10.3.3 Limitations of the BMP Tests
Current limitations to the BMP practices are correctly identified in the works of
Hafner; see (Koch et al. 2019, 2020b; Justesen et al. 2019; Hafner and Astals 2019;
Hafner et al. 2018b). Among them, leakages, the difficulty to describe correctly
co-digestion mixtures, and the toxicity of substances added to the digested mixture,
make it not possible to extend the results obtained in batch mode to industrial
reactors working in continuous way.
Also in (Koch et al. 2019), a nice graphical approach on how to understand
mistakes in experimental procedures from the curves showing the methane yield
trend is proposed.
Usually, a normal BMP curve can be recognized based on four basic aspects:
1. The increase of methane production has to be monotonic and the slope should not
be negative
2. The increase had to be quite steep in the first part with a reduced initial lag; then
after the first increase, a steady state should be reached and an almost constant
value of CH 4 production
3. The slope has not undulate
4. The progression of the curve has to be of first order (Brulé et al. 2014) or based on
the equation by Monod (Koch and Drewes 2014), but not the one describing the
Gompertz (Gompertz 1825; Zwietering et al. 1990) model. If the curve resembles
that of the Gompertz model, this can indicate probably the probable existence of
design and/or execution errors.
The main three errors which are taken into account in (Koch et al. 2019) (see also
Fig. 10.6) are as follows:
– Incorrect inoculum storage
– Incorrect inoculum dilution
– Errors in the choice of inoculum to substrate ration (ISR)
Inoculum storage plays a fundamental role in permitting the activation of the used
inoculum when inserted in the BMP reactor or bottle (De Vrieze et al. 2015; Koch
et al. 2017). The most important parameter to perform a good storage procedure is
temperature and not time (Li et al. 2014; Hagen et al. 2015; Wang et al. 2016).
In fact, storage at low temperatures can induce a lag phase in the experiment,
while storage at ambient temperature is recommended (Li et al. 2014; Hagen et al.
2015).
Dealing with inoculum dilution with deionized water, some studies suggest that
this can have a beneficial effect and contribute to avoid inhibition from unknown
substances contained in the substrate (Holliger et al. 2016; Angelidaki et al. 2009).
The volatile solid concentration in the original inoculum has to be around
10–30gVS/L (Raposo et al. 2011, 2012). These values are also very similar to the
values that the final mixture should have. On the other hand, an excessive dilution of
the inoculum can also have drawbacks and underestimate the final production of
328
PietroBartocci et al.
Current limitations to the BMP practices are correctly identified in the works of
Hafner; see (Koch et al. 2019, 2020b; Justesen et al. 2019; Hafner and Astals 2019;
Hafner et al. 2018b). Among them, leakages, the difficulty to describe correctly
co-digestion mixtures, and the toxicity of substances added to the digested mixture,
make it not possible to extend the results obtained in batch mode to industrial
reactors working in continuous way.
Also in (Koch et al. 2019), a nice graphical approach on how to understand
mistakes in experimental procedures from the curves showing the methane yield
trend is proposed.
Usually, a normal BMP curve can be recognized based on four basic aspects:
1. The increase of methane production has to be monotonic and the slope should not
be negative
2. The increase had to be quite steep in the first part with a reduced initial lag; then
after the first increase, a steady state should be reached and an almost constant
value of CH 4 production
3. The slope has not undulate
4. The progression of the curve has to be of first order (Brulé et al. 2014) or based on
the equation by Monod (Koch and Drewes 2014), but not the one describing the
Gompertz (Gompertz 1825; Zwietering et al. 1990) model. If the curve resembles
that of the Gompertz model, this can indicate probably the probable existence of
design and/or execution errors.
The main three errors which are taken into account in (Koch et al. 2019) (see also
Fig. 10.6) are as follows:
– Incorrect inoculum storage
– Incorrect inoculum dilution
– Errors in the choice of inoculum to substrate ration (ISR)
Inoculum storage plays a fundamental role in permitting the activation of the used
inoculum when inserted in the BMP reactor or bottle (De Vrieze et al. 2015; Koch
et al. 2017). The most important parameter to perform a good storage procedure is
temperature and not time (Li et al. 2014; Hagen et al. 2015; Wang et al. 2016).
In fact, storage at low temperatures can induce a lag phase in the experiment,
while storage at ambient temperature is recommended (Li et al. 2014; Hagen et al.
2015).
Dealing with inoculum dilution with deionized water, some studies suggest that
this can have a beneficial effect and contribute to avoid inhibition from unknown
substances contained in the substrate (Holliger et al. 2016; Angelidaki et al. 2009).
The volatile solid concentration in the original inoculum has to be around
10–30gVS/L (Raposo et al. 2011, 2012). These values are also very similar to the
values that the final mixture should have. On the other hand, an excessive dilution of
the inoculum can also have drawbacks and underestimate the final production of
328
PietroBartocci et al.