Environmental Engineering Reference
In-Depth Information
Experimental reactor set-up
An example of a continuous biogas fermentation trial is illustrated in
Fig. 3.8, which is a comparably simple set-up based on the BMP test (Fig.
3.5). The gas measuring device shown was designed and assembled in a
laboratory setting, but there are different types of devices on the market (e.g.
gas meters, micro gas counters). This set-up is only suitable for liquid
substrates and substrates with small particles as feeding is done via a hose.
In biogas research institutions, more sophisticated digesters that are
especially designed for the needs of the biogas process are generally used.
An example of a digester for solid substrates can be seen in Fig. 3.9.
Experimental details for continuous biogas trials
The substrate used for the experiment should be characterised for pH, TS,
VS, TKN and NH 4 -N; COD determination is optional, depending on the
substrate type. Sometimes, if the substrate is easily storable and a large
cooling chamber is available, it is possible to use the same substrate batch
throughout the whole experiment. Normally though, different substrate
batches will be used during the experiment, so a representative sample of
every substrate batch has to be taken.
Feeding of the digesters can be done manually (once or twice a day).
However, feeding on weekends is often problematic, so feeding automation
can be advantageous. Apart from that, the digester effluent has to be
collected regularly, either automatically or by hand. Process monitoring
includes the amount and composition of the biogas, fermentation
temperature recorded online and pH, often measured offline. For a good
measurement, the temperature of the sample during the pH measurement
has to be recorded. Nevertheless, the pH may vary due to CO 2 degassing.
Therefore, it is more accurate to measure the pH online; for this, a
sophisticated pH adapter is necessary in which the electrode can be taken
out for cleaning during a running fermentation. Effluent samples are taken
regularly from the digester to determine TS, VS, TKN and NH 4 -N. As with
the substrate, COD determination is optional. Also, P and K can be
analysed to estimate the fertiliser value of the digestate.
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Start-up for continuous biogas trials
The start-up phase of an anaerobic digestion process, in other words the
time needed to reach a stable process, can often be problematic and time
consuming. A normal start-up can take 2-3 months, followed by 1-
3 months of steady-state fermentation. A prolonged start-up period can be
necessary if the microbes need to adapt to the substrate (e.g. a strong change
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