Environmental Engineering Reference
In-Depth Information
3.3.5 Chemical oxygen demand (COD)
Chemical oxygen demand is a parameter that indicates the total chemically
oxidisable material in the sample and therefore indicates the energy content
of a feedstock. In this analysis the sample is refluxed in a boiling mixture of
sulphuric acid and a known excess of potassium dichromate (K 2 Cr 2 O 7 ). A
silver sulphate catalyst can be added for improved oxidation performance.
Apart from that, mercury nitrate can be added to counteract the interference
of chloride ions by forming complexes with them. As a next step, the
remaining unreduced potassium dichromate is titrated with ferrous
ammonium sulphate, which allows the determination of the consumed
oxygen equivalents. (Standards DIN 38 414 and APHA 5220 B apply.)
The COD represents the maximum chemical energy present in the
feedstock. Since microbes convert chemical energy to methane, this is also
the maximum energy that can be recovered as biogas, though losses for the
energy demand of the microbes themselves have to be subtracted, as well as
for material that is not degradable by anaerobic microorganisms (e.g.
lignocellulosic material). The main advantage of this method compared to
the VS/ODM determination is that volatile components can also be
determined provided that good cooling is applied and the condenser is
working when adding sulphuric acid.
The COD analysis can give quite high errors due to sample inhomogene-
ity and the large number of sample treatment steps required (dilution,
weighing and titration). It should therefore preferably be carried out by
experienced personnel. For typical biogas feedstocks, which contain high
concentrated organic matter and bulky material, drying and milling of
samples can improve reproducibility. However, as mentioned earlier, a loss
of volatile substances can occur. Raposo et al. (2008a) present a modified
method for improving the COD determination of solid substrates and
solutions with high suspended solid content. It is also important to be aware
of the toxic components (potassium dichromate, mercury nitrate) that are
used in COD measurements, as well as their proper disposal.
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3.3.6 Nitrogen content
The nitrogen content of a feedstock can be determined by the total Kjeldahl
nitrogen (TKN) determination (ISO 5663; ISO 11261; APHA 4500-N org B).
In this analysis, organic nitrogen is converted to ammonia nitrogen by
boiling the feedstock sample in the presence of sulphuric acid and a catalyst.
After that, a base is added and ammonia is distilled from the alkaline
solution to an acid solution (usually boric acid) where ammonia is absorbed
quantitatively. The amount of ammonia can then be determined by different
methods, for example potentiometric acid titration or the photometric
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