Environmental Engineering Reference
In-Depth Information
determines the amount of time necessary to decompose a specific feedstock
and thus the necessary retention time of the feedstock inside the digester.
Low molecular weight carbohydrates, volatile fatty acids and alcohols are
digested in hours; proteins, hemicelluloses and lipids in days, while cellulose
needs several weeks to be decomposed in anaerobic conditions. Feedstock
substrates consisting of fats and oils, known for their very high methane
yields, require longer retention times and larger digester volumes compared
with substrates rich in carbohydrates and proteins. In practice, for economic
reasons, digesters are operated with the shortest retention times and the
highest methane yields possible.
2.3.3 Impurities with disturbing effects
Together with the supplied feedstock, various unwanted components can be
accidentally supplied to the biogas plant. Once they enter the digester, their
presence can cause perturbations of the normal operation. Common problems
are reduction of the active volume of the digester (caused by sedimentation of
sand on the bottom of the digester), process failure through foaming, phase
separation and floating layers, or even damage to machinery such as pumps,
caused by metallic impurities or other disturbing components.
The most common disturbing material is sand, often supplied with animal
manure. Light materials such as straw and wood particles may cause
floating layers and perturbations of the fluid dynamics. The presence of
straw can also have disturbing effects, although this depends on particle size:
small-particle straw does not disturb the process and can improve the
methane yield considerably (Steffen et al., 1998). Inorganic materials such as
glass and metal scrap, polymeric compounds like plastics (often supplied
with biogenic wastes) and salts and fatty compounds present in some
industrial wastes are also considered disturbing components (Table 2.6).
Once they occur, disturbing effects are difficult to control. For this
reason, all feedstock types must be carefully selected and those containing
disturbing components must be avoided or properly pre-sorted before being
fed to the digester. The classic example is organic household waste, which is
best separately collected (source separation) in order to obtain the required
purity and guarantee trouble-free AD and high-quality end products.
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2.3.4 Inhibitors
Some compounds in the feedstock (and thus supplied to the digester with the
feedstock) can have a negative effect on the microbiology inside the digester,
causing imbalance or complete cessation of microbiological activity in the
worst case (see Table 2.6). These are named inhibitors and their inhibitory
effect depends to a large extent on their concentration in the feedstock
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