Sludge is a generic term for solids separated from suspension in a liquid by a variety of processes. Most commonly sludge refers to solid waste extracted in the process of sewage treatment; the term sewage sludge is used commonly. When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter will settle out in about an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be "fresh" before anaerobic processes become active. Once anaerobic bacteria take over, the sludge will become putrescent in a short time and must be removed from the sedimentation tank before this happens.
This is commonly accomplished by two different ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where decomposition by anaerobic bacteria takes place resulting in liquefaction and a reduction in the volume of the sludge. After digesting for an extended period of time, the result is called "digested" sludge and may be disposed of by drying and then landfilling. Alternately, the fresh sludge may be continuously extracted from the tank by mechanical means and passed on to separate sludge digestion tanks which operate at higher temperatures than the lower story of the Imhoff tank and as a result digest much more rapidly and efficiently. Excess solids from biological processes such as activated sludge can also be referred to as sludge, although more often called “biosolids,” a public relations term that is increasingly used by water professionals in the United States. Sludge has limited value as a soil conditioner and if derived from municipal wastewater treatment plants it will contain toxic materials. Often thought to consist of only "human waste," sewage sludge in fact contains all materials from cities which the treatment can remove from wastewater. After the 1991 Congressional ban on ocean dumping, the U.S. Environmental Protection Agency (EPA) instituted a policy of disposing of sludge on agricultural land. EPA promoted this policy by presenting it as recycling. But with more and more incidents of illness reported, there has been increasing concern among scientists about the disposal of sewage sludge on land.
Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.