History and future of domestic biogas plants in the developing world

Abstract

Technologies which recover biogas do so by harnessing anaerobic degradation pathways controlled by a suite of microorganisms. The biogas released acts as an environmentally sustainable energy source, while providing a method for disposal of various wastes. Biogas contains 50–70% methane and 30–50% carbon dioxide, as well as small amounts of other gases and typically has a calorific value of 21–24 MJ/m3. Various appliances can be fuelled by biogas, with stoves offering an application appropriate for deployment in developing countries. Widespread dissemination of biogas digesters in developing countries stems from the 1970s and there are now around four and 27 million biogas plants in India and China respectively. These are typically small systems in rural areas fed by animal manure. However, in many other countries technology spread has foundered and/or up to 50% of plants are non-functional. This is linked to inadequate emphasis on maintenance and repair of existing facilities. Hence for biogas recovery technology to thrive in the future, operational support networks need to be established. There appear to be opportunities for biogas stoves to contribute to projects introducing cleaner cookstoves, such as the Global Alliance for Clean Cookstoves. Beyond this, there remains potential for domestic plants to utilise currently underexploited biogas substrates such as kitchen waste, weeds and crop residues. Thus there is a need for research into reactors and processes which enable efficient anaerobic biodegradation of these resources.

Full text

History and future of domestic biogas plants in the developing world
Tom Bond ⁎, Michael R. Templeton
Environmental and Water Resource Engineering, Department of Civil and Environmental Engineering, Skempton Building, Imperial College London, London SW7 2AZ, UK
abstractarticle info
Article history:
Received 4 June 2010
Revised 28 September 2011
Accepted 28 September 2011
Available online 26 October 2011
Keywords:
Biogas
Recovery
Developing countries
Cookstoves
Technologies which recover biogas do so by harnessing anaerobic degradation pathways controlled by a suite
of microorganisms. The biogas released acts as an environmentally sustainable energy source, while provid-
ing a method for disposal of various wastes. Biogas contains 50–70% methane and 30–50% carbon dioxide, as
well as small amounts of other gases and typically has a calorific value of 21–24 MJ/m
3
. Various appliances
can be fuelled by biogas, with stoves offering an application appropriate for deployment in developing coun-
tries. Widespread dissemination of biogas digesters in developing countries stems from the 1970s and there
are now around four and 27 million biogas plants in India and China respectively. These are typically small
systems in rural areas fed by animal manure. However, in many other countries technology spread has foun-
dered and/or up to 50% of plants are non-functional. This is linked to inadequate emphasis on maintenance
and repair of existing facilities. Hence for biogas recovery technology to thrive in the future, operational sup-
port networks need to be established. There appear to be opportunities for biogas stoves to contribute to pro-
jects introducing cleaner cookstoves, such as the Global Alliance for Clean Cookstoves. Beyond this, there
remains potential for domestic plants to utilise currently underexploited biogas substrates such as kitchen
waste, weeds and crop residues. Thus there is a need for research into reactors and processes which enable
efficient anaerobic biodegradation of these resources.
© 2011 International Energy Initiative. Published by Elsevier Inc. All rights reserved.
Introduction to Biogas
Microbially-controlled production of biogas is an important part of
the global carbon cycle. Every year, natural biodegradation of organic
matter under anaerobic conditions is estimated to release 590–800 mil-
lion tons of methane into the atmosphere (ISAT/GTZ, 1999a). Biogas re-
covery systems exploit these biochemical processes to decompose
various types of biomass, with the liberated biogas potentially providing
an energy source. There is a distinction between anthropogenic anaero-
bic processes which recover the energy within biogas and those which
do not. Examples of the first category are bioreactors designed specifi-
cally for substrates, including sewage, agricultural, industrial and munic-
ipal waste, containing a high proportion of anaerobically-degradable
biomass. In developing countries the expansion of biogas recovery sys-
tems has been based upon small-scale reactors designed for digestion
of cattle, pig and poultry excreta. Meanwhile, landfill sites and municipal
wastewater treatment plants where anaerobic processes produce biogas
which is released into the atmosphere, either before or after combus-
tion, belong to the second category. Biogas contains 50–70% methane
and 30–50% carbon dioxide, depending on the substrate (Sasse, 1988)
as well as small amounts of other gases including hydrogen sulphide.
Methane is the component chiefly responsible for a typical calorific
value of 21–24 MJ/m
3
(Dimpl, 2010)oraround6kWh/m
3
.Biogasis
often used for cooking, heating, lighting or electricity generation. Larger
plants can feed biogas into gas supply networks. The activities of at least
three bacterial communities are required by the biochemical chain
which releases methane. Firstly, during hydrolysis, extracellular en-
zymes degrade complex carbohydrates, proteins and lipids into their
constituent units. Next is acidogenesis (or fermentation) where hydro-
lysis products are converted to acetic acid, hydrogen and carbon dioxide.
