Analysis and characterisation of biogas feedstocks - The Biogas: Science, Production and Applications - page 76

CO 2 leaves the biogas plant via the digestate (buffer capacity). A high pH
will even increase CO 2 solubility. This can have an influence on the CH 4
concentration in the measured biogas. Last but not least, non-degradable
organic components cause large deviations from the theoretical oxidation
state in many biomass feedstocks.

3.5.3 Nutrient demand and availability

Macronutrients

The composition of anaerobic microbial biomass can be estimated as 50%
C, 20% O, 10% H, 11% N, 2% P and 1% S. In a biogas feedstock, the
minimal demand for macronutrients in anaerobic digestion should be
1000:5:1:1 (COD degradable :N:P:S) if the substrate is easily available for
methanogens. If large amounts of the feedstock still have to be broken down
by hydrolytic microbes, the demand for macronutrients rises to 350:5:1:1
(COD degradable :N:P:S) (Bischofsberger et al., 2005).
According to Braun (1982), the C:N ratios in biogas feedstocks (or
feedstock mixtures) should range between 16:1 and 45:1. Bischofsberger
et al. (2005) give a comparable range of C:N ratios for waste digestion 10:1
to 45:1 for the hydrolysis step and 20:1 to 30:1 for the methanogenesis step.
However, according to Braun (1982): experiments with a very low C:N ratio
of 4.2:1 (and a consequently large excess of nitrogen) have also been carried
out successfully. Experiences of the current authors showed the same, but
attention should be drawn to possible difficulties of process stability at high
ammonia concentrations (see Section 3.5.4).
In general, it can be assumed that in most biogas feedstocks there is
sufficient nitrogen and phosphorous available. Limitations of macronutrients
(P, N) occur especially in monodigestion (e.g. industrial wastewaters). As
countermeasures, macroelements will have to be added to these feedstocks, or
else they will need to be co-digested with N- or P-rich feedstocks.

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Micronutrients

The microbes in anaerobic digestion require certain micronutrients.
According to Bischofsberger et al. (2005), the essential trace elements are
Ni, Co, Mo, Fe, Se and Wo for methanogenic bacteria and Zn, Cu and Mn
for hydrolytic bacteria. In co-digestion of various biogas feedstocks,
sufficient micronutrients are often available. This is especially true if
manure is among the substrates. In monodigestion, where only one type of
substrate is used, the probability of trace element limitation is higher. While
the trace element content of substrates can be analysed quite easily (see
Section 3.4.1), the bioavailability of these trace elements is very difficult to