Ever taken a dip in a swimming pool, and wondered why the water tastes funny? That's because of the chlorine added to the water, as a disinfectant.
Why disinfect swimming pool water?
The water in a swimming pool may be contaminated with bacteria from the source. Also as we go swimming in it, bacteria (especially a species called Escherichia coli) from our bodies escape into it. Chlorinating the water kills these bacteria.
How does chlorination work?
Chlorination can be done in two ways. In the first, liquefied chlorine gas is added directly to the water. It dissolves to form hydrochloric acid and hypochloric acid. In the second method, sodium hypochlorite salt is dissolved in water.
In both cases, a hypochlorite ion is produced, which breaks up under the effect of sunlight to release oxygen free radicals. The oxygen free radical reacts with all organic matter it encounters, killing the bacteria present in water. Some chlorine molecules that fail to ionize in water also react with organic matter.
During chlorination, it is not safe to enter the water. The oxygen and chlorine cause whitening and loss of hair, and drying of the skin. That’s why swimming pools are closed during chlorination. Once the concentration of hypochlorite has dropped to 3 parts per million (ppm), the pool can be used.
Hypochlorite ions produced by adding chlorine to water break up under the effect of sunlight to release oxygen free radicals that kill bacteria.
Is chlorination effective?
Chlorine (gas) is known to kill almost all kinds of bacteria, as well as viruses and protozoa. It is most effective when added at night. This is because it gets more time to act; in bright sunlight the hypochlorite ion breaks up very quickly. Also, it leaves behind no contaminants in the water, except the harmless chloride ion. Finally, it is an affordable method, especially for a huge volume of water like a swimming pool.
Are there better methods?
Other methods are now being preferred for decontamination, due to the drawbacks of using chlorine. As the hypochlorite ion decomposes very quickly under sunlight, one must repeatedly use chlorine in a country like India. Also, unreacted chlorine leaves an unpleasant taste in the mouth. Chloramine (NH2Cl) is now preferred increasingly, as it is equally effective, and stable for much longer.