Wastewater Treatment — UPSC Mains Model Answer

The activated sludge process relies on aerobic microbial communities — primarily heterotrophic bacteria — to metabolise dissolved organic matter through oxidation. Microorganisms assimilate organic compounds as carbon and energy sources, converting them into biomass, carbon dioxide, and water. This biological oxidation is the primary mechanism responsible for Biochemical Oxygen Demand (BOD) reduction in secondary treatment.

Efficient organic matter removal depends on maintaining adequate dissolved oxygen levels, sludge retention time, and food-to-microorganism ratios within aeration tanks. Sludge recirculation ensures a sustained microbial population, while clarifiers separate treated effluent from settled biomass. Deviations in these parameters directly compromise treatment efficiency and effluent quality.

India's Central Pollution Control Board prescribes effluent discharge standards that secondary treatment must meet before release into inland surface waters. The National Mission for Clean Ganga and urban local body mandates under AMRUT have expanded sewage treatment plant capacity, yet a significant gap between sewage generated and treated remains a governance challenge.

The activated sludge process does not effectively remove nutrients such as nitrogen and phosphorus, nor does it address micropollutants and pharmaceuticals increasingly found in urban wastewater. Excess sludge disposal itself becomes an environmental liability, necessitating integrated sludge management policies alongside treatment upgrades.