What is the purpose of secondary (biological) treatment?

• A. Remove dissolved and suspended organic material.
• B. Disinfect the water.
• C. Remove microbes.
• D. Remove inorganics and volatiles.

Answer: (A)

Secondary (biological) treatment uses microorganisms to oxidize and break down the remaining organic matter in wastewater. Its main purpose is to remove dissolved and suspended organic material, reducing the biochemical oxygen demand and improving effluent quality. The microbes need oxygen to metabolize organics, turning them into carbon dioxide, water, and new biomass. This stage targets organic material rather than inorganic compounds or pathogens. Disinfection is a separate step after secondary to kill remaining microbes, and inorganic or volatile compounds aren’t the primary target of this biological process.