Water Disinfection

the technology employed to eliminate infectious disease-causing bacteria and viruses from drinking water. Water disinfection methods can either be chemical (also called reagent) or physical (also called nonreagent). Chemical methods of disinfection include the use of chlorine, ozone, and ions of heavy metals; physical methods include disinfection with ultraviolet rays and ultrasonics. Before disinfection, water usually undergoes purification, during which a significant portion of microorganisms are removed, as well as the ova of helminths.

Chemical methods. In order to achieve a stable disinfectant effect using chemical methods, it is necessary to determine the proper dosage of the reagent and to ensure sufficient duration of the reagent’s contact with the water. The dosage of the reagent is determined by trial disinfection or by mathematical estimation. A factor is included in the calculation to account for a surplus of chlorine or ozone; this guarantees the destruction of microorganisms that may enter the water some time after the disinfection process is completed.

The use of ozone as a disinfectant is based on ozone’s ability to decompose in water, with the resultant formation of atomic oxygen. Oxygen destroys the enzyme systems of microbial cells and oxidizes certain compounds that impart an unpleasant odor to water, for example, humic bases. The concentration of ozone necessary for disinfection depends on the degree of contamination of the water and lies in the range of 1–6 mg/l with a dosage administration time of 8–15 min; the quantity of surplus ozone must not exceed 0.3–0.5 mg/l, since a higher dosage imparts a specific odor to the water and causes corrosion of water pipes. From a hygienic standpoint, ozonation is one of the best methods of water disinfection. However, because the procedure involves a large expenditure of electrical energy, the use of complex apparatus, and the need for highly qualified technical supervision, ozonation has found application only in centralized water supplies.

Copper, silver, and other heavy metals can be used for disinfection of drinking water owing to their oligodynamic properties, that is, their ability to exert a bactericidal effect even in small concentrations. Compounds containing bromine and iodine were commonly used in the early 20th century; these substances have greater bactericidal properties than chlorine but also require the use of more complex technology. At present, iodates are used to disinfect drinking water predominantly in sparsely populated areas and in regions with widespread endemic goiter.

Chemicals may also be used for the small-scale disinfection of drinking water. Preparations manufactured for this purpose have high bactericidal effects that ensure disinfection within 30 min after initial contact with the water. These preparations are harmless to humans, can be stored for long periods of time without a loss of activity, and dissolve rapidly without changing the organoleptic properties of water. An added advantage is that these substances do not react with the materials out of which vessels for storing water are made. The most common preparations for small-scale disinfection are organic chloramines and organic iodine-containing compounds. One tablet of halazone (a chloramine) contains 3 mg of active chlorine and disinfects 750 ml of transparent, colorless water within 30 minutes; the same quantity of turbid, colored water requires 2–3 tablets. Halazone is unreliable when used in water that is heavily contaminated with organic products. Other disadvantages are its slow rate of solution and its chlorine aftertaste. Halazone bisulfate tablets (halazone in combination with sodium bisulfate) are more effective due to the increased bactericidal properties of chlorine in a weakly acidic medium. Considerably more effective than halazone are organic iodine-containing compounds in combination with tartaric acid. These compounds dissolve rapidly in water, releasing 3 mg of active iodine, whose mild taste is no longer discernible 30–40 min after initial contact with the water.

Physical methods. The most commonly used physical agent for disinfecting drinking water is ultraviolet radiation. Its bactericidal action is related to the effect of ultraviolet light on cell metabolism—especially on the enzyme systems of the bacterial cell. Ultraviolet rays destroy both endosporous and nonendosporous forms of bacteria and do not change the organoleptic qualities of the water. In order that ultraviolet irradiation be effective, the water must be colorless and transparent. A disadvantage inherent in the irradiation method is that the bactericidal effect does not continue once the source of ultraviolet light is shut off. Because of this short-term effect and because the water must be colorless and transparent, disinfection by ultraviolet rays is used mainly with subterranean waters and underground streams. A combination of ultraviolet light and small doses of chlorine is used for disinfection of open water sources.

Ultrasonics disinfect drinking water by inducing cavitation— the formation of spaces in the cytoplasm of the bacterial cell that represent large fluctuations in intracellular pressure; these fluctuations lead to the rupture of the cell membrane and the subsequent destruction of the cell. The bactericidal effect of ultrasonics is dependent on the frequency of the sonic vibrations.

The most common and reliable physical method of small-scale water disinfection is boiling. In addition to destroying such organisms and biological substances as bacteria, viruses, bacteriophages, and antibiotics—all of which are often contained in open water sources—boiling removes gases found in the water and diminishes the water’s hardness. Boiling has little effect on the taste of water.

The effectiveness of disinfection is tested by measuring the saprophytic microflora content—specifically, the Escherichia coli content—of the water in conduit pipes. The E. coli content is a reliable measure, since all known causative agents of the water-borne infectious diseases that affect humans (cholera, typhoid fever, dysentery) are more sensitive than the E. coli bacilli to the effects of chemical and physical disinfection. Water is considered fit for consumption when it contains no more than three E. coli bacilli per liter. At water supply stations that make use of chlorination or ozonation, the amount of residual chlorine or ozone is checked every hour or half-hour as an indirect indicator of the effectiveness of the disinfection process.