Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242

Contents lists available at SciVerse ScienceDirect

Renewable and Sustainable Energy Reviews

j ourna l ho me pa ge: www.elsev ier .com/ locate / rse r

Anaerogenera

TasneemCentre for Pollu

a r t i c l

Article history:Received 20 JuReceived in reAccepted 18 FAvailable onlin

Keywords:MethaneBiogasGlobal warminLandllsManureMiningRumenWetlands

Contents

1. Bioga1.1.

2. Bioga3. Sourc4. Huma

4.1. 4.2. 4.3. 4.4. 4.5. 4.6. 4.7.

5. Oppo6. Steps 7. Factor

7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8.

CorresponE-mail add

1364-0321/$ doi:10.1016/j.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3229A brief history of biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3229

s and global warming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3230es of methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3230n-related sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3231Landlls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3231Natural gas and petroleum systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3231Coal mining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3231Livestock enteric fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3231Handling manure management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3231Wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3231Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3232

rtunities of methane capture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3232associated with the generation of biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3233s which inuence anaerobic digestion of an organic substrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3234Specic surface of the substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234C/N ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234Dilution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3235Loading rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3236Retention time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3236Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3236

ding author. Tel.: +91 9443265262; fax: +91 4132655263.ress: prof.s.a.abbasi@gmail.com (S.A. Abbasi).

see front matter 2012 Elsevier Ltd. All rights reserved.rser.2012.02.046bic digestion for global warming control and energytionAn overview

Abbasi, S.M. Tauseef, S.A. Abbasi

tion Control and Environmental Engineering, Pondicherry University, Pondicherry 605014, India

e i n f o

ne 2011vised form 13 February 2012ebruary 2012e 28 March 2012

g

a b s t r a c t

Anaerobic digestion often generates biogas an approximately 3:1 mixture of methane and carbondioxide which has been known to be a clean fuel since the late 19th century. But a great resurgenceof interest in biogas capture hence methane capture has occurred in recent years due to the rapidlygrowing spectre of global warming. Anthropogenic causes which directly or indirectly release methaneinto the atmosphere, are responsible for as much as a third of the overall additional global warming that isoccurring at present. Hence the dual advantage of methane capture generating energy while controllingglobal warming have come to the fore.

This paper presents an overview of the natural and the anthropogenic sources that contribute methaneto the atmosphere. In this context it underscores the urgency with which the world must develop andenforce methods and practices to enhance methane capture.

2012 Elsevier Ltd. All rights reserved.

T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242 3229

7.9. Mixing/agitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32367.10. Pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32377.11. Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32377.12. Solid residue/slurry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3237

8. Anaerobic digesters/reactors/fermenters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32379. Low-rate and high-rate anaerobic reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323710. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3239

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3239References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3239

1. Biogas

When organic matter such as food, plant debris, animalmanure, sewage sludge, and biodegradable portions of municipalsolid waste undergoes decomposition in absence of free oxygen,it normally generates a gas which consists of 4070% methane, therest being mostly carbon dioxide with traces of other gases [13].If ignited, this gas burns cleanly (i.e. gives off no soot or foul smell)similar to Lural gas). Thand imprecdecompositit is also a rthe word bcombustiblis generateBiogas has (Table 1).

It must bonly gas pothe two, minvolved inditions, andsuch as hydmethane [5nature and generation referred as

1.1. A brief

Generathas been wthe phrase wa bluish moground [7].of methanepresent abotion of the w

scholar Pliney, who noted it around 50 BC. There is evidence thatin the years around 10 BC, biogas was used in Assyria for heatingbaths but little information is available about later years [8].

In the 17th century, Van Helmont also recorded that decayingorganic material produced ammable gases. In 1776, Volta resolvedthat there was a direct connection between how much organicmaterial was used and how much gas the material produced [9,10].That this combustible gas is methane was established by the work

ted i41ham

of mroug

isolceticlose

mic dioxndingals oen, cen thed thsumown984 Lof hotion

for tt byas w

Thesee prermyear re cothe 1uel mped c

Table 1Comparison of

Fuel

Petrol Natural gasLiqueed naLiqueed peKerosene Diesel CNGBiogas

a Natural gab Direct CO2PG (liqueed petroleum gas) or CNG (compressed nat-is gas is commonly called biogas which is an inexact

ise term because the gas which is produced by aerobicion (carbon dioxide) is also biogas in the sense thatesult of biodegradation just as the other biogas is. Butiogas has come to be used exclusively to denote thee CH4CO2 mixture (besides traces of other gases) thatd by the anaerobic decomposition of organic matter.good caloric value, though lesser than LPG and CNG

e mentioned that a mixture of CH4 and CO2 is not thessible by anaerobic degradation of organic matter. Ofethane is produced only if methanogenic bacteria are

the anaerobic decomposition [4]. Under different con- with other species of anaerobic microorganisms, gasesrogen and hydrogen sulde may be generated instead of,6]. But methanogenic bacteria occur very commonly inin most instances anaerobic digestion does result in theof the predominantly CH4CO2 mixture which is widelybiogas.

history of biogas

ion of methane by anaerobic digestion of organic matteritnessed since time immemorial and has given rise toill-o-the-wisp which has been inspired by the sight of

ving light that is sometimes seen at night on soft wet We know now that this light is formed due to ignition

released by the anaerobic digestion of organic matterve or in wet soil. One of the rst records of the descrip-ill-o-the-wisp phenomena are attributed to the Roman

conducing 180

Becmationwas th1890s,gen, aof celludue tocarbonskis materihydroghydrogassumThis asnow kn

In 1tation producmentson it buon biogdung. could b

In Gin the and moup till fossil fdevelo

the caloric values of various fuels [165,166].

