ISIS Report 02/10/06
Biogas from biological wastes tops renewable energies as it also prevents
carbon emissions and environmental pollution. It is at the heart of a burgeoning
eco-economy in China, but certain constraints need to be addressed for
its full potential to be realized. Prof. Li Kangmin and Dr.
A fully referenced
version of this article is posted on ISIS members’ website, or is available for download here
What is biogas?
Biogas is a combustible
mixture of gases produced by micro-organisms when livestock manure and other
biological wastes are allowed to ferment in the absence of air in closed containers
 (Dream Farms). The
major constituents of biogas are methane (CH4, 60 percent or more
by volume) and carbon dioxide (CO2, about
35 percent); but
small amounts of water vapour, hydrogen sulphide (H2S), carbon
monoxide (CO), and nitrogen (N2)
are also present.
The composition of biogas varies according to the biological material. The
methane content of biogas produced from night soil (human excreta), chicken
manure and wastewater from slaughterhouse sometimes could reach 70 percent
or more, while that
from stalk and straw of crops is about 55 percent. The concentration of H2S
in biogas produced from chicken manure and molasses could be as high
as 4 000mg/m3, and from alcohol wastewater
even higher at 10 000 mg/m3. Biogas is mainly used as fuel,
like natural gas, while the digested mixture of liquids and solids ‘bio-slurry’
and ‘bio-sludge’ are mainly used as organic fertiliser for
crops. But there are numerous other uses for biogas, bio-slurry and bio-sludge
There’s evidence that biogas
was used to heat bath water in Assyria during 10 BC; and the first digestion
plant to produce biogas from wastes was built in a leper colony in Bombay
India in 1859  (Sustainable
Food System for Sustainable Development).
China is one of countries in the world to have used biogas technology early
in its history. By the end of the nineteenth century, simple biogas digesters
had appeared in the coastal areas of southern China. Mr. Luo Guorui invented
and built an eight cubic metre Guorui biogas tank in 1920, and established
the Santou Guorui Biogas Lamp Company. In 1932, he moved the Company to Shanghai
and changed his firm’s name to Chinese Guorui Biogas Company with many branches
along the Yangtze River and in the southern provinces. Chinese Guorui Biogas
Digester Practical Lecture Notes was published in 1935 , the first
monograph on biogas in China and in the world. That was the first wave of
biogas use in China.
The second wave of biogas use in China originated in Wuchang in 1958 in a
campaign to exploit the multiple functions of biogas production, which simultaneously
solved the problems of the disposal of manure and improvement of hygiene.
The third wave of
biogas use occurred between the late 1970s and early 1980s when the Chinese
government considered biogas production an effective and rational use of natural
resources in rural areas. Biogas production not only provided energy, but
also environmental protection and improvement of hygiene, and was an important
aspect of modernization of agriculture. Some 6 million digesters were set
up in China, which became the biogas capitol of the world, attracting
many from the developing countries to learn from it. The ‘China dome’ digester
became the standard construction to the present day (Fig. 1) , especially for small-scale
domestic use. But many
new types of rural household digesters have also been built based on water
pressure, as for example, the plug flow auto-cycle
rural digester, the up-flow small
scale digester, the fender digester, and recently, the pulse
flow anaerobic reactor.
Figure 1. China dome digester
China’s 2003-2010 National Rural Biogas Construction Plan was announced
in 2003. The proposal was to increase biogas use by 11 million to a total
of 20 millions households by 2005, to make one in ten farmer’s households
a biogas user; although the rate would reach 15 percent in some areas. By
2010, China would increase biogas-using households by a further 31 million
to a total of 50 million, so the rate of use would reach 35 percent. From
2003, a government subsidy of 1 000 Yuan (about US$ 150) would be provided
for each biogas digester.
Why use biogas?
The main reason for using anaerobic digestion, which generates biogas
as a by-product, is to treat wastes. According to the government’s Chinese Ecological White Paper issued in
2002 , the total
amount of livestock and poultry wastes generated in the country reached 2.485
billion tonnes in 1995, some 3.9 times the total industrial solid wastes.
These wastes are precious resources if used properly, but constitute major
pollution when discharged into rivers and lakes. It is estimated that less
than 10 percent of the wastewater in China is currently treated, and that 10 million ha of farmland are
seriously polluted by organic wastewater and solid wastes as well.
