Environmental Engineering Reference
In-Depth Information
4.3.2 Chemical pre-treatment
Chemical pre-treatment has been investigated using a range of different
chemicals, mainly acids and bases of different strengths under different
conditions. The use of temperature and chemicals together (thermochemical
pre-treatment) is described in a later section.
Alkali treatment
As mentioned previously in this chapter, lignocellulosic materials are
resistant to hydrolysis due to their structure and composition. Alkali pre-
treatment removes the acetate groups from hemicellulose, which makes the
hemicelluloses more accessible to hydrolytic enzymes. This enhances
digestability. Alkali addition also causes swelling of lignocelluloses,
although this is a secondary effect (Kong et al., 1992). Lignin is also partly
solubilised by alkali pre-treatment, and this allows more access to cellulose
and hemicellulose. Alkali treatment can be carried out with different
concentrations of lime, sodium hydroxide (NaOH) and potassium hydro-
xide (KOH).
There have been several reports of alkali treatment being effective for
solid-state anaerobic digestion. He et al. (2008) showed an increase in biogas
yield from rice straw of 27.3-64.5% using 6% NaOH for 3 weeks at ambient
temperature. Liew et al. (2011) carried out simultaneous pre-treatment and
methanisation using 3.5% NaOH on fallen leaves and showed that with an
optimised substrate to inoculum ratio, the methane yield increased by
21.5%. Interestingly, when a sub-optimal substrate to inoculum ratio with
too much substrate was used, the control tests produced extremely low levels
of biogas, but the alkaline pre-treated tests increased methane yields 22-fold.
These studies demonstrated that alkali pre-treatment can increase gas yield
from hemicellulose-rich substrates and dissolve lignin complexes. However,
it is important to note that alkali pre-treated substrates have high pH values.
The above-mentioned experiments were carried out using small-scale batch
tests but, during continuous fermentation, alkali pre-treatment leads to
increased pH and salt build-up. The pH increase affects the ammonium-
ammonia balance and inhibits methanisation, and high concentrations of
cations like Ca
2+
,K
+
or Na
+
lead to an inhibition of anaerobic digestion
due to osmotic pressure (Chen et al., 2008). However, the pH increase may
be beneficial for substrates with low pH or high lipid content (e.g. as
demonstrated by Beccari et al. (2001) with olive oil mill effluent and
Ca(OH)
2
). This pre-treatment technology was deemed economically
unattractive due to the high costs of bases (Chang et al., 1997).
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