Disinfection of drinking water

Disinfection of drinking water - removal from water of pathogens. There are several methods of water disinfection (see diagram). As a rule, to obtain adequate and sustainable results for drinking water disinfection it should be subjected to pre-treatment (see water Purification).
Chlorination is the most common way to treat drinking water. Most use chlorine and chlorine dioxide; in technical and economic terms, the advantage is given liquid chlorine and hypochlorite (bleach). At interaction of chlorine or bleach with water are formed in it hypochlorous acid (НОCl) and the free ion (Hcl-); then hypochlorous acid dissociates with the formation of hypochlorite-ion battery (ОCl-). Chlorine contained in hypochlorous acid and hypochlorite ion reacts with those present in the water with organic substances and binds them. This is mainly determined by the so-called chloroplast disinfected water. Free (active) chlorine or its active compound destroy the enzyme system of microbial cells. To achieve the disinfecting effect a certain dose of chlorine and sufficient duration of contact with the water. The duration of the contact on the water pipes must be at least 30 minutes Required dose of chlorine determine trial chlorination of water to be disinfected. Approximately for trial chlorination you can take the following chlorine dose: for filtered surface (and clarified underground) water 0,5-1 mg/L. If the water is very dirty, the dose should be increased accordingly.
In simple chlorination value is necessary dose is determined mainly by chloroplast and comes with a surplus of not less than 0,3 mg/l to provide warranty disinfection. When the water source is heavily polluted (see water supply Sources), for more reliable disinfection is double chlorination - before treatment and after it. If in water there are substances (phenols and other), which even in small concentrations, can at chlorination to give it a unpleasant smell and taste, to prevent this to the water previously add ammonia or ammonium salt (preammonization water). Thus chloroplast water decreases, while maintaining active chlorine longer.
Installations and devices for dosing of chlorine or its compounds)added to water - chlorinators - everywhere, except small water, place in a special room or in a separate building - chlorination plant (Fig. 1).

Fig. 1. The plan of premises chlorination: I - tambour; II - the interim storage for bottles; III - dosing for liquefaction of hargasha; IV - duty room; V - tambour; 1 - spare cylinders with chlorine; 2 - ventilation strut; 3 - a window; 4 - cylinders on the scale; 5 - vacuum-chlorinators; in - sludge trap; 7 - a washbasin.

Chlorination of drinking water, however, has its drawbacks: the need for careful dosing of chlorine, as even a small dose reduction sharply reduces the effectiveness of disinfection of drinking water, and excess doses gives the water chlorine odor; the possibility of specific hlorfenamina smells; toxicity of chlorine and the need for special measures of transportation, storage, etc.
During the disinfection of drinking water, especially high doses of chlorine, is dechlorination, carried out by physical means using filters with active carbon (0.5-2.5 m, grain, coal, 1.5 - 2.5 mm, filtering rate of 20 - 30 m3/hour), or chemically - in tanks with the use of sodium thiosulfate, sulfur dioxide, sodium sulfite, etc. that neutralizes chlorine (with mandatory requirements calculation neutralizing substances).
Ozonization is the most promising method of drinking water disinfection in connection with the reduction of energy consumption for generation of ozone in a special apparatus (ozonizers). The air passing through the ozonizer, subjected to electric discharge high voltage, due to which the significant part of the oxygen (O2) turns into ozone (O3). From ozone air enriched with ozone, goes in the tank where it is mixed with water to be disinfected. Disinfecting effect of ozone is associated with deoxidation ozone molecules and efficiency of the atom of oxygen, which is accompanied by the appearance in the water oxidation potential, much higher than at chlorination. In contact with water for 8-15 minutes a number O3required for disinfection of drinking water depends on the degree of contamination, composition and properties of water and varies from 1 to 6 mg/l and more. To achieve a reliable effect decontamination dose of residual ozone in water should 0.3-0.5 mg/l to exceed etnopovedenie water.
Excess ozone in water does not cause unpleasant odours and tastes in the water; on the contrary, ozonization significantly improves organoleptic properties. Therefore, from the hygienic point of view ozonation is one of the best methods O. p. C. Disadvantages O. p. C. ozone; high consumption of electricity, the complexity of the equipment, the need of qualified technical supervision.
Ozonation applies only to O. p. C. with centralized water supply (Fig. 2).

Fig. 2. The plan station of ozonization of water, working on the principle of counter flow water supply and ozonized air: 1 - coastal well; 2 and 4 - pumps; 3 - water treatment plants (coagulation, sedimentation, sand filter); 5 - pressure tank; 6 - sterilizer; 7 - ozonizer; 8 - filter; 9 - dryer air; 10 - air-purge drum; 11 - tank for clean water.

In addition to chlorination and ozonation, to chemical method of disinfection of drinking water and is using the oligodynamic properties of heavy metals (copper, silver and other) because of their ability to exert a bactericidal effect in extremely small concentrations. The use of silver was recommended for disinfection of water in swimming pools.
Physical methods greatest practical application of received O. p. C.
ultraviolet germicidal light. As sources bactericide radiation used mercury-quartz lamps of high pressure and organo-mercury lamps low pressure; 70% of the radiation power recent falls in the region of wavelengths 250-260 MMK, having the highest bactericidal action. O. p. C. this method does not change the properties and composition of water. Ultraviolet rays act on the cellular metabolism and especially on the enzymatic activity of the bacterial cell. One of the important conditions of the effectiveness of exposure is clarity and transparency of water. O. p. century bactericidal rays produced in installations tray-type with non-immersed lamps or in the pressure units with immersed in water sources of radiation (Fig. 3).

Fig. 3 Installation for water disinfection by ultraviolet rays (Akha-1): - section; - the scheme of movement of water in the cell; 1 - inspection window; 2 - body; 3 - partitions; 4 - water supply; 5 - mercury-quartz lamp PortPC-7; quartz case.


Disinfection of drinking water high-intensity ultrasound (10-30 W/cm2), bactericidal properties of which are connected with occurrence in water cavitation bubbles and pulses huge pressures. O. p. century ultrashort waves, especially centimeter wavelength (3-10 cm), bactericidal activity of which as believe, is caused by a sharp rise in temperature mass bacterial cells. O. p. C. radioactive radiation, with the specific mechanism of bactericidal action as other reagentless methods O. p. century, is still at a stage of preliminary studies and technical tests.
When monitoring the effectiveness of O. p. C. assume that the agents of enteric bacterial infections (cholera, typhoid, dysentery and other propagating through water, less resistant to chemical and physical means used to O. p. century than saprophytic microorganisms, usually located in the water. Therefore, when O. p. C. tend not to difficult achieved and unjustified its sterilization, and only to the destruction of pathogenic microbes that are harmful to health. The water is disinfected, if it remains no more than 100 microbes in 1 ml and not more than three of coliform bacteria in 1 l of water. In this case, all pathogens, as less resistant, may be deemed to be killed in the process O. p. C. This requirement was included in the standard quality of drinking water. At waterworks, where the water is disinfected with chlorine or ozone, every hour or half an hour) to verify the content of residual chlorine in the water (or ozone as an indirect indicator of the reliability of disinfection of drinking water.
In recent decades it has been spread by water enteric viruses (enteroviruses) and their etiological role in several diseases (infectious hepatitis likely, polio and other). Enteroviruses were more resistant than pathogenic bacteria and E. coli. So when epidemiological risk O. p. century should be based on the higher of residual chlorine (ozone), as usual levels of E. coli in these cases does not meet hygienic requirements.