Fundamentals of Microbiology

4.4.5 Trickling filters

In this method sewage is sprayed over a bed of rocks or molded plastic. The rocks provide enough surface area for microbial activity and the space between rocks are large enough to provide air to the bottom. A slimy gelatinous film of aerobic microorganisms grow on rock surface and metabolize the organic matter in water when it passes along into CO₂ and water. Trickling filters remove 80 to 85% of BOD.