170 Organic waste recycling: technology and management
Figure 4.8 Mean daily biogas production at hydraulic retention times of 30, 50, and 70
days from pilot small-scale digesters. Each point is the mean of four digesters.
Floating gas holder digester (Indian design)
This type of digester, designed by the Khadi and Village Industries Commission
(KVIC), India, consists of a cylindrical well, most commonly made from bricks,
although chicken wire mesh reinforced concrete has been used. Pressure in the
digester remains constant because the gas produced is trapped under a floating
cover on the surface of the digester which rises and falls on a central guide
(Figure 4.9 a & b). The cover is usually constructed of mild steel, although, due
to corrosion problems, other materials such as ferrocement, bamboocement,
different kinds of plastic and fiberglass have been used. A major cause for the
loss of heat from this kind of digester is its cover. The digester may be buried
under the ground to prevent heat loss, leaving the gas holder more or less above
the ground
This digester is fed semi-continuously through a straight inlet pipe, and
displaces an equal amount of slurry through an outlet pipe. When ratios of
height to diameter are high, a central partition wall is built inside the digester to
prevent short circuiting of the substrate.
This design is extensively used in India and fed with cattle dung only. If
agricultural residues are being used or mixed with animal wastes, the residues
should be chopped to small pieces to prevent blockage. This design is simple to
build and maintain, and does not require an experienced builder. It is used
throughout the world and both it and the Chinese-type fixed-dome digesters are
the most common types of digester used for treating organic wastes.
Table 4.10 briefly compares the various aspects of the Chinese and Indian-
type biogas digester systems.
30 days50 days70 daysOperation period (days)Biogas production (m3 day-1)