chapter  9
34 Pages

Incineration Systems for Sludge Wastes

Assisted by the massive construction grant program of the Water Pollution Control Act, a

large number of primary and secondary municipal and industrial wastewater treatment

plants were brought on-line in the United States between 1970 and 1990. The plants

generate large quantities of waste solids (402). These ‘‘biosolids’’ include several waste

streams:

‘‘Primary’’ sludge (produced by gravity settling in the first stage of treatment)

‘‘Biological’’ (‘‘secondary’’) sludge produced in the activated sludge process

‘‘Advanced’’ wastewater treatment sludge from tertiary treatment process

Other treatment plant solids such as the ‘‘screenings’’ recovered by the mechanical

removal of large-dimension solids, ‘‘grit’’ recovered by settling sand and other

coarse granular solids, and ‘‘scum’’ or ‘‘skimmings’’ recovered as the floatable

solids skimmed from clarifiers

At the points of their generation in the treatment plant, these wastes vary in solids content

over a very wide range (from 1% to 8% solids). If incineration is to be considered as an

affordable process alternative, the sludge must first be dewatered to 20% solids or more.

Note that in accordance with common usage, the relative proportions of dry solids and

moisture in sludge are referenced in most of the wastewater treatment literature as ‘‘percent

solids’’ rather than ‘‘percent moisture.’’