In the fixed dome type digester biogas plant, the digester and gas-collector (gas dome) are
enclosed in the same chamber. This type of construction is suitable for batch type biogas plant. The
digester is conveniently built at or below ground level in comparatively cooler zone. The construction
of the digester is with locally available materials like, bricks, tera-cota. The pressure inside the digester
increases as the biogas is liberated. The biogas gets collected in the upper portion of the digester in a
dome shaped cavity. The outlet pipe is provided at the tope of the fixed dome. Alternatively the gas
collector (gas holder) is a separately installed chamber. The digester tank and gas collector chamber are

separated by a water seal tank.
The arrangement of a separate gas collector is preferred as the tapping of gas from the gas
holder does not affect the pressure and the digestion process in the main digester. The water seal tank
prevents the return of the gas from the gas collector to the digester chamber.
An additional displacement chamber may be provided for provid-ing space to the displacement
slurry in the digester due to gas pressure in the upper dome of the fixed type digester. The fixed dome
type digester can be fed on daily basis with small quantities of the slurry. The excess slurry in the
digester gets accommodated in the displacement chamber. The level of the slurry in the main digester
and the displacement collector can vary in accordance with the pressure and volume of the biogas in the
fixed type of dome. The pressure in the fixed dome and the displacement gas collector are almost the

same as they are connected by the outlet from the main digester.
Floating Gas Holder Type. In this design a dome made floats above the slurry in the disaster.

In the Fig. 2.4, The disaster tank is of cylindrical masonry construction. The floating dome is of fabricated
steel construction. The dome guide shaft provides the axial guide to the floating dome. As the gas
is collected in it. The sliding bearing provides smooth sliding surface and guide to the floating dome.
The gas generated in the slurry gets collected in the dome and the dome arises. The water seal tank
provides separation between the gas in the dome and the outlet gas.

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