only feasible with homogenous substrate and a dry matter content below 5%. Mechanical
agitation becomes problematic above 10% dry matter.
Ballon plants
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.
Advantages: Standardized prefabrication at low cost; shallow installation suitable for
use in areas with a high groundwater table; high digester temperatures in warm
climates; uncomplicated cleaning, emptying and maintenance; difficult substrates like
water hyacinths can be used.
Disadvantages: Low gas pressure may require gas pumps; scum cannot be removed
during operation; the plastic balloon has a relatively short useful life-span and is
susceptible to mechanical damage and usually not available locally. In addition, local
craftsmen are rarely in a position to repair a damaged balloon.
Balloon biogas plants are recommended, if local repair is or can be made possible and the
cost advantage is substantial.
Horizontal plants
Horizontal biogas plants are usually chosen when shallow installation is called for
(groundwater, rock). They are made of masonry or concrete.
Advantages: Shallow construction despite large slurry space.
Disadvantages: Problems with gas-space leakage, difficult elimination of scum.
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.
Advantages: Low cost of installation (as little as 20% of a floating-drum plant); high
potential for self help approaches.
Disadvantages: Short useful life; serviceable only in suitable, impermeable types of
soil.
Earth-pit plants can only be recommended for installation in impermeable soil located above
the groundwater table. Their construction is particularly inexpensive in connection with plastic
sheet gas-holders.
17