Environmental Engineering Reference
In-Depth Information
mostly remain in the digestate. The reason for this is that only a limited
amount of chemical contaminants are anaerobically degradeable. In
addition, the contaminant concentration based on TS will even increase,
as TS are degraded in the biogas plant. Therefore, in order to increase the
quality of the digestate, the input materials of biogas digesters should be
tightly regulated. In many countries, there is an additional legislative issue.
If substances that are legally considered wastes enter a biogas plant, the
whole digestate is to be treated according to waste legislation. If, for
example, municipal food waste is mixed with grass silage, which is not a
waste pre-digestion, then the digestate from the mixture is then considered a
waste. This applies even if there are no pollutants in the digestate. In
countries such as Denmark, digestate produced from feedstock mixtures
that comprise up to 25% organic wastes (with the exception of stomach and
intestine contents from slaughterhouses, which are equivalent to animal
manure) is considered animal manure, and its use is controlled by manure
and slurry regulations. If the amounts of wastes co-digested are above this
limit, the digestate is considered waste and its use (as fertiliser for example)
is governed by sewage sludge regulations.
Biogas subsidies
Biogas is subsidised by some countries due to its high production costs.
Different subsidies may be granted for crop digestion as compared to waste
digestion. The main idea behind this strategy is that crops have to be
purchased as substrates whereas gate fees are often received for waste
material. Countries like Germany and Austria have lists of biogas
feedstocks that are permitted for an extra subsidy in crop digestion.
Although mixtures are permitted in some cases, the type of feedstock should
generally fit to the corresponding subsidy scheme.
3.3 Essential laboratory analysis of feedstocks
The most important types of analysis for biogas feedstocks are now
described. A summary of the different norms and standards for the analyses
can be found in Section 3.7 at the end of this chapter.
3.3.1 Sample taking and preparation
For biogas feedstocks, standards VDI 4630 and ISO 566713 give detailed
information on different issues relating to sampling. The quality of many
feedstocks can differ depending on the time and sampling location.
Especially for heterogeneous feedstocks, the sample taker's experience and
knowledge of the overall process are of high importance. According to
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