Caloric value (CV) (approximate)

10 800 kcal per kg

8600 kcal per m3

tural gas 13 140 kcal per kg troleum gas 10 800 kcal per kg

10 300 kcal per kg 10 700 kcal per kg 8600 kcal per m3

5000 kcal per m3

s European Union mix.emissions (emission factor, gCO2e/kWh).ndependently by John Dalton and Humphrey Davy dur-808 [11].p in 1868 and Popoff in 1875 reported that the for-

ethane during the decomposition of organic matterh a microbiological process [12,13]. Omelianski, in theated microbes responsible for the release of hydro-

acid and butyric acid during methane fermentation [14]. He also reported that methane perhaps formedroorganism-mediated reaction between hydrogen andide [15]. Later, in 1910, Sohngen seconded Omelian-s [14]. He also reported that fermentation of complexccurs through oxidationreduction reactions to formarbon dioxide and acetic acid. He demonstrated thaten reacts with carbon dioxide to form methane. He alsoat acetic acid through decarboxylation forms methane.ption remained highly controversial for decades but is

to be essentially correct [15].ouis Pasteur produced 100 L of biogas from the fermen-rse dung collected from Paris roads. He claimed that this

rate should be sufcient to cover the energy require-he street lighting for Paris. Parisians did not follow up

1987 the street lamps of Exeter, in Britain, were runninghich was produced from wastewater rather than horse

developments suggested that more and more biogasoduced by anaerobic digestion of a variety of wastes [8].any methane gas was rst sold to the public gas works1923. In the following years the practice became moremmon in Europe. The popularity of biogas steadily rose950s when increasing availability and falling prices ofade biogas energy less and less attractive, especially inountries [8]. In developing countries, however, the use

Indirect emission factor(kgCO2e/GJ, net CV basis)

12.51

5.55a

20.008.00

13.3414.138.360.246b

3230 T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242

Table 2Top ve methane-emitting countries: 2005 [167].

Country Kt of CO2 equivalenta % of world totala

China 1,333,098.1 18.7IndiaUnited StateEuropean UnBrazil

a These metagriculture an

of biogas aspatronized increasingly

2. Biogas a

An entirdigestion hthe impactthe world hwarming ishyped-up pback), but a[9,1623].

It is alsogreenhousemore globacess organicthe waste tatmospherein the openis dumped of lakes andthe wastewleries and othese emit global warmof the decasinks whichbalance betposheric mefor thousandue to anthtributed to each molectimes greattive forcing(nitrous oxistitute rougon 100-yr gtics reveal tin methanetribution ocomparison

3. Sources

Methaneas stand-al(human-relactivities i(enteric feagriculture treatment/d[39,41,42].

Table 3Global estimates of natural methane sources [28].

Natural sources Average methaneux (Tg CH4/yr)

Range

nds 174 10023130 1050

tes 22 2029s, estuaries and rivers 9.1 2.315.6gical 9 414nimals 8 215tes 5 45res 3 25frost 0.5 01

260.6 157.3352.6

ns for global anthropogenic methane emissions (based on data from [168]).

Methane emission (TgCH4)

2010 2020 2030

es from natural gas and oil systems 75.96 85.19 93.89es from coal mining activities 24.54 32.08 37.63

nary and mobile combustion 11.65 13.80 16.92ss comenerg

lturec fermltivat

re managricu

rial prrial pr

lling of solid waste 38.05 40.72 43.34water 21.42 23.54 25.30waste sources 0.73 0.73 0.73

339.30 373.56 405.81

atural as well as anthropogenic sources. By now a concen- of about 1.8 ppm, on a mole fraction in dry air basis, hasuilt up in the atmosphere [43]. It presents atmospheric molen of 2.5 times higher than that observed in ice cores dated10001750 and is higher than that observed throughoutisting ice-core record, which spans the past 800,000 years]. The atmospheric increase of methane since 1750 implies583,977.6 8.2s of America 548,073.7 7.7ion 535,846.8 7.5

492,160.7 6.9

hane emissions are those stemming from human activities such asd from industrial methane production.

a source of energy has remained popular, and has beenby the governments. China and India in particular, have

expanding biogas programmes [16].

nd global warming

ely new dimension to the implications of anaerobicas been added in recent years. This has occurred afters of global warming have become apparent and afteras arrived at an almost complete consensus that global

neither a gment of some peoples imagination, nor anossibility (as a lot of people believed till a few years

very real and a very serious threat to the entire world

now a well-accepted fact that methane is a powerful gas, each molecule of methane causes about 25 timesl warming than a molecule of CO2 [24]. If we do not pro-

waste and recover methane from it but, instead, allowo rot in the open we will let the methane escape into

to cause global warming [17]. The dung or rumen lying, the biodegradable part of municipal solid waste whichhere and there; the dead plants decaying at the bottom

ponds; the human excreta or sewage disposed on land,aters high in COD of food processing, tanneries, distil-ther industries discharged in public swears, etc.,all ofmethane [2528]. Consequently they all contribute toing. Methane is anyway generated in nature as a result

y of plant and animal matter but there are also natural remove excess methane [29,30]. Due to this naturalween the sources and the sinks of methane, the tro-thane levels have hovered around 700 parts per billionds of years [3135]. But the extra methane generatedropogenic activities over the last 200 years has con-the rise of troposheric methane levels by 150% [36]. Asule of methane has global warming (GW) potential 25er than the GW potential of a molecule of CO2, the radia- by methane along with other non-CO2 Kyoto gasesde, hydrouorocarbons, peruorocarbons and SF6) con-hly one-third of total CO2 equivalent emissions basedlobal warming potentials [37]. The IEA (2012) statis-hat for the year 2005 (Table 2) China leads the world

emissions, followed by India and the USA. The con-f different sources of methane to global warming, in

to sources of CO2 and N2O, is represented in Fig. 1.

of methane

is emitted, either as a component of biogas or

WetlaLakesTermiOceanGeoloWild aHydraWild Perma

Total

Table 4Projectio

Sector

EnergyFugitivFugitivStatioBiomaOther

AgricuEnteriRice cuManuOther

IndustIndust

WasteLandWasteOther

Total

from ntrationbeen bfractioto AD the ex[44,45one emission, from a variety of both anthropogenicated) and natural sources [3840]. Anthropogenicnclude fossil fuel production, animal husbandryrmentation in livestock and handling of manure),(especially rice cultivation), biomass burning, andisposal systems for biodegradable liquid/solid wastesTables 3 and 4 show the global estimate of methane

Fig. 1. Relativthe world andbustion 9.70 10.24 10.91y sources 0.02 0.02 0.02

entation 90.74 100.15 108.98ion 34.87 34.88 35.20agement 11.28 11.87 12.55ltural sources 20.02 20.02 20.02

ocessocesses 0.32 0.32 0.32e contributions of different sources of greenhouse gas emissions in in Asia.

