Anaerobic digestion in rural China

Simple reactors provide energy and fertilizer for Chinese farms and villages. Another benefit includes improved household sanitation.

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 time. 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 10oC. 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.

[Author Affiliation]

Paul Henderson is an Environmental Engineer with the city of Vancouver, Canada. He recently 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.