Wastewater Treatment

The goal of wastewater treatment is to lower the Biochemical Oxygen Demand (BOD) — the amount of oxygen that would be consumed if all the organic matter in a litre of the water were oxidised by bacteria — since a higher BOD means greater polluting potential. Primary treatment physically removes solids: floating debris is screened out by sequential filtration, and grit (soil and small pebbles) settles out by sedimentation, leaving a primary sludge and a clarified effluent that is passed on for secondary treatment. This effluent then undergoes secondary (biological) treatment in large aeration tanks that are constantly agitated and pumped with air, where aerobic microbes flourish and consume most of the remaining organic matter, forming floc — mesh-like masses of bacteria bound with fungal filaments — which sharply reduces the BOD. The floc settles in a secondary settling tank as activated sludge; a small part is recycled back into the aeration tank as an inoculum, while the larger remaining part is pumped to anaerobic sludge digesters, where anaerobic bacteria further digest the sludge and release a biogas mixture of methane, hydrogen sulphide and CO2 that can be used as an energy source. Tertiary treatment is the final polishing stage that removes remaining inorganic nutrients such as nitrogen and phosphorus and disinfects the water — commonly using UV light, a chemical-free method that can fully replace chlorination and inactivates pathogens, including chlorine-resistant organisms like Cryptosporidium and Giardia, without altering water quality otherwise. India's National River Conservation Plan (enacted 1995) and river-specific programmes such as the Ganga Action Pl …