Q.The purpose of biological treatment of waste water is to ________.

Biological treatment of wastewater uses microbes to decompose organic waste, and its main purpose is to reduce the water's Biochemical Oxygen Demand (BOD) before discharge.

In secondary (biological) sewage treatment, aerobic microorganisms (in activated sludge tanks/aeration tanks) consume and break down the dissolved and suspended organic matter in wastewater, converting it into microbial biomass, CO2 and water. This significantly lowers the wastewater's BOD -- the amount of oxygen microbes would otherwise consume decomposing that organic matter -- which is important because a high-BOD effluent released untreated into a river/lake would deplete dissolved oxygen and harm aquatic life. Sedimentation (options a and c) is …