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.

Because the earth is not round ,but it may seem round when viewed from ,our planet but true some points earth is not round was an oblate spheroid -a square that it squashed as globe however is not even a perfect oblate spheroid.in fact isaac newton's first proposed that earth was not perfectly round but it was an oblate spheroid instead.that mean a square that is squashed as its poles and swollen at the