ABSTRACT
Among the sludges generated from wastewater treat-
ment plants, there are two types: (a) municipal sewage
sludge and (b) industrial sludge, depending on the
sources of the wastewater. By taking a municipal waste-
water treatment plant as an example, there is primary
sludge obtained from gravity settling at the beginning of
the process, secondary sludge generated from biological
treatment (usually activated sludge), see Figure 38.1,
and also tertiary sludge generated by processes such as
chemical precipitation and filtration [5]. The sludge
produced may undergo further biological digestion ei-
ther aerobically or anaerobically in order to reduce the
volume as well as the pathogens in the sludge [6].
The digestion of raw sludge will also convert some of
the organic carbon into methane or CO2.
