Family-Size Biogas Plant

Straws and most other biomass have a tendency to float on the water and form a scum layer. Tests in India have shown that these types of materials will generate biogas when they have been in contact with digester fluid for at least three days. This can be achieved by filling the digester with a quantity of wet compost. The compost will float. New material is added from the bottom of the digester. This material will push the compost upward but will stay under the digester liquid, as the compost being wet is relatively heavy. The result­ing digested material forms a thick mat. The digested material can be removed once per year after opening the plant (Oosterkamp, 2003).

A plant suitable to convert maize stalks and other biomass into biogas is of the water jacket floating drum plant type. The water jacket reduces the emission of methane. In fixed dome plants 10% of the methane production is lost. The main modification is the use of a large 0.3 m diameter inlet pipe. Through this pipe the waste biomass can be pushed down into the digester proper. The floating drum can be removed and digested material taken out from the top.

Wet Digesters

Most digester systems in Europe are manure based (wet) and/or use maize silage for their feed. Some of these wet systems add about 10% straws (VS). Two or more sequentially linked digesters give about 15% more gas yield than a single continuously stirred digester (Angelidaki et al., 2005). The wet systems have the disad­vantage that solids (10—15%) need to be kept in suspen­sion by an impeller. With a high percentage of grasses and straw scum layers are formed that need to be removed mechanically after opening the digester.