The facultative bacteria mediating these reactions exhaust residual oxy-
gen in the digester, thus producing suitable conditions for the final step:
methanogensis, where obligate anaerobic bacteria control meth ane pro-
duction from acidogenesis products. Anaerobic digesters are typically
designed to operate in the mesophilic (20–40 °C) or thermophilic
(above 40 °C) temperature zones. Sludge produced from the anaerobic
digestion of liquid biomass is often used as a fertiliser. Biogas recovery
technologies have been failures in many developing countries, with
low rates of technology transfer and longevity and a reputation for
being difficult to operate and maintain. Thus the objectives of this re-
view were to identify the factors underlying successful andunsuccessful
operation of domestic biogas plants and to investigate the future chal-
lenges, which, once overcome, wouldenable sustained expansion of bio-
gas technology.
Energy for Sustainable Development 15 (2011) 347–354
⁎Corresponding author at: Pollution Research Group, School of Chemical Engineer-
ing, Howard College Campus, University of KwaZulu-Natal, 4041, Durban, South Africa.
Tel.: +27 0760 447 643; fax: + 27 031 260 3241.
E-mail address: tomgbond@hotmail.com (T. Bond).
0973-0826/$ –see front matter © 2011 International Energy Initiative. Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.esd.2011.09.003
Contents lists available at SciVerse ScienceDirect
Energy for Sustainable Development

History of biogas production
There are suggestions that biogas was used for heating bath
water in Assyria as long ago as the 10th century B.C. and that anaer-
obic digestion of solid waste may well have been applied in ancient
China (He, 2010). However, well documented attempts to harness
the anaerobic digestion of biomass by humans date from the mid-
nineteenth century, when digesters were in constructed in New
Zealand and India, with a sewage sludge digester built in Exeter,
UK to fuel street lamps in the 1890s (University of Adelaide,
2010). In Guangdong Province, China, commercial use of biogas
has been attributed to Guorui Luo. In 1921, he constructed an
8m
3
biogas tank fed with household waste and later that decade
founded a company to popularise the technology (He, 2010). The
first German sewage treatment plant to feed biogas into the public
gas supply began to do so in 1920, while in the same country the
first large agricultural biogas plant began operating in 1950. The
spread of biogas technology gained momentum in the 1970s,
when high oil prices motivated research into alternative energy
sources. The fastest growth of biogas use in many Asian, Latin Amer-
ican and African countries was in the 1970s and the first half of the
1980s (Ni and Nyns, 1996). During that period the Chinese govern-
ment promoted “biogas use in every rural family”and facilitated the
installation of more than seven million digesters (He, 2010)(Fig. 1).
From the second half of the 1980s, while biogas technology found
more applications in industrial and urban waste treatment and en-
ergy conservation, its dispersion into rural areas slowed. In China,
by the end of 1988, only 4.7 million household biogas digesters
were reported (Ni and Nyns, 1996). Particularly since the turn of
this century there has been another rapid increase in the number
of plants (Fig. 1)(He, 2010) and in 2007 there were 26.5 million
biogas plants (Chen et al., 2010) the overwhelming majority house-
hold systems with volumes from 6 to 10 m
3
. Meanwhile, in 1999
there were over three million family-sized biogas plants in India
(Fig. 2) and by the end of 2007, the Indian government had provid-
ed subsidy for the construction of nearly four million family-sized
biogas plants (Indian Government, 2007). The National Project on
Biogas Development (NPBD) has run since 1981–1982 and pro-
motes its own digester designs while providing financial support
and various training and development programmes. Subsidies from
state and central governments to install household bioreactors ran-
ged from 30% to 100% in the 1980s–1990s (Tomar, 1995).
Biogas appliances and the need for clean cookstoves
In developing countries, cookers/stoves, lamps, refrigerators and
engines are appliances commonly fuelled by biogas (ISAT/GTZ,
1999a). Biogas can be converted into electricity using a fuel cell,
though this is still considered a research area due to the need for
very clean gas and the cost of fuel cells (Dimpl, 2010). In contrast,
using biogas to fuel a combustion engine and in turn an electric gen-
erator is a proven means of producing electricity, given the wide
availability of suitable generators. For example, in Pura, India a well-
studied community biogas digester was used to fuel a modified diesel
engine and run an electrical generator (Reddy, 2004). As hydrogen
sulphide can corrode engine components it is typical to control its
presence in the outlet flow from the digester. Contacting biogas
with ferrous salts in a closed filter is a common method to achieve
this. Alternatively a small amount of air can be injected into the di-
gester headspace in order to facilitate biochemical hydrogen sulphide
oxidation (Dimpl, 2010). Biogas burns with a clean, blue flame and
stoves have been considered the best means of exploiting biogas in
rural areas of developing countries (ISAT/GTZ, 1999b)(Fig. 3). Due
to the physiochemical properties of biogas, commercial butane and
propane burners are not suitable for biogas without modification.
Since six litres of air are required to combust one litre of biogas, as-
suming a methane composition of 60%, compared with 31 L and
24 L of air for butane and propane respectively, commercial appli-
ances need larger gas jets when burning biogas. Removing water is
achieved by cooling, such as in an underground pipe, to condense
the moisture.
The efficiency of biogas stoves has been quoted as 20%–56% (Itodo
et al., 2007; ISAT/GTZ, 1999b), though such figures are strongly affect-
ed by operating conditions and stove design. Moreover, many health
benefits can result from the switch from traditional to cleaner fuels.
