Which treatment stage is primarily focused on enhancing water quality through biological processes?

The stage primarily focused on enhancing water quality through biological processes is secondary treatment. This phase employs microorganisms to break down organic matter and pollutants in the wastewater. By utilizing bacteria and other microorganisms, secondary treatment effectively reduces biochemical oxygen demand (BOD) and suspended solids, which are key indicators of water pollution.

During this stage, the activated sludge process is commonly used, where air is introduced to the wastewater in aeration tanks to facilitate microbial growth. As the microorganisms metabolize the organic materials, they form flocs that can be settled out in a secondary clarifier, further improving the clarity and quality of the effluent.

In contrast, other treatment stages serve different purposes. Preliminary treatment focuses on the removal of large solids and debris, while primary treatment involves sedimentation to remove heavier solids and oils. Tertiary treatment may include advanced filtration or disinfection processes to further polish the effluent and remove remaining contaminants, but it does not primarily rely on biological processes to achieve water quality enhancement.