Environmental Engineering Reference
In-Depth Information
4.2
Large-scale TDH unit (source: R. Scheuchel GmbH, Ortenburg,
Germany).
retention time is 20minutes. The hydrolysed substrate is cooled down to
almost process temperature using the heat exchanger. Finally, the pressure is
released (Dinglreiter, 2007). Figure 4.2 shows such a unit.
Various studies have shown that thermal pre-treatment increases biogas
yield only up to a certain temperature, and gas production decreases below
this temperature. DiStefano and Ambulkar (2006) note the maximum
temperature as 175
C for sewage sludge. Using TDH to pre-treat crops, the
maximum temperature is 220
8
C (Dinglreiter, 2007). Thermal pre-treatment
of brewers' spent grains shows a lower gas yield with pre-treatment above
160
8
C as compared with untreated substrate (Bochmann et al., 2010). The
maximum temperature depends on the composition of the substrate and
also on the retention time of pre-treatment.
8
Ultrasonic treatment
Ultrasonic treatment is less suitable as a pre-treatment technology than for
post-treatment of the liquid effluent from anaerobic digesters. The
frequency of ultrasound is over 20 kHz; using these frequencies causes
cavities or liquid-free bubbles to form and then implode, producing
shockwaves in a process called cavitation. These forces cause the disruption
of microbial cell walls in the liquid. In general, this technology is used for
the treatment of sewage sludge from wastewater treatment plants, but the
effect on biogas production from this ultrasonicated substrate is very low.
Ultrasonic treatment only disintegrates microbiological biomass and not the
input material (Onyeche et al., 2002).
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