What is the purpose of a secondary clarifier?

The purpose of a secondary clarifier is primarily to settle out biomass that is produced during the secondary treatment processes of wastewater treatment. In the secondary treatment phase, biological processes are utilized to break down organic matter in the wastewater. This process produces a mixture of treated water and suspended solids, which include microorganisms, organic matter, and other particulates.

The secondary clarifier plays a crucial role by allowing this mixture to remain undisturbed long enough for the heavier solids, or biomass, to settle to the bottom of the tank due to gravity. This process effectively separates the treated water from the settled solids, resulting in a clearer effluent that can either be further treated or discharged into the environment. The settled solids can then be returned to the biological treatment system or taken for further processing, depending on the specific design of the treatment plant.

Thus, the correct answer highlights the clarifier's function in removing biomass, ensuring that the treated water meets regulatory standards before discharge or additional treatment stages.