According to the World Health Organisation (WHO) over three billion
people worldwide continue to use solid fuels, including wood, dung,
agricultural residues and coal, to supply their energy needs (WHO,
2011). Cooking with solid fuels on open fires or with traditional stoves
results in high levels of air pollution, due to pollutants such as small
particles and carbon monoxide. Indoor air pollution, a significant pro-
portion generated from traditional cooking stoves, is thought to be re-
sponsible for 2.7% of the total global burden of disease (WHO, 2011).
The prevalence of traditional fuels is illustrated by figures from
Bangladesh where mud-constructed stoves are used by over 90% of
0
200
400
600
800
1000
1200
1400
1600
1800
2000
1973 1991
Biogas plants 104
Year
1976 1979 1982 1985 1988 1994 1997 2000 2003 2006
Fig. 1. Number of biogas plants in China (Chen et al., 2010; Zeng et al., 2007).
348 T. Bond, M.R. Templeton / Energy for Sustainable Development 15 (2011) 347–354

all families and have a thermal efficiency of only 5–15% (Hossain,
2003), compared with 29% for an improved biomass stove and 58%
for an improved natural gas stove developed in the same country
(Akter Lucky and Hossain, 2001). In rural India cooking is estimated
to comprise 60% of overall energy consumption (Ravindranath and
Hall, 1995). Various programmes have developed more efficient and
cleaner cooking stoves (for example, Bailis et al., 2007; Dutta et al.,
2007) and this area is currently subject to increasing research mo-
mentum. India recently launched the National Biomass Cookstoves
Initiative (NCI), with the aim of providing cleaner biomass cook-
stoves, of comparable cleanliness and efficiency to those run on
fuels such as liquefied petroleum gas (LPG), to all households current-
ly using traditional cookstoves (Venkataraman et al., 2010). Further-
more, in September 2010, the United Nations announced the Global
Alliance for Clean Cookstoves, which has the target of delivering
100 million clean cookstoves by 2020 (Smith, 2010). To achieve
these goals, formidable technological and dissemination challenges
will need to be overcome (Venkataraman et al., 2010; Smith, 2010).
Although the principal strategy of these schemes is to introduce im-
proved cookstoves to combust traditional biomass fuels they appear
to represent an opportunity for renewed interest in biogas stoves
and by extension domestic digesters.
Fig. 2. Common digester designs in the developing world. Top left: fixed dome digester (Chinese type). Top right: floating cover digester (Indian type). Below: balloon or tube digester.
Source: Plöchl and Heiermann (2006), based on Gunnerson and Stuckey (1986).
Fig. 3. Biogas stove, Java, Indonesia (picture: Elisa Roma, University of KwaZulu-Natal, South Africa).
349T. Bond, M.R. Templeton / Energy for Sustainable Development 15 (2011) 347–354

Design of biogas plants
Many different types of biogas reactors are used throughout the
world. In general designs used in developing countries for digestion
of livestock waste are classified as low-rate digesters, being simpler
than those in more temperate regions and lacking heating and stir-
ring capability. This is also related to climate, since unheated plants
and those without insulation do not work satisfactorily when the
mean temperature is below 15 °C (ISAT/GTZ, 1999a). Three major
types of digesters are used in developing countries for livestock
waste: the Chinese fixed dome digester, the Indian floating drum di-
gester and balloon (or tube) digesters (Plöchl and Heiermann,
2006)(Fig. 1). Such digesters are usually sized to be fed by human
and animal waste from one household and to deliver the energy de-
mand of the household. In practice this means digester volumes are
between 2 and 10 m³ and that they produce around 0.5 m³ biogas
per m³ digester volume (Dutta et al., 1997; Akinbami et al., 2001;
Omer and Fadalla, 2003). Floating drum digesters are normally
made from concrete and steel, whereas fixed dome digester are con-
structed with various available materials, such as bricks. Balloon (or
tube) digesters are fabricated from folded polyethylene foils, with
porcelain pipes as inlet and outlet. The principle behind these digest-
er designs is very much the same. Feedstock enters through the inlet
pipe either directly or after a mixing pit. Substrate retention times of
20–100 days are used with such mesophilic digesters (Sasse, 1988).
Biogas is collected above the slurry before leaving through an outlet
pipe for utilisation. Even a pit in the ground can be used as a digester
provided the biogas can be captured. There have been efforts to pro-
mote low-cost batch-fed digesters fed by weeds and various biomass
sources which use a gas-proof membrane above a pit and a 120 day
substrate retention period (Lichtman et al., 1996).
Biogas substrates
Although in theory any type of biomass can be degraded to biogas,
the dramatic growth in biogas technology in China and India has been
based upon pig and cow manure, respectively. Cattle dung is
especially suitable as a substrate due to the presence of methanogenic
bacteria in the stomachs of ruminants. Biogas production to provide a
five-member family with two cooked meals a day is 1500–2400 L
(ISAT/GTZ, 1999b). Taking the lower value, this indicates a minimum
of one pig, five cows, 130 chicken or 35 people are required to provide
enough biogas to cook for a family of five (Table 1). This correlates
with practical experience, as it has been reported rural households
in India require four to five cattle to feed a 2 m
3
biogas plant, around
the smallest available (Dutta et al., 1997).
