Analysis and characterisation of biogas feedstocks - The Biogas: Science, Production and Applications - page 77

determine. An exacerbating factor is the precipitation of many trace
elements by hydrogen sulphide. Speece (1996) describes simple tests for trace
element limitation. If trace element limitation occurs in a biogas process,
trace element solution will often have to be added.

3.5.4 Inhibition and toxicity by feedstock components

Some substances in biogas feedstocks can inhibit or be toxic to anaerobic
microbes. Inhibition means a decrease or cessation of growth, whereas
toxicity provokes death of microbes. It is important to know if inhibitory or
toxic substances are present in biogas feedstocks. It is possible to adapt the
microbial consortium to inhibitory substances. In the following text only a
short overview of the most important inhibitory substances is given; for
further details on inhibition see the works of Chen et al. (2008),
Bischofsberger et al. (2005), Gerardi (2003), Speece (1996) and Braun (1982).

Ammonia inhibition

Ammonia is produced in anaerobic digestion from nitrogen-rich substrates.
It is known that the undissociated form of ammonia nitrogen, NH 3(aq) , is the
inhibitory component in nitrogen-rich biogas fermentations. The concen-
tration of NH 3(aq) depends strongly on temperature and pH. In different
literature sources, different inhibitory concentrations are given. According
to Bischofsberger et al. (2005), inhibition starts at 1.5 to 3.0 gNH 4 -N/l.
However, there are reports that substantially higher concentrations could be
tolerated without a decrease in degradation performance: 5.0 gNH 4 -N/l
(Braun, 1982) and 8.5 gNH 4 -N/l (Speece, 1996). Comparable results have
been obtained by the authors of this chapter. Chen et al. (2008) give an even
broader range of concentrations at which about 50% reduction of methane
production occurred: 1.4 to 14 gNH 4 -N/l. In addition to the influence of
temperature and pH on ammonia inhibition, the adaptation of microbes to
high ammonia concentrations is also an important factor. In practice, high
nitrogen feedstocks frequently pose problems on process stability in biogas
plants. Rapid changes from low-nitrogen feedstocks to high-nitrogen
feedstocks can be especially problematic. For a stable anaerobic process
at high ammonia concentrations, the following parameters are a prerequi-
site: good adaptation of the microbes, good trace element availability and
low to medium hydrogen sulphide concentrations.

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Hydrogen sulphide inhibition

Hydrogen sulphide is produced by the anaerobic degradation of sulphur
components. As with ammonia, the undissociated form, H 2 S (aq) is known to