SICHUAN'S HOME-SCALE BIOGAS DIGESTERS

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Other site considerations include the need to place the fermenting pit away from an area with trees and large roots . . . and to position it in such a way that the soil above the chamber will receive sunlight, to allow the unit to take advantage of direct solar gain. The composition of the foundation material is also critical, since the presence of expansion clay, mixed soils, ground water, or solid rock could call for the use of a different construction method or relocation of the pit.

Finally, each unit must be sized to handle adequately both the incoming waste and the fuel needs of the family or community that it serves. The Chinese have found that a household with 3 to 7 persons, and perhaps a pig or two, requires a digester of 200- to 425-cubic-foot internal volume, based upon the assumption that each family member uses about ten cubic feet of gas-for cooking and lighting—per day and that the daily yield of fuel from a typical small-scale pit digester is approximately 15% of the volume of the fermenting liquid .

. . . AND COST-EFFECTIVE

In order to cut the cost of constructing the units and to conserve materials, the builders try to use what's available locally when fabricating each pit. Most systems are built by hand . . . and indigenous rock, lime, clay, and homemade brick are incorporatedwhenever possible-to provide structural support or to cement components together. (It's interesting to note that, over the years, Chinese workers have developed a variety of concrete grades made from coal and lime slag, crushed brick and tile, ash, clay, cinder, or sandin short, they use many substances which we would often consider waste—and feel that the resulting construction material compares favorably with the "real thing".)

Normally, a pit is first excavated, and then the digester's masonry "shell" is laid up around a wooden or steel mold. Occasionally, though, the container is cast right in place. In any event, a typical unit consists of a central liquid-manure and gas storage tank, connected to a slanted inlet pipe that extends into the slurry. Opposite this—and separated by a small retaining wallis an outlet vat containing about one-tenth the volume of the main tank, from which effluent fertilizer can be withdrawn through a covered hatch. The gas itself travels along a pipe fastened to a removable cover at the top of the domed tank lid, and is used as needed (a simple water-in-tube manometer is employed to measure internal pressure). At some locations, ponds are built on the area directly over the tank, to seal potential gas leaks, to store solar heat, and to grow aquatic vegetation.

The Chinese digester's simple design maximizes its utility. Because the vat is broader than it is high, it is not likely to interfere with deep ground water supplies . . . is easy to excavate a site for . . . and has more potential for gas production than would a taller unit, because of its greater surface area. Moreover, the steepangled inlet pipe can't clog, and the force of the fresh waste material entering the system stirs up the fermenting matter . . . which helps speed decomposition. Additionally, the position of the outlet chamber allows only "sanitary" effluent to escape, since parasitic eggs and bacteria settle to the bottom of the tank, where they're eventually destroyed. Finally, the cleanout cover allows for the necessary semiannual inspection and manual emptying of the entire vat.

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