Analysis and characterisation of biogas feedstocks - The Biogas: Science, Production and Applications - page 61

phenate method (ISO 5663 and APHA 4500-N org B; see Section 3.7). The
ammonia nitrogen (NH 4 -N) determination is quite similar to the TKN
determination but the first boiling step for organic nitrogen degradation is
left out (DIN 38 406(E5); APHA 4500-NH 3 ).
Determination of TKN in a sample is important, primarily to evaluate if
there is sufficient nitrogen available for the growth of anaerobic bacteria. In
most cases there will be excessive nitrogen in the biogas reactor, so
determination of the TKN content in a biogas feedstock helps to estimate
nitrogen concentrations in the biogas reactor. As a rough estimation, it can
be assumed that 60-80% of the TKN will be degraded to ammonia during
anaerobic digestion, though deviations can occur depending on the specific
substrate. This is important to know, because ammonia inhibition can occur
if the ammonia concentration in the reactor exceeds certain levels. Details
on recommended C:N ratios are described in Section 3.5.3 and details on
ammonia inhibition can be found in Section 3.5.4. Biogas feedstocks with
high nitrogen concentrations are generally protein-rich materials such as
slaughterhouse waste, stillage, rape seed cake and residues from meat
processing.
It should be noted though, that a TKN determination does not determine
all nitrogen present. Substances such as nitrates or nitrites are not
determined, but these substances occur mostly in negligible concentrations
in biogas feedstocks. In most cases, the NH 4 -N concentration of a biogas
feedstock gives only limited information, since ammonia content rises
strongly during anaerobic degradation of the proteins. However, the ratio of
NH4-N to TKN in a feedstock can provide information on the amount of
available protein.

3.3.7 Biochemical methane potential (BMP)

Biochemical methane potential tests are mainly used to determine the
possible methane yield of a feedstock. These tests also provide information
on the anaerobic degradability of a feedstock, including the degradation
rate. In addition, a first rough evaluation of the presence of inhibitory
components can be made. Standardised anaerobic degradation tests are
given in EN 11734, DIN 38 414 (S8) and VDI 4630. The BMP test presented
here is mainly based on DIN 38 414 (S8) and is slightly simplified according
to Braun (1982, 2007). To make the test more practical, the expensive
eudiometer gas measuring devices are replaced by simple water displacement
bottles. In addition, a bottle with an alkaline solution is placed after the
digester vessel in order to absorb the produced carbon dioxide and to allow
direct methane measurement.

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