Figure 6: Installation of a Shanghai fixed-dome system near
Shanghai, PR China
Photo: L. Sasse
Floating-drum plants
The drum
Figure 7: Floating-drum plant in Mauretania
Photo: gtz/GATE
In the past, floating-drum plants were
mainly built in India. A floating-drum
plant consists of a cylindrical or dome-
shaped digester and a moving, floating
gas-holder, or drum. The gas-holder
floats either directly in the fermenting
slurry or in a separate water jacket. The
drum in which the biogas collects has an
internal and/or external guide frame that
provides stability and keeps the drum
upright. If biogas is produced, the drum
moves up, if gas is consumed, the gas-
holder sinks back.
Size
Floating-drum plants are used chiefly for digesting animal and human feces on a continuous-
feed mode of operation, i.e. with daily input. They are used most frequently by small- to
middle-sized farms (digester size: 5-15m3) or in institutions and larger agro-industrial estates
(digester size: 20-100m3).
Advantages: Floating-drum plants are easy to understand and operate. They provide
gas at a constant pressure, and the stored gas-volume is immediately recognizable by
the position of the drum. Gas-tightness is no problem, provided the gasholder is de-
rusted and painted regularly.
Disadvantages: The steel drum is relatively expensive and maintenance-intensive.
Removing rust and painting has to be carried out regularly. The life-time of the drum
is short (up to 15 years; in tropical coastal regions about five years). If fibrous
substrates are used, the gas-holder shows a tendency to get "stuck" in the resultant
floating scum.
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