According to the Chinese
Academy of Sciences Geography and Resources China Natural Resources and Environment Data Bank, the total
annual production of manure and night soil could theoretically generate about
130 billion m3 of methane, equivalent to 93 million tonnes
of coal . While only 50 percent
of the theoretical production can be realised in rural areas, 80 percent of
the industrial wastewater can also be used to produce methane.
The COD (chemical oxygen
demand, a measure of pollutant concentration) of wastewater from a distiller
often reaches 40 000 mg/litre while aerobic treatment only permits COD below
1 000 mg/litre, which means having to dilute the wastewater 40 times. With
anaerobic digestion, 90 percent of the pollutants can be readily removed,
thereby greatly reducing pollution to farmland, rivers and lakes .
During the 10th
Five Year Plan, the government invested 35 billion Yuan to promote an ecological
model based on biogas. It devoted
great effort to develop 2 200 biogas engineering projects for wastes from
intensive animal husbandry and poultry treating more than 60 million tonnes
of manure a year. In addition, it installed 137 000 digesters to treat sewage
The second main reason for
anaerobic digestion is that methane is a major greenhouse gas, second to carbon
dioxide in amount generated, but with a global warming potential 22 times
that of carbon dioxide. Using biogas not only removes polluting wastes, but
also mitigates global warming  (Dream Farm 2 - Story So Far).
The methane flux from exposed slurry is 3.92 mg per square metre per hour,
compared with 10.26 mg per square metre per hour from compost in rice fields
. Methane mitigation saves
carbon emissions and can be traded as carbon credits under the Clean Development
Mechanism of the Kyoto Protocol for climate change  (Biogas Bonanza for Third World
Using biogas also solves
the most serious problem of energy supply in rural areas, where people traditionally
forage for fuel wood in forest. A 10m3
digester in rural areas can save 2 000 kg of fuel wood, which is equivalent
to reforesting 0.26-4 ha . Africa lost 64 million ha of forest between
1990 and 2005, more than any other continent, and fuel wood gathering was
a major cause of forest depletion .
Biogas methane provides
fuel for cooking, not only saving the forests and also the women fetching
and carrying heavy loads of fuel wood. Unlike firewood, biogas burns without
smoke, thus also saving women and children from respiratory distress and disease
. Biogas can be used to generate electricity, prolonging the active hours
of the day and enabling the family to engage in social or self-improvement
activities or to earn extra income.
The anaerobic digester solves
sanitation problems by taking in human as well as animal manure, improving
home and farm hygiene and the general environmental conditions.
Finally, anaerobic digestion not only yields biogas but also bio-slurry and
bio-dregs rich in nutrients, minerals and biologically active compounds that
form excellent organic fertiliser for crops and fodder for pig and fish (see
Rich fertiliser and feed from anaerobic digestion
The liquid and solids in the digester is a treasure trove of valuable
biological resources .
These include major nutrients for crops such as nitrogen (N), phosphorus (P)
and potassium (K), as well as trace elements that can stimulate seed germination
and growth. Also present are biologically active compounds such amino acids,
growth hormones, gibberelin, sugars, humic acid, unsaturated fatty acids,
vitamins, cellulase and other enzymes, and antibiotics that may suppress the
growth of pathogens, which benefit both plants and animals. The slurry and
solids can be used as fodder for livestock and fish. The solid phase will
include the micro-organisms responsible for fermenting the wastes and producing
methane, which would have multiplied in the digest, constituting a rich source
of protein when the digested slurry or dregs are used as fodder.
The digested slurry can be used as organic manure in the sowing season and
as a source of water in other seasons. Seeds submerged in slurry germinate
better and the seedlings grow stronger. Used as a spray for plants, the slurry
inhibits disease and boosts yields.
The digested slurry can be used to feed fish, the dosage depending on the
transparency of the fishpond (an indication of how much organic nutrient is
present). It can also be fed to pigs as an additive to speed up growth and
shorter the rearing period by 25 percent, saving feeds by 15 percent. When
fed to boiler and layers, the slurry from cow, chicken and pig manure increased
the rate of egg laying by 14 percent, 9 percent and 7 percent respectively.
The solid dregs from the digester have high levels of humic acid and can
be used as a soil conditioner or as substrate for culturing mushrooms. They
can also be used to culture earthworms to be fed to chickens. Chickens fed
earthworms lay 15 to 30 percent more eggs .
