Environmental Engineering Reference
In-Depth Information
up. In addition, the water in the water displacement bottle should be
acidified so that hardly any CO
2
will dissolve in it (DIN 38 414 (S8)).
Evaluation of BMP test
Gas production of the blanks (i.e. batch trials with only inoculum and no
substrate) should not exceed 20% of the total gas production in the BMP
tests (VDI 4630). Otherwise, the results will have too much uncertainty.
Daily gas production can be obtained by subtracting the gas production of
the blanks from the daily gas production of the tests with samples. After
that, based on the ideal gas equation (equation 3.1), the measured gas
volumes have to be calculated at normal volumes (T
0
= 273K and
p
0
= 1013 hPa). Therefore, the ambient pressure (p) and the ambient
temperature (T) - more precisely, the temperature of the gas in the
displacement bottle - have to be known. For a more accurate calculation,
the additional hydrostatic pressure of the displaced water (normally
approximately 10 cm) will have to be added to the ambient pressure (p).
On the other hand, the vapour pressure at ambient temperature can be
subtracted from the ambient pressure (p).
V
p
T
0
V
0
¼
½
3
:
1
p
0
T
As a next step, depending on the daily gas production, a degradation
curve can be drawn (Fig. 3.7). The degradation rate can be evaluated
according to the steepness of the graph. Very steep curves mean very good
availability and degradability, while flat curves mean slow feedstock
availability or slight inhibition. If degradable organic matter is added and
no (or negative) net biogas production occurs, this is a sign of very strong
inhibition or toxicity of the feedstock. In reality, variations of these curves
often occur. In general, it is preferable not to base the BMP on fresh weight
because the water content of a feedstock can vary. For solid feedstock and
feedstock with very limited volatile components, the BMP is often based on
VS. For high-strength wastewater and feedstock with many volatile
components, it is preferable to base the BMP on the COD. For comparison
with other data, it is best to provide both.
Due to the complexity of this method of analysis and the many
influencing factors, there is a strong possibility of error and inaccuracy.
Apart from the representativity and inhomogeneity of the sample, the type
and condition of the inoculum can have a very big influence on the results.
Other influencing factors are the inoculum to substrate ratio, gas tightness
of the whole set-up and the accuracy of VS or COD measurements. For a
rough evaluation,
the results can be compared to literature data. In
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