2.3.1 Fixed Dome Digester

The fixed dome digester, the Chinese model biogas plant, was first built in China in 1936 (UN, 1984). It consists of an underground digester and a fixed (non-movable) gas holder on the top and is typically constructed with concrete, bricks, and cement plaster to create a gas-tight dome. When gas is produced, it displaces slurry into a compensating chamber. A compensation chamber, also called a compensation tank or a compensation reservoir, is an additional chamber connected to the fixed dome digester. Its purpose is to provide a space for accommodating the variations in gas volume during digestion.

The gas pressure increases with the volume of gas stored. When gas production is low, there will be little gas in the digester’s headspace, and the gas pressure will be low. The fixed dome-type plant has a longer lifespan of 20 years. The digesters can be constructed in various capacities, 6 – 200m³, applicable for households, institutional buildings, and community systems.

Here is a table highlighting the pros and cons of using fixed dome digesters.

To address the issue of expert design and tightness, there are also prefabricated digesters, made of fibreglass or low-density polyethene, that can be placed underground, with similar working principles to a fixed dome digester. The prefabricated digester provides the capacity (1 – 6m³).

Further advantages are that the prefabricated modules save time during the installation and can be easily relocated. It is especially suitable for refugee camp contexts if permanent construction is not allowed locally.

Investment costs can be high, especially if digesters need to be imported. A limited range of sizes is available; multiple digesters may be required for larger volumes. In addition, the components are quite large and may pose a challenge regarding transportation.

For further information, please click on the Materials tab at the top of the page.