As the name implies, this type of digester has a gas-collecting dome that is fixed. The digester is normally constructed using bricks and mortar and ends with a solid fixed dome in the shape of an igloo. Although this is the most well known digester design and the most widely used, it has a number of inherent disadvantages.
The main disadvantage is the fact that it can only ever produce gas of variable pressure. As the biogas is generated it rises and accumulates in the brick dome from where it is piped to point of use. The volume and rate of gas production is dependent on the type and frequency of material fed into the digester as well as on the temperature. This means that the amount and pressure of gas available will continuously vary, making it less efficient to run any type of biogas equipment such as gas water heaters, lights and generators.
The second disadvantage is related to the actual construction of the digester which requires a very high level of skilled labour. Constructing a dome using bricks and mortar is a task suitable only to a very experienced brick layer and is also time consuming. A critical aspect of a digester is that it has to be constructed and sealed in such a way that it is airtight - any crack in the structure will allow the biogas to escape. History has shown that this aspect has been the biggest cause of failure of fixed dome digesters due to the development of cracks as the cement cures and/or as a result of differential settlement of the structure. More than 50% of this type of digester has a functional life span of more than 3 years.