Biogas use in Reconstruction
Practical Action
Alternative (short-term) Biogas Plants
However, in more short-term scenarios other digesters such as the bag or balloon digester (a type
of Plastic Bio-Digester) and the plug flow digester (a type of Earth-pit plant) may be appropriate,
since they are easier and quicker to construct. This corresponds with a shorter useful life span
compared with more advanced technologies.
Bag or Balloon Digester
A balloon plant is a type of Bio-Digester that 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 and therefore safety valves are required; if higher gas
pressures are needed, a gas pump is required as well.
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. Balloon biogas plants are recommended if local repair is or can be made possible and the
cost advantage is substantial.
Advantages: Standardised prefabrication at low cost, low construction sophistication, ease of
transportation, 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. There is only little scope for the creation of local
employment and, therefore, limited self-help potential.
Variations: A variation of the balloon plant is the channel-type digester, which is usually covered
with plastic sheeting and a sunshade (fig.2-E). Balloon plants can be recommended wherever the
balloon skin is not likely to be damaged and where the temperature is even and high.
Figure 2: Sections of
various biogas digester
plant concepts:
(A) Floating-drum plant
(B) Fixed-dome plant
(C) Fixed-dome plant
with separate gas holder
(D) Balloon plant
(E) Channel-type digester
with plastic sheeting and
sunshade.
Source: Biogas Plants, L.
Sasse, GATE, 1988, p.
14
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