T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242 3231

Table 5Global methane emissions from enteric fermentation in 2004 [62].

Region/country Emissions (million tonnes CH4 per year by source)

Dairy cattle Other cattle Buffaloes Sheep and goats Pigs Total

Sub-Saharan Asiaa India China Central and west Asia an North Ameri Western Eur Oceania and Eastern Euro Other develo

Total

Livestock proGrazing Mixed Industrial

a Excludes C

anthropogethirds of pre

It is estimare related

Methanepermafrostnon-wetlanand wildre

The extecantly fromsuch as climagement prcapture [47nicant effethe key biolhuman-relation of techsuch as lanaffects the e

4. Human-

4.1. Landl

Methaneable compoanaerobic camount of mcontent of tthe site [49]of methanecountries, fbiggest antall methane

4.2. Natura

Natural losses occusion, and diin conjuncttion, and stoemissions [

al m

thane Mini thi

ng of

vesto

ong heepne as

(largnveranimtatioanimve prrom 4 tha

livesriculued trosse

andli Africa 2.30 7.47 0.000.84 3.83 2.401.70 3.94 5.250.49 5.12 1.25

South America 3.36 17.09 0.06d North Africa 0.98 1.16 0.24ca 1.02 3.85 0.00ope 2.19 2.31 0.01

Japan 0.71 1.80 0.00pe and CIS 1.99 2.96 0.02ped 0.11 0.62 0.00

15.69 50.16 9.23

duction system4.73 21.89 0.00

10.96 27.53 9.230.00 0.73 0.00

hina and India.

nic emissions of 340 50 Tg CH4 yr1, or nearly two-sent total emissions, assuming a constant lifetime [45].ated that more than 60% of global methane emissions

to these anthropogenic activities [24]. is also released in nature from wetlands, gas hydrates,

, termites and other rumens, oceans, freshwater bodies,d soils, and other sources such as degrading vegetations [46].nt of methane emission from a source can vary signi-

one country or region to another; depending on factorsate; manner of industrial, agricultural and waste man-actices; and extent of provision available for methane]. Temperature and moisture have a particularly sig-ct on the anaerobic digestion process, which is one ofogical processes that cause methane emissions in bothted and natural sources [16,47]. Also, the implementa-nologies to capture and utilize methane from sourcesdlls, coal mines, and manure management systemsmission levels from these sources [47].

related sources

ls

is generated in landlls and open dumps as biodegrad-

4.3. Co

Mestrata.unlockhandli

4.4. Li

Amfalo, smetharumention coby the fermenby the digestisions fin 200ted byand Agcontinhave c

4.5. Hnent of the waste contained in them decomposes underonditions (i.e. in absence of free oxygen) [48]. Theethane evolved depends on the quantity and moisture

he waste and the design and management practices at. Landlls are among the largest human-related sources

in developed countries [50]. In some of the developedor example the USA, landll also happens to be thehropogenic source of methane, accounting for 34% of

emissions [50].

l gas and petroleum systems

gas is largely made up of methane. Hence methaner during the production, processing, storage, transmis-stribution of natural gas [51]. Because gas is often foundion with oil; the production; renement, transporta-rage of crude oil also leads to similar fugitive methane

52,53].

Livestocdecompositteria exiteddeposited oform, produand holdingswine oper[62,66].

4.6. Wastew

In the cdomestic amatter, susptaminants, whenever aoften with 1.82 0.02 11.610.88 0.07 8.020.91 0.01 11.821.51 0.48 8.850.58 0.08 21.171.20 0.00 3.580.06 0.11 5.050.98 0.20 5.700.73 0.02 3.260.59 0.10 5.660.18 0.00 0.91

9.44 1.11 85.63

2.95 0.00 29.586.50 0.80 55.020.00 0.30 1.04

ining

lies trapped in coal deposits and in the surroundingng operations, in both underground and surface mines,s methane, leading to its release [54,55]. In addition,

the coal after mining results in methane emissions [39].

ck enteric fermentation

domesticated livestock, ruminant animals (cattle, buf-, goat, and camel) produce signicant amounts of

part of their normal digestive processes [5658]. In thee fore-stomach) of these animals, microbial fermenta-ts feed into products that can be digested and utilizedal [5961]. This microbial fermentation process (entericn) produces methane as a by-product, which is exhaledal. Methane is also produced in smaller quantities by theocesses of other animals, including humans, but emis-these sources are insignicant [47]. It was estimatedt about a million tonnes of methane was being emit-tock ([62]; Table 5). Considering that, as per the Foodtural Organization (FAO) the production of livestock haso increase dramatically [62], this gure is expected tod 90 million tonnes by now.

ng manure managementk manure keeps releasing methane due to the anaerobicion of organic material contained in the manure by bac-

along with the manure by the animal [6365]. Manuren elds and pastures, or otherwise handled in a dryces signicant amounts of methane. Manure lagoons

tanks, which are commonly used at larger dairy andations, also release signicant quantities of methane

ater treatment

ourse of treatment of biodegradable wastewater fromnd industrial sources for removing soluble organicended solids, pathogenic organisms, and chemical con-methane is produced and is released to atmospherenaerobic conditions develop [6770]. This may happenthe sludge that separates during sedimentation due to

3232 T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242

Table 6A summary of reports on methane emissions from paddy elds in India, indicating wide variations between site to site.

State/region Specic site Period and frequency of observations Methane emissions(mg/m2 h)

Reference

New DelhiNew Delhi n Orissa 4; 3 h

Uttar Prades one da

New Delhi h

Orissa 09.3

Orissa 4;

Orissa

New Delhi

Orissaembe1530

Orissa embe

New Delhi

Delhi

New Delhi

Assam

Orissa

Orissa

Assam Gujarat

Orissa Orissa

Punjab

Lucknow Uttar Prades

Orissa

Punjab

India

the high BOof dissolvedconditions, sions can besludge undreleased un

4.7. Agricul

Methaneconditions elds oodeenvironmenof organic [76,77]. Thecultivar, agNational Physical Laboratory 0, 15, and 30 min National Physical Laboratory Every 15 min for 30 miNot stated June to November, 199

over a 24 h periodh Institute of Agricultural Sciences,

Banaras Hindu UniversityAt 4 h intervals during month.

