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.

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on

The answer is c i think