Destruction of disinfection byproducts and their precursors in swimming pool water by combined UV treatment and ozonation

Both UV treatment and ozonation are used to reduce different types of disinfection byproducts (DBP) in swimming pools. UV treatment is most common as it is particularly efficient in removing the repulsive chlorine like smelling chloramines (combined chlorine). UV treatment of a pool water increased chlorine reactivity and formation of chlor-organic DBP such as trihalomethanes. Based on the similar selective reactivity of ozone and chlorine we hypothesized that the created reactivity towards chlorine by UV treatment of dissolved organic matter in pool water might also be expressed as an increased reactivity towards ozone and that ozonation might saturate the chlorine reactivity created by UV treatment and mitigate the increased DBP formation. By experimentally treating pool water samples, we found that UV treatment makes pool water highly reactive to ozone. The created reactivity towards chlorine decreases dose dependently with ozone dosage prior to contact with chlorine. Furthermore, the kinetics of ozone in UV treated pool water changed drastically from a half-life in excess of 20 min to complete consumption in less than 2 min. We discovered that ozonation in UV treated pool water induced formation of some DBPs that are not commonly reported in pool water where trichloronitromethane is noteworthy as it is genotoxic. Therefore, we performed repeated treatment experiments consisting of combined UV/ozone treatment interchanged with chlorination for 24 h the genotoxicity created during the ozone treatment was removed by the UV treatments. Based on the experimental results, a treatment system is proposed with a UV system placed in a side stream to the recirculation flow followed by an ozone dosing point and a small reaction chamber with a volume that allow 1-2 min reaction time before returning to the main recirculation flow. This side-stream UV/ozone treatment will improve the swimming pool water quality.