Joyce E. McCormick, age 12, of Florence, South Carolina, for her question:
How is water purified?
Most of the water in our streams and lakes is not fit to drink. This was true, even before our dreary age of pollution. You need to consume five pints of water a day, either in drinks or food. And if you live in the United States, you probably use another 550 pints for daily duties such as showers and baths, laundry and dish washing. All this water must go through elaborate purification systems before it reaches our faucets.
Our household water supplies come from outside sources such as rivers and lakes. The quality of this water varies in different regions. The natural source may contain lots of floating debris, a fishy tang or other unpleasant features. All natural sources contain assorted bacteria, some of which may be unwholesome or downright dangerous to human health. One thing is certain, in every region the source of the water supply has its own unique set of problems. All these problems are analyzed and treated at your local water purifying plant.
Each local purifying plant is tailored to treat the local water problems. Most systems pipe the supply through a basic series of improvements, plus a few extras. Almost every city chlorinates its water supply, even when no other purifying is needed. The addition of small quantities of chlorine puts a speedy end to bacteria that might carry serious diseases. Chlorine, however, adds an unpleasant flavor of its own. This may be removed by spraying jets of the water through the air. Most unpleasant tastes and odors are removed by contact with the oxygen.
All water purifying plants perform these two basic operations. And almost all of them also have more to do. Raw water from the natural source usually carries an assortment of floating debris. This problem is solved in several stages. The right amount of alum, aluminum sulphate, may be added to the water. This chemical acts as a coagulant, forming tiny gobs of jelly called flocs. Fragments of mud and other debris stick to the flocs and so do many bacteria. Then the water, with its load of captured impurities, is allowed to stand quietly while the flocs settle to the bottom.
The calm water, now minus its debris, is led quietly away to be filtered through a bed of gravel and sand several feet thick. This screens out stray flocs and bits of debris that refused to settle. The filtered water is clean and sparkling bright. However, most plants add a second dose of chlorine, just in case any harmful bacteria have survived. The sparkling water is now safe to drink, though it may be guilty of various unpleasant odors or flavors. Activated carbon may be added to remove these. A final spray through the air removes other unpleasant features and also adds the refreshing, zesty quality of properly purified water.
Some treatment plants need to add more chlorine or more alum, or some other coagulant. Some plants add a little lime to reduce rusting in metal plumbing pipes. And up to date water systems now add traces of fluorine to reduce tooth decay in young people and to build stronger bones in older people. Water supplies that dissolve limestone chemicals refuse to form foamy lather with the soap. As a rule, this hard water is treated by softening chemicals in the home.