Freshwater protozoa scurry about feeding on bacteria and sewage in water collected from a municipal sewage treatment plant. The larger ones are ciliates belonging to to the Colpidium genus and measure about 50 microns in length. Microorganisms like these, including bacteria and many metazoans, perform an important function by decomposing organic matter found in sewage water and helping remove contaminates during the aerobic treatment process. Recorded under Nomarski DIC.