Biomass with a carbon: nitrogen ratio between 20 and 30 has been
reported to produce optimised biogas composition (das Neves et al.,
2009). Substrates with either excessive carbon or nitrogen can result
in poor bioreactor performance and biogas with high carbon dioxide
content. Straw and urine are examples of biomass resources with
high carbon and nitrogen levels respectively. Particularly in Europe
there has been interest in cultivated energy crops as biogas sub-
strates. These include maize (Zea mays), rye (Secale cereale), triticale
(Triticum X Secale), sugar beet (Beta vulgaris) and barley (Hordeum
vulgare), while hemp (Cannabis sativa) and alfalfa (Medicago sativa)
also show promise (Plöchl and Heiermann, 2006)(Table 1). As can
be seen, plants such as barley and maize have biogas yields similar
to animal waste. However, yields from rice straw and rice straw
hull, both potentially useful substrates in the developing world, are
lower at 0.18 and 0.014–0.018 m
3
/kg DM (dry matter) respectively.
It is believed that fresh human excreta is suitable for biogas pro-
duction, whereas sludge collected from septic tanks, pit latrines, etc.
is not (Klingel et al., 2002). This is most likely because both aerobic
and anaerobic processes contribute to the decomposition of biode-
gradable waste in pit latrines, leaving a residual of biologically-inert
solids after a certain residence time (Foxon et al., 2009). Another im-
portant property is solids content. Slurry with a solids content of 5%
to 10% is appropriate for use in low-rate domestic digesters (Sasse,
1988). Because of this, where cow manure is the feedstock, an equal
amount of water is normally added to the digester simultaneously
(ISAT/GTZ, 1999b). When public toilets supply digesters, water used
for flushing or cleaning should be limited to 0.5–1.0 L per bowl
(ISAT/GTZ, 1999b). Several studies have found that the use of multi-
ple substrates often has synergistic effects in that biogas production
is higher than would be expected on the basis on methane potential
of feedstock components (Shah, 1997). This is illustrated by data
showing biogas yields for cattle manure, sewage and a 50:50 mix of cat-
tle manure and sewage were 0.380, 0.265 and 0.407 m
3
/kg DM respec-
tively after 40 days' digestion (Shah, 1997). Consequently, co-digestion
is often beneficial and the focus of much recent research activity, often
Table 1
Biogas production from selected substrates (Amon et al., 2004; Chanakya et al., 2005;
Gunaseelan, 2004; Heiermann and Plöchl, 2004; Linke et al., 2003; Maramba, 1978;
Oechsner et al., 2003; Plöchl and Heiermann, 2006; Sasse, 1988; Sathianathan, 1975).
Substrate Daily production
(kg/animal)
% DM Biogas yield
(m
3
/kg DM)
Biogas yield
(m
3
/animal/day)
a
Pig manure 2 17 3.6–4.8 1.43
Cow manure 8 16 0.2–0.3 0.32
Chicken manure 0.08 25 0.35–0.8 0.01
Human
excrement/sewage
0.5 20 0.35–0.5 0.04
Straw, grass ~80 0.35–0.4
Water hyacinth 7 0.17–0.25
Maize 20–
48
0.25–0.40
b
Barley 25–
38
0.62–0.86
Rye 33–
46
0.67–0.68
Triticale 27–
41
0.68–0.77
Sugar beet 22 0.76
Hemp 28–
36
0.25–0.27
b
Alfafa 14–
35
0.43–0.65
Rice straw 87 0.18
Rice straw hull (husks) 86 0.014–0.018
Baggase 0.165 (m
3
/kg organic DM)
Leaf litter 0.06 (m
3
/kg)
DM = dry matter. a = based on mean biogas yield (m
3
/kg DM). b = calculated from
methane yield based on biogas of 55% methane.
Table 2
Advantages and disadvantages of biogas technology, based on ISAT/GTZ (1999c).
Advantages Disadvantages
Improved sanitation Laborious operation and maintenance
–Reduced pathogens Limited lifespan (~ 20 years for many plants)
–Reduced disease transmission Construction costly
Low cost energy source:
cooking, lighting etc.
Less suitable in cold regions
Low cost fertiliser: improved
crop yields
Less suitable in arid regions
Improved living conditions Negative perception where low functionality
of existing plants
Improved air quality Requires reliable feed source
Reduced greenhouse
emissions
Requires reliable outlet for treated sludge
Reduced nitrous oxide
emissions
Poor hygiene of sludge from mesophilic
digestion
Less demand for alternative
fuels
High construction costs relative to income of
many potential users
–Conservation of woodland
–Less soil erosion
–Time saved collecting firewood
350 T. Bond, M.R. Templeton / Energy for Sustainable Development 15 (2011) 347–354

with combinations of sewage, municipal waste and industrial waste
(Dereli et al., 2010; Lee et al., 2009; Shanmugam and Horan, 2009;
El-Mashad and Zhang, 2010).
Advantages and disadvantages of biogas technology
Anaerobic digestion of human and animal waste provides sanita-
tion by reducing the pathogenic content of substrate materials
(Table 2). Hence biogas installation can dramatically improve the
health of users. This is particularly the case where biogas plants are
linked to public toilets and/or where waste is no longer stored openly.
