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Water disinfection

The aim of water disinfection is to destroy living and spore forms of pathogenic organisms as well as to prevent their secondary growth in the water supply. The microorganisms present in water are partially removed with suspended solids by sedimentation and filtration. The complete removal of microorganisms requires special means, which are used in physical chemical methods.

Physical methods of water disinfection include boiling, pasteurization, ultraviolet irradiation, application of ultrasound or gamma rays, and ultrafiltration. Boiling and pasteurization are the simplest methods of water disinfection. They can be applied only in households and in some types of food industry. These methods are not used in water supply practice.

Ultraviolet rays effectively destroy microorganisms. Sources of UV radiation are high-pressure mercury lamps or low-pressure argon quartz lamps. Disinfection using UV radiation is a method requiring considerable investment and operating costs and should be accompanied by final chlorination. It is used for small quantities of clear water, because the presence of suspended solids causes dispersion of radiation and reduces disinfection efficiency.

Chemical disinfection methods involve introducing substances into the water that destroy bacteria. The effects of chemical agents depend on their type and amount, physico-chemical composition of water, contact time of the chemical agent with water, quantity and type of microorganisms in the water, and water temperature. Most chemical methods involve the use of strong oxidizing agents such as chlorine, sodium hypochlorite, chlorine dioxide, chloramines, ozone, bromine, and iodine.

The most common method and the cheapest method of water disinfection is chlorination. The water is most often chlorinated using chlorine gas – added to the disinfected water in the form of chlorine water, which is prepared and dosed from chlorinators.

Ozone, thanks to its strong oxidizing properties, is very often used as a disinfection agent when chlorination fails due to the formation of chlorine derivatives of organic compounds or compounds that give water taste and smell. One of the disadvantages of ozone is its low persistence, and thus the danger of secondary bacterial growth in the water supply. Therefore it is recommended that water should be chlorinated after ozonation.