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Anaerobic Digestion in Rural China
by J. Paul Henderson
No centralized waste management systems are in place to handle
agricultural by-products, human wastes, animal manures and
food residuals generated by the 840 million Chinese - 70 percent
of the country's population - who live on farms or in villages.
Instead, since the 1970s, The People's Republic of China has
been promoting underground, individual, anaerobic digesters
to process rural organic materials. This strategy has resulted
in approximately five million household anaerobic digesters
installed in China today.
In rural China, nonrecyclable inorganic wastes tend to accumulate
on unused pieces of land scattered throughout the countryside.
Management practices for organics, on the other hand, are more
varied. Food residuals and many agricultural by-products are
fed to farm animals as supplements to commercial feed. Households
that do not have digestion systems use unprocessed human and
animal manure as fertilizer. Waste from toilets and animal
pens is stored in open pits until it is scooped out and delivered
to the field. Another method for dealing with this material
is simply to pile up the manure, cover it with soil and allow
it to compost without turning.
Households with an anaerobic digestion system mainly utilize
human and animal manures along with agricultural by-products
such as grain stalks (primarily rice), sweet potato vines,
and weeds. Other organics such as spoiled food, grain husks
and weeds are added in small quantities. Thirty years ago,
when digesters first were being promoted, rice stalks were
one of the main materials processed, mostly because of an insufficient
amount of other organics. The early digesters also were large
and used mechanical equipment for cleaning. Experience has
shown that grain stalks tend not to break down very well in
household systems, and cause the formation of a crust of up
to one meter thick inside the digester. The crust needed to
be removed in an annual clean out and hindered digester efficiency.
The only way to remove the crust and the accumulated material
was to enter the digester from a port at the top of the reactor.
Not only was this difficult work, but it also was dangerous
due to the potential for explosions from the biogas, and absence
of oxygen.
Today, in Sichuan Province for example, most agricultural
families raise two to four pigs for market at a time due to
an increase in meat consumption. In situations like these,
the combination of human waste and animal manure provides sufficient
feedstock to meet the majority of the energy needs of the family.
Grain stalks are either burned directly as fuel during winter
when gas production drops or are sold as a paper feedstock.
 Reactor
Design and Operation
Reactor design has evolved over tie. It used to be a cylindrical
main reactor with a domed top that accepted waste material
through a port connected to the bottom. Early reactors had
a separate effluent storage container that was connected by
a pipe attached below the water line. A port was constructed
at the top to allow the reactor to be cleaned out. To maintain
a gas seal, the lid had to be heavy making it hard to remove.
The influent port was relatively large and located outside
so that a variety of materials - including human and animal
manure - could be manually fed into the reactor. Modern designs
have simplified construction of the system. There is no longer
a port at the top of the reactor. The effluent chamber and
reactor are now connected, while toilets and pigsties generally
are directly connected to the influent port. In the standard
modern design, effluent is removed from the reactor at the
top of the water column, meaning that supernatant is collected
rather than sludge. Additionally, no mixing of the system occurs
when effluent is removed. In some systems, a vertical cylindrical
pull-rod port is added to the base of the effluent port. Effluent
is removed by moving the pull-rod - simply a wooden shaft with
a metal disk on the bottom - up and down in the port. A bucket
can be placed directly under the pull-rod port, simplifying
effluent removal, while the movement of the wooden shaft provides
some mixing in the reactor.
Head space volume above the reactor essentially is fixed,
although the volume increases slightly with increasing pressure
since the effluent port liquid level moves up and down with
pressure changes. Additionally, if effluent is not regularly
removed from the system, the increasing liquid volume also
reduces head space volume. As a result, gas pressure delivered
into the home is not constant, causing variation in heat produced
by cooking elements and variation of gas lamp light intensity.
 To
resolve this problem, some systems are constructed with a separate
gas storage chamber with a floating cover to maintain constant
pressure regardless of gas volume. This modification consists
of a cup shaped concrete storage container floating upside
down in a tank of water. The cup moves up and down in the tank
with changes in gas volume. Another advantage of this type
of storage system is that during cold weather, moisture in
the gas condenses in the storage container rather than in the
gas line. Condensate in supply lines can block the lines or
be another cause of oscillating gas pressure.
Construction of the reactors is completed by technicians
trained by the local government and members of the household.
The technicians are local residents who construct reactors
as a sideline to their regular work. The basic construction
materials for the reactors are concrete and bricks, easily
available and commonly used in rural China. Most reactors are
built in conjunction with the construction of new pigsties
and toilet facilities. Construction time for a reactor is approximately
one week, and the total cost including materials and the technician's
time is approximately $80 US.
The modern automatically feeding systems are very simple
to operate and maintain. Waste from the pigsty and toilet flows
directly into the reactor. The toilets are basic Asian squat
style facilities with the plumbing feeding into the reactor
influent port. Many of the homes have no running water and
- in all cases - the toilets are nonflushing and water only
is added manually for cleaning. Ideally, according to some
sources, daily infeed to a six cubic meter reactor should be
approximately 30 kilograms of feces and other organics plus
approximately 50 kilograms of water and urine. The net solids
of the material entering the reactor should be approximately
eight percent.
Manually fed systems require more work. Waste material must
be transferred from the initial storage pits and delivered
to the influent port of the reactor. Although material is supposed
to be collected from the effluent port and flushed back into
the influent port to promote mixing of the reactor, few farmers
actually do this. Effluent is removed as required.
Biogas and Effluent
From the farmers' point of view, the primary reason for constructing
a digester system is to produce biogas which is approximately
60 percent methane. Gas production is temperature dependent,
with production being inhibited at mean ambient temperatures
less than 10°C. More than half of China's rural population
are in areas where mean ambient temperatures exceeds 10°C
eight to 12 months of the year.
The gas primarily is used for cooking and lighting. A digester
can provide approximately 60 percent of a family's energy needs.
All of the kitchens have a traditional fuel stove in addition
to the gas burner and during winter months, when gas production
drops, straw, firewood and coal are used for cooking.
Effluent from the reactors is an odorless, dark colored slurry,
primarily used as an agricultural fertilizer. Other applications
include a feed supplement for pigs, mushroom growing media,
fertilizer for fish ponds, worm rearing media (the worms are
then fed to chickens), and media for soaking seeds prior to
germination.
Paul Henderson is an Environmental Engineer with the city
of Vancouver, Canada. He recent- ly conducted a six month
study of waste management systems in the Peoples' Republic
of China, funded by the Canadian International Development
Agency, the Sichuan Provincial Commission of Science and
Technology, and the Solid Waste Association of North America.
The author especially wishes to thank the Mianyang, Sichuan
Province, Municipal Biogas Office for its assistance.
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