Analysis and characterisation of biogas feedstocks - The Biogas: Science, Production and Applications - page 64

Substrate/inoculum ratio

The ratio of substrate to inoculum is an important factor in BMP tests,
particularly in terms of degradable material (volatile solids) load in the
sample and inoculum. According to VDI 4630, the ratio of VS sample to
VS sludge should be 0.5. If the amount of feedstock sample is very low, the

effect of any occurring degradation in the inoculum sludge will be
disproportional and can lead to errors. If the amount of feedstock sample
is very high, there is a high probability of acidification. This occurs if the
accumulation of acids in the hydrolytic phase exceeds buffer capacity,
thereby lowering the pH in the test. This is a problem because little
methanogenesis can take place below pH 6.5-6.0. However, a buffer
solution can be added to decrease the probability of acidification (EN 11734;
Raposo et al., 2006). According to Raposo et al. (2006, 2008b), a
considerable change in the ratio of substrate to inoculum can cause
differences in the results. In addition, if water displacement bottles are used,
the amount of sample should be adapted to the biogas measuring capacity
of the water displacement bottles. If necessary, larger water displacement
bottles can be used.

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Gas measurement details and alternatives

For accurate results, the set-up (Fig. 3.5) should be tested for gas tightness
before the experiments. To do this, the closed set-up can be run with a slight
overpressure so that the water level rises some centimetres in the exit hose of
the water displacement bottle. The water level can be marked and then
checked after a few hours. If the water level stays near the mark, the set-up
should be sufficiently gas tight. The effect of very small leaks can be
amplified in set-ups that work under larger overpressure (e.g. water
displacement or pressure measurement). Different methodologies are given
for gas volume measurement. In EN 11734, gas production is calculated by
measuring the build-up of overpressure in a closed set-up. In
DIN 38 414 (S8), gas production is measured by water displacement in a
eudiometer. Other viable options given in VDI 4630 are gas collector tubes,
collection in gas bags or continuous gas measurements by micro gas
counters or gas meters. In general, gas measuring methods such as water
displacement and pressure measurement only allow limited total gas
production. For continuous gas counters there are practically no limits, so
decisive up-scaling of the BMP tests is also possible.
The set-up illustrated in Fig. 3.5 is designed only for methane
measurement. If the biogas (including its other gases) is to be measured,
the bottle containing the alkaline solution has to be removed from the set-

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