Environmental Engineering Reference
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for AD instability by Marchaim and Krause (1993).VFA concentration is an
excellent parameter for early indication of organic overloading and is easily
measured by gas chromatography or high-performance liquid chromato-
graphy after all particulates are removed from the sample (Holm-Nielsen
et al., 2008) Pind et al. (2003) presented a novel in-situ filtration technique
for animal slurry or manure. An on-line headspace gas chromatographic
method was described by Boe et al. (2005, 2010).
10.2.7 Other important analytics and process information
Besides VFAs, the measurement of alkalinity has also proved to be useful in
the on-line monitoring of the fermentation process. A correlation between
VFA accumulation and alkalinity levels was presented by Hawkes et al.
(1994), who suggested that the monitoring of bicarbonate alkalinity would
serve as an effective tool for early warning of organic overloading. Increased
concentration of VFAs will consume alkalinity before changes of pH are
noticeable, and it is thus an alternative to pH measurements for well
buffered systems (Lahav and Morgan, 2004). Some researchers suggest
measuring the ratio of VFAs to bicarbonate alkalinity (VFA/ALK): the
lower the ratio, the better is the balance of oxygenic and methanogenic
bacteria (Barampouti et al., 2005).
Other parameters that might be considered in developing biogas process
monitoring schemes include ammonium. Although considered a nutrient,
the compound in its un-dissociated form of 'free ammonia' can become
toxic and inhibitory to the fermentation process at some concentrations
depending on the pH (Koster, 1986) and concentrations could therefore be
monitored. Likewise, depending on pH and alkalinity conditions of the
system, the concentrations of hydrogen sulphide can exhibit a similar effect.
O'Flaherty et al. (1998) describe hydrogen sulphide toxicity threshold
alkalinity values from 200 to 500mg/l.
Appropriate process information regarding the anaerobic digester process
(biomass, substrates, intermediates, digestate and nutrients) not only
facilitates making better control decisions but also optimises productivity,
in turn leading to decreased variability and potentially improved biogas
quality. Utilisation of diverse substrates and demand for high biogas yields
requires highly efficient process optimisation. Several on-line techniques are
well described in the literature (Vanrolleghem and Lee, 2003; Steyer et al.,
2006; Holm-Nielsen, 2008).
A growing number of biogas plants in Europe have shown an increased
interest in on-line monitoring and control. This has led to more focused
research to be carried out in the field of instrumentation, control and
automation of anaerobic processes.
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