Indian Agricultural Research Institute,New Delhi

JulyNovember 2004;Interval of 15 min for 1

Central Rice Research Institute, Cuttack JuneOctober 1994; 9.015.0015.30

Not stated June to November, 1999.009.30, 15.0015.30

Central rice research institute JanuaryMay 19979.009.30, 15.0015.30

Indian Agricultural Research Institute,New Delhi

JulyOctober

Research farm of Central Rice ResearchInstitute, Cuttack

19951998JanuaryMay, JulyDec09.0009.30 and 1500

Central rice research institute, Cuttack JanuaryMay, JulyDec

9.009.30, 15.0015.30

Indian Agricultural Research Institute,New Delhi

4 yearsEvery 10 min for 20 min

Indian Agricultural Research Institute(IARI)

JulyOctober 19941997Every 0, 10, and 20 min

Indian AgriculturalResearch Institute, New Delhi

105 days; every 20 min for 40

Kahikuchi, Guwahati 4 months;Every 15 min for 45 min

Central Rice Research Institute (CRRI),Cuttack

JanuaryMay, JuneDecembe

Balianta June to November, NovembeFebruary;0, 5, 15, 30 min interval.

Lambhavel, Central Gujarat FebruaryApril every 15 day

9.005.00 for every 2 hCentral Rice Research Institute, Cuttack Every 15 min for 60 min, twicCentral Rice Research Institute (CRRI),Cuttack,

June to December, 2005;9.009.30, 15.0015.30

Punjab Agricultural University,Ludhiana

2005200611 am12 noonAt 11, 16, 25, 38, 56, 83, 94 adays Every 0, 15 and 30 min

Lucknow h Environmental eld station of the

National Botanical Research Institute,Lucknow.

JulyOctober, 2007;

Central Rice Research Institute, Cuttack 7 am to 9 am, 3 pm to 5 pm;At 0, 15, 30 min interval

Punjab Agricultural University,Ludhiana

2005200611 am12 noon

D of the sludge; this rapidly leads to the total depletion oxygen in the sludge and development of anaerobicresulting in methane emissions [46,71,72]. These emis-

avoided by treating the wastewater and the associateder aerobic conditions or by capturing methane that isder anaerobic conditions [47,73].

ture

is produced during agriculture whenever anaerobicdevelop. This happens most signicantly in the paddyd for rice cultivation [24,74,75]. Flooded soils are idealts for methane production because of their high levels

substrates, oxygen-depleted conditions, and moisture level of emissions varies with soil conditions, type ofricultural practices, and climate. Tables 6 and 7, which

summarizeindication is possible modifying tcultivation

5. Opportu

Whereaates nearlylisted in thethat is usutities of biothere is litactivities, aproper soil 0.060.62 [169]41.73 [170]

intervals [171]

y of each 2.148.23 [172]

804020,920 (IARI Soil)104710910 (Raipur soil)

[172]

0 h and 13.16 [173]

192.08843.75 [174]

8.15 [175]

15.627.2 [176]

r; h

4.1316.36 [177]

r 1997 0.125 [177]0.59 [178]

25571833 [178]

min 24503720 [179]

974011310 [180]

r, 2000 3085840877 [181]

r to 0.291.36 [182]

883018630 [183]s Site I 105.67720.64

Site II 201.59430.94[184]

e a day 17.59 [185]1.172.52 [185]

nd 108

0.04 and 0.93 [186]

315.1204.7 [187]8.52914.44 [187]

0.180.35 [188]

2.200.56 [77]

1.50.6 CO2eq [189]

data pertaining to rice paddies of India, provide anof the very wide variation in methane emissions thatbetween one paddy eld and the other. By suitablyhe agricultural practices, methane emissions from ricecan be signicantly reduced [78,79].

nities of methane capture

s coal mining, and production of natural gas/oil gener- pure methane, the other ve anthropogenic activities

preceding section produce the methaneCO2 mixtureally called biogas. Of these ve activities, the quan-gas exhaled by livestock are difcult to control and

tle that can be done about it. Of the remaining fourgriculture can be made a lesser emitter of methane byand water management, and proper choice of cultivar,

T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242 3233

Table 7Seasonal integrated ux (Esif) of methane as estimated at various sites; this again reects very high inter-site variation.

State/region Specic site Period and frequency ofobservations

Methane emissionsEsif = mg/m2

Reference

Uttar PradesOrissaOrissa h to Ju

r to F

Assam as doice a dular inr

Assam Assam Assam

minAssam

m and, 15, 3

Andhra PradDelhi Kerala OrissaWest Bengal

Assam

Assam g wasce a dular inin

Assam min

India 6 to 2

to minimizdigestion [8handling ovide opporbut also capsource.

Well-estanimal maavailable towastewaterand by carefugitive bioin the biogacollection eis 7085% [other formsof supplyinovercome [

6. Steps as

Anaerobwastes in tthe breakdoprocess [93

1. Large pro(such asmonomeis broughtive and

se prt-chic, bcetoh Banaras Hindu University, Varanasi Not stated CRRI Farms, Cuttack 1 year Chilka, Gahirmatha, Anshupa 19972000; Marc

October, Novembe15, and 30 min

Amalopam, Tezpur University 2 years sampling wonce in 7 days, twand 2 pm) and reg15 min for an hou

Amalopam, Tezpur University do Amalopam, Tezpur University do Kahikuchi, under lowerBrahmaputra valley zone

Once in a weekevery 0, 15, 30, 45

II SitesAmalopam, Tezpur University

AprilJuly, 2006at 7-day intervalstwice a day (at 9 aat a intervals of (045 min)

esh NRSA, Hyderabad Not stated National Physical Laboratory 1 year RRL, Trivandrum Not stated Balianta, near Bhubasnewar Not stated

IRPE, GabberiaLakshmikantapur

Not stated

Titabar Farms-AAU, Jorhat (UpperBrahmaputra Valley)

