2.3.1 Fixed Dome Digester

Left: Cross-section View Fixed Dome Digester (UPM), Right: Construction of Fixed Dome Digester in Lesotho (Source: TED, Lesotho)

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.

The initial costs are relatively low.To achieve a gas-tight construction, certain specific technical skills are necessary.
If well-constructed, long service life.Fluctuating gas pressure can complicate its utilization.
No parts that move or rust.The produced gas is not visible without the use of a gas pressure device.
Constructing underground saves space and safeguards the digester against temperature changes.A special sealant, such as an engine oil mixture or acrylic emulsion, is necessary for the interior plastering of gasholders.
 Gas leaks are more likely to happen if the construction work is not done by experienced masons.
 Require precise plans for the levels, and excavating through bedrock may pose challenges and high costs.
 It is challenging to repair the reactor once if located underground.
Adopted from: (Vögeli Y., 2014), (GTZ )
Pros & Cons of 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).

Left Isometric View of Fibreglass Digesters (UPM), Right: Fibreglass digester in Refugee Camp in Cox’s Bazar (UPM)

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.

Further Reading:

  • Fixed dome plants (Link)