2.2. The biogas plant

Each biogas plant consists of the digester and the gasholder. The size of the digester depends on the amount of substrate occurring daily at given temperatures. The size of the gasholder depends on the daily gas production and mainly on the gas consumption times. For reasons of economy and construction, it is not feasible to store the gas over several days. For household plants, the digester volume (VD) is about five times as great as the volume of the gasholder (VG). The gas is fed directly from the gasholder to the burner through pipes. For reasons of costs and gas pressure, it is an advantage when the point at which the substrate occurs and where the biogas plant normally stands is not too far from the place where the gas is used. Saving biogas in pressurised containers or in bottles in liquid form is not economical.

There are three types of plant which would basically and technically be suitable for households. These are the floating-drum plant, the plant with a plastic balloon-type gasholder and the fixed-dome plant. Only the fixed-dome plant has proven successful for dissemination programmes for household plants as it is hard-wearing and requires little maintenance.

The technology of fixed-dome plants today - in contrast to earlier years - is no longer a great problem. The type of plant which has asserted itself in German development cooperation is the one developed by CAMARTEC in Tanzania with the so-called "strong ring" and the gas-tight dome plastered with the aid of cement agents. When this has been erected technically perfectly its service life is practically unlimited. Periods of 15 - 20 years are most frequently assumed for economic efficiency calculations. However, specific training of masons is necessary if this model is to be built to operate correctly. In the fixed-dome plant, the gas is stored in the upper area of the digester. The gas then pushes part of the substrate into a compensation chamber. This then flows back into the digester as soon as gas is taken out. The compensation chamber is consequently the same size as the inner gasholder. The gas pressure then falls or rises depending on the quantity of gas stored. The gas pressure occurs due to the deference in level between the level of liquid in the digester and that in the compensation chamber. To avoid gas pressure which is too high, the maximum size of a fixed-dome plant with a digester-gasholder ratio (VD:VG) of 5:1 at about 50 m³ VD. During construction, a household fixed-dome plant requires an area of about 7 by 4 metres. After construction has been finished, the plant will only take up a space of about 5 m².

For the floating-drum plant, which was previously preferred, the demands on quality in production can be met more easily. The corrosion on the steel gas dome which can hardly be avoided and the higher costs mean that this model is less suitable for dissemination. Even Indian entrepreneurs specialised in the construction of floating-drum plants are now beginning to build fixed-dome plants. Experiments on replacing the steel by ferrocement, fibre glass or other materials currently have to be seen as unfruitful. Floating-drum plants today are only considered for digester volumes over 50 m³. The steel drum then floats in a water bath.

The plant with plastic foil is the most low-cost in its production but, due to its short service life, incurs high operation costs. Biogas dissemination programmes still using these plants need a secure service structure. In individual cases however, the foil plant has proven itself for larger plants (VD > 100 m³ (e.g. Ferkessedougou/lvory Coast).