Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term "septage" also refers to sludge from simple wastewater treatment but is connected to simple on-site sanitation systems, such as septic tanks.

After treatment, and dependent upon the quality of sludge produced (for example with regards to heavy metal content), sewage sludge is most commonly either disposed of in landfills, dumped in the ocean or applied to land for its fertilizing properties, as pioneered by the product Milorganite.^{[citation needed]}

The term "Biosolids" is often used as an alternative to the term sewage sludge in the United States, particularly in conjunction with reuse of sewage sludge as fertilizer after sewage sludge treatment. Biosolids can be defined as organic wastewater solids that can be reused after stabilization processes such as anaerobic digestion and composting. Opponents of sewage sludge reuse reject this term as a public relations term.

Sewage sludge treatment is the process of removing contaminants from wastewater. Sewage sludge is produced from the treatment of wastewater in sewage treatment plants and consists of two basic forms — raw primary sludge and secondary sludge, also known as activated sludge in the case of the activated sludge process.

Sewage sludge is usually treated by one or several of the following treatment steps: lime stabilization, thickening, dewatering, drying, anaerobic digestion or composting. Some treatment processes, such as composting and alkaline stabilization, that involve significant amendments may affect contaminant strength and concentration: depending on the process and the contaminant in question, treatment may decrease or in some cases increase the bioavailability and/or solubility of contaminants. Regarding sludge stabilization processes, anaerobic and aerobic digestion seem to be the most common used methods in EU-27.

When fresh sewage or wastewater enters a primary 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. Most commonly, the fresh sludge is continuously extracted from the bottom of a hopper-shaped tank by mechanical scrapers and passed to separate sludge-digestion tanks. In some treatment plants an Imhoff tank is used: sludge settles through a slot into the lower story or digestion chamber, where it is decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge.

The secondary treatment process also generates a sludge largely composed of bacteria and protozoa with entrained fine solids, and this is removed by settlement in secondary settlement tanks. Both sludge streams are typically combined and are processed by anaerobic or aerobic treatment process at either elevated or ambient temperatures. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then landfilling.