Editing Bioreactor (section)

===Sewage treatment===
Conventional [[sewage treatment]] utilises bioreactors to undertake the main purification processes. In some of these systems,  a chemically inert medium with very high surface area is provided as a substrate for the growth of  biological film.  Separation of excess biological film takes place in  settling tanks or cyclones. In other systems [[Water aeration#Aeration methods|aerator]]s supply oxygen to the sewage and biota to create [[activated sludge]] in which the biological component is freely mixed in the liquor in "flocs". In these processes, the liquid's [[biochemical oxygen demand]] (BOD) is reduced sufficiently to render the contaminated water fit for reuse. The biosolids can be collected for further processing, or dried and used as fertilizer. An extremely simple version of a sewage bioreactor is a septic tank whereby the sewage is left in situ, with or without additional media to house bacteria. In this instance, the biosludge itself is the primary host for the bacteria.{{citation needed|date=October 2019}}