This is still the most commonly used disinfection method in our country. However, it is recently come to understanding that chlorinating the water could result in development of some harmful substances including carcinogen compounds.

The reason why chlorine is still widely used despite this fact could be that it is cheap, easy to apply, and remains for a long while after application.

The main factors effecting disinfection are contact time, concentration of the disinfecting substance, organism count, and other determinants.

Calcium hypochlorite Ca(ClO)2 and sodium hypochlorite NaClO are the most commonly used ones. Calcium hypochlorite Ca(CIO)2, also known as chlorine of lime, contains 34% chlorine.

Chlorine gas is highly poisonous. 30 ppm (parts per million) causes coughing, 60 ppm is hazardous, and 1000 ppm causes sudden death. Special applications are necessary for areas disinfected with chlorine gas.

Liquid and granular chlorines are readily available in the market with various degrees of purity. Chlorine is administered to the water in different concentrations regarding its condition. This is usually done with dosing pumps. Chlorine should be left in the water sufficiently. A small dose of chlorine cannot disinfect, and a higher dose is a health hazard.

Dissolving chlorine:
If chlorine gas is higher than PH 3, it first transforms into Cl2+ H2O ↔ HOCl + H+ Cl-, and then into HOCl ↔ H+ + OCl- ion. HOCl (hypochlorous acid) displays 40 to 80 times more disinfecting effect than OCl (hypochlorite) ion. Free chlorine could be administered to water as hypochlorite salt, and the reactions would be as following: Calcium hypochlorite Ca(CIO)2 + 2H2O ↔ 2HOCl + Ca(OH)2 or sodium hypochlorite NaClO + H2O ↔ HOCl + NaOH, and they transform into disinfecting HOCl (hypochlorous acid) ion. HOCl, easily reacting with nitrogen compounds that could be in the water, creates respectively chloramine (mono), dichloramine, and nitrogen-chloramine compounds. These are called chlorine compounds, and they are hazardous to health. HOCl reacting with F++, Mn++, H2S or similar substances is less effective in disinfecting.

Removing excess chlorine in water:
Activated carbon and sulfur dioxide could be utilized to remove excess chlorine or compound chlorine from water. It is thus possible to disinfecting first, and then removing compound chlorines.