<P> "Biosolids" is a term often used in wastewater engineering publications and public relations efforts by local water authorities when they want to put the focus on reuse of sewage sludge, after the sludge has undergone suitable treatment processes . In fact, biosolids are defined as organic wastewater solids that can be reused after stabilization processes such as anaerobic digestion and composting . The term "biosolids" was introduced by the Water Environment Federation in the U.S. in 1998 . However, some people argue that the term is a form of "propaganda" (or at least a euphemism) with the aim to hide the fact that sewage sludge may also contain substances that could be harmful to the environment when the treated sludge is applied to land, for example environmental persistent pharmaceutical pollutants . </P> <P> Thickening is often the first step in a sludge treatment process . Sludge from primary or secondary clarifiers may be stirred (often after addition of clarifying agents) to form larger, more rapidly settling aggregates . Primary sludge may be thickened to about 8 or 10 percent solids, while secondary sludge may be thickened to about 4 percent solids . Thickeners often resemble a clarifier with the addition of a stirring mechanism . Thickened sludge with less than ten percent solids may receive additional sludge treatment while liquid thickener overflow is returned to the sewage treatment process . </P> <P> Water content of sludge may be reduced by centrifugation, filtration, and / or evaporation to reduce transportation costs of disposal, or to improve suitability for composting . Centrifugation may be a preliminary step to reduce sludge volume for subsequent filtration or evaporation . Filtration may occur through underdrains in a sand drying bed or as a separate mechanical process in a belt filter press . Filtrate and centrate are typically returned to the sewage treatment process . After dewatering sludge may be handled as a solid containing 50 to 75 percent water . Dewatered sludges with higher moisture content are usually handled as liquids . </P> <P> Sludge treatment technologies that are used for thickening or dewatering of sludge have two products: the thickened or dewatered sludge, and a liquid fraction which is called sludge treatment liquids, sludge dewatering streams, liquors, centrate (if it stems from a centrifuge), filtrate (if it stems from a belt filter press) or similar . This liquid requires further treatment as it is high in nitrogen and phosphorus, particularly if the sludge has been anaerobically digested . The treatment can take place in the sewage treatment plant itself (by recycling the liquid to the start of the treatment process) or as a separate process . </P>

Where is water released when it is finished being treated