Editing Activated sludge (section)

===Bioreactor and final clarifier===
The process involves air or [[oxygen]] being introduced into a mixture of screened, and primary treated sewage or industrial wastewater ([[wastewater]]) combined with organisms to develop a biological [[flocculation|floc]] which reduces the [[organic matter|organic]] content of the [[sewage]]. This material, which in healthy sludge is a brown floc, is largely composed of [[Saprotrophic bacteria]] but also has an important [[protozoa]]n flora component mainly composed of [[amoeba]]e, [[Spirotrich]]s, [[Peritrich]]s including [[Vorticella|Vorticellids]] and a range of other filter-feeding species. Other important constituents include motile and sedentary [[Rotifer]]s. In poorly managed activated sludge, a range of [[mucilage|mucilaginous]] filamentous bacteria can develop - including ''[[Sphaerotilus natans]]'', ''[[Gordonia (bacterium)|Gordonia]]'',<ref>{{cite journal | vauthors = Oerther DB, de los Reyes FL, Hernandez M, Raskin L | title = Simultaneous oligonucleotide probe hybridization and immunostaining for in situ detection of Gordona species in activated sludge | journal = FEMS Microbiology Ecology | volume = 29 | issue = 2 | pages = 129–136 | date = 1999 | doi = 10.1111/j.1574-6941.1999.tb00604.x | doi-access = free }}</ref> and other microorganisms -  which produces a sludge that is difficult to settle and can result in the sludge blanket decanting over the weirs in the settlement tank to severely contaminate the final effluent quality. This material is often described as sewage fungus but true fungal communities are relatively uncommon.

The combination of wastewater and biological mass is commonly known as ''mixed liquor''. In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated [[supernatant]] is run off to undergo further treatment before discharge. Part of the settled material, the [[sludge]], is returned to the head of the [[aeration]] system to re-seed the new wastewater entering the tank. This fraction of the floc is called ''return activated sludge'' (R.A.S.).

The space required for a sewage treatment plant can be reduced by using a [[membrane bioreactor]] to remove some wastewater from the mixed liquor prior to treatment. This results in a more concentrated waste product that can then be treated using the activated sludge process.

Many sewage treatment plants use [[axial flow pump]]s to transfer nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification.  These pumps are often referred to as internal mixed liquor recycle pumps (IMLR pumps). The raw sewage, the RAS, and the nitrified mixed liquor are mixed by [[submersible mixer]]s in the anoxic zones in order to achieve denitrification.