Environmental Engineering Reference
In-Depth Information
process stability and efficiency. The ratio between CH
4
and CO
2
is usually
stable during the process; it might nevertheless change when the anaerobic
fermentation is disturbed. This is because the ratio also depends on other
factors such as substrate quality and composition, process temperature and
pH.
10.2.5 Biogas production in wastewater treatment plants
In cases where biogas production is not the main aim of implementing the
anaerobic digestion process, i.e. in wastewater treatment plants, organic
matter removal is an important indicator. The effectiveness of the process
can be expressed as TS, VS, COD or biological oxygen demand (BOD)
removal (El-Gohary et al., 1999; Perez et al., 2001; Steyer et al., 2002). The
BOD gives important information about easily biodegradable organic
substances, which can be correlated to total biogas production. However, in
samples with a significant content of organic polymers, polysaccharides, fats
and proteins, underestimation of BOD may occur (Liu et al., 2004).
10.2.6 Volatile fatty acid monitoring as a useful process
control tool
Organic overloading can be defined as an input of COD that exceeds the
degradation capacity of the microorganisms (Moletta et al., 1994). This
parameter can also be measured to ensure effective substrate feeding of the
reactor. Volatile fatty acids (VFAs) are intermediates that may accumulate
during the fermentation process; they thus become the first indicator of
anaerobic digestion imbalance (Punal et al., 2003). VFA measurement can
be a very useful tool for process monitoring both in low and high buffered
systems, whereas pH and alkalinity can be only applied to the first type.
VFAs measurement seems to be the best parameter for indicating
instabilities since the concentration of these compounds gives a signal on
the state of the internal environment and progress of the digestion process.
Each individual component of the VFAs can give valuable information and
become an indicator for early warning of process imbalances. Different
researchers have suggested a diverse choice of acids as an indicator of stress
level in the event of a failure in the AD of biomass: iso-butyric and iso-
valeric (Hill and Bolte, 1989) and n-butyric and iso-butyric (Ahring et al.,
1995). Propionic acid may also be a process indicator. During overloading,
when the partial pressure of hydrogen increases, the degradation of
propionate is affected before other VFAs (Boe, 2006). Hansson et al.
(2003) proposed the use of propionate as a process indicator, and the ratio
of propionic to acetic acid was found to be viable and a sensitive indicator
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