1 year

Tezpur 6 months samplinonce in 7days, twiand 2 pm) and reg15 min for an 45 m

North Bank PlainAgroclimatic Zone, Tezpur

9.0014.00 hevery 0, 15, 30, 45

2003 to 2004; 200

e development of conditions favorable for anaerobic082]. It is the remaining three activities landlls,

f manure, and wastewater treatment which pro-tunities to not only reduce fugitive biogas emissions

2. Theshorpion

3. In a

ture much of the generated biogas for use as energy

ablished technology exists for generating biogas fromnure [8386]. Likewise several types of reactors are

anaerobically digest different types of biodegradables to obtain biogas [8790]. By using these technologies,ful management of manure and wastewater to reducegas emissions, a major portion of methane generateds can be captured. A reasonable assumption for the gasfciency for a properly planned gas collection system91,92]. Municipal solid waste (MSW), phytomass, and

of biodegradable solid waste have enormous potentialg biogas but there are technological problems yet to be17,20].

sociated with the generation of biogas

ic digestion involves bacterial fermentation of organiche absence of free oxygen. The fermentation leads town of complex biodegradable organics in a four stage,94] (Fig. 2):

tein macromolecules, fats, and carbohydrate polymers cellulose and starch) are cracked into water solublers (amino acids, long-chain fatty acids, and sugars). Thist about by exoenzymes (hydrolase) present in faculta-obligatory anaerobic bacteria.

these ferdioxide,

4. Methanosume thto produby methway (4Cway (CO(4CH3OH

Methylaverted. Aceyield calcul

Theoretition:

VB = C1(1

where VB isbioreactor (

Biogas, methane anduces somconsumed dioxide is clarger amoutent for thecan also getthe overall 20,775 [190]30,175 [191]

ne, July toebruary; 0,

0.834.71 [192]

ne foray (at 9 amterval of

6435 [193]

1170 [193]10,600 [193]Pre-monsoon 7510Monsoon 16,390

[194]

2.00 pm)0 and

Site I 1380Site II 960

[194]

5020 [195]1080 [195]3027 [195]5090 [195]18,010 [195]

8160 [195]

done foray (9 amterval of

10,565 [196]

813013,000 [196]

007 34.38 23.26 [197]

oducts are then fermented during acidogenesis to formain (C1C5) volatile fatty acids, principally lactic, pro-utyric, and valeric acid.genesis, homoacetogenic microorganisms consume

mentation products and generate acetic acid, carbonand hydrogen.genic organisms, which are strictly anaerobic, con-e acetate, hydrogen, and some of the carbon dioxidece methane. Three biochemical pathways are usedanogens to achieve this [95]: (a) acetotrophic path-H3COOH 4CO2 + 4CH4); (b) hydrogenotrophic path-2 + 4H2 CH4 + 2H2O); (c) methylotrophic pathway

+ 6H2 3CH4 + 2H2O).

ted substrates other than methanol can also be con-totrophic pathway is the primary one, hence theoreticalations are often made using this pathway [96].cally, methane formation follows an exponential equa-

eC2tB )

the biogas yield (m3 d1), tB is residence time in thed), and C1 and C2 are constants.in theory, should contain equal volumes (5050) ofd carbon dioxide. However, acetogenesis typically pro-e hydrogen, and for every four moles of hydrogenby hydrogenotrophic methanogens a mole of carbononverted to methane [97]. Fats and proteins can yieldnts of hydrogen leading to higher typical methane con-se substrates. In certain conditions, these molecules

converted to products other than methane. Therefore,biogas yield and methane content varies for different

3234 T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242

Adopted from

substrates, methane cobut more of

Wherevdecomposinanaerobic dtermites anprincipally tors so thatend producgas, the restsulde andclean fuel, cof carbon d

of suitable generators [109,110]. A wide variety of substrates canbe used to generate biogas (Fig. 3).

tors which inuence anaerobic digestion of anc sub

senceis eson aially ally me wond argane of

ion oted b7. Facorgani

Prewater digestiessentespeciare throute amicroo

SomoperatrecounFig. 2. The steps involved in anaerobic digestion. [97].

biological consortia and digester conditions [97]. Thentent of biogas can range from 40 to 70% (by volume)ten than not it is in 5565% range [98108].er biogas is generated be it from organic matterg under anaerobic conditions in the open, or in captiveigesters, or in the guts of large ruminant animals, or byd some other smaller organisms these four steps areinvolved. If the process is properly controlled in reac-

it proceeds optimally as per these stages, the principalt, the biogas, contains 4070% (by volume) of methane

being carbon dioxide and traces of ammonia, hydrogen hydrogen [8]. This biogas, which is a convenient andan either be used directly with or without the removalioxide or can be converted into electricity with the help

7.1. Specic

Greater microorgantion. If the sbe commin

7.2. C/N rat

The relaorganic maratio. C/N rmum for an

If the C/by the metlonger avaimaterial. As

If the C/Nin the form pH value rismethanoge

Animal wferred feed24 [120]. Plthe C/N ratiC/N rationshas a C/N ra

To mainlevels, matelow C/N rat

7.3. Dilutio

Water sherate a slurdiluted too and may nobe difcult of the digelevels of slu[16].

7.4. pH

Optimumthe input mperiod of dand the pHstrate

of adequate quantities of nitrogen, micronutrients, andsential if an organic substrate is to undergo anaerobicnd generate methane-rich biogas [111,112]. These arethe requirements of microorganisms named in Table 8,ethanogenic bacteria. Because these microorganismsrkers who take the fermentation along the desired

t optimum pace, generating conditions which help theseisms ensures success of the process [16,113].

the aspects which have to be kept in view for successfulf an anaerobic digestion process for obtaining biogas areelow.

surface of the substrate

the specic surface of the substrate, more efciently theismsubstrate contact; consequently faster the diges-ubstrate is in the form of large pieces of solids it shoulduted.

