In a trickling filter A. Filtration process is used B. Biological action is used C. Neither A nor B D. Both A and B

The correct answer is: Both A and B.

A trickling filter is a type of biological wastewater treatment system that uses a bed of rocks or other media to support a biofilm of microorganisms. The microorganisms break down organic matter in the wastewater, and the filtered water is then discharged to the environment.

The filtration process in a trickling filter removes suspended solids from the wastewater. The biological action in a trickling filter removes dissolved organic matter from the wastewater.

Both the filtration process and the biological action are important for the proper functioning of a trickling filter.