Environmental Engineering Reference
In-Depth Information
4.3.3 Biological pre-treatment
Microbiological pre-treatment
Microbial pre-treatment, also known as pre-acidification or multi-stage
fermentation, is a simple kind of pre-treatment technology in which the first
steps of anaerobic digestion (hydrolysis and acidogenesis) are separated
from acetogenesis and methanogenesis. A two-stage digestion system is
common for carrying out this kind of pre-treatment. The concept of
carrying out digestion in separate vessels is similar to the multiple chambers
of ruminant digestive systems. The pH value of the first digester (the pre-
acidification step) should lie between 4 and 6, thereby inhibiting methane
production (Deublein and Steinhauser, 2008; Thauer, 1998). This inhibition
causes volatile fatty acids (VFAs) to accumulate. The gas produced during
this pre-acidification step has high concentrations of carbon dioxide and
hydrogen. The production of H
2
goes hand in hand with the production of
fatty acids and is an important indicator to evaluate the pre-acidification
step. The extent of H
2
production is most strongly influenced by pH: H
2
production at pH 6 is initially high and then stops and at pH 4 is lower but
prolonged and greater overall (Liu et al., 2006). Antonopoulou et al. (2008)
demonstrated in continuous fermentation tests that H
2
concentration was
35-40%v/v of the total gas amount of the pre-acidification step.
Microbiological pre-treatment has a very positive effect on the degrada-
tion rate of substrates in anaerobic digestion. In general, cellulose,
hemicellulose and starch-degrading enzymes work best between pH 4 and
6 at temperatures from 30 to 50
C, so this pre-acidification step increases the
degradation rate by creating an optimal environment for hydrolytic
enzymes, particularly for carbohydrate degradation. Liu et al. (2006)
achieved an additional biogas yield of 21% at a hydraulic retention time of
approximately 30 days. This was caused by higher degradation through
increased hydrolytic enzyme activity.
Another positive effect of this pre-treatment method is on the methane
concentration in the biogas. In addition to H
2
and VFA production, CO
2
is
formed during the pre-acidification step. CO
2
can be present in three forms:
at higher pH values in the form of the carbonate ion CO
3
2
; at neutral pH as
HCO
3
; and in acidic environments as CO
2
. Due to the low pH, most of the
carbonate is in the form of CO
2
, which is volatile and is released into the
hydrolysis gas produced from the pre-acidification step. This means that for
the methanogenesis step, a higher CH
4
concentration is present in the gas
phase. Nizami et al. (2012) produced a biogas with 71% methane content in
a two-phase system digesting grass silage. The same grass silage produced a
biogas with 52% methane content in a wet single-stage system.
In large-scale biogas plants, pre-acidification systems are offered by
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