What are aeration tanks primarily used for in secondary treatment?

• A. To remove heavy metals from the wastewater
• B. To promote the growth of microorganisms that digest organic material
• C. To filter out remaining solids
• D. To hold treated effluent before release

Answer: (B)

Aeration tanks are primarily utilized in secondary treatment processes to enhance the biological decomposition of organic materials present in wastewater. During this phase, air or oxygen is introduced into the tank, creating a conducive environment for microorganisms, such as bacteria and protozoa, to thrive. These microorganisms consume organic pollutants and break them down into simpler, less harmful substances.

The function of promoting microbial growth is central to the secondary treatment process, as it effectively reduces the biochemical oxygen demand (BOD) of the wastewater, leading to cleaner discharge into the environment. Additionally, the aeration process supports the formation of activated sludge, which can be further processed to remove solids from the treated water.

Other options, while relevant to wastewater treatment in different contexts, do not align with the primary function of aeration tanks in secondary treatment. For instance, the removal of heavy metals is typically associated with different methods like chemical precipitation or adsorption, while filtering out remaining solids is usually accomplished by clarifiers or filters after biological treatment has occurred. Holding treated effluent before release pertains more to final storage or distribution rather than the aeration process itself.