Biogas - Digester types
In this chapter, the most important types of biogas plants are described:
• Fixed-dome plants
• Floating-drum plants
• Balloon plants
• Horizontal plants
• Earth-pit plants
• Ferrocement plants
Of these, the two most familiar types in developing countries are the fixed-dome plants and
the floating-drum plants. Typical designs in industrialized countries and appropriate design
selection criteria have also been considered.
Fixed-dome plants
The costs of a fixed-dome biogas plant are relatively low. It is simple as no moving parts
exist. There are also no rusting steel parts and hence a long life of the plant (20 years or
more) can be expected. The plant is constructed underground, protecting it from physical
damage and saving space. While the underground digester is protected from low
temperatures at night and during cold seasons, sunshine and warm seasons take longer to
heat up the digester. No day/night fluctuations of temperature in the digester positively
influence the bacteriological processes.
The construction of fixed dome plants is labor-intensive, thus creating local employment.
Fixed-dome plants are not easy to build. They should only be built where construction can be
supervised by experienced biogas technicians. Otherwise plants may not be gas-tight
(porosity and cracks).
The basic elements of a fixed dome plant (here the Nicarao Design) are shown in the figure
below.
Figure 1: Fixed dome plant Nicarao design: 1. Mixing tank with inlet pipe and sand trap.
2. Digester. 3. Compensation and removal tank. 4. Gasholder. 5. Gaspipe. 6. Entry hatch,
with gastight seal. 7. Accumulation of thick sludge. 8. Outlet pipe. 9. Reference level. 10.
Supernatant scum, broken up by varying level.
Source: TBW
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