In China, the area of small-scale biomethane digester is about 6-8 m2. Annual output is 300 m3. Cost of each biomethane digester is 1500-2000 RMB.

In spite of providing energy, biomethane in China also has the following characteristics: 1) environment friendly. For an 8-10 m3 biomethane digester, dejecta of 5-8 pigs or 2000-3000 chickens can be used. 2) The residues in the digester can also be used for fertilizer. 3) economically. It saves money of buying electricity or save labor of seeking firewood. And the woods can be saved.

8.1.3. Process of small-scale biomethanation

The process of small-scale biomethanation includes feedstock collecting, pretreatment, fermentation, treatment and purifying, storage and transportation, where the fermentation digester is the main equipment. The digesters are required to be airtight and impervious to make sure they are anaerobic. The temperature in the digester should be maintained at 20-40°C. There should be enough manure in the digester. Appropriate water (about 80%) content and pH (7-8.5) are required.

Batch fermentation and semi-continuous fermentation are usual technologies for small-scale biomethanation. In batch fermentation technology, all the feedstock is added at the first. The biomethane generates fast at the beginning and then decreases. This technology is easy for management, but the biomethane generation rates are different. In semi-continuous fermentation technology, 1/4 — 1/2 feedstock was added at the first. When the biomethane generation slows down, more feedstock is added to make the biomethane generation work in order.