CHLORINE

The chlorine is a chemical element of the group of halogenous, like the fluorine, the bromine, the iodine and the astatine. In the nature it can be found in gas form forming bivalents chlorine molecules (Cl₂).

It was discovered by the Swedish chemistry Carl Cheele in 1774. Humphry Davy put the name of chlorine which is derived from a Greek word that means green, in honour to the dark green colour of this gas.

It can become liquid to - 35º C, is therefore easily liquefiable due to it is usually transported in liquid state by the use of pressurized bottles.

Like compound, in the nature it can be found in great amount taking part to the common salt or sodium chloride (NaCl), that in liquid state is dissociated in its ions Cl - and Na + .

The chlorine is a quite effective and economic disinfectant for the treatment and purification of water. It can be in water dissolved as a gas, like sodium hypochlorite or like calcium hypochlorite.

However, the chlorine contribution reacts with the organic matter of the water forming a series of compounds derived from chlorine that can be very annoying and bad odour. Between these compounds, the most detrimental are trihalomethanes with a cancerigenic character for the human health. Between all of them the most important is the trichlormethane or chloroform (CHCl₂), that was traditionally used as analgesic but nowadays it is not used due to its toxicity. These toxic compounds are associated to risks of colon cancer and damages in the kidney and the liver. It can also be formed other detrimental subproducts like volatile organic compounds, chlorites, chloroacetic acids or chloride of cyanogen.

The chlorination process can be easily understood in the following graph in which it can be appraised several good defined phases:

In phase AB all the chlorine that is added is used for combining with the organic matter reason why the residual chlorine level consequently is zero.

At phase BB', the residual chlorine level increases, but all this chlorine is combined in form of chloramines, which are products with a low disinfectant power that produce a disagreeable odour. These compounds are the causes of the called odour to swimming pool .

From B' to C the chlorine added is used to destroy the chloramines, reason why the measured residual chlorine diminishes until a minimum point in C called breakthrough point. From this point, all the chlorine added is used to increase the residual chlorine that would be like free chlorine and it would have more disinfectant power than the combined one that chloramines form.

For that reason, the breakthrough point must be surpassed to have residual free chlorine in the swimming pool and the combined one can be the minimum possible.

A way to eliminate all these chlorine sub products (trihalomethanes, chloramines and all type of compounds derived from chlorine) is to replace the prechlorination before the breakthrough point C by another oxidating agent like ozone that it does not form these sub products. Ozone oxidizes all the organic matter present in the water completely so the small amount of chlorine we add later will be residual free chlorine immediately so it leaves its property disinfectant to the water.