Organic Waste Recycling

Biofuels production 165

equal to the mean cell residence time (șc) in the digester. To provide longer șc in
the digesters, attached-growth bacteria have been employed in which the
anaerobic bacteria, attached to artificial media or settled as blanket in the
digesters, decompose the organic wastes. Descriptions of the dispersed-growth
digesters and attached-growth digesters are given below.

Dispersed-growth digesters

Combined digester and gas holder - fixed dome (Chinese design)

In this type of digester (Figure 4.5) the gas storage volume is directly above the
digesting contents of the reactor. The volume of the digester equals the volume
of slurry and gas combined. The small-capacity digesters of this type (6-12 m^3 )
are suitable for single family or a group of families. The larger sizes (50 m^3 ) are
designed for community gas requirements. The roof, walls and bottom of the
reactors are constructed either of bricks (in-situ) or precast concrete. The inlet
opening and displacement tank are of lime clay. Both the top and the bottom are
hemispherical, and are joined together by straight sides. If the digester is
constructed from bricks the inside surface is sealed by many thin layers of
mortar to make it water and gas tight.

Figure 4.5 Fixed-dome digester

This digester is totally buried underground to ensure uniform temperature
distribution, to save space, and to make use of soil support. The inlet pipe is
straight and ends at mid level in the digester. The outlet is also at the same level,
and consists of a fairly large storage tank. A manhole is provided at the top to
gain access to digester during cleaning times. When digester is in operation, this
manhole is covered and sealed with clay.
The gas produced during digestion is stored under the dome and displaces
some of the digester contents into the effluent chamber; this condition creates

INLET GAS OUTLET

REMOVABLE MANHOLE

COVER SEALED WITH CLAY LOOSE COVER

DISPLACEMENT TANK

SLURRY

GAS 1000 mm

OUTLET PIPE