biofertilizers.

The anaerobic treatment of solid organic waste is not as wide-spread as the aerobic process, mainly due to the longer time re-quired to achieve biostabilization (Fernandez et al., 2010). Theprocess is also sensitive to high levels of free ammonia resultingfrom anaerobic degradation of the nitrogen rich protein compo-nents (Fountoulakis et al., 2008). The specific activity of methano-genic bacteria has been found to decrease with increasingconcentrations of ammonia (Chen et al., 2008).

Recent advancements in bioreactor designs have increased theuse of anaerobic digestion for the treatment of solid organic waste.To date, a number of novel bioreactor designs have been developedwhere anaerobic digestion can be performed at a much higher ratethan the conventional methods. Many factors, including the typeand concentration of substrate, temperature, moisture, pH, etc.,may affect the performance of the anaerobic digestion process inthe bioreactor (Behera et al., 2010; Jeong et al., 2010). This manu-script reviews the potential of anaerobic digestion for recycling so-lid organic waste.

There are some components in wastes (metals and some recal-citrant organic compounds, often toxic) that will not be brokendown and will therefore become concentrated in the residue.Although this is an important consideration and must be consid-ered when applying digestions, it is a huge topic and will not be re-viewed here.

2. Anaerobic digestion

With the introduction of both commercial and pilot anaerobicdigestion plant designs during early 1990s, anaerobic digestion oforganic waste has received worldwide attention (Karagiannidisand Perkoulidis, 2009). It is a process by which almost any organicwaste can be biologically transformed into another form, in the ab-sence of oxygen. The diverse microbial populations degrade organ-ic waste, which results in the production of biogas and otherenergy-rich organic compounds as end products (Lastella et al.,2002; Lata et al., 2002). A series of metabolic reactions such ashydrolysis, acidogenesis, acetogenesis and methanogenesis are in-volved in the process of anaerobic decomposition (Park et al., 2005;Charles et al., 2009).

Anaerobic digestion is applicable for a wide range of materialincluding municipal, agricultural and industrial wastes, and plantresidues (Kalra and Panwar, 1986; Gallert et al., 1998; Chenet al., 2008). Furthermore, this process has some advantages overaerobic process due to a low energy requirement for operation

and a low biomass production (Wang et al., 1999; Steyer et al.,2002; Angenent et al., 2004; Kim et al., 2006), and it is considereda viable technology in the competent treatment of organic wasteand the simultaneous production of a renewable energy (De Baere,2006; Jingura and Matengaifa, 2009).

The anaerobic digestion of organic waste is also an environmen-tally useful technology. Ward et al. (2008) described the benefits ofthis process to reduce environmental pollution in two main ways:the sealed environment of the process prevents exit of methaneinto the atmosphere, while burning of the methane will releasecarbon–neutral carbon dioxide (no net effect on atmospheric car-bon dioxide and other greenhouse gases).

On the other hand, the anaerobic process has some disadvan-tages such as long retention times and low removal efficienciesof organic compounds (Park et al., 2005). The chemical composi-tion and structure of lignocellulosic materials hinders the rate ofbiodegradation of solid organic waste. It has been documented thathydrolysis of the complex organic matter to soluble compounds isthe rate-limiting step of anaerobic processes for wastes with a highsolid content (Chulhwan et al., 2005; Mumme et al., 2010). Conse-quently, various physical, chemical and enzymatic pre-treatmentsare required to increase substrate solubility and accelerate the bio-degradation rate of solid organic waste (Torres and Llorens, 2008;Charles et al., 2009).

3. Anaerobic co-digestion

Co-digestion is a waste treatment method in which differentwastes are mixed and treated together (Agdag and Sponza,2007). It is also termed as ‘‘co-fermentation’’. Co-digestion is pref-erably used for improving yields of anaerobic digestion of solid or-ganic wastes due to its numeral benefits. For example, dilution oftoxic compounds, increased load of biodegradable organic matter,improved balance of nutrients, synergistic effect of microorgan-isms and better biogas yield are the potential benefits that areachieved in a co-digestion process. Co-digestion of an organicwaste also provides nutrients in excess (Hartmann and Ahring,2005), which accelerates biodegradation of solid organic wastethrough biostimulation. Additionally, digestion rate and stabiliza-tion are increased (Sosnowski et al., 2003; Lo et al., 2010). Jinguraand Matengaifa (2009) described the following multiple benefits ofco-digestion: the facilitation of a stable and reliable digestion per-formance and production of a digested product of good quality, andan increase in biogas yield.

