Environmental Engineering Reference
In-Depth Information
Thermophilic anaerobic digestion is generally more efficient in terms of organic matter
removal and methane production than the mesophilic process (Gavala, H.N. et al., 2003).
Figure 9 . Flow diagram of anaerobic digestion for power generation (modified from Kumar, 2000)
Telles et al. (2002) evaluated the performance of a slow rate anaerobic digester in treating
secondary sewage sludge. The digester was fed by secondary sewage sludge without any
previous thickening, having a concentration of volatile suspended solids (VSS) of 24-29 gL
-1
.
The operation of anaerobic digestion at room temperature was stable, with no noticeable scum
or foaming problems. The COD reduction in these experiments reached 29, 21 and 45% in the
sludge. Anaerobic digestion of Kraft waste activated sludge (or secondary sludge) was tested
with a pilot-scale digester for sludge reduction and biogas production (Puhakka et al., 1992).
With sludge containing 38% lignin, 40% reduction of the sludge and a biogas production of
0.5 m
3
-biogas/kg sludge removed were achieved. In these tests, 13 g NaOH/kg sludge was
added to maintain the optimum pH in the system for the maximum sludge reduction
efficiency. Fein et al. (1989) reported that the anaerobic digestion process for the treatment of
Kraft mill primary sludge could be significantly more economical than the conventional
landfilling. Stahl et al. (2004) also reported a pilot trial using anaerobic digestion to pre-treat
wastewater and to digest untreated paper mill effluent, which resulted in a much lower
amount of organics entering the activated sludge system, thus substantially reducing the
quantity of the secondary sludge from the aerobic treatment operation. Anaerobic digestion
has been widely adopted for the treatment of municipal sewage sludge before final disposal
