Drinking water disinfection

Disinfection of drinking water – removal of pathogens from the water. There are several ways to disinfect water (see diagram). As a rule, to obtain sufficient and sustainable results of drinking water disinfection, it must be pre-cleaned (see Water Purification).

Chlorination is the most common way to treat drinking water. Chlorine and chlorine dioxide are often used; technically and economically, liquid chlorine and hypochlorite (bleach) are preferred. When chlorine or hypochlorite interacts with water, hypochlorous acid (HCl) and free ion (HCl-) are formed in it; then hypochlorous acid dissociates to form hypochlorite ion (OCl-). Chlorine contained in hypochlorous acid and hypochlorite ion reacts with organic substances present in water and binds them. This is mainly determined by the so-called chlorine absorption of disinfected water. Free (active) chlorine or its active compounds destroy the enzyme system of the microbial cell. To achieve a disinfecting effect, a certain dose of chlorine and a sufficient duration of its contact with water are necessary. The duration of contact on the water mains must be at least 30 minutes. The required dose of chlorine is determined by trial chlorination of the water to be disinfected. Roughly for trial chlorination, the following doses of chlorine can be taken: for filtered surface (and clarified groundwater) 0.5-1 mg / l. If the water is highly polluted, the dose should be increased accordingly.

With simple chlorination, the magnitude of the required dose is determined mainly by chlorine absorption and is taken with an excess of at least 0.3 mg / l to ensure warranty disinfection. When the water source is heavily polluted (see Water supply sources), double chlorination is carried out for more reliable disinfection – before cleaning and after it. If there are substances in the water (phenols, etc.), which, even in low concentrations, can give an unpleasant odor and taste to it during chlorination, then to prevent this, ammonia or ammonium salts are preliminarily added to the water (pre-ammonization of water). At the same time, the chlorine absorption of water decreases, the time of preservation of active chlorine in it is extended.

Installations and apparatuses for dosing chlorine (or its compounds) added to water — chlorinators — everywhere, except for small water mains, are placed in a special room or in a separate building — chlorination (Fig. 1).

Chlorination of drinking water, however, has its drawbacks: the need for careful dosing of chlorine, since even a small dose reduction drastically reduces the effectiveness of drinking water disinfection, and an excess dose gives the smell of chlorine to the water; the possibility of the appearance of specific chlorophenol odors; chlorine toxicity and the need for special measures for its transportation, storage, etc.

When drinking water is disinfected, especially with large doses of chlorine, dechlorination is carried out by physical means using filters with activated carbon (height 0.5-2.5 m, grain of coal 1.5-2.5 mm, filtering speed 20-30 m3 / hour) or by chemical means – in tanks using sodium thiosulfate, sulfur dioxide, sodium sulfite, etc., which neutralize chlorine (with the obligatory calculation of the need of neutralizing substances).

Ozonization is the most promising method of drinking water disinfection due to cheaper electricity needed to produce ozone in special devices (ozonizers). The air passing through the ozonizer is exposed to a high-voltage electrical discharge, due to which a significant portion of the oxygen in the air (O2) is converted to ozone (O3). From the ozonizer, ozone-enriched air is sent to tanks, where it is mixed with water to be disinfected. The disinfecting effect of ozone is associated with the deoxidation of the ozone molecule and the release of an oxygen atom, which is accompanied by the appearance of oxidative potential in water, which is much higher than during chlorination. Upon contact with water for 8-15 minutes. the amount of O3 needed to disinfect drinking water depends on the degree of contamination, the composition and properties of water, and ranges from 1 to 6 mg / l and more. To achieve a reliable decontamination effect, the dose of residual ozone in water should be 0.3-0.5 mg / l higher than the ozone absorption of water.

Excessive ozone in water does not cause unpleasant odors and taste in water; on the contrary, ozonation very significantly improves its organoleptic properties. Therefore, from a hygienic point of view, ozonation is one of the best methods for O. p. Disadvantages O. p. ozone; high power consumption, hardware complexity, the need for qualified technical supervision.

Ozonation is applied only for O. p. with centralized water supply (Fig. 2).

In addition to chlorination and ozonation, the use of oligodynamic properties of heavy metals (copper, silver, etc.) also applies to chemical methods of disinfecting drinking water.

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