Sodium hypochlorite - NaClO, is obtained by chlorination of an aqueous solution of sodium hydroxide (NaOH) with molecular chlorine (Cl2) or electrolysis of a solution of sodium chloride (NaCl). For the purposes of chlorination, existing methods of water treatment with chlorine or other chlorine agents containing active chlorine can be combined into two main groups:
Pre-chlorination of water is most often used as a means to improve some water purification processes (for example, coagulation and deferrization), and also as an effective way to neutralize some toxic compounds in wastewater treatment. In this case, excess chlorine is spent on the oxidation of various water impurities, is sorbed by coagulant flakes, oxidizes microorganisms capable of immobilization and development on the surface of equipment and pipelines, as well as in the thickness of the filter load, etc. As a rule, large doses of chlorine are used in prechlorination, and the stage there is no dechlorination of water, since the excess amount of chlorine is usually completely removed at other stages of the water purification process.
Final chlorination of water (post-chlorination) is the process of water disinfection, which is carried out after all other methods of its treatment and, therefore, is the final stage of water purification. If the water is not treated other than disinfection, then in this case it will be post-chlorination.
Postchlorination can be carried out with both small doses of chlorine (normal chlorination) and higher doses of chlorine (overchlorination). If, when using chlorination, other disinfectants are used together, then it is called combined chlorination.
