It is difficult to accept one particular type of digester for household biogas production. The design of the digesters is diversified based on the availability of sub­strate, geographical location, and climatic conditions. For example, a digester designed in mountainous re­gions has less gas volume in order to avoid gas loss. For tropical countries, it is recommended to have di­gesters underground due to the geothermal energy (Bin, 1989). Of all the different digesters developed, the fixed dome model developed in China and the floating drum model developed in India sustained to perform well until today (Rajendran et al., 2012). Recently, plug flow digesters are gaining attention due to its portability and easy operation.

Fixed Dome Digesters

The fixed dome digesters (Figure 1.11) is also called "hydraulic" or "Chinese" digesters and it is the most frequent model developed and used in China for biogas production (Rajendran et al., 2012). In this case, digester is filled through the inlet pipe until the level reaches the base level of the expansion chamber. Biogas that is pro­duced is accumulated at the upper part of the digester called storage part. The difference in the levels between the slurry inside the digester and the expansion chamber develops pressure inside due to accumulation of biogas. This accumulated biogas requires space and presses the substrate apart and enters into the expansion chamber. The slurry flows back into the digester straight away after

gas is released (Adeoti et al., 2000). Fixed dome digesters are usually built underground and the size of the digester depends on the place, number of households, and the amount of substrate available every day. Generally size of these digesters normally varies between 5 m3 and 150 m3 in various parts of Asia (Tomar, 1994). Instead of having a digester for each home, a large-volume digester is used to produce biogas for 10 to 20 homes, and is called community-type biogas digesters. In countries where houses are clustered as in Africa, these types of biogas digesters are more viable (Adeoti et al., 2000).