Horizontal plants
Horizontal biogas plants are usually chosen when shallow installation is called for (groundwater, rock). They are made of masonry or concrete.
Earth-pit plants
Masonry digesters are not necessary in stable soil (e.g. laterite). It is sufficient to line the pit with a thin layer of cement (wire-mesh fixed to the pit wall and plastered) in order to prevent seepage. The edge of the pit is reinforced with a ring of masonry that also serves as anchorage for the gas-holder. The gas-holder can be made of metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden frame that extends down into the slurry and is anchored in place to counter its buoyancy. The requisite gas pressure is achieved by placing weights on the gas-holder. An overflow point in the peripheral wall serves as the slurry outlet.
Ferrocement plants
The ferro-cement type of construction can be applied either as a self-supporting shell or an earth-pit lining. The vessel is usually cylindrical. Very small plants (Volume under 6 m3) can be prefabricated. As in the case of a fixed-dome plant, the ferrocement gasholder requires special sealing measures (proven reliability with cemented-on aluminium foil).
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Fixed-dome type
The fixed-dome biogas plant consists of one lower segment (for the digester) and a hemisphere over it (for both digester and gas holder). The mixing tank is connected to the digester by a 15-cm asbestos cement pipe. Through the outlet hole provided in the digester, the slurry is pushed into the outlet tank and overflows through another hole provided in the outlet tank.
Floating-drum type
The floating-drum biogas plant consists of a deep well-shaped underground digester connected by inlet and outlet pipes. A mild-steel gas storage drum, inverted over the slurry, rises and falls around a guide pipe corresponding to the accumulation and withdrawal of gas.
Bag or Balloon type
Made of rubberized nylon fabric, the bag-type biogas plant is a portable unit, which can conveniently be placed at any location. The appropriate model is selected on the basis of technical requirements such as location, distance between kitchen and cattle shed, availability of dung and water, preferences of the beneficiaries, and so on. A balloon plant consists of a heat-sealed plastic or rubber bag (balloon), combining digester and gas-holder. The gas is stored in the upper part of the balloon. The inlet and outlet are attached directly to the skin of the balloon. Gas pressure can be increased by placing weights on the balloon. If the gas pressure exceeds a limit that the balloon can withstand, it may damage the skin. Therefore, safety valves are required.
If higher gas pressures are needed, a gas pump is required. Since the material has to be weather- and UV resistant, specially stabilized, reinforced plastic or synthetic caoutchouc is given preference. Other materials which have been used successfully include RMP (red mud plastic), Trevira and butyl. The useful life-span does usually not exceed 2-5 years.
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