Environmental Engineering Reference
In-Depth Information
genesis becomes the rate-limiting step (Borjesson and Mattiasson, 2007).
Examples of parameters that affect the various steps in the AD processes to
various degrees are the composition of the input materials, various pre-
treatment of feedstocks, feeding rate and temperature control of the process
(Mata-A ´ lvarez et al., 2000; Angelidaki, 2002).
A fine balance must be maintained between these groups of bacteria and
processes to secure a productive yield. In the case of process imbalance, an
accumulation of hydrogen or other intermediates is likely to occur that
would result in inhibition and metabolic shifts (Legin et al., 2003). Process
imbalances normally have three major causes: hydraulic overloading,
organic overloading or the presence of inhibitory concentrations of toxic
materials in the reactor such as ammonia and heavy metals (Legin et al.,
2003). Proper understanding and manipulation of these stages and their
respective intermediates or products is the key to suitable monitoring
scheme solutions for the biogas plant.
10.2.2 Basic process monitoring
Currently, many biogas plants neither continuously measure processes in
depth nor utilise in-depth process data when they have been generated. In
rare cases, data are collected, but only intended to serve as routine
monitoring and control of parameters rather than for real-time process
control. Often, process control in these plants is limited to daily evaluation
of easily measurable parameters such as temperature, pH or biogas
production rate, volume and quality. Additionally, at regular intervals
(weekly or monthly) off-line measurements are carried out for total
suspended solids, volatile organic acids, ammonia nitrogen and volatile
fatty acids (VFAs). The number of measurements is often limited due to the
high costs of chemical analysis, time consumption and lack of trained
personnel. However, even with recent development of less expensive
laboratory analysis such as fast photometric tests for ammonia nitrogen
and VFAs, these measurements cannot give a fast response to facilitate
process imbalances during the AD process. The AD process is continuous -
there is no on-and-off button like at natural gas grid connected systems.
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10.2.3 Advances in process monitoring
More sophisticated and robust techniques are required for on-line and
continuous monitoring of fermentation. Such innovative solutions would
provide a better control tool, increase the stability and yield possibilities of
the process, and allow the fermentation process to run at more optimal
conditions. It would effectively increase the economy of biogas plants.
In order to meet increasing economic and technical operational targets,
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