The term filters is not strictly correct as no straining or filtering action is involved. The trickling filter is designed to bring the settled wastewater liquid into contact with the biological slime growth. Each filter is filled with special stones which range from 50 to 80 mm in diameter. These are termed filter media. This term is also applied to wood, plastic discs and other materials which can provide a surface for algae growth.
Rotary distributors apply the flow evenly over the filter media.
Each arm contains a number of nozzle openings and the arms are driven round by the jet action as it discharges through the nozzles. The spraying of the wastewater saturates the liquid with oxygen and aerobic conditions are maintained in the bed of stones. The stones become coated with a microbial flora called a zoogloeal slime. This slime consists of bacteria, fungi, protozoa, algae and nematodes. This trick of nature allows a natural biological process to convert pollutants in a liquid into slime solids which will settle out and can be removed in the secondary sedimentation process.