Anaerobic Digestion for Environmental Processes
Associated Problems
As with all processes, there are several problems associated with anaerobic digestion.
These problems must be understood and kept under control to ensure maximum efficiency and
safety. From the onset of the process, several factors which could severely affect the outcome
of digestion are encountered. As mentioned before, the methanogenic bacteria are very
fastidious and can only operate at a particular temperature and pH. In order to avoid
extremely long HRT's it is often necessary to preheat the influent, using additional energy. It
is also important to maintain alkalinity in the reactor to prevent the pH from dropping far
below 6.6.
Within the reactor system, foaming and the formation of scum layers can become a
slight problem, especially during the initial start-up. Foaming occurs in the reactor when there
is an excess amount of organics, or when there is an imbalance between the different types of
bacteria. An imbalance in the reactor occurs if there is too much grease in the system,
extreme temperature fluctuations, high alkalinity, low total solids, or excessive or inadequate
mixing. This imbalance results in foaming of the reactor and production of a scum layer. To
control the production of scum and foam one must remove grease from the sludge influent
before entering the system, ensure there is a continuous mixing speed, and temporarily reduce
sludge feed to control temperature fluctuations (ASD, 28).
Explosions occur when there is the proper mixture of air, heat, and gas. Because all
these factors are present in the treatment facility, safety precautions are necessary. Since local
heat sources can not be eliminated, mixing the digester gas with the surrounding air must be
avoided. It is essential that the digester has a tight lid which does not allow any air to leak in
or gas to leak out. When handling digester gas, it also must be contained at the proper
pressure (ASD, 56).
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