Water-jacket floating-drum plants
Water-jacket plants are universally
applicable and easy to maintain. The
drum cannot get stuck in a scum layer,
even if the substrate has a high solids
content. Water-jacket plants are
characterized by a long useful life and a
more aesthetic appearance (no dirty
gas-holder). Due to their superior
sealing of the substrate (hygiene!), they
are recommended for use in the
fermentation of night soil. The extra cost
of the masonry water jacket is relatively
modest.
Figure 8: Water-jacket plant with external guide
Material of digester and drum
frame. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3
Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas
The digester is usually made of brick,
concrete or quarry-stone masonry with
pipe
Source: Sasse, 1984
plaster. The gas drum normally consists
of 2.5 mm steel sheets for the sides and
2 mm sheets for the top.
It has welded-in braces which break up surface scum when the drum rotates. The drum must
be protected against corrosion. Suitable coating products are oil paints, synthetic paints and
bitumen paints. Correct priming is important. There must be at least two preliminary coats
and one topcoat. Coatings of used oil are cheap. They must be renewed monthly. Plastic
sheeting stuck to bitumen sealant has not given good results. In coastal regions, repainting is
necessary at least once a year, and in dry uplands at least every other year. Gas production
will be higher if the drum is painted black or red rather than blue or white, because the
digester temperature is increased by solar radiation. Gas drums made of 2 cm wire-mesh-
reinforced concrete or fiber-cement must receive a gas-tight internal coating. The gas drum
should have a slightly sloping roof, otherwise rainwater will be trapped on it, leading to rust
damage. An excessively steep-pitched roof is unnecessarily expensive and the gas in the tip
cannot be used because when the drum is resting on the bottom, the gas is no longer under
pressure.
Floating-drums made of glass-fiber reinforced plastic and high-density polyethylene have
been used successfully, but the construction costs are higher compared to using steel.
Floating-drums made of wire-mesh-reinforced concrete are liable to hairline cracking and are
intrinsically porous. They require a gas-tight, elastic internal coating. PVC drums are
unsuitable because they are not resistant to UV.
Figure 9: Floating-drum plant in Burkina Faso
Photo: gtz/GATE
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