Biofuels in Perspective

184 Biofuels
Figure 10.9 Schematic representation of a conventional UASB reactor for anaerobic digestion of wastes
with a low suspended solids concentration.
in the digestion section is captured by the separator, so that unhindered settling can take
place in the upper zone. To avoid blocking of the biogas outlet and allow separation of
biogas bubble from sludge particles, a gas chamber is introduced under the separator element.
The settled sludge particles on the separator elements eventually slide back into the
digestion zone. Thus, the settler enables the system to maintain a large sludge mass in the
reactor, while an effluent essentially free from the suspended solids is discharged. Recently
several variants of the UASB concept have been developed, basically to make more efficient
use of the reactor volume of the digestion section of the reactor (EGSB, Expended
Granular Sludge bed reactor), and/or to increase the efficiency of the sludge retention in the
settling section. Under favourable operational conditions, the UASB reactor can receive an
organic volumetric load of 20 kgCOD/m 3 .d. UASB reactors and its variants are currently
used in a variety of liquid waste from agro business (stillage, paper mill, fruit canning effluents,
tanneries, slaughterhouses) and industry (breweries, pharmaceutical, petrochemical
wastes)
10.4.2 Different Process Configurations for Wet Digestion Fermenters
If the suspended solids concentration of the material to be digested is not low, biogas can
be produced by wet- or dry-fermentation processes (see Figure 10.10). Today about 90
% of German agricultural biogas plants are operated by wet fermentation, which allows
for a continuous treatment of liquid, pasty or solid substrates. The typical total solids
content of the digester content is in a range of between 8 and 10 % TS which permits
conventional pumps and mixers to be used. Dry-fermentation processes are operated with
solid substrates with a resulting total solids content in the digester of at least 20 % TS. These
processes are mainly operated discontinuously without any mixing of the solid substrate
(Weiland, 2004b). Dry fermentation is of increasing interest because an extra technology