io

tive proportions of carbon and nitrogen present in anterial is expressed in terms of the carbon/nitrogen (C/N)atio in the range of 16:125:1 is considered to be opti-aerobic digestion [114117].N ratio is too high, the nitrogen is consumed rapidlyhanogens to meet their protein requirement and is nolable to react on the left-over carbon content in the

a result the biogas production gets depressed [118]. ratio is too low, nitrogen is liberated and accumulates

of ammonia. This increases the pH of the material. Whenes higher than 8.5 it begins to exert a toxic effect on thenic bacteria [22,119].aste, such as cow dung, which has been the most pre-

in low-rate biogas systems, has an average C/N ratio ofant materials contain a high percentage of carbon and soo is high [102]; for example rice straw and sawdust have

of 70 and >200, respectively (Table 9). Human excretatio of about 8 [121].tain the C/N level of the digester material at optimumrials of high C/N ratio can be mixed with materials ofio [122124].

n

ould be added, if necessary, to the raw material to gen-ry which is neither too thick nor too thin. If a material ismuch, the solid particles may settle down in the digestert get degraded properly. If the slurry is too thick, it mayto stir and may impede the ow of gas to the upper partster [103,104]. Different systems can handle differentrry density, generally in the range of 1025% of solids

biogas production is achieved when the pH value ofixture is between 6.7 and 7.5 [8,125]. During the initialigestion, large amounts of organic acids are produced

of the mixture decreases. As digestion continues and

T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242 3235

Table 8Micro-organisms involved in anaerobic digestion.

Stage Bacteria

Stage I(C6H10O5)n + nH2O = n(C6H12O6)

Stage IIC6H12O6 + 2H2O = 2CH3COOH + 4H2 + CO2 Bacteriodes, Clostridium, Butyrivibrie, Eubacterium, Bidobacterium, LactobacillusC6H12O6 + 2H2 = 2CH3CH2COOH + 2H2OC6H12O6 = CH3CH2CH2COOH + 2CO2 + 2H2C6H12O6 = 2CH3CHOHCOOHC6H12O6 = 2CH3CH2OH + 2CO2

Stage IIICH3CHOHCOOH + H2O = CH3COOH + CO2 + 2H2 Desulfovibrio, Syntrophobacter wolinii, SyntrophomonasCH3CH2OH + H2O = CH3COOH + 2H2CH3CH2CH2COOH + 2H2O = 2CH3COOH + 2H2CH3CH2COOH + 2H2O = CH3COOH + CO2 + 3H2

Stage IV4H2 + CO2 = CH4 + 2H2O Methanobacterium formicicum, M. bryantii; Methanobrevibacter ruminantium, M. arboriphilus,

Methanospirilum hungatei; Methanosarcina barkeri2CH3CH2OH + CO2 = 2CH3COOH + CH42CH3(CH2)2COOH + 2H2O + CO2 = 4CH3COOH + CH4CH3COOH = CH4 + CO2

the concentration of ammonia increases, due to the digestion ofnitrogen, the pH value increases. When the methane gas productionstabilises, the pH remains between 7.2 and 8.2 [126].

When plant material is fermented in a batch system, the aceto-genesis/fermentation stage is rapid, producing organic acids whichreduce the pH and inhibit further digestion [127]. In general a dropin pH and a rise in the proportion of CO2 in the biogas are indica-tors of a disturbance in the digestion process. In such situations,reduction in pH can usually be controlled with the addition of lime[128].

7.5. Temperature

Different species of methanogenic bacteria function optimally inthree different temperature ranges: 5065 C, 2040 C, and

3236 T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242

Table 9C/N ratio of some biodegradable materials.

Raw material C/N ratio

Duck dung Human excrChicken dunGoat dung Pig dung Sheep dung Cow dung Water hyaciMunicipal soElephant duMaize strawRice straw Wheat strawSaw dust

carried outdigesters oppsychrophi

The mesbut the mesanaerobic dis considere

Althougmore efciecontrol andfavorable en

7.6. Loadin

This is anthe digestiousually is [1can happenIt may causleading to ssystem has[16].

7.7. Retenti

Retentiostrate) anddigester to substrate ranaerobic rlow substrachieve higbelow.

Hydraulidenote subis the timedegraded, sthe digester

Solids reto denote mbecause monot necessamicroorganover, it is participatesorganics door solid-fegas eld [1

microorganisms. So the use of the term solid, instead of microor-ganism in the context of microorganism retention time can be asource of confusion.

Nevertheless it is a part of the established jargon and hence weo adhere to it. Solids retention time (ST) is the duration for

active microorganisms reside in a digester. relatoorgresench d

ust suantite th

This ses [eater

onlyim to

whistra

ass th giveeste

conv tankorgasam

in thrs, rendeaero39].tion

xicit

eral the r. Smsiumher vy mre esgher s so4,144ncesg thtion 8eta 8g 10

12181924

nth 25lid waste 40ng 43

6070

90>200

in the mesophilic mode [100] with lesser number oferating in thermophilic mode and much lesser in the

lic mode [131].ophilic temperature range is between 20 C and 40 Cophilic temperature considered to be most suitable forigestion is 35 C [132]. In thermophilic digestion 55 Cd to be ideal [132,133].h thermophilic anaerobic digestion process is generallynt than the mesophilic process, it is more difcult to

also needs extra energy inputs [134], leading to a lessergy balance than mesophilic anaerobic digestion.

g rate

important process control parameter especially whenn is carried out in continuous mode which is how it35]. Overloading can easily lead to system failure. This

if there is inadequate mixing of the waste with slurry.e a signicant rise in volatile fatty acids concentration,harp drop in pH. When this happens feed rate to the

to be reduced for a while till the process re-stabilizes

on time

n time is the duration for which organic material (sub- microorganisms (solids) must remain together in aachieve the desired extent of degradation. Shorter theetention time required to achieve this objective in aneactor, more efcient the reactor [136]. But to achieveate retention times it is necessary to simultaneouslyh microorganism (solids) retention times as explained

c retention time (HRT): The term commonly used to

will alswhich

Theto micrisms pfood eaone mthe quconsum(F/M). procesin a gr[139].

Theas we away bythe subisms pat anyin a dig

In stirred(microat the wordsdigesteor susprate anHRT [1conven[107].

7.8. To

Minamongdigestemagnebut hig

Healead abut hisuch ateria [substaushincentrastrate retention time is hydraulic retention time. This which an organic material, sought to be aerobicallypends in a digester from the instant of its entry in to

to its exit.tention time (SRT): Solids is the term commonly usedicroorganisms in a digester. It is not a precise termst digester feeds contain suspended solids which arerily made up of live biomass. Hence solids other thanisms are also present in an anaerobic digester. More-the volatile solids content in any substrate which

in anaerobic digestion (non-volatile or refractory not). Hence terms such as high solids digestioned digestion are also commonly used in the bio-7,27,106] wherein solids is not meant to denote

[4,145].

