Environmental Engineering Reference
In-Depth Information
several plant constructers, varying from continuous to batch pre-acidifica-
tion systems. Continuous pre-acidification is offered, for example, by the
companies AAT and Enbasys from Austria. Substrates are fed continuously
in a two-reactor CSTR system. The daily removal of material to feed the
second reactor is balanced by a feed of fresh material to the first reactor.
Plug-flow reactors are also in use. This technology guarantees the treatment
of the requested retention time, which is not given in the CSTR system.
Batch pre-acidification digesters are completely emptied after a retention
time of a few days and refilled with new substrates.
Enzyme addition
The purpose of enzyme addition is to break down polymers in the substrate,
particularly lignocelluloses. A cocktail of enzymes is typically used, and may
include cellulases, xylanases, pectinases and amylases. Enzymes can be
applied in three different ways: by direct addition to the vessel of a single-
stage anaerobic digestion; by addition to the hydrolysis and acidification
vessel (first stage) of a two-stage system (see the previous section on
microbiological pre-treatment); or by addition to a dedicated enzymatic pre-
treatment vessel. The addition of enzymes to anaerobic digestion has been
analysed in different studies. Romano et al. (2009) analysed the effect of
enzyme addition on anaerobic digestion of a type of pasture grass, wheat
grass. The enzyme addition showed a positive impact on solubilisation of
the substrate. In this study, no additional gas yield was measured, but a
slightly faster degradation rate was found in a single-stage system. In a
different study, an additional gas yield was achieved using two-stage
digestion of brewers' spent grains with enzyme addition in the acidification
stage (Bochmann et al., 2007). Higher VFA production was also achieved
through enzyme addition. Ellenrieder et al. (2010) analysed the addition of
single enzymes like cellulose, amylase or pectinase to maize and grass silage,
but no additional benefit on gas yield was determined.
4.3.4 Combined processes
Steam explosion
The principle of steam explosion is related to thermal pre-treatment. The
substrate is heated in a closed system to a temperature of 160 to 220
C,
causing a rise in pressure. After a process specific retention time (between 5
and 60 minutes), pressure is released abruptly. This sudden drop in pressure
causes intracellular water to evaporate. Cell walls are disrupted, causing
substrates to lose their structure. Due to the long retention time and high
temperatures, the Maillard reaction occurs and Maillard products are
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