The treatment of sewage is a microbiological issue because of the need to remove coliform and other enteric or pathogenic microbes from the sewage. The treatment of sewage using the activated slude process adds another microbiological dimension, because it is the industrial control of microbial predator prey relationships that results in clean effluent. Images are from the Avonmouth sewage treatment works in SW England from the 1980s. The treatment of sewage aims to remove inorganic and organic particles, bacteria and dissolved organic matter from the sewage to produce a clean effluent. Incoming sewage is first physically filtered by a grid, and then the flow of sewage is slowed to allow grit and other particles to settle out. The resulting material in suspension is passed into the activated sludge plant where it is violently aerated. Floc-associated and suspended bacteria consume the dissolved organic matter. Protozoa, especially peritrich ciliates, attached to the flocs, eat the suspended bacteria. The result is that the suspended bacteria and the dissolved organic matter are converted into sedimentable flocculent matter. This can be removed, and digested to produce methane that helps power the process, while the effluent can be used for industrial processes, or after final polishing, be available for other uses. This image was provided by D. J. Patterson. Copyright: D. J. Patterson, image used under license.