Spring 2021 EN

Biogas Journal | Spring_2021 English Issue
Diagram 2: Diagram of the two main buffer areas in the microbial biogas
process, the acid buffer of the short-chain organic fatty acid metabolites (VOA)
and the alkalinity/”lime reserve” (TIC) of the hydrocarbonate buffer
VOA I TIC [mg Hac-Equ L -1 ]
VOA buffer
GC-VOA method
VOA / TIC alkalinity
Previous VOA/TIC method
Hydrogen carbonate buffer
buffer
buffer
Unrecorded fatty acids
(previous VOA / TIC method)
Besides the VOA and TIC diagram, there is the phosphate buffer with a pK value of 6.8, the ammonium
buffer with a pK value of 9.24 is more or less outside of that. In the final titration values
of 4.4, 5.0 and 5.75, fatty acids are “cut off” and are not detected, which is something that
cannot happen in the GC-VOA method which is separately calibrated for each fatty acid.
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Digester contents are a mixture of
buffer solutions
Thus, biogas digesters are basically a mixture
of buffer solutions that can normally
be titrated with sulphuric acid in order to
obtain a rough estimate of the buffer quantities
of short-chain organic acids (C 1
-C 6
)
and also of the carbonate buffer (TIC value)
they contain by means of the titration
amount changed in that way (see Diagram
2). Incidentally, the carbonate buffer is
also an essential component of physiologically
important blood buffer.
The titration end points usual for the VOA/
TIC value are shown in Diagram 2. The TIC
value according to Nordmann (titration up
to a pH value of 5.0) is converted and given
as a calcium carbonate equivalent or “calcium
reserve” in CaCO 3
milligrams (mg)
per liter. The VOA value (titration of pH 5.0
to 4.4) states the acetic acid equivalents
in mg/l or ppm (parts per million). Half a
dozen variations can be found in literature,
for example, Ripley 1086 (=APHA 1992)
titrates from pH 5.75 to 5.0 instead of
from pH 5.0 to 4.4 (see Diagram 2).
In the Biogas Journal published in 2006,
Rieger and Weiland were the first to compare
the VOA/TIC values according to Nordmann
(or McGhee) with concentrations of
short-chain organic acid metabolites determined
by titration by using an HPLC
analysis method that could be validated
and they found that the values correlate. At
the same time, GC and HPLC analytics are
of equal value, each fatty acid is calibrated
individually.
The VOA titration values of 500 to 5,000
acidic acid equivalents (mg/kg), however,
were more a point cloud with a mean value
of 2,000 that was not forced to the zero
point, while the values determined very accurately
according to the concentration via
HPLC lay on a straight line that was forced
to the zero point. It is important to keep
in mind that in a titration from an alkaline
range (pH ≈ 8.0) to 5.75 or 5.0, a part of
the TIC and also the VOA buffer is not recorded
(see Diagram 2). Thus, the resulting
TIC value in the second titration from
pH = 5.0 to 4.4 according to Nordmann
(McGhee) or in Ripley from pH = 5.75 to
5.0 is added to the VAO value.
Thus, an excessively high erroneous VOA/
TIC initial value is suggested, particularly
in greatly buffered liquid manure biogas
systems with low fatty acid values, that is
not forced to the zero point, such as in digestion
towers. As a result, it usually only
starts at 0.2 with Nordmann, although the
parallel concentration of fatty acids is still
within an acceptable range.
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