It has been observed that co-digestion of mixtures stabilizes thefeed to the bioreactor, thereby improving the C/N ratio anddecreasing the concentration of nitrogen (Cuetos et al., 2008).The use of a co-substrate with a low nitrogen and lipid contentwaste increases the production of biogas due to complementarycharacteristics of both types of waste, thus reducing problemsassociated with the accumulation of intermediate volatile com-pounds and high ammonia concentrations (Castillo et al., 2006).

Several studies have shown that mixtures of agricultural, muni-cipal and industrial wastes can be digested successfully and effi-ciently together (Table 1). A stimulatory effect on synthesis ofmethane gas has been observed when industrial sludge was co-digested with municipal solid waste (Agdag and Sponza, 2007).The co-digestion of municipal solid waste with an industrial sludgeratio of 1:2 yielded the highest amount methane gas, compared tomunicipal solid waste alone. Similarly, in a two-phase anaerobicdigestion system, Fezzani and Cheikh (2010) recorded the highestmethane productivity when a mixture of olive mill wastewater andolive mill solid waste was co-digested. The process has also beenuseful in obtaining a valuable sludge which can eventually be usedas a soil amendment after minor treatments (Gomez et al., 2006).

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4. Anaerobic bioreactors

Anaerobic bioreactors have potential application for rapiddigestion of solid organic waste constituents to reduce the envi-ronmental load as compared to conventional sanitary landfills(Agdag and Sponza, 2007). Bioreactor design has been found toexert a strong influence on the performance of a digester (Williamand David, 1999). As described earlier (Table 2), a variety of newbioreactor designs have been developed in recent years, whichfacilitate a significantly higher rate of reaction for the treatmentof waste (Bouallagui et al., 2003; Mumme et al., 2010; Xing et al.,2010). According to Ward and his co-workers, an anaerobic biore-actor should be designed in a way that allows a continuously highand sustainable organic load rate with a short hydraulic retentiontime and has the ability to produce the maximum level of methane(Ward et al., 2008).

Several types of bioreactors are currently in use but the threemajor groups of bioreactors commonly in use include batch reac-tors, a one stage continuously fed system and a two stage or mul-ti-stage continuously fed system. Batch reactors are the simplest,filled with the feedstock and left for a period that can be consid-ered to be the hydraulic retention time, after which they are emp-tied. Anaerobic batch reactors are useful because they can performquick digestion with simple and inexpensive equipment, and alsoare helpful in assessing the rate of digestion easily (Parawiraet al., 2004;Weiland, 2006). On the other hand, batch reactors havesome limitations such as high fluctuations in gas production aswell as gas quality, biogas losses during emptying the bioreactorsand restricted bioreactor heights (Linke et al., 2006). The secondtype of bioreactors is known as ‘one-stage continuously fed sys-tems’, where all the biochemical reactions take place in one biore-actor. The third type of bioreactors are ‘two-stage’ or ‘multi-stagecontinuously fed systems’, in which various biochemical processessuch as hydrolysis, acidification, acetogenesis and methanogenesis

take place separately (Ward et al., 2008). The two-stage system isconsidered a promising process to treat organic wastes with highefficiency in term of degradation yield and biogas production(Fezzani and Cheikh, 2010). According to Demirer and Chen(2005), a two stage system allows the selection and enrichmentof different bacteria in each phase. The complex organic materialsare degraded by acidogenic bacteria to volatile fatty acids and alco-hols, which are then easily metabolized into methane and carbondioxide by methanogens or archaea. Further, this type of systemincreases the stability of the process by controlling the acidificat-ion phase through optimization of the hydraulic retention timeto prevent overloading and the build-up of toxic material. The bio-mass concentration and other conditions can also be optimizedindependently for each stage (Demirer and Chen, 2005). All theabove three types of bioreactors, along with a variety of methaniz-ers such as continuously stirred tank bioreactor, tubular bioreactor,anaerobic sequencing batch bioreactor, up flow anaerobic sludgeblanket and anaerobic filters are applied for the treatment ofdifferent types of waste (Bouallagui et al., 2005).

