Bacterial aerobic oxidation of polluted water in biological oxidation ponds is done to purify it. Presence of bacteria helps in

• coagulation and flocculation of colloids.
• oxidation of carbonaceous matter to CO2.
• nitrification or oxidation of ammonia derived from breakdown of nitrogeneous organic matter to the nitrite and eventually to the nitrate.
• all (a), (b) and (c).