For samples which are in liquid phase, these have to be mixed well before
sampling. In the case in which the sample is taken using a valve, the material
which exits first has to be excluded. If the sample is taken in pipes, a vertical pipe
is preferred than a horizontal one, to avoid the deposits of sediments.
10.2.3 Sample Transport, Storage, and Preparation
The sample is then put in sealed vessels. It has to be preserved by cooling it to 4
C
during transport and storage, which is preferable to be quite short. If the storage time
is forecasted to be long, it is preferable to store the sample at À20
C and so freeze it
to preserve its original characteristics. Physical impurities can be separated from the
sample, previously taking note of their masses. For some kinds of analysis, preparation of the sample can be done through drying or milling. To improve the sample
blending performance, water can be added.
10.2.4 pH Measurement
The pH defines the basicity or acidity of an aqueous solution. Usually, it is measured
in liquid raw materials, using a standard potentiometric electrode (standards EN
12176 (1998) and APHA 4500-H+ B (National Environmental Methods Index,
4500-H+B n.d.)). For solid samples, before the measuring, they can be mixed with
water. The typical pH values in anaerobic fermentation are generally slightly above
neutral. During the process, the pH is maintained constant by the buffer capacity
inside the reactor, which is due to mainly the presence of CO 2 (which is adsorbed in
gaseous form into the liquid), ammonia (which is present in liquid phase), and water,
that is also present inside the reactor. If the pH value is very high or too low (e.g.,
pH 7.5), in this case the buffer capacity is probably not sufficient to
control effectively the pH, so it can be needed a neutralization step before feeding the
feedstock to the biogas plant. It is quite frequent to have a small acidification of the
feedstock, for example; in that case, the pH value can be controlled with the addition
of bases (e.g., Ca(OH) 2 , Na 2 CO 3 , NaOH) in the reactor (Bischofsberger et al. 2005).
The pH is measured initially on the substrate before using it in an anaerobic digestion
test to know if it has to be corrected and if acidification is already ongoing. Then, pH
has to be measured also during the anaerobic tests to see how the process is
developing and if the buffer capacity of the substrate is properly working (see
Fig. 10.2). Usually substrates like food waste can cause in general acidification of
the mixture in the reactor; for this reason, also two-phase anaerobic digestion can be
performed.
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sampling. In the case in which the sample is taken using a valve, the material
which exits first has to be excluded. If the sample is taken in pipes, a vertical pipe
is preferred than a horizontal one, to avoid the deposits of sediments.
10.2.3 Sample Transport, Storage, and Preparation
The sample is then put in sealed vessels. It has to be preserved by cooling it to 4
C
during transport and storage, which is preferable to be quite short. If the storage time
is forecasted to be long, it is preferable to store the sample at À20
C and so freeze it
to preserve its original characteristics. Physical impurities can be separated from the
sample, previously taking note of their masses. For some kinds of analysis, preparation of the sample can be done through drying or milling. To improve the sample
blending performance, water can be added.
10.2.4 pH Measurement
The pH defines the basicity or acidity of an aqueous solution. Usually, it is measured
in liquid raw materials, using a standard potentiometric electrode (standards EN
12176 (1998) and APHA 4500-H+ B (National Environmental Methods Index,
4500-H+B n.d.)). For solid samples, before the measuring, they can be mixed with
water. The typical pH values in anaerobic fermentation are generally slightly above
neutral. During the process, the pH is maintained constant by the buffer capacity
inside the reactor, which is due to mainly the presence of CO 2 (which is adsorbed in
gaseous form into the liquid), ammonia (which is present in liquid phase), and water,
that is also present inside the reactor. If the pH value is very high or too low (e.g.,
control effectively the pH, so it can be needed a neutralization step before feeding the
feedstock to the biogas plant. It is quite frequent to have a small acidification of the
feedstock, for example; in that case, the pH value can be controlled with the addition
of bases (e.g., Ca(OH) 2 , Na 2 CO 3 , NaOH) in the reactor (Bischofsberger et al. 2005).
The pH is measured initially on the substrate before using it in an anaerobic digestion
test to know if it has to be corrected and if acidification is already ongoing. Then, pH
has to be measured also during the anaerobic tests to see how the process is
developing and if the buffer capacity of the substrate is properly working (see
Fig. 10.2). Usually substrates like food waste can cause in general acidification of
the mixture in the reactor; for this reason, also two-phase anaerobic digestion can be
performed.
312
PietroBartocci et al.