Biodigesters are also classified as ‘‘wet’’ or ‘‘dry’’ solid wastedigesters. According to Ward et al. (2008), wet bioreactors have to-tal solids of 16% or less, while dry bioreactors contain 22–40% totalsolids, with the intermediate rating termed ‘semi dry’, whileaccording to Karagiannidis and Perkoulidis (2009), dry systemscontain 30–40% dry matter where as wet systems contain 10–25% dry matter. Another type of bioreactor based on operatingtemperatures (i.e., thermophilic or mesophilic) are also available(Kuo and Cheng, 2007; Karagiannidis and Perkoulidis, 2009).

5. Biogas yield from anaerobic digestion of solid organic waste

The process of biogas generation from solid organic waste is of-ten carried out by several different anaerobic bacteria (Jingura and

Table 1Relative biogas production rates and methane yield from the co-digestion of solid organic waste.

Substrate Co-substrate Biogas productionrate (l/d)

Methane yield(l/kg VS*)

Comments References

Cattle excreta Olive mill waste 1.10 179 The co-digestion system produced337% higher biogas than that ofexcreta alone

Goberna et al. (2010)

Cattle manure Agricultural waste andenergy crops

2.70 620 Significant increase in biogasproduction from the co-digestion wasobserved

Cavinato et al. (2010)

Fruit and vegetable waste Abattoir wastewater 2.53 611 The addition of abattoir wastewaterto the feedstock increased biogasyield up to 51.5%

Bouallagui et al. (2009a)

Municipal solid waste Fly ash 6.50 222 Application of fly ash significantlyenhanced biogas production rates ofthe municipal solid waste

Lo et al. (2010)

Municipal solid wastes Fat, oil and grease wastefrom sewage treatmentplants

13.6 350 Co-digestion resulted in an increaseof 72% in biogas production and 46%methane yield in comparison withmunicipal solid waste

Martin-Gonzalez et al. (2010)

Pig manure Fish and bio-diesel waste 16.4 620 Highest biogas production rate wasobtained by a mixture of wastes

Alvarez et al. (2010)

Potato waste Sugar beet waste 1.63 680 Co-digestion improved methane yieldup to 62% compared to the digestionof potato waste alone

Parawira et al. (2004)

Primary sludge Fruit and vegetable waste 4.40 600 Co-digestion produced more biogasas compared to primary sludge alone

Gomez et al. (2006)

Sewage sludge Municipal solid waste 3.00 532 Biogas production of the mixturesincreased with increasingproportions of the municipal solidwaste

Sosnowski et al. (2003)

Slaughter house waste Municipal solid waste 8.60 500 Biogas yield of the co-digestionsystems doubled that of the slaughterhouse waste digestion system

Cuetos et al. (2008)

* VS: Volatile solids.

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Matengaifa, 2009). Several reports indicate that anaerobic diges-tion of the organic fraction of solid waste yields promisingamounts of biogas (Table 3). Biogas is generally composed of 48–65% methane, 36–41% carbon dioxide, up to 17% nitrogen, <1% oxy-gen, 32–169 ppm hydrogen sulphide and traces of other gases(Ward et al., 2008). Unlike fossil fuel, biogas does not contributemuch to the greenhouse effect, ozone depletion or acid rain (Nathand Das, 2004). This is one of the main reasons that anaerobicdigestion may play a very crucial role in meeting energy challengesof the future generation.

