Water supplied to cities and towns is usually taken from rivers or lakes and contain various kinds of pollutants, and so has to be purified. First, the raw water is passed through screens to separate coarse solids and it is then fed into settling tanks where the suspended solids settle out. Some times, substances such as alum are added to aid the settling process.
From the settling tanks the water is sent to sand filter beds -large concrete tanks with thick layers of coarse and fine sand at the bottom. The sand filters remove the remaining suspended matter from the water. Finally, the clear water is treated with chlorine gas in a closed tank to kill bacteria. The purified water is then pumped into tanks for distribution. For industrial use, water is sometimes passed through ion exchangers to remove ions like magnesium and calcium present in hard water.