Which commonly used method of secondary sewage treatment involves spraying the sewage over a bed of rocks or molded plastic? view available hint (s) ?

Trickling filter method

Trickling filter in waste water treatment is a fixed bed, biological reactor that works under aerobic conditions. Pre settled wastewater is continuously sprayed over the filter which is made out of rocks, gravel and molded plastic and it is further purified as it trickles downward. As the water moves through the pores of the filter, organics are degraded by the biofilm covering the filter material.