The biogas yield is affected by many factors including type andcomposition of substrate, microbial composition, temperature,moisture and bioreactor design, etc. Hernandez-Berriel et al.(2008) studied the methane production from biodegradation ofmunicipal solid waste. They found that the process reached the on-set of the methanogenic phase at day 63 and the methane produc-tion rate was greater at a moisture level of 70%. However, adecrease in biogas production was observed in the case of fruitand vegetable waste due to rapid acidification of these wastes,resulting in a lowering of the pH in the bioreactor. Moreover, pro-duction of larger volatile fatty acids from such waste under anaer-obic conditions inhibits the activity of methanogenic bacteria. Theaddition of co-substrates such as abattoir waste water and acti-vated sludge to fruit and vegetable waste can enhance biogas pro-duction up to 52% (Bouallagui et al., 2009a). Behera et al. (2010)examined methane production from food waste leachate (FWL)

in simulated landfill bioreactors (lysimeters) for a period of 90 dayswith four different inoculum–substrate ratios (ISRs). The maxi-mum methane yield was achieved in the lysimeter at ISR of 1:1.Based on the results obtained from this study, the authors

Table 2Different types of bioreactors used for anaerobic digestion.

Bioreactor type Type of substrate Organic loadingrate (kg/m3/d)

Comments References

Anaerobic sequencing batchbioreactor

Fruit and vegetablewaste and abattoirwastewater

2.6 A decrease in biogas production wasobserved due to high amount of freeammonia at high organic loading rate(OLR)

Bouallagui et al. (2009b)

Continuously stirred tankreactors

Municipal solid waste 15 The reactor showed superior processperformance as the OLR progressivelyincreased up to 15 kg/m3/d

Angelidaki et al. (2006)

Full-scale anaerobicdigester

Industrial food waste 17 Methane yield of 360 l/kg feed waste with40 days retention time was observed

Ike et al. (2010)

Integrative biologicalreactor

Kitchen waste 8.0 Integrative biological reactor showed thebest performance and biogas productionrate was higher than the single reactor

Guo et al. (2011)

Laboratory-scale semicontinuous reactors

Municipal solid wasteand press water frommunicipal compostingplant

20 The reactor performance for biogasproduction was higher up to 20 OLR butfurther increase in OLR did not affect thebiogas production

Nayono et al. (2010)

New starch basedflocculant-anaerobicfluidized bed bioreactor

Primary treatedsewage effluent with orwithout refractoryorganic pollutants

43 The efficiency and microbial activity athigh OLR was higher than conventionalanaerobic fluidized bed bioreactor

Xing et al. (2010)

Rotating drum mesh filterbioreactor

Municipal solid waste 15 The reactor proved to be stable andhelpful in mixing the waste at high OLR,which is usually not possible inmechanically stirred digesters

Walker et al. (2009)

Self mixing anaerobicdigesters

Poultry litter 16 Self mixing at high OLR and highbiomethanization of the poultry litter wasobserved

Rao et al. (2011)

Submerged anaerobicmembrane bioreactor

Sewage sludge, foodwaste and livestockwastewater

1.8 The reactor showed unstable performanceduring the initial stage, but performedsuperior after acclimation formation

Jeong et al. (2010)

Two-phase anaerobic semi-continuous digester

Olive mill wastewaterand olive mill solidwaste

14 The best performance in terms of methaneproductivity, soluble COD and phenolremoval efficiencies and effluent qualitywas observed

Fezzani and Cheikh (2010)

Two stage anaerobichydrogen and methaneproduction reactor

Organic waste 3.0 Compared to a single-stage methanogenicreactor, 11% higher energy was achieved

Luo et al. (2011)

Up flow anaerobic solid-state bioreactor

Mixture of maize silageand straw

17 The UASS reactor showed the highestmethanogenic performance for thedigestion of solid biomass

Mumme et al. (2010)

Table 3Biogas yield recorded from anaerobic digestion of the solid organic waste.

Substrate Methane yield(l/kg VS*)

References

Municipal solid waste 360 Vogt et al. (2002)Fruit and vegetable wastes 420 Bouallagui et al. (2005)Municipal solid waste 530 Forster-Carneiro et al. (2007)Fruit and vegetable waste,

and abattoir wastewater850 Forster-Carneiro et al.

