The advantage of continuous-feed plants is that the bacteria receive a regular supply of substrate
and are therefore able to generate a more constant supply of biogas. The problem is that buoyant
constituents tend to form a stiff layer of scum that impedes biogas production and may even plug up
the plant. That drawback can be countered by installing suitable agitators and lengthening the
The digester inlet, outlet and, to the extent applicable, the agitator must be designed to work
together in ensuring the proper retention time, i.e. to avoid short-circuit flow, because the gas
production rate would otherwise stay well below the optimum level.
Continuous-feed biogas plants are sized on the basis of the desired retention time for the organic
material, in combination with the digester load, which in turn is a function of the prevailing
temperature and type of substrate (cf. chapter 4.3).
5.3 Biogas plants of simple design
There are two basic types of tested biogas plants that have gained widespread acceptance in
- floating-drum plants in which the metal gasholder floats on the digester, and
- fixed-dome plants in which gas storage is effected according to the displacement principle.
5.3.1 Floating-drum plants
A floating-drum biogas plant essentially consists of a cylindrical or dome-shaped digester and a
movable, floating gasholder, or drum. The drum in which the biogas collects has an internal or
external guide frame that provides stability and keeps the drum upright. Braces can be welded into
the drum as a means of breaking up the scum layer when the drum is rotated. The digester is
usually made of brick, concrete or quarrystone masonry with rendering, while the gasholder is
normally made of metal.
Floating-drum plants are used chiefly for digesting animal and human excrements on a continuous-
feed mode of operation, i.e. with daily input. They are used most frequently by:
- small-to-midsize family farms (digester size: 5 - 15 m³)
- institutions and large agroindustrial estates (digester size: 20-100 m³).
Advantages: Floating-drum plants are easy to understand and operate. They provide gas at a
constant pressure, and the stored volume is immediately recognizable.
Drawbacks: The steel drum is relatively expensive and maintenance-intensive due to the necessity
of periodic painting and rust removal. If fibrous substrates are used, the gasholder shows a
tendency to get "stuck" in the resultant floating scum.
Floating-drum plants can be recommended as a mature, easy-to-operate, functionally capable
means of producing biogas, particularly when reliability is deemed more important than
Floating-drum plants with gasholder in the digester (cf. fig. 5.6)