Environmental Engineering Reference
In-Depth Information
3.6
Conclusions
The abundance and variety of possible feedstocks for biogas production
demonstrates that there is a need for detailed characterisation and
evaluation of each specific feedstock type. For first estimates as to whether
a feedstock is suitable or not, a principle understanding of the anaerobic
digestion process is necessary. Typical biogas feedstocks should be
degradable and have a considerable water content, but the amount of
fibrous material (e.g. lignocellulosic material) should be rather low. If
process production data of the feedstock are available, they can be used for
a first evaluation of the feedstock. However, in most cases detailed
laboratory analysis will be necessary. Essential analysis includes the
determination of pH, total solids/dry matter, volatile solids/organic dry
matter, chemical oxygen demand, total Kjeldahl nitrogen, ammonia
nitrogen and biochemical methane potential. In many cases it will be
sufficient to determine either the VS or the COD for the estimation of the
energy content of a feedstock. The BMP test is the core analysis method for
biogas feedstock characterisation. However, it can show a high level of
uncertainty, and is much less informative than continuous fermentation
trials. Such continuous fermentation trials give the best information on the
performance of a biogas feedstock in a biogas plant. A big disadvantage,
however, is that they are quite expensive due to the large amount of analysis
required and the duration of the experiment.
To evaluate limitation by macroelements, the presence of nitrogen,
sulphur and phosphorous can be determined. In practice, most biogas
feedstocks provide sufficient macroelements because of the low demand of
anaerobic microbes (compared with aerobic microbes). However, a lack of
microelements (trace elements) can occur, especially in monodigestion.
Classic trace element analysis (e.g. ICP-OES) can give valuable information
on the concentration of these elements in a feedstock, but no information on
the bioavailability of these elements. Nitrogen and sulphur analysis will
provide information on possible ammonia and hydrogen sulphide inhibition.
The most important step in feedstock characterisation is to combine the
information from the different analyses in order to obtain an overall picture
on the suitability of a certain feedstock (or feedstock mixture) for a biogas
plant. In total, process stability issues (inhibition, toxicity), fertiliser value of
the digestate and the overall economics of a biogas project have to be
evaluated. In short, reliable feedstock analyses and characterisation will
always play a very important role in the biogas business.
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