Rapid public health improvements following biogas implementation
have been observed in rural China, with reductions in schistosomiasis
and tapeworm of 90–99% and 13% respectively (ISAT/GTZ, 1999c;
Remais et al., 2009). Solid retention times of 3 weeks at mesophilic
conditions are enough to kill pathogens leading to typhoid, cholera,
dysentery, schistosomiasis and hookworm (Sasse, 1988). However,
for eliminating other pathogens mesophilic anaerobic processes are
rather ineffective, with typically 50% inactivation of helminth eggs
and modest reductions of tapeworm, roundworm, E. coli and Entero-
cocci (Feachem et al., 1983; Sasse, 1988; Gantzer et al., 2001). Thus,
the WHO suggests pathogen reduction by mesophilic anaerobic
digestion is insufficient to allow subsequent use of human excreta
as fertiliser (WHO, 2006). Moreover, pervasive health benefits are as-
sociated with a switch to a cleaner cooking fuel. In Guatemala, an as-
sociation between domestic use of wood fuel and reduced birth
weight, independent of key maternal, social, and economic confound-
ing factors has been documented (Boy et al., 2002). Of over 1,700
women and newborn children, the percentage of low birth weights
was 19.9% for open fire users, compared with 16.0% for those using
electricity or gas.
However, while the construction costs of biogas plants vary be-
tween different countries they are often high relative to the income
of farmers and other potential users. Recent studies undertaken in
Thailand (Limmeechokchai and Chawana, 2007) and Kenya (Mwirigi
et al., 2009) identified the high investment costs as a major barrier to
technology uptake and in seven Asian and African countries farmers
classified as medium or high income comprised nearly 95% of those
adopting biogas technology (Ni and Nyns, 1996). In Kenya it has
been suggested that without alternative financial capital it was diffi-
cult for farmers to fund biogas systems and respectively 46% and
57% of fixed-dome and flexible-bag plant owners received subsidies
covering over 25% of the construction costs (Mwirigi et al., 2009).
Assessing the economic impact of biogas systems can be complex,
since it often requires allocating a monetary cost to fuels without a
defined market value. Nevertheless, one of the main drivers for the
spread of biogas technology in Asia has been to reduce pressure on
woodland as a fuel source. The success of such strategies is illustrated
by a study in Sichuan province, China, where installation of biogas
systems decreased household usage of coal and wood by 68% and
74% respectively (Remais et al., 2009). These energy savings were suf-
ficient to recoup the construction costs within 2–3 years. It is though
worth noting that no new biogas systems were installed without gov-
ernment subsidies. Similarly, surveys undertaken in the Southern
Province of Sri Lanka have found that the introduction of biogas for
cooking has resulted in an 84% fall in firewood consumption (de
Alwis, 2002). Such reduced burning of wood is also likely to have
health benefits (see above). Increased agricultural yields of 6–10%
and sometimes up to 20% have been recorded through use of biogas
slurry as fertiliser (ISAT/GTZ, 1999c). An agricultural disposal route
also provides a means to utilise nutrients, notably nitrogen and phos-
phorus, which would be wasted without reuse. Although rarely eval-
uated, with lower dependence of fossil fuels and wood come
environmental benefits in terms of reduced deforestation, soil erosion
and greenhouse gas emissions. Methane is the second most important
greenhouse gas (after carbon dioxide). Over 100 years it has a global
warming potential over 20 times that of carbon dioxide (USEPA,
2010). Hence, through combustion of methane and its conversion to
carbon dioxide, less global warming results. Agricultural production
contributes around 33% of total anthropogenic methane emissions,
mostly from ruminant animals and rice cultivation. It has been esti-
mated biogas technology could potentially reduce global anthropo-
genic methane emissions by around 4% (ISAT/GTZ, 1999c). Another
possibility is reduced emissions of nitrous oxide (N
2
O) (Table 2),
now regarded as the biggest manmade threat to the ozone layer
(Ravishankara et al., 2009) and which has a global warming potential
over 300 times that of carbon dioxide. Recent estimates indicate food
production (60%) is the largest anthropogenic source of N
2
O, with
synthetic fertiliser and animal waste management being the largest
individual contributors to this category (Syakila and Kroeze, 2011).
Nitrous oxide can be formed during both nitrification and denitrifica-
tion processes, with nitrite a precursor in both cases. Anaerobic diges-
tion of animal waste is believed to be a feasible strategy to mitigate
N
2
O emissions, although insufficient to reverse the increasing emis-
sions arising from animal production (Oenema et al., 2005). Certainly
anaerobic digestion of animal manure can be expected to reduce
emissions from biological oxidation of ammonia (i.e. nitrification
pathway). Furthermore, reduced demand for synthetic fertilisers
caused by increased use of digested biomass as fertiliser could reduce
emissions. However, discussing the impact of greenhouse gas
emissions is complex, as ideally emissions for the complete
disposal/treatment/reuse cycle need comparing across relevant sce-
narios for disposal of animal waste (including digestion, burning as
a fuel and no anthropogenic disposal). For example, during digestion
of cattle slurry, it was observed that greenhouse gas emissions (com-
prising CH
4
,N
2
O and NH
3
) from slurry stores were more important
than after field application of digested manure (Clemens et al., 2006).
