Environmental Engineering Reference
In-Depth Information
3.5 Simplified experimental set-up to determine BMP. Carbon dioxide
in biogas is absorbed in an alkaline solution and the residual gas
volume is measured by water displacement.
Experimental set-up of simplified BMP test
The set-up of the simplified BMP test is shown in Fig. 3.5. For every
feedstock sample, a triplicate BMP test should be carried out. In addition, in
every BMP test row, a triplicate of blanks (only inoculum) has to be set up. If
BMP tests are carried out for various feedstock samples in one experiment,
one blank triplicate is sufficient. To set up a BMP test, the feedstock sample
and the inoculum (ratios are discussed later) are weighed and filled into a
glass vessel of capacity 0.5-1.0 l. A magnetic stirrer is added for mixing and,
if desired, the gas phase can be flushed with nitrogen before closing. Then the
vessel is placed in a climate chamber or a water bath. To determine methane
concentration without carbon dioxide, the digester vessel is connected to a
bottle of alkaline solution (2-4mol/l NaOH), which is then connected to the
water displacement bottle (1-2 l). The water volume in the water displace-
ment bottle has to be larger than the expected gas production volume. A tube
leaves the displacement bottle to a recipient, where the daily production
volume can be retrieved and measured. Initially, the volume should be read
daily. After a decrease in gas production, the volume can be read two or
three times per week. The majority of gas is normally produced during the
first week. After 20-30 days the biological degradation is normally almost
finished. The BMP tests can be stopped when daily gas production is lower
than 1% of total gas production. At the end, a pH measurement of every
BMP test should be carried out to check if acidification has occurred or if
alkaline solution has been drawn back to the digester flask due to negative
pressure. In either case, the BMP test will have to be repeated.
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