During the process of water purification, water which comes to the filtration tank has impurities and suspended particles. Filtration tank which has three layers: fine sand layer at the top, a coarse sand layer in the middle and gravel at the bottom. In this process, the layers of sand and gravel act as filters. When water passes though these layers, even the smallest of suspended particles are removed.

Water from a river or a lake is brought through canals or long pipes to the water work where it is mixed with required quantities of alum and soda lime solutions. The substances react with one another to form aluminium hydroxide, a jelly-like, sticky solid. It is then pumped into big settling tanks, where most of the suspended impurities settle down in two or three days. The clear water still containing some suspended matter is passed through successive filters of boulders, gravel, coarse sand and fine sand. The clear water from the filters is chlorinated and then passed to the reservoirs for distribution in the city.