(2007)Swine manure 337 Ahn et al. (2009)Municipal solid waste 200 Walker et al. (2009)Food waste leachate 294 Behera et al. (2010)Rice straw 350 Lei et al. (2010)Maize silage and straw 312 Mumme et al. (2010)Jatropha oil seed cake 422 Chandra et al. (2011)Palm oil mill waste 610 Fang et al. (2011)Household waste 350 Ferrer et al. (2011)Lignin-rich organic waste 200 Jayasinghe et al. (2011)Swine manure and winery

wastewater348 Riano et al. (2011)

Food waste 396 Zhang et al. (2011)

* VS: Volatile solids.

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concluded that the bioreactors with efficient leachate collectionand gas recovery facilities could be effective to treat non-hazard-ous liquid organic wastes for energy recovery. Likewise, Lee andhis co-workers (2009b) suggested that anaerobic digestion ofFWL in bioreactor landfills or anaerobic digesters (with a preferredcontrol of alkalinity and salinity) could be a sustainable solution toconvert biomass to biogas and achieve high biodegradabilitypotential.

6. Role of microorganisms in anaerobic digestion

The anaerobic digestion process can be catalyzed by a variety ofmicroorganisms that convert complex macromolecules into lowmolecular weight compounds. An inoculum source is crucial forthe optimization of the waste/inoculum ratio (Lopes et al., 2004;Forster-Carneiro et al., 2007). Sludge is commonly used as inocu-lum for the treatment of waste (Forster-Carneiro et al., 2007; Donget al., 2009); although naturally selected strains or artificiallymixed strains of microorganisms are also employed. In addition,cell aggregates in the form of flocs, biofilms, granules, and mats,with dimensions that typically range from 0.1 to 100 mm may alsobe used in the treatment system (Jeong et al., 2010).

A wide variety of microbial communities have been reported tobe involved in the anaerobic decomposition process. Fricke et al.(2007) reported that organic material is most likely decomposedby heterotrophic microorganisms. Lee et al. (2009a) reported thatClostridium species are most common among the degraders underanaerobic condition. However, it is very unusual for a biologicaltreatment to rely solely on a single microbial strain and generallya microbial consortium is responsible for the anaerobic digestionprocess (Fantozzi and Buratti, 2009). According to Ike et al.(2010), a group of microorganisms such as actinomyces, Thermo-monospora, Ralstonia and Shewanella are involved in the degrada-tion of food waste into volatile fatty acids, but Methanosarcinaand Methanobrevibacter/Methanobacterium mainly contribute inmethane production. Similarly, Charles et al. (2009) reported thepresence of Methanosarcina thermophila, Methanoculleus thermo-philus, and Methanobacterium formicicum during anaerobic diges-tion. Using denaturing gradient gel electrophoresis and DNAsequencing techniques, Trzcinski et al. (2010) found hydrogeno-trophic species (mainly, Methanobrevibacter sp., M. formicicumand Methanosarcina sp.) active in methane synthesis. An increasein methane content was also observed with the increase in thenumber of hydrogenotrophic species (Trzcinski et al., 2010). How-ever, high concentration of organic acid like acetic acid(>5000 mg L�1) and butyric acid (>3000 mg L�1) in the biodigesterhas been found to inhibit the growth of microorganisms and con-sequently the production of energy rich compounds (Kim et al.,2008).

7. Factors affecting anaerobic digestion

The anaerobic digestion of organic material is a complex pro-cess, involving a number of different degradation steps. The micro-organisms that participate in the process may be specific for eachdegradation step and thus could have different environmentalrequirements.

7.1. Temperature

Many researchers have reported significant effects of tempera-ture on the microbial community, process kinetics and stabilityand methane yield (Dela-Rubia et al., 2002; Bouallagui et al.,2009b; Riau et al., 2010). Lower temperatures during the processare known to decrease microbial growth, substrate utilization

rates, and biogas production (Kim et al., 2006; Trzcinski andStuckey, 2010). Moreover, lower temperatures may also result inan exhaustion of cell energy, a leakage of intracellular substancesor complete lysis (Kashyap et al., 2003). In contrast, high tempera-tures lower biogas yield due to the production of volatile gasessuch as ammonia which suppresses methanogenic activities(Fezzani and Cheikh, 2010).