Experience with domestic biogas technology
Asia
Worldwide, effective and widespread implementation of domestic
biogas technology has occurred in countries where governments have
been involved in the subsidy, planning, design, construction, opera-
tion and maintenance of biogas plants. There are several such coun-
tries in Asia, where in particular China and India have seen massive
campaigns to popularise the technology. Surveys in various regions
of India have found the proportion of functional plants to be from
40% to 81% (Dutta et al., 1997; Bhat et al., 2001). It should be noted
that, although not always stated, digester age is a significant factor
in performance, with, on average, higher functionality being associat-
ed with younger digesters as well as more recent designs (Tomar,
1995). In Madhya Pradesh state digesters surveyed at various times
were built from 1974–93, with a major installation push in 1981–
1982 driven by the NPBD. In 1981–1982 functionality was found to
be only 30% improving to 81% in 1985–1986. An analysis of several
studies considered overall around 60% of biogas plants in the mid-
1990s were functional, though that figure rose to over 80% if only re-
cently installed plants were considered (Tomar, 1995). In the mid-
1990s a large survey of 24,501 plants in Madhya Pradesh found 53%
of plants were functional; 48% of defects were technical, the majority
in the digester foundations, inlet–outlet chambers and digester walls,
with 13% of defects operational and 21% resulting from incomplete in-
stallation (Tomar, 1995). Floating drum plants had a higher propor-
tion of functionality relative to fixed dome plants, while only a very
small number of community plants were operating effectively. One
of a limited number of areas experiencing a higher degree of function-
ality is the Sirsi block of the Uttara Kannada district, Karnataka state,
southern India. Here, of 187 household plants in eight villages, 100%
were found to be operating satisfactorily (Bhat et al., 2001). In the
study area, 37% of digesters were installed in 1985–1989, 36% in
351T. Bond, M.R. Templeton / Energy for Sustainable Development 15 (2011) 347–354

1990–1994 and 27% in 1995–1999, thus age was not the key determi-
nant of efficacy. Reasons given for the success of biogas dissemination
were free servicing and the presence of competing entrepreneurs
who assisted householders in all phases of plant construction and in-
stallation, including the procurement of subsidies. Other relevant fac-
tors, some particular to the Sirsi block, were a demand for biogas
plants (i.e. more applicant households than administered subsidies),
warranties for plant performance, while availability of cattle manure,
household incomes and literacy rates were above the national
average.
Over 60,000 biogas plants had been installed in Nepal by 1999
(Singh and Maharjan, 2003), while a total of 24,000 domestic biogas
plants were installed in Bangladesh from 1971 to 2005 (Alam,
2008), while there are also over 2000 biogas plants sited on poultry
farms (Dimpl, 2010). The Bangladeshi government has been heavily
involved in the dissemination of biogas plants through the country,
with subsidies offered for plant construction. A survey of 66 plants
in the country found that 3% were functioning without defect, 76%
were defective but functioning and 21% were defective and not func-
tioning (Alam, 2008). In Sri Lanka it is believed there are up to 5000
biogas plants (de Alwis, 2002). A survey in 1986 found that 61% of
plants were functional. However, by 1996 another investigation
found that only around 29% of household plants were operational,
with a multitude of reasons given for failure (de Alwis, 2002). There
was also a large degree of geographical variability: the percentage
of operational (household and other) plants was between 34% and
65% in districts where over 10 plants were surveyed, though underly-
ing causes were not discussed. In Pakistan, the Ministry of Petroleum
and Natural Resources commissioned 4137 biogas plants between
1974 and 1987 (Mirza et al., 2008). However, after the government
withdrew financial support the program essentially failed. As well as
the lack of subsidies, a lack of technical training, high cost and inade-
quate community participation were identified as contributory factors
to this decline (Mirza et al., 2008). In another scheme, the Pakistan
Council of Renewable Energy Technologies (PCRET) installed 1200 bio-
gas plants from 2001 with 50% of the cost borne by the user. It is
reported that presently there are 5357 biogas units installed in the coun-
try. By 1982, there were already 1000 biogas plants in Thailand, with the
Ministry of Public Health central to their propagation. However, by 2000
these activities had largely ceased, with the diffusion of various designs
proving unsuccessful (ISAT/GTZ, 1999d), although subsequently larger
biogas systems have become popular in livestock farms as a means to
treat wastewater or slurry, with a total of capacity of 60,210 m
3
installed
in 2001 (Limmeechokchai and Chawana, 2007).
By 2007, there were 26.5 million biogas plants in China (Chen et
al., 2010). Household biogas digesters are especially prevalent in the
Yangtze River Basin, with Sichuan Province having the largest num-
ber of biogas plants, at 2.94 million. The rapid development of biogas
through the country is linked to accumulated technical knowledge,
the availability of fermentation materials, and strong state support,
including financial. Nonetheless, of the seven million household bio-
gas tanks installed during the 1970s, around half had already been
abandoned by 1980. Various technical issues were cited for their fail-
ure, such as gas leakage, insufficient feedstock, blockages and lack of
maintenance (He, 2010). Some 60% of biogas digesters in China's
rural areas were believed to be operating normally in 2007 (Chen et
al., 2010). The lack of attention paid to plant maintenance is a major
reason for failure, while qualified technical support is in short supply.
Such trends reflect an emphasis on plant construction rather than op-
eration, maintenance or repair (Chen et al., 2010).