7.9. Mixing

Mixing cess stabilitof mixing arstop the forgradients w

Very raptoo slow acircuiting [on the contionship between HRT and SRT, and the importance of foodanism ratio: At any given temperature, the microorgan-t in a digester can only consume a limited amount of

ay. Hence in order to digest a given quantity of substratepply adequate number of microorganisms. The ratio of

y of substrate and to the quantity of bacteria available toat substrate is called the food-to-microorganism ratioratio is the controlling factor in all biological treatment137,138]. A lower than adequate F/M ratio will result

percentage of the substrate being converted to biogas

way in which F/M ratio can be kept adequately low even reduce HRT (to enhance digester efciency), is to nd a

ch SRT is kept high. In other words to nd ways by whichte passes through the digester quickly but microorgan-rough much more slowly. This situation can ensure thatn time more quantities of microorganisms are presentr than substrate (hence low F/M ratio).entional low-rate digesters and in the continuously

reactors (CSTRs), there is no provision to retain solidsnisms) [16,93]. Hence the solids pass out of the digesterse rate as the substrate-to-be-degraded does. In otherese systems HRT = SRT. On the other hand, in high-rateetention of microorganisms by way of attached growthd growth systems enables SRT HRT. In a typical high-bic digester, SRT is about three times higher than the

Some attempts have been made to enhance SRT inal CSTRs by providing biolm support systems in them

y

ions, especially of heavy metals, and detergents arematerials that inhibit the normal growth of bacteria in aall quantities of minerals (sodium, potassium, calcium,, ammonium and sulfur) stimulate the bacterial growth,concentrations may be inhibitory [4,125].etals such as copper, nickel, chromium, zinc, and

sential for bacterial growth in very small quantities,quantities have a toxic effect [140143]. Detergents

ap, antibiotics, organic solvents also inhibit the bac-]. Recovery of digesters following inhibition by toxic

can only be achieved by cessation of feeding ande contents or diluting the contents to push the con-of inhibitory substances to below the toxic level

/agitation

is required to maintain uid homogeneity, hence pro-y, within a digester [103,104,146148]. The objectivese to combine the incoming material with the bacteria, tomation of scum, and to avoid pronounced temperatureithin the digester.id mixing can disrupt the bacterial community while

stirring can cause inadequate mixing and short-149]. The extent of mixing required is also dependentent of the digestion mixture [144].

T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242 3237

7.10. Pathogens

Certain pathogenic bacteria (e.g., Salmonella, Escherichia coli, Lis-teria) and viruses present in municipal solid waste can pose risk ofinfection toare sensitivtrol occurs wtemperaturthat 90% rethermophilmesophilic [151]. For cbefore or astipulated b[152]. Pasteization (130former. Motion [3].

7.11. Light

Light dotion. Hencedigestion ch

7.12. Solid

After thresidue or dand screenbefore bein

The puriof the slurry

8. Anaerob

Before digesters/reterms basicably. In thedigest it, reon this factbial action anaerobic happens is cal/biochemthe vessel inan anaerob

A biogdigester/ferare employother termsfor waste tr[16].

It is necematter degbic and is cBut, as descdegradation[103]. Thosseveral speria which adigester/feranaerobic. Oanaerobic.

-ra

biogping

(1 mne sicted

ily due. Th

in a for uhem

mixof 40

VRq

(1)

VR is reactor volume and q is volumetric ow rate of thets.

typical xed dome digester; it is believed that the Chinese were the rstis concept. As the digestion occurs, biogas is generated which collects under

dome and pushes some of the slurry to the overow tank. When the gas out for use, its pressure inside the dome is reduced and some of the slurryfrom the overow tank. the workers handling the waste [150]. Such pathogense to temperature, hence most effective pathogen con-hen anaerobic digestion is performed at thermophilic

es and at long retention times. Bendixen [151] foundduction of a Salmonella population was achieved atic temperature (53 C) within a mere 0.7 h whereas atconditions (35 C) well over 2 days were necessaryertain types of wastes, a separate pasteurization stepfter anaerobic digestion at 70 C for 60 min has beeny the European Union Animal Byproducts Regulationurization (70 C) is an effective alternative to steril-C); however, bacterial spores are not reduced in the

reover, pasteurized digestate is prone to recontamina-

es not kill methanogens but strongly inhibits methana- light should be blocked from entering the anaerobicamber.

residue/slurry

e anaerobic degradation is nearly complete, the solidigestate is removed and is normally cured aerobically

ed for items such as glass shards, and plastic piecesg disposed on land.ty of the material fed into the system dictates the quality

that is produced.

ic digesters/reactors/fermenters

proceeding with a brief description of anaerobicactors/fermenters, it must be claried that all the threeally mean the same thing and can be used interchange-

anaerobic process the bacteria eat the substrate andleasing methane, CO2, etc. The term digestion is based. The anaerobic process releases gases due to micro-as happens in fermentation. Hence it is also calledfermentation or just fermentation. And since whatessentially a biological process with associated chemi-ical reactions, it can be rightly called a reaction. Hence

which anaerobic digestion is carried out can be calledic reactor.as digester is also an essentially anaerobicmenter/reactor. This term is used for systems whiched primarily for biogas production as distinct from

which are applied to systems which are primarily usedeatment and in which biogas is but a major by-product

ssary to stress upon one more aspect. The step in organicradation which leads to methane is purely anaero-ontrolled by a consortium of methanogenic bacteria.ribed earlier, there are other steps of organic matter

which must occur before the methanogenesis stepe steps do not involve strict anaerobes but, rather,cies of cellulolytic, acidogenic, and acetogenic bacte-re aerobic or facultative. In the so-called anaerobicmenter/reactor all degradation is, therefore, not trulynly the decisive step, of methane generation, is strictly

9. Low

Thedevelo1000 Lfrom ois collethe davolumstoreddrawn

In cpoorly(HRT)

