Swimming Pool Chemicals Explained

Disinfectants

Many disinfectants are used in pools and spas around the world, including halogen-based compounds (chlorine, bromine, iodine), ozone, and ultraviolet light with hydrogen peroxide. Those used most often are chlorine, bromine, and iodine, and each has advantages and limitations.

Chlorine—Pools can be disinfected with chlorine-releasing compounds, including hypochlorite salt compounds. Calcium hypochlorite is inexpensive and popular for cold-water pools, but not suitable for hot pools and spas because it will promote scaling on heat exchangers and piping. Chlorine levels can be rapidly reduced with high use and regular checks should be made to ensure maintenance of disinfection. Some adjustment of pH is required for most forms of chlorine disinfection. When chlorine gas is used, a fairly high alkalinity needs to be maintained to remove the acid formed during dosing [5]. Sodium hypochlorite is a liquid chlorine, and has a pH of 13, causing a slight increase in the pH of the pool water, which should be adjusted with an acidic mixture. The sun’s rays will degrade sodium hypochlorite. Chlorinated isocyanate is available in three forms—granular, tablet, and stick. The granular form contains 55%–62% available chlorine and the stick and tablet form contain 89% available chlorine.

Bromine—Bromine needs to be used at levels twice those of chlorine to achieve similar disinfection. Bromine is available as the sodium or potassium salts. In the presence of ammonia, bromine rapidly forms relatively unstable ammonia bromamines that possess disinfection efficiencies comparable to that of free bromine. It is also unnecessary to destroy ammonia bromamines because they do not produce irritating odors.

Iodine—Potassium iodide is a white, crystal chemical. This chemical needs an oxidizer, such as hypochlorite, to react with organic debris and bacteria. Iodine does not react with ammonia, hair, or bathing suits, or cause eye irritation, but it can react with metals, producing greenish-colored pool water [6].

Ozone—Ozone is a very powerful oxidant and is effective against viruses. It can only be generated at the point of use and commercial generation units are safe for use. Ozone dosing is only practical where there is water circulating off-pool because adequate ozone-water mixing is essential for maximum oxidation. Ozone generators may be of the ultraviolet lamp or corona discharge type. The ultraviolet lamp efficiency reduces with time and the lamp and associated activated charcoal filter will need replacement.

Ultraviolet Light—Ultraviolet light, like ozone, is sometimes used for off-pool water disinfection. Ultraviolet light has no effect on pH or color and has little effect on the chemical composition of the water. However, color, turbidity, and chemical composition of the water can interfere with ultraviolet light transmission. The water must be adequately treated before ultraviolet light exposure. Hydrogen peroxide is often used for this purpose as it is relatively safe in low concentrations, is nonflammable, and produces oxygen and water as end products. For the ultraviolet light plus hydrogen peroxide system to be effective, it must operate 24 hours a day. Ultraviolet light disinfection is not pH dependent, but the addition of hydrogen peroxide results in slightly acidic conditions.

Silver-copper Ionization—Sanitizing can be accomplished by using an ionizing unit that introduces silver and copper ions into the water by electrolysis, or by passing an electric current through a silver and copper electrode. The limiting factors in using this system in the pool and spa are cost, slow bactericidal action, and potentially high contaminant levels caused by bather loads. Also, black spots can form on pool surfaces if the proper parameters of water chemistry are not maintained. An approved chemical disinfectant must be used with an ionizing unit.

The effective use of halogen disinfectants is based on the pH, hardness, and alkalinity of the water. Improper pH, hardness, and alkalinity levels in the pool can render high levels of disinfectant useless in killing disease-causing organisms. Table 14.1 summarizes water-quality problems that affect pools and suggests corrective actions.

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