Many uses for biogas
Biogas can be used directly for cooking and for co-generation of electricity
and heat, which is especially feasible when the biogas is used at or near
the site of generation.
Biogas methane can also be used as fuel for vehicles, and is the cleanest
biofuel available. Cars run on biogas methane have been voted environmental
cars of the year in 2005. Thousands of them are already operating in Sweden,
which has hundreds of filling stations supplied by community biogas digesters
 (Organic Waste-Powered Cars).
Biogas can be used in ovens and lamps to heat greenhouses and at the same
time increase the carbon dioxide concentration to boost photosynthesis in
the greenhouse plants and increase yields. Experiments in Shanxi Province
have shown that increasing carbon dioxide four-fold between 6 and 8 am boosts
yields by 67.2 percent . Similarly, a biogas lamp gives both light and
warmth to silkworm eggs increasing their rate of hatching as well as cocooning
over the usual coal heating .
Biogas methane can also be used to make methanol, an organic solvent and
important chemical for producing formaldehyde, chloromethane, organic glass,
and compound fibre .
Finally, biogas can be used to prolong storage of fruit and grain . An
atmosphere of methane and carbon dioxide inhibits metabolism, thereby reducing
the formation of ethylene in fruits and grains. It also kills harmful insects,
mould, and bacteria that cause diseases.
Some notable examples of biogas use in China 
In 1996, grain production reached 504.3 million tonnes in China.
It was hard to sell the grains. Nanyang in Henan Province had 6.7 million
ha of wheat crop (1 percent of wheat cropland in China), and a record yield
of 9.5 tonnes/ha that year. Nanyang also had 1.5 million tonnes of shop worn
grains. So, Tianguan Alcohol Factory expanded its operation to consume 1.75
million tonnes of shop worn grains/year to produce denatured alcohol as fuel
for automobiles, and used the dregs of the distiller to produce biogas in
a 30 000 m3
digester, supplying more than 20 000 households or 20 percent of the population.
Nanyang became a biogas city in China.
of Shaoxing Country, Zhejiang Province produces 28 000 pigs, 10 000 ducks,
1 million ducklings and 100 000 chickens each year. In 2001, they spent 1.2
million Yuan to build digesters to treat 30 tonnes of livestock and poultry
wastes and nightsoil. This produced enough biogas for more than 300 households
plus 7 200 tonnes of organic fertilizer each year.
Hongzhi Alcohol Corporation
Limited located in Mianzhu in Sichuan Province is the largest
alcohol factory in south-western China, and produces alcohol for human consumption.
It runs a service using industrial organic wastewater, sewage and dregs to
produce biogas, paid for by industry and residents in cities, but provided
free to farmers. The company also built a biogas power plant generating 7
million kilowatts per hour. The city of Mianzhu treats 98 percent of municipal
sewage including wastewater from hospitals through digesters with a total
capacity of 10 000 m3. The treated water
reached national discharge standards, greatly improving the environment.
Biogas the basis of eco-economy
Up to the end of 2005, China has 17
million digesters with annual production of 6.5 billion m3 biogas , mostly
in rural areas, with 50 million people enjoying the benefits of biogas technology.
The annual production of biogas is projected to reach 25 billion m3 by 2020. Biogas
could provide energy to one quarter of households in rural areas.
Biogas is at the centre of a burgeoning eco-economy in China. As animal husbandry
goes intensive, there are many large or medium size livestock and poultry
farms in the suburbs of cities. An example is Fushan farm in Hangzhou, with
32.47 ha paddy fields, 4 ha tea trees, 13.7 ha water shields and 7.3 ha fishponds.
It also produces 30 000 laying hens, 150 000 broilers, and 8 000 pigs a year,
with 15 tonnes of solid waste and 70 tonnes of wastewater discharged daily,
a huge amount of pollution. But using biogas digesters to deal with the pig
and poultry wastes, biogas energy becomes available for processing tea and
heating the chicken coop, and there’s fodder for fish and pigs and fertilisers
for tea trees and the paddy fields, and no pollution is exported to surrounding
areas . This eco-farm has now moved to the outskirts far from the city
because of its malodour, however. It is possible to use a combination of multiple
micro-organisms to deodorize pig manure or chicken manure. Also, the large
amounts of water in slurry could be reused to wash away wastes in hog houses
as a water-saving measure.