HRT =

wherereactan

Fig. 5. Ato use ththe xedis takenreturns Fig. 4. A oating dome biogas digester.

te and high-rate anaerobic reactors

as digesters used by farmers in India, China and other countries basically contain a large chamber of volume,3) or more. In it animal dung mixed with water is fedde each day and the overow of partially digested slurry

in a sump at the other side each day. The volume ofng-water slurry feed is about 1/401/50 of the reactore biogas is generated continuously and is temporarilyxed or a oating dome (Figs. 4 and 5) from where it isse through a pipe tted with an on-off control.ical engineering parlance these are semi-batch anded reactors [153,154] with a hydraulic retention time50 days. The HRT value is derived from:

3238 T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242

Fig. 6. Exampxed reactor;

For a dcowdungw

HRT = 20040

If the sam

HRT = 20050

It has bemost procethe operaticost of any the HRT of smaller woles of retained biomass digesters in which microorganisms are retained for long times ev(b) UASB; (c) uidized bed reactor.

igester of 2000 L volume, fed at the rate of 40 L ofater slurry per day (d):

0 L

(L d1)= 50 d

e digester is fed 50 L of cowdungwater slurry,

0 L

(L d1)= 40 d

en established [136] that 7080% of the total cost ofsses is made up of the cost of the concerned reactors;onal cost is only of the order of 2030%. Hence if theprocess is to be reduced then, other things being equal,its reactants must be reduced because lower the HRT,uld be the size of the reactor that would be needed.

A very digesters manaerobic dpared to thaerobic prothe world. Ideveloped anow referrethat anaerostirred and that anaeroof the order

This sloimpedimenadvantage form of a clen as digester feed keeps passing out; hence SRT HRT: (a) anaerobic

high HRT of 4050 days is needed in the low-rateentioned above to accomplish signicant extent ofigestion. But this requirement of HRT is too high com-e aerobic activated sludge process and other high-ratecesses which have been commonly employed all overn the 1950s the anaerobic activated sludge process wass a parallel to the aerobic activated sludge process. It isd as a rst generation high-rate process [155]. But evenbic activated sludge reactor, which was continuouslyalso heated to maintain it at temperatures of 35 C (sobic digestion could occur at a faster rate) needed HRTs

of 1015 days [155].wness of anaerobic digestion process was the majort in the widespread use of the process in spite of thethat the process generated a useful by-product in theean fuel.

T. Abbasi et al. / Renewable and Sustainable Energy Reviews 16 (2012) 3228 3242 3239

Then one after another breakthroughs occurred in anaerobicreactor design beginning with the introduction of anaerobic l-ter by Young and McCarty [156]. Anaerobic bafed reactor (ABC),UASB (upow anaerobic sludge blanket) rector, downow xedlm reactoreactor, andduced one athe introdufeature of ameans to rreactor evethe reactordigesters. Tsame time aimplying m

Minimize mizing q a

Maximizemeans bythe digestems by solid supsuspendeket reactoquality anbearing slinuent [

Minimize enhancin

Enhance tloading, ing. This(substrateorganics. will engaga less conin high-so

The masby:

L = C1HRT

where C1 isFurther

rate digesteprocess to and compoand range o[16,164].

A logicadigesters hat all?

The answaste energin rural androle. They ers even atdigesters wwhich low-high-rate dof technicalrate digestegreater adv

10. Summary

The paper briey recapitulates the origin and the denition ofthe terms biogas and anaerobic digestion. It then reviews the

of bihaneo geextrcan b

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age.pdarchagricultcCartaffordentwd int

iomedbbasi 12, 1

bbasi hytomenewabbasi ermanbbasi Tle and

bbasi ntrol 11;15

bbasi ean eudge b12;16rs, expanded/uidized bed reactor, diphasic/triphasic anaerobic sequencing batch (ASB) reactors were intro-fter another by different scientists within a decade of

ction of the anaerobic lter [85,157163]. The commonll these reactors is that they utilize one or the otheretain active mass of anaerobic microorganisms in then as the waste-to-be-treated is made to travel through

at much faster rate than in the low-rate anaerobichis enables low HRTs to be maintained while at thechieving high SRTs (solid retention times); solids hereicroorganisms. In essence the endeavor has been to:

HRT: This can be achieved by minimizing VR and maxi-s in Eq. (1).

SRT: This can be accomplished by nding ways and which microorganisms are retained much longer inter (Fig. 6). This is achieved in attached growth sys-providing anchors to microorganisms in the form ofport systems as in anaerobic lters. It is achieved ind growth systems like upow anaerobic sludge blan-rs by developing a highly active sludge of good settlingd providing other means so that the microorganism udge does not get washed out along with exiting treated22].food-to-microorganism (F/M) ratio: this is achieved byg SRT/HRT ratio, as above.he digester loading: Whereas HRT represents volumetricthe so called digester loading represents mass load-

aspect is important because different digester feedss) may contain different concentrations of digestibleHence at identical HRTs a more concentrated substratee more microorganisms and produce more biogas thancentrated substrate. This aspect is brought to the forelids or dry anaerobic digesters [17,27].

s loading rate, normally expressed in kg m3 d1 is given

(2)

the concentration (usually expressed as kg m3).improvements in the design and operation of high-rs over the years have enabled the anaerobic digestionbe used for wastewaters of widely different strengthssitions. The problems associated with process stabilityf applicability have also been solved to a large extent

l question may be asked at this stage: if high-rateave so many virtues why are low-rate digesters used

wer is that in their context, for conversion of animaly at a small-scale and in a dispersed manner required

suburban settings, low-rate digesters have a usefulare economically viable and are net energy produc-

the small scale at which they are operated. High-rateould not be economically viable at the small scales atrate digesters are successfully utilized. This is becauseigesters need much more rigorous, and higher, level

supervision than low-rate biogas plants. Hence low-rs will continue to serve a useful purpose even as ever

ancements occur in high-rate digester technology.

impactof metbut alswhich waste

Ackno

SMNew DDepart

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[21] Aclsl20ogas on global warming and deals with the implications capture as a means to not only prevent global warmingnerate a fairly clean source of energy. The factors withaction of biogas from different types of biodegradablee maximized are then covered.

gements

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