Residual Disinfection and pH Control In swimming pool treatment, disinfection or sanitization is essential to rid the pool of bacteria and control nuisance organisms like algae which may occur in the pool, ¡ltration equipment, and piping. There are a number of available disinfectant compounds available, including chlorine, bromine and ozone dosing systems, of which chlorine is the most common. Chlorine Chlorine is a strong oxidizing agent that destroys organic pollutants and bacteria. Part of the chlorine combines with compounds containing nitrogen forming chloramines, while the rest remains active, continuing it’s disinfecting action. Combined chlorine is the quantity of chlorine that has already combined with nitrogen containing compounds. It is much less eective as a disinfectant than free chlorine. The addition of combined chlorine, and free chlorine gives total chlorine. A pool manager needs to aim for the perfect balance where free and total chlorine are proportionally equal, and thus to keep the combined chlorine levels near zero. The presence of chloramines is undesirable because of the distinctive ‘swimming pool smell’ as well as irritation to the eyes and mucous membranes caused by combined chlorines like dichloramines. Commercial chlorine for disinfection may be available as a gas (Cl2), a liquid like sodium hypochlorite or bleach (NaOCl) or in a solid state like calcium hypochlorite, chlorohydantoins or chlorocyanuric acid compounds. These compounds, once dissolved in water, establish equilibrium between the hypochlorous acid (HOCl) and the hypochlorite ions (OCl¯). Although both forms are considered free chlorine, it is the hypochlorous acid that provides the strongest disinfecting and oxidizing characteristic of chlorine solutions. The amount of hypochlorous acid in chlorinated water depends upon the pH value of the solution. Changes in pH value will eect the HOCl equilibrium in relation to the hydrogen and hypochlorite ions. As depicted by the graph, HOCl decreases and OCl¯ increases as pH increases. At a low pH, almost all the free chlorine is in the molecular form HOCl, and at a pH of around 7.5, the ratio between HOCl and OCl¯ is 50:50. Since the ionic form OCl¯ is a slow acting sanitizer while the molecular HOCl is fast acting, it is important to measure pH regularly. As a general rule a pH of about 7.2 is recommended to maintain fast acting disinfection conditions. Bromine In many countries bromine sanitizing has been introduced as an alternative for chlorine, although it is not as strong. The advantage of bromine lies in its stability at higher temperatures (advantageous for heated pools and hot tubs), and its maintained disinfection power at a higher pH. Furthermore, there is very little reaction between bromine and nitrogen compounds reducing the unpleasant odor, and eye irritation problems. The main disadvantage of bromine is the slower acting disinfecting power, making it less suitable for larger pools. The Signi¡cance of Pool and Spa Water Testing 10 Photometers 10.55 www.hannainst.com | benchtop
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