Generally, anaerobic digestion is carried out at mesophilic tem-peratures (El-Mashad et al., 2003). The operation in the mesophilicrange is more stable and requires a smaller energy expense(Fernandez et al., 2008; Ward et al., 2008). Castillo et al. (2006)found that the best operational temperature was 35 �C with an18 day digestion period while a little fluctuation in temperaturefrom 35 �C to 30 �C caused a reduction in the rate of biogas produc-tion (Chae et al., 2008). Overall, a temperature range between 35–37 �C is considered suitable for the production of methane and achange from mesophilic to thermophilic temperatures can causea sharp decrease in biogas production until the necessary popula-tions have increased in number. Briski et al. (2007) reported thatfor biodegradation, the temperature must be below 65 �C becauseabove 65 �C denaturation of enzymes occurs. However, thermo-philic conditions have certain advantages, such as a faster degrada-tion rate of organic waste, higher biomass and gas production, lesseffluent viscosity and higher pathogen destruction (Zhu et al.,2009). Ward et al. (2008) has shown optimal growth temperaturesfor some methanogenic bacteria: 37–45 �C for mesophilic Methan-obacterium, 37–40 �C for Methanobrevibacter, 35–40 �C for Metha-nolobus, Methanococcus, Methanoculleus, Methanospirillum andMethanolobus, 30–40 �C for Methanoplanus and Methanocorpuscu-lum and 50–55 �C for thermophilic Methanohalobium andMethanosarcina.

7.2. pH

A range of pH values suitable for anaerobic digestion has beenreported by various researchers, but the optimal pH for methano-genesis has been found to be around 7.0 (Huber et al., 1982; Yangand Okos, 1987). Agdag and Sponza (2007) reported a very narrowrange of suitable pH (7.0–7.2) in the industrial sludge added biore-actors during the last 50 days of the anaerobic incubation. Simi-larly, Ward et al. (2008) found that a pH range of 6.8–7.2 wasideal for anaerobic digestion. Lee et al. (2009b) reported that met-hanogenisis in an anaerobic digester occurs efficiently at pH 6.5–8.2, while hydrolysis and acidogenesis occurs at pH 5.5 and 6.5,respectively (Kim et al., 2003). From the batch experiments, itwas shown that the appropriate pH range for thermophilic acido-gens was 6–7 (Park et al., 2008). Dong et al. (2009) suggested thatthe hydrogen production will be at a maximum if the initial pH of abiosystem is maintained at 9. However, similar results can also beachieved at pH 5–6 (Kapdan and Kargi, 2006). Liu et al. (2008)showed that the most favorable range of pH to attain maximal bio-gas yield in anaerobic digestion is 6.5–7.5.

7.3. Moisture

High moisture contents usually facilitate the anaerobic diges-tion; however, it is difficult to maintain the same availability ofwater throughout the digestion cycle (Hernandez-Berriel et al.,2008). Initially water added at a high rate is dropped to a certainlower level as the process of anaerobic digestion proceeds. Highwater contents are likely to affect the process performance by dis-solving readily degradable organic matter. It has been reportedthat the highest methane production rates occur at 60–80% ofhumidity (Bouallagui et al., 2003). Hernandez-Berriel et al. (2008)studied methanogenesis processes during anaerobic digestion atdifferent moisture levels i.e., 70% and 80%. They found that the

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onset of the methanogenic phase took place around day 70 in bothcases, at 70% and 80% moisture. However, bioreactors under the70% moisture regime produced a stronger leachate and conse-quently a higher methane production rate. At the end of the exper-iment, 83 ml methane per gram dry matter were produced at the70% moisture level, while 71 ml methane per gram dry matterwere produced with the 80% moisture. Nonetheless, bioreactorsfrom both moisture regimes showed similar ratios (0.68) of bio-chemical oxygen demand (BOD) to chemical oxygen demand(COD).