Other developing countries
Elsewhere in the world, the situation is mixed. As with Asia it is
not straightforward to quantify and compare causes of digester fail-
ure (e.g. technical, economic, lack of feedstock) between countries
owing to the incomplete reporting of these parameters. Moreover,
in many countries the number of plants constructed is under 1000,
therefore the availability of operational and technical support is
much less than in those Asian countries with more widespread expe-
rience of the technology. One review found the number of operational
rural digesters was 50–75% of the total in various developing coun-
tries and in Latin America the number of plants installed from 1985
to 1992 was only one-seventh of those installed from 1982 to 1985
(Ni and Nyns, 1996). In the Ivory Coast, Tanzania and Costa Rica
non-technical reasons comprised respectively 69%, 25% and 50% of
total failures (Ni and Nyns, 1996). Part of the explanation is that the
routine operation and maintenance of the digesters is usually labori-
ous. In particular it has been noted that biogas technology has had
very little success in sub- Saharan Africa, except Tanzania and Burundi
where some hundreds of plants have been constructed (Akinbami et
al., 2001). Figures from 1993 indicate the African countries with the
highest numbers of biogas plants were Zimbabwe (N100), Burundi
(N136), Kenya (N140) and Tanzania (N600) (Akinbami et al., 2001).
Meanwhile, a survey in Kenya in 1995 estimated that about 850 do-
mestic biogas plants were installed (Gitonga). However, only 25% of
installed plants were operational, with many abandoned plants, giv-
ing a negative image of biogas technology. In Tunisia, governmental
bodies, with French and German involvement, made efforts to pro-
mote biogas technology in the Sejenane region from 1982. After one
of the partners withdrew its support in 1992, despite continued sup-
port from state organisations, biogas dissemination almost completely
halted (ISAT/GTZ, 1999d).
Applicability of biogas plants in the developing world
Based on the above, some recommendations can be made regard-
ing suitable circumstances for installation of biogas plants. Particular-
ly relevant here are factors listed by Deutsche Gesellschaft für
Technische Zusammenarbeit (GTZ), the German government techni-
cal assistance agency, which constrain effective implementation of
biogas plants in the developing world (ISAT/GTZ, 1999c)(Table 3).
Conversely it is possible to define a set of conditions comprising an
ideal situation for biogas systems fed by animal manure (ISAT/GTZ,
1999c). Low rate digesters work best in tropical regions, especially
where the temperature is above 20 °C year round. As seen, the meth-
ane generating potential of various substrates imposes limits on
biogas production and consequently digester sizing (Table 1), with
1500–2400 L of biogas considered sufficient to supply cooking re-
quirements for a family of five. Thus ideal conditions for a house-
hold-digester comprise a daily supply of at least 30 kg/day of dung,
with full stabling of animals on concrete floors (facilitating transfer of
Table 3
Factors constraining successful implementation of biogas technology ISAT/GTZ
(1999c).
Excluding factors Critical factors
Climate too cold or too dry Low income of the target group
Irregular or low gas demand Unfavourable macro- and micro-
economics
Under 20 kg dung/day available or under
1000 kg live weight of animals per
household in indoor stabling or 2000 kg
in night stabling
Good supply of energy throughout
the year, therefore only moderate
economic incentives for biogas
technology
Irregular gas demand
No stabling or livestock in large pens Gas appliances not available
No building materials available High building costs
No or very little water available Low qualification of builders
Integration of biogas plant into the
household and farm routines not
possible
Institution has only limited access to
the target group
No suitable institution can be found for
dissemination
No substantial government interest
352 T. Bond, M.R. Templeton / Energy for Sustainable Development 15 (2011) 347–354

dung to the digester) and perhaps supplemented by other substrates.
The equivalent requirement if human excreta were the substrate
would be with a daily supply of at least 14 kg human faeces (equivalent
to 28 people, calculation using data in Table 1). Other ideal conditions
are that the use of organic fertiliser is already established, the biogas
plant can be located close to the stable and point of gas consumption,
the cost is moderate relative to income of the target group, that financ-
ing is secure and that efficient dissemination and support networks
exist, including government support (ISAT/GTZ, 1999c). Overall this
suggests household biogas plants are most advantageous in rural
areas, with both reliable feedstock and an established outlet for pro-
duced sludge and a sustainable support network for users.
Potential for spread of domestic digesters
Even in those countries with an established record in installing
small-scale livestock digesters there remains potential for continued
spread of these systems. While the introduction of biogas technology
can have a multitude of environmental and public health benefits
(Table 2) those arising from biogas stoves appear especially relevant
as an avenue to promote biogas technology in the near future. In partic-
ular, biogas stoves can make an important contribution to those high-
profile projects which aim to reduce air pollution through the acceler-
ated introduction of cleaner cookstoves—the Global Alliance for Clean
Cookstoves and NCI. The potential maximum number of household
livestock digesters in India has been estimated as 12–17 million,
based on the availability of cow manure (Ravindranath and Hall,
1995; MNES, 1999); compared with current levels of around four mil-
lion. Meanwhile, in Bangladesh since it is thought 80% of the manure
from the 22 million cattle in the country could be made available for
biogas production (Hossain, 2003), this indicates a potential maximum
of around 3.5 million household plants based on the value of five cows
per digester. This would represent a massive increase from the current
number of over 25,000 biogas plants in Bangladesh (Dimpl, 2010). Sim-
ilarly, given the number of cattle and buffalo in the country, it was es-
timated that the 60,000 plants installed in Nepal by 1999 represented
only 4% of the total potential (Singh and Maharjan, 2003). Meanwhile,
in Nigeria it was calculated that biogas production from the 12 million
cattle in the country could potentially reach 3.3 million m
3
/day (Itodo
et al., 2007). In China the current rapid expansion of rural biogas plants
shows no sign of slowing (Fig. 1)(He, 2010). Since the annual produc-
tion of dry livestock and poultry excrement in the country is estimated
at 1467 million tons, of which 1023 million can be collected (Chen
et al., 2010), this suggests considerable scope for continued expansion
based on existing designs and government support. Indeed, it has been
calculated that only 19% of biogas potential has been utilised in rural
China (Chen et al., 2010).
