Environmental Engineering Reference
In-Depth Information
has the specific putrefaction smell, indicating that the process of stabilization has been
compromised. Thus the process reaction rate and correspondingly the reactor organic
loading rates are determined by the rate of biological degradation of the methanogens.
In most cases, anaerobic digestion is performed at a recommended temperature of
33ºC to36ºC (mesophilic regime), but in some cases, the design might provide a
termophilic regime of 53ºC to56ºC, which has the advantage of a higher organic loading
rate, lower reactor volumes required, and achieves an almost complete pathogen removal.
The mesophilic regime provides a partial pathogen removal effect.
One of the most attractive aspects of the anaerobic digestion process is the production
of biogas - a mixture of methane and carbon dioxide (Fig. 8.2), as a result of the organic
mineralization. Under normal process conditions, the methane content in the biogas is
about 70%. Another advantage is the low volume of excess sludge formed during the
process. The biogas is a valuable energy source, which could be reused in the plant for
maintaining the necessary temperature in the reactor. The excess amount could be
transformed to electricity and used to supplement the energy supply in the vicinity.
The design of the reactor and the corresponding reactor volumes are determined on the
basis of loading rates, which account for both - the acid formation stage and the
methanogenesis to be performed in the same reactor. Most of the organic matter is
usually hydrolyzed before the sludge reaches the reactor, along
Figure 8.2. Fermentative pathways of
anaerobic digestion of organic matter
in sludge.
the sewer lines and during the sedimentation process. For a proper operation, a delicate
balance needs to be maintained between the acid formers and the methanogens, which
usually is controlled by the organic and hydraulic loading rates, the pH, the temperature
and the volatile acids concentrations in the reactor.
