Sludge is a generic term for solids separated from suspension in a liquid. This 'soupy' material usually contains significant quantities of 'interstitial' water (between the solid particles). Commonly sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes.

When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter will settle out in 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. The sludge will become putrescent in a short time once anaerobic bacteria take over, and must be removed from the sedimentation tank before this happens.

This is accomplished in one of two ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where it is decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then land filling. More commonly with domestic sewage, the fresh sludge is continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and, as a result, digest much more rapidly and efficiently.

Sewage sludge is produced from the treatment of wastewater and consists of two basic forms — raw primary sludge (basically faecal material) and secondary sludge (a living ‘culture’ of organisms that help remove contaminants from wastewater before it is returned to rivers or the sea). The sludge is transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime/alkaline stabilisation. Some treatment processes such as composting and alkaline stabilization involve significant amendments may dilute contaminant concentrations; depending on the process and the contaminant in question, treatment may decrease or in some cases increase the bioavailability and/or solubility of contaminants.  In general, the more effectively a wastewater stream is treated, the greater the resulting concentration of contaminants into the product sludge.

The treatment process reduces the water content of the sludge. The basic principal is that the dirtier the water is after the sludge is removed, the less toxic the sludge is going to be. The toxicity of the sludge will additionally vary dependent on the source of the waste water. Varying combinations of domestic and industrial customers will affect the composition of the sludge collected this has been proven when random samplings of treated sludge are found to be filled with heavy metals, as well as chemical residues that are not removed by the treatment process. Newer, innovative treatments will one day remove more pathogens than today; as treatment process currently in use do not remove 100% of the pathogens, and in many cases pathogen regrowth after spreading is significant.

http://en.wikipedia.org/wiki/Sludge