However, for the long-term spread of biogas recovery technolo-
gies reliance on animal manure will need to be overcome. Thus a
reoccurring theme of recent literature is the need for small-scale
plants which digest alternative substrates. In China, there is demand
for household anaerobic systems which allow efficient digestion of
crop residues and straw (Chen et al., 2010). Although the high car-
bon: nitrogen ration of straw, specifically in the form of lignocellu-
loses, is thought to make straw rather resistant to anaerobic
digestion, laboratory tests have found a biogas yield of 0.35–
0.4 m
3
/kg DM (Table 1). Only 0.5% of total crop residues in China
are currently utilised for biogas generation (Liu et al., 2008)and
when co-digested with other substrates such as animal manure they
are normally limited to under one-third of the total substrate mass.
Alternatively, pre-silage and fermentation are sometimes used to
raise biogas generation. There has also been interest in additives or
digester designs which promote efficient biodegradation of straw.
Furthermore, designs incorporating solar-powered heating and
water saving devices have been proposed to allow dissemination
into colder and more arid regions of China (Chen et al., 2010). The
digestion of weeds in a plug-flow-like plant designed to produce 6–
8m
3
/day of biogas with a retention time of 36 d has been investigat-
ed in India (Chanakya et al., 2005). However, while the design was an
engineering success in the sense it produced an adequate amount of
biogas, the women feeding the digester were required to spend 1.3–
2 h per day collecting vegetation, compared with 2.5–3 h per week
when gathering firewood for cooking. In India, wastes such as sew-
age, municipal solid waste, and crop residues such as rice husks and
bagasse (sugarcane waste) have potential for biogas generation.
However, while biogas yields from some tropical plant residues ap-
proach those of energy crops, Table 1 shows yields from rice straw,
rice straw husks and bagasse are relatively low at 0.18, ~ 0.018 and
0.165 respectively (Plöchl and Heiermann, 2006). Hence, these crop
residues may have a limited usefulness as biogas sources.
Meanwhile, one potential barrier to digestion of sewage and ani-
mal excreta is that mesophilic anaerobic digestion does not by itself
produce sludge of suitable hygienic quality for use as fertiliser, if
that is to be the disposal/reuse route. The WHO suggests post treat-
ment is required to meet its health guidelines for reuse of human ex-
creta in agriculture (WHO, 2006). European legislation is stricter and
states that anaerobic digestion of animal waste must include pasteur-
isation for 1 h at 70 °C if sludge is being applied to land subsequently
(EC, 2002). As indicated by this regulation, thermophilic digestion of
sewage sludge provides a good level of hygiene (Gantzer et al.,
2001). Consequently, for biogas digesters to deliver improved sanita-
tion, designs incorporating additional treatment stages may be re-
quired. Municipal solid waste (MSW) in developing countries is
typically rich in organic material (up to 70%) and thus a suitable bio-
gas substrate (Müller, 2007; Vögeli and Zurbrügg, 2008). The diges-
tion of MSW has attracted attention in Southern India, where
kitchen waste from households and restaurants, market waste and
waste from slaughterhouses is utilised in urban digesters of various
sizes (domestic and larger). A few systems co-digest toilet waste
(Vögeli and Zurbrügg, 2008).
Conclusions
Biogas technology offers a unique set of benefits. It can improve
the health of users, is a sustainable source of energy, benefits the en-
vironment and provides a way to treat and reuse various wastes—
human, animal, agricultural, industrial and municipal. It has come a
long way since the 1970s, with China and India supplying models of
how to disseminate small biogas plants in rural areas. In other devel-
oping countries, the proportion of functional plants is often 50% or
less. This reflects a need for investment in operational validation,
maintenance and repair if the technology is to thrive. Experience sug-
gests considerable government involvement is requested for these
support networks to be continued over time. The current drive to re-
duce indoor air pollution by promoting cleaner cookstoves would ap-
pear to present biogas stoves with renewed development
opportunities. At the same time, domestic biogas digesters have num-
ber of challenges to overcome for continued proliferation in the 21st
century. Designs which deliver lower cost, improved robustness,
functionality, ease of construction, operation and maintenance
would aid the market penetration of biogas plants. Furthermore, to
move beyond a dependence on livestock manure there is a need for
small-scale bioreactors which efficiently digest available substrates
in both rural and urban situations. On a domestic level these include
kitchen waste, human excreta, weeds and crop residues.
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