In primary treatment the run-off should be drained through screens, grit chambers and sedimentation tanks to remove toxic chemicals from the water. In secondary treatment the waste water the primary treatment is brought into contact with sludge heavily laden with micro-organisms. Afterwards the air is passed to give oxygen to the micro-organisms which convert the organic matter into low – energy compounds. The sludge material settles down in the tank and the clean water can be used after treating with chlorine gas. After primary and secondary treatments, pollutants such as phosphorus and nitrogen are removed, so that the water may be harmless to human health. It is called ‘Advanced Waste Treatment’.