7.4. Substrate/carbon source

The rate of anaerobic digestion is strongly affected by the type,availability and complexity of the substrate (Ghaniyari-Benis et al.,2009; Zhao et al., 2010). Different types of carbon source supportdifferent groups of microbes. Before starting a digestion process,the substrate must be characterized for carbohydrate, lipid, proteinand fiber contents (Lesteur et al., 2010). In addition, the substrateshould also be characterized for the quantity of methane that canpotentially be produced under anaerobic conditions. Carbohy-drates are considered the most important organic component ofmunicipal solid waste for biogas production (Dong et al., 2009).However, starch could act as an effective low cost substrate for bio-gas production compared to sucrose and glucose (Su et al., 2009). Itwas reported that the initial concentration and total solid contentof the substrate in the bioreactor can significantly affect the perfor-mance of the process and the amount of methane produced duringthe process (Fernandez et al., 2008).

7.5. Nitrogen

Nitrogen is essential for protein synthesis and primarily requiredas a nutrient by the microorganisms in anaerobic digestion(Kayhanian and Rich, 1995). Nitrogenous compounds in the organicwaste are usually proteins which are converted to ammonium byanaerobic digestion (Sawayama et al., 2004). In the form of ammo-nium, nitrogen contributes to the stabilization of the pH value inthe bioreactor where the process is taking place. Microorganismsassimilate ammonium for the production of new cell mass. A nutri-ent ratio of the elements C:N:P:S at 600:15:5:3 is considered suffi-cient for methanization (Fricke et al., 2007). Ammonia in highconcentration may lead to the inhibition of the biological processand it inhibitsmethanogenesis at concentrations exceeding approx-imately 100 mM(Fricke et al., 2007). Sterling et al. (2001) found thatthe amount of ammonia in the digester may also affect the produc-tion of hydrogen and removal of volatile solids. Total biogas produc-tion was unaffected by small increases in ammonia nitrogen whilehigher increases reduced the biogas production by 50% of the origi-nal rate. In the fluidized-bed anaerobic digester, themethane forma-tion decreased at ammonium concentrations of greater than6000 mg NH4–N/l. It was reported that methanogenic activity is de-creased by 10% at ammonium concentrations of 1670–3720 mgNH4–N/l, while by 50% at 4090–5550 mg NH4–N/l, and completelyzero at 5880–6000 mg NH4–N/l (Sawayama et al., 2004).

7.6. C/N Ratio

The C/N ratio in the organic material plays a crucial role inanaerobic digestion. The unbalanced nutrients are regarded as animportant factor limiting anaerobic digestion of organic wastes.For the improvement of nutrition and C/N ratios, co-digestion of or-ganic mixtures is employed (Cuetos et al., 2008). Co-digestion offish waste, abattoir wastewater and waste activated sludge withfruit and vegetable waste facilitates balancing of the C/N ratio.Their greatest advantage lies in the buffering of the organic loading

rate, and anaerobic ammonia production from organic nitrogen,which reduce the limitations of fruit and vegetable waste diges-tion. The C/N ratio of 20–30 may provide sufficient nitrogen forthe process (Weiland, 2006), and Bouallagui et al. (2009a) sug-gested that a C/N ratio between 22 and 25 seemed to be best foranaerobic digestion of fruit and vegetable waste, whereas,Guermoud et al. (2009) and Lee et al. (2009b) reported that theoptimal C/N ratio for anaerobic degradation of organic waste was20–35.

8. Conclusions

The preceding review clearly indicates that anaerobic digestionis one of the most effective biological processes to treat a widevariety of solid organic waste products and sludge. The primeadvantages of this technology include (i) organic wastes with alow nutrient content can be degraded by co-digesting with differ-ent substrates in the anaerobic bioreactors, and (ii) the processsimultaneously leads to low cost production of biogas, which couldbe vital for meeting future energy-needs. However, different fac-tors such as substrate and co-substrate composition and quality,environmental factors (temperature, pH, organic loading rate),and microbial dynamics contribute to the efficiency of the anaero-bic digestion process, and must be optimized to achieve maximumbenefit from this technology in terms of both energy productionand organic waste management. The use of advanced moleculartechniques can further help in enhancing the efficiency of this sys-tem by identifying the microbial community structure and func-tion, and their ecological relationships in the bioreactor. Thistechnology has tremendous application in the future for sustain-ability of both environment and agriculture, with the productionof energy as an extra benefit.

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