Environmental Engineering Reference
In-Depth Information
addition, sampling in a pipe is preferably carried out in a vertical pipe or a
pipe with turbulent flow so that problematic sediments can be avoided.
Sample transport and storage
Clean re-sealable sampling vials made of inert plastic, glass or steel should
be used. After sampling, the vials have to be labelled. If poor biologic
stability of the sample is assumed, samples must be cooled to 4
C during
transport. All samples should be stored in a cooling chamber at 4
8
8
C until
analysis. Obviously, short storage times before analysis are preferable to
long storage times. If longer storage times are expected, samples can also be
stored at
20
8
C, although this might produce changes in the degradability of
the substrate.
Sample preparation
Physical impurities can be sorted out from the sample, but their amount and
mass have to be documented. Samples are sometimes dried before analysis
but this is only really suitable when non-volatile substances are being
measured as it can cause the loss of some volatile components and therefore
a false result. For some tests, milling or cutting the sample can improve
analysis accuracy due to increased homogeneity of the sample, but milling of
wet samples is often only possible after drying. An alternative way to mill
organic fibres without drying (and without the associated losses) is during
cooling with liquid nitrogen, but this is very rarely applied in the biogas field
due to very high costs. Mixing in a blender is another alternative, although it
is limited by high fibre content. Water can be added to the sample to
improve blending performance.
3.3.2 pH value
The pH value determines the acidity or basicity of an aquaeous solution. Its
unit is the negative logarithm of the concentration of hydronium ions (H
+
).
The pH value can be determined in a liquid feedstock with a standard
potentiometric electrode (standards EN 12176 and APHA 4500-H
+
B (see
Section 3.7)). For semi-solid or solid feedstocks, the sample can be mixed
with water and then analysed. Quite a wide range of pH values of biogas
feedstocks is acceptable due to the usually high buffer capacity of the
anaerobic digestion broth. The pH value in anaerobic fermentation is
normally slightly above neutral. The buffer capacity depends mainly on CO
2
concentration in the gas phase, ammonia concentration in the liquid phase
and water content in general. If the pH in the feedstock is too high or too
low so that the buffer capacity is exceeded and the pH in the reactor is
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