Environmental Engineering Reference
In-Depth Information
Animal slurries can also be a valuable part of the digestion mix but,
because of their relatively low methane yield, the proportion used has to be
carefully selected so as not to reduce the overall productivity of the digestion
plant.
Micronutrients
Although the majority of farm-based digesters using energy crops as a
feedstock co-digest with animal slurry, this is not always the case. Where
there is mono-digestion of a crop then this may be at risk of a deficiency in
micronutrients or an imbalance in macronutrients. In fact there are few
mono-substrates that are likely to satisfy the full complement of
micronutrients demanded by methanogens and syntrophicacetogens
(Weiland, 2010; Demirel and Scherer, 2011). It is therefore not surprising
that the supplementation of maize digesters with Fe, Co and Ni has
improved biogas yields (Hinken et al., 2008), while Se and Mo have also
been found to be deficient (Lebuhn et al., 2008). Optimum TE mixes for
maize silage have been worked out in a series of trials using a synthetic
model substrate (Pobeheim et al., 2010, 2011). In some respects, however, it
is unwise to base any dosing strategy on a fixed TE recipe without knowing
the baseline concentrations in the digester as a result of TEs entering as part
of the feedstock. TEs can be toxic as well as stimulatory to the anaerobic
consortium and the concentration at one loading may be insufficient at a
higher loading (see Section 6.7.1). Although it is possible to analyse the TE
content of the digestate and then refer to guideline values, it is usually more
sensible to follow the recommendations of Zitomer et al. (2008) and set up a
simple series of test bottles to which individual TEs are added: an increase in
biogas production of greater than two standard deviations in any variant
will quickly indicate a probable TE deficiency.
Trace element deficiencies are more likely to occur when crops or crop
residues are digested without ruminant animal manure. Quite simply,
farmers usually give their animals dietary supplements that contain the TEs
necessary to stimulate the anaerobic consortia in the gut or rumen of the
animals, and most of these elements pass out with the animal faeces (Sager,
2007). Slurry may not be seen as a good energy producer in AD processes,
but it is certainly an excellent source of those elements that are likely to
improve biogas production. Improvements in biogas production have also
been reported in digestion plant treating grass-clover silage (Jarvis et al.,
1997) and sugar beet pulp due to the addition of cobalt. In our laboratory,
as part of the EU 6th Framework project CROPGEN (www.cropgen.soton.
ac.uk), digesters fed on a maize as a mono-substrate showed a requirement
for cobalt at high loadings and this was supplemented as part of a TE mix
(Gonzalez-Gil et al., 2001).
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