Sludge Treatment and Disposal 229Cooling air dischargeCombustion zoneFigure 10-16. Multiple-hearth incinerator. (Courtesy of Nichols Engineering and
Research Corp.)
The fluidized bed incinerator is full of hot sand suspended by air injection; the sludge
is incinerated within the moving sand. Owing to the violent motion within the fluid
bed, scraper arms are unnecessary. The sand acts as a “thermal flywheel,” allowing
intermittent operation. Despite a flurry of interest during the previous decade, sludge
incineration is no longer considered the best available technology by many state regu-
latory agencies because of environmental concerns about atmospheric emissions and
ash disposal.
The second method of disposal, land disposal, is becoming more popular, par-
ticularly in areas where there are restrictions on industrial contaminants entering the
wastewater treatment. (Sludges contaminated with industrial chemicals may not be
suitable for land application.) The ability of land to absorb sludge and to assimilate it
depends on such variables as soil type, vegetation, rainfall, and slope. In addition, the
important variable of the sludge itself will influence the capacity of a soil to assimilate
sludge. Generally, sandy soils with lush vegetation, low rainfall, and gentle slopes