Chlorine For Water Treatment
Why is Such a Dangerous Chemical Added to Water?
Chlorination of drinking water supplies has probably saved many millions of lives, and could save millions more.
It has been estimated that 1.2 billion people in the world lack access to safe supplies of drinking water. Many thousands of people in these areas, mainly children, die from water related diseases. Water is a vital part of life, 67% of our bodies are water, and access to an adequate supply of water is indispensable for every human being. One of the greatest accomplishments in Western society has been the disinfection of water supplies with chlorine.
US Troops
In fact the idea of adding chlorine to water supplies was first suggested by US Army Major Carl Darnall in 1910. He was working on ways to prevent US troops from getting ill while in the field, and one of the major sources of illness was the drinking water. As further work was done, it was found that the most effective way of killing the micro-organisms which cause disease was by chlorination.
Chlorine Chemistry
Chlorine is a very reactive element found in the seventh group of the periodic table. It has seven electrons in its outer shell, which means it will readily react wit anything which will donate an electron. Chlorine is very abundant on Earth, mainly in the form of sodium chloride (salt). To produce chlorine from a solution of sodium chloride, an electrical current is passed through it, producing chlorine gas and leaving behind a solution of sodium hydroxide, also known as caustic soda.
Hypochlorous Acid
Chlorine is added to drinking water supplies either as the gas itself, or as sodium hypochlorite (bleach) or calcium hypochlorite solid. In all three cases the result is a solution containing hypochlorous acid (HOCl), which, depending on the pH of the environment, will dissociate to form H+ and OCl- ions. This chemical has a good ability to attack pathogens (agents that cause disease) because they penetrate their protective membrane.
Important Functions
Chlorine performs two important functions when added to drinking water. First it kills the bacteria, viruses and other micro-organisms in the water, so that it is safe to drink. Second it remains at a residual level in the water so that any potentially harmful organisms that may enter the water during transport, storage and distribution will also be removed.
Emergency
So in an emergency, when water supplies have been contaminated, or piped water is not available there are some simple solutions. Boiling water is the simplest; one minute of vigorous boiling will kill most common pathogens in water. Secondly, though, it is possible to chlorinate water by the addition of a small quantity of ordinary household bleach, which normally gives instructions for the quantities to be used on the labels.
