La déchloramination des eaux de piscines par irradiation UV. Étude bibliographique

Theoretical and practical aspects of the dechloramination of swimming pool water by UV irradiation

Laboratoire de Chimie et Microbiologie de l'Eau (UMR CNRS 6008), École Supérieure d'Ingénieurs de Poitiers, Université de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France

Auteurs de correspondance : joseph.delaat@esip.univ-poitiers.fr florence.berne@univ-poitiers.fr

Résumé

Cette étude a eu pour but de faire le point des connaissances sur la déchloramination des eaux de piscines par irradiation UV. L'étude bibliographique donne, pour les composés présents dans les eaux de piscines (chloramines, chlore libre, THM, ions nitrate), les spectres d'absorption UV/visible, les rendements quantiques de photolyse à 254 nm, les doses UV à 254 nm nécessaires pour obtenir un rendement d'élimination de 50 % ainsi que des informations concernant les mécanismes de photolyse. Ces données indiquent en particulier que la photodécomposition des chloramines résulte d'une série de réactions radicalaires encore assez mal connues et nécessite des doses UV relativement élevées. La déchloramination des eaux de piscines par irradiation UV sera aussi accompagnée d'une décomposition du chlore libre.

Sur le plan appliqué, l'étude montre que plusieurs déchloraminateurs UV ont actuellement été agréés en France par la Direction Générale de la Santé. La déchloramination des eaux de piscines par irradiation UV permet une diminution significative de la concentration en chlore combiné mais conduirait à une augmentation importante de la concentration en THM et parfois en trichloramine dans l'eau des bassins. En l'état actuel des connaissances, des analyses et des études complémentaires s'avèrent nécessaires afin de préciser et d'expliquer l'impact de la déchloramination UV sur la qualité des eaux de piscines.

Abstract

Swimming pool water must be continuously treated with chlorine gas or sodium hypochlorite in order to inactivate pathogenic microorganisms and to prevent the spread of waterborne illness. Along with its disinfectant properties, chlorine reacts with contaminants introduced by bathers (urine, sweat, dander...) to give rise to the formation of many disinfection by-products (THMs, haloacetic acids, haloacetonitriles, chloral hydrate...) and of inorganic and organic chloramines which are referred to combined chlorine. Among these chloramines, trichloramine is very volatile and causes significant eye and respiratory irritation in swimmers and pool attendants. In France, the concentrations of combined chlorine in water and of trichloramine in the atmosphere of indoor swimming pools must not exceed 0.6 mg.L^-1 and 0.5 mg.m^-3, respectively. The UV dechloramination process is now used to reduce the concentration of combined chlorine in water but the photochemical reactions involved in this process are not known. In order to better understand the possible photochemical reactions that may occur during UV dechloramination, the rates and mechanisms of photodecomposition of chloramines and of free chlorine as well as of THM and nitrate will be reviewed in the first part of this paper. The second part of the paper shall present some practical aspects of UV dechloramination and the observed effects on the concentration of disinfection by-products.

Chloramines absorb UV-C light. At 253.7 nm, the molar absorption coefficients of NH₂Cl, NHCl₂, NCl₃ and CH₃NCl₂ reported in literature are nearly equal to 371, 118, 470 and 211 M^-1.cm^-1, respectively. A UV dose of 40 mJ.cm^-2 at 253.7 nm should lead to a 3–4% decay of NH₂Cl, NHCl₂, and CH₃NCl₂ and ≈ 10% decay for NCl₃. For a given UV dose, the rate of photolysis of NCl₃ is faster at 222 nm than at 253.7 or at 280 nm. In the presence of dissolved oxygen, the photodecomposition of NH₂Cl can lead to the formation of nitrite (≈ 0.4 mol.mol^-1 of NH₂Cl decomposed).

Free chlorine species at neutral pH also absorb UV radiation at wavelength below 350 nm. At 253.7 nm, the molar absorption coefficients of HOCl and ClO^- are equal to 59 and 66 M^-1.cm^-1, respectively. The rate of photodecomposition and the quantum yields of photolysis of free chlorine depend on the concentration of free chlorine and on the water quality parameters because the primary photo-products of HOCl and ClO^- (hydroxyl radicals and chlorine atoms) can initiate a complex reaction mechanism of decomposition of free chlorine. The formation of hydroxyl radicals also explains that non-reactive organic compounds toward chlorine in the dark can be degraded by chlorine under UV irradiation.

UV irradiation may also decompose brominated trihalomethanes in swimming pool water but significant decompositions should require high UV doses (i.e. a 50% decomposition of CHBr₂Cl for a UV dose of 4550 mJ.cm^-2 at 253.7 nm.). Nitrate ion which absorbs predominantly in the 200–240 nm wavelength range can lead to the formation of nitrite when medium pressure lamps are used. The formation of nitrite can be reduced if quartz sleeves with a cut-off of wavelengths lower than 240 nm are used. During UV dechloramination of swimming pool water, a very fast oxidation of nitrite into nitrate by residual free chlorine is also expected.

UV-dechloramination of swimming pool waters is widely used worldwide. The UV-dechloraminator is inserted in the circulating water circuit between the sand filter and the injection point of disinfectant. A complete UV-dechloramination equipment includes a radiation chamber with one or several UV lamps inserted in quartz tubes, a UV sensor in order to control UV intensity, an automatic wiper for the removal of deposits on the tubes and the UV sensor, and a microprocessor in order to control and to optimize the UV treatment. In France, several UV-dechloraminators (equipped with low pressure or medium pressure UV lamps) have already been certified by the Direction Générale de la Santé. The UV-dechloramination process leads to a 60–80% decrease of the combined chlorine concentration. However, UV dechloramination was found to increase the concentration of trihalomethanes (up to 390%) and sometimes of NCl₃. Additional studies are needed in order to confirm and to better understand the effects of UV irradiation on the concentration of disinfection by-products during the UV dechloramination of swimming pool water.