Northern China has cold winters but sufficient sunshine. Digesters do not
operate below 10 C, and pigs raised in winter eat but do not fatten. People
also lack fresh vegetables in winter. All these problems are solved with a
four-in-one eco-model that provides a greenhouse to plant vegetables, a shed
to raise pigs, a digester underneath the pig shed and a toilet in the big
green house adjoining the pig shed . The pigs grow well with manure flowing
into the digester together with human excreta. The digester works well because
the temperature could be kept above 10 C, and it greatly improves the living
conditions of farmers. The digester provides biogas as energy, slurry and
dregs as fertilisers, and the pigs produce carbon dioxide to enrich the greenhouse
to produce plenty of quality vegetables.
In southern China, a five-in-one model incorporates pigs, digester, fruit
orchard, light trap, and fishponds . The pig manure flows into a digester
to be fermented. Biogas is harvested to provide energy for cooking and lighting.
The digested slurry is used as fertilizer for the fruit gardens and feed for
pig and fish. The light-trap hangs above the fishpond to attract and kill
pests, which become additional fish feed. This model is practised especially
in Guangxi Province in southern China, where a yellow sticky board (a kind
of fly paper) is hung in the orchard for additional pest control .
Constraints and prospects
China is building new socialist villages in its current
11th five-year plan. The guidelines are to develop production and
to clean the environment, to innovate to save resources. Developing biogas
fits in well with this programme. The major constraint is the lack of technical
capacity for running and maintaining the biogas digester.
However, a new breed of biogas farmer workers has appeared. For example,
Mr. Liu Zijian in Guangxi Province is playing a major role in his village.
Liu first built an eight cubic metre digester and renovated the toilet, pigsty
and kitchen at the same time, saving 7 540 Yuan (~US$ 1 000). He has worked
out effective ways of getting the digester to start producing methane, and
how to ensure success. Indeed, he has built 130 digesters without a single
failure. The whole village has 165 digesters serving 72 percent of villagers.
Nevertheless, any improvement in the design of biogas digesters to make installation
and maintenance easier will encourage wider uptake. Up to the end of 2005,
Shanxi Province held 40 biogas technical training courses and trained 6 000
farmers, 4 037 of which gained National Biogas Professional Technician Certificate
. In order to popularise the biogas technology, a training course is held
twice a month lasting 7 days and costing 600 Yuan per person (280 Yuan by
correspondence). After completing the course, participants would be given
a professional technical grade certificate that is recognized by the state
The Pearl River Delta in Guangdong Province
is famous for its dyke-pond system, which integrates fish farming with crops
cultivated on the dykes between fishponds. Here, the uptake of biogas digesters
is poor, at least partly because when electrification arrived, according to
Prof. Deng Hanzeng of Guangzhou Institute of Geography, it was just much easier
to operate the switch rather than maintain a digester. Prof. Deng was part
of a team of scientist led by Prof. Zhong Gongfu who, in the 1980s, pioneered
the study of the dyke-pond system developed by farmers in the Pearl River
Delta and elsewhere over the past two thousand years . They would very
much like to see biogas digesters and other sustainable, zero-emission and
zero-waste practices revitalised and adopted in the region.
Another important factor to encourage widespread adoption of biogas technology
is an appropriate legal framework. For example, a comprehensive ecological
tax reform law came into effect in Germany in March 1999 that raised taxes
on energy sources tied to carbon emissions, and exempted renewables from taxation.
In February 2000, the German parliament passed a Renewable Energies Sources
Act that included payments for excess green energy generation fed back into
the power grid . In that event, the meters run backwards, reducing the
customer’s electricity bills. These policies help make green energy cost-effective,
and are essential to the ultimate success of green power programmes.
To maximise the benefits of biogas as energy, China also needs advanced technologies
to purify and compress methane and to build new engines that use methane effectively
and efficiently , to run cars as well as farm machinery.
The prospects for biogas as renewable energy are excellent, provided the
constraints can be removed. Anaerobic digesters can also treat crop and food
wastes as well as the algal blooms and water hyacinths that clog up polluted
rivers and turn them into biogas energy.
Li Kangmin is Professor of Asian Pacific Regional Research and
Training Centre for Integrated Fish Farming, and China’s
representation on the International Organization of Biotechnology and Bio-engineering.
He teaches integrated fish farming and has researched and published extensively
on ecological agriculture and local circular economy, including eutrophication
control, surface aquaponics, vermiculture and biogas development.