Environmental Engineering Reference
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that are essential for anaerobic microbes are missing (or in very low
concentrations). However, this analysis gives no direct information on the
bioavailability of the trace elements. Lack of trace elements can often occur
in monodigestion (e.g. maize, stillage). Trace element analysis also gives
information on whether heavy metals exceed toxic concentrations in biogas
feedstocks. Apart from that, this test can also assess nutrient content for the
evaluation of digestate use as fertiliser.
3.4.2 Sulphur content
Total sulphur content can be determined according to ISO 11885 by ICP-
OES. Biogas feedstock sample preparation is similar to trace element
analysis (see Section 3.4.1). However, the wavelength of sulphur is very low
compared with the other elements, which can sometimes produce problems
in the accuracy of the detection.
Sulphur determination in biogas feedstocks is not always essential.
However, high sulphur concentrations will lead to high H 2 S concentrations
in the biogas, which can lead to microbial inhibition and are of issue for end
use of biogas in, for example, gas engines. If the feedstock has a high
nitrogen content, the sulphur content of the feedstock should be analysed.
The reason for this is that NH 3 and H 2 S have synergistic negative effects
(Chen et al., 2008) and even a moderate sulphur content in biogas feedstocks
can become problematic. Substrates that can be problematic due to high
sulphur content are stillages or industrial wastewaters with high sulphate
concentrations.
3.4.3 Phosphorous content
Total phosphorous content can be determined according to ISO 6878,
DIN 38 414 (S12) and APHA 4500-P. As a first step, biogas feedstock
samples are dried and milled. The sample is then digested in a mixture of
sulphuric acid and nitric acid in order to solubilise all existing phosphorous.
With addition of an acid complexing agent, the blue complex antimony
phosphorous molybdate is formed, which can be measured quantitatively by
photometric extinction measurements. An alternative way of determining
total phosphorous is described in EN 13346 (sludges) and ISO 11885
(wastewater). After digestion of the sample, total phosphorous content can
be analysed by AAS, ICP-OES or ICP-MS.
In most biogas feedstocks (e.g. crops or organic wastes), it can be assumed
that sufficient phosphorous is available for microbial growth and this
analysis is thus rarely carried out. However, phosphorous analysis of a
feedstock can be a valuable indicator for the fertiliser value of the digestate
after anaerobic digestion.
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