Sewage 'fungus' is actually a bacterium (Sphaerotilus natans) found in the serious secondary stage of treatment in sewage treatment works. It feeds on a diet of sugars and nutrients found in raw human sewage and is found in flowing waters where raw sewage has flowed past for a week or more. It is an indicator that raw sewage has been sustained in a river for long periods.

Richard Knowles explains the signs of sewage fungus seen in March 2021 at Emma's Dyke, just below the main outfall of Witney Sewage Treatment Works.