Effects of UV-based treatment on water and air chemistry in chlorinated indoor pools

Mehrnaz Zare Afifi, Purdue University

Abstract

Although swimming is known to be beneficial in terms of cardiovascular health, as well as some forms of rehabilitation, swimming is also known to present risks to human health, largely in the form of exposure to microbial pathogens and disinfection byproducts (DBPs). Chlorination is the most common method of disinfection for swimming pools. The two primary goals of applying chlorine in water treatment are inactivation of microbial pathogens and oxidation of soluble organic compounds. However, important drawbacks of swimming pool chlorination include limited efficacy against some microbial pathogens, particularly Cryptosporidium parvum and Giardia lamblia, and the formation of potentially harmful DBPs. Acute and chronic human health problems associated with exposure to DBPs in swimming pools have been identified. In recent years, combined UV/chlorine treatment has been applied with increasing frequency to swimming pool water because UV irradiation is known to be effective for inactivation of some chlorine-resistant microbial pathogens (e.g., C. parvum and G. lamblia), and it has the potential to promote photodecay of some disinfection by-products (DBPs) (e.g., inorganic chloramines). However, it has also been reported in the literature that UV-based treatment has the potential to promote formation of some DBPs (e.g., CNCl and CNCHCl2). Promotion of these and other reactions also results in increases of free chlorine consumption rates. Because some residual chlorine compounds and DBPs are volatile, their presence in pools implies a potential to influence air quality in the vicinity of pools, especially for indoor pool facilities. Hence, UV irradiation has the potential to alter water and air chemistry in indoor pools. To date, observations of the effects of UV-based treatment on water chemistry in the presence of residual chlorine and DBP precursors in swimming pools have been largely anecdotal and incomplete. In addition, there are uncertainties about the UV source type (i.e., low-pressure [LP] or medium-pressure [MP] Hg lamps) to use as a secondary swimming pool water treatment process. This study was conducted to further examine this issue by exploring water and air chemistry in a chlorinated indoor swimming pool under three different operating conditions: conventional chlorination (1st year) which served as a control, chlorination augmented by MP UV irradiation (2 nd year), and chlorination augmented by LP UV irradiation (3 rd year). The UV systems were installed in an existing recirculation system as a secondary disinfectant. Water samples were collected from the pool five days per week during the academic year and once per week during summer for measurement of pH, temperature, total alkalinity, free and combined chlorine, eleven volatile DBPs, and urea concentration. In addition, for observation of air quality in the target indoor pool, measurements of gas-phase NCl 3 concentration and corrosion rates of four metal alloys were conducted. The aim of this study was to examine formation and degradation of volatile DBPs, so as to improve our understanding of the changes in water and air chemistry that will result from UV irradiation of chlorinated swimming pool water. After installation of MP UV, the concentrations of dichloroacetonitrile (DCAN), and dibromochloromethane (CHBr2Cl) increased, whereas the concentrations of dichloromethylamine (CH3NCl2), monochloramine (NH2Cl), dichloramine (NHCl2), chloroform (CHCl 3), bromoform (CHBr3) and cyanogen bromide (CNBr) decreased. LP UV irradiation also decreased CHCl3 (more efficiently than MP UV) and CNBr concentrations; while CHBr3, CHBr2Cl concentrations were increased compare to the control year. The cyanogen chloride (CNCl) and trichloramine (NCl3) concentrations observed in the first year and after inclusion of both UV sources (MP and LP UV) of the study were not statistically significantly different. In addition, changes in the concentrations of DCAN, CH3NCl2, NH2Cl, and NHCl2 were statistically insignificant after inclusion of LP UV, as compared to the control year. A large fraction of the existing literature regarding swimming pool air quality has focused on trichloramine (NCl3). For this work, gas-phase NCl3 was analyzed by an air sparging-DPD/KI method. The results showed that gas-phase NCl3 concentration depends on the bather loading and liquid-phase NCl3 concentration. However, an apparent “negative” concentration of gas-phase NCl3 was observed after inclusion of UV sources as a secondary water treatment, which cannot be interpreted literally, and appears to be attributable to a source of interference in the assay. Thus, this study indicates a limitation of this analytical method for measuring trichloramine in the gas-phase by applying combined UV/ chlorine water treatment in indoor pools. Among four different corrosion coupons (carbon steel, galvanized, aluminum, and 304-stainless steel) which were installed in different locations of the target swimming pool, carbon steel had the highest corrosion rate. After installation of UV sources, the corrosion rates of these metal alloys changed, as compared to the control year; particularly after inclusion of LP UV as a secondary water treatment, the corrosion rate for all metal alloys increased significantly, which again confirmed that UV irradiation would alter water and air chemistry in indoor pools. Urea is the dominant organic-N compound in human urine and sweat, and is known to be one of the important precursors for producing NCl3 in swimming pools. Results of daily measurements of urea indicated a link between bather load and urea concentration in the pool. Urea concentration was unaffected by UV-based treatment. Collectively, these results imply an overall improvement in water quality as a result of the inclusion of the both UV systems; and MP UV was more efficient than LP UV for reducing most of the eleven volatile DBPs concentration in this pool. However, a need exists to standardize the application of UV systems in recreational water settings. The results of this study are also expected to benefit UV/chlorine applications in other water treatment settings, such as treatment of drinking water, wastewater, or in water reuse facilities.

Degree

Ph.D.

Advisors

Blatchley, Purdue University.

Subject Area

Civil engineering|Environmental engineering

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