Sphaerotilus natans is an aquatic periphyton organism associated with polluted water. It forms colonies commonly known as "sewage fungus", but later identified as tightly sheathed filamentous bacteria.

Sphaerotilus natans is often associated with a buoyant floc (or "bulking sludge") causing poor solids separation in activated sludge clarifiers of secondary sewage treatment.[4] Metal surfaces covered with S. natans may experience accelerated corrosion if the slime creates a barrier causing differential oxygen concentrations.[6] S. natans slimes may reduce quality of paper produced by paper mills using recycled water streams.

https://en.wikipedia.org/wiki/Sphaerotilus_natans