The proper treatment and disposal of sludge require knowledge of the origin of the solids to be handled, as well as their characteristics and quantities. The type of treatment employed and the method of operation determine the origin of sludge. In sewage treatment plants, sludge is produced by primary settling, which is used to remove readily settleable solids from raw wastewater. Biological sludges are produced by treatment processes such as activated sludge, trickling filter, and rotating biological contractors. Chemical sludges result from the use of chemicals to remove constituents through precipitation; examples of precipitates that are produced by this process include phosphate precipitates, carbonate precipitates, hydroxide precipitates, and polymer solids.

Sludge is characterized by the presence or absence of organic matter, nutrients, pathogens, metals and toxic organics. These characteristics are an important consideration for determining both the type of treatment to be...