Environmental Engineering Reference
In-Depth Information
wilting and is typically carried out for one or two days. Wilting for more
than 48 hours, however, can lead to WSC losses and mould development.
Field wilting is not always possible owing to weather conditions, but ensiling
of unwilted wet crops can be carried out by an experienced worker or
improved by using additives such as acids to bring about a more rapid pH
drop, starter cultures to give the LAB a head start or preservatives such as
nitrite to inhibit unwanted microorganisms (Wilkinson, 2005; McDonald
et al., 1991).
Sugar concentration is also essential, as the production of acids during
fermentation is dependent on sugar. The major WSCs of grasses are glucose,
fructose, sucrose and fructans, and a WSC concentration below 30 g per
kilogramme of fresh crop weight results in poor fermentation (Wilkinson,
2005). For whole-crop maize silage, WSC is not an issue as sugar is released
from the corn grains, but in some grass species the WSC concentration can
be very low, particularly when cut late in the season. The concentration of
WSC is higher when grass is harvested in the afternoon (Nizami et al., 2009).
Wilting (for less than 48 hours) can be used to increase the sugar
concentration as it removes water by evaporation. More carbohydrate is
available in the plant material, but as cellulose, hemicelluloses and pectin,
which cannot be used by LAB. Sugar may be released from these structural
carbohydrates by using hydrolytic enzyme additives. An alternative
commonly used additive for crops with low WSC is molasses (Wilkinson,
2005; McDonald et al., 1991).
The buffering capacity of plants is also important and is affected by the
number of weak acid salts such as citrate present in the plant material, which
can recombine with the H + formed by the fermentation. This varies between
crop types, but is often lower in mature crops. The presence of ammonia in
the plant also has a buffering effect, as ammonia mops up the H + ions to
form ammonium. Ammonia concentration in the plant is related to the use
of fertiliser and can be reduced if fertiliser is applied early in the season and
there is a long wait before harvest. Acid additives can be used to help
ensiling of crops with high buffering capacities (Wilkinson, 2005).
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4.2.3 Relevance for biogas production
Many different crops can be ensiled, but the most relevant for biogas
production are whole-crop maize and grass, which make up 48% and 4%
respectively of the fresh weight of biomass used for anaerobic digesters in
Germany, the biggest biogas producer in Europe. In addition, over 90% of
biogas plants in Germany use maize silage to some extent and over 35% use
grass silage (Gemmeke et al., 2009). Grass silage is expected to be of
increasing importance for biogas production in Europe owing to the
abundance of grasslands and the controversy of using arable land for energy
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