Removal of urea from swimming pool water by UV/VUV: The roles of additives, mechanisms, influencing factors, and reaction products

To discover an applicable technology for urea abatement from swimming pool water (SPW), this study compared the performances of seven ultraviolet (UV)-based technologies on urea removal, including UV alone, UV coupled with hydrogen peroxide (UV/H2O2), sulfite (UV/Na2SO3), potassium persulfate (UV/K2S2O8), a combination of UV and vacuum UV (UV/VUV), and UV/VUV in tandem with either H2O2 (VUV/H2O2) or potassium persulfate (VUV/K2S2O8). Among them, UV and UV/Na2SO3 showed little removal ability, and UV/H2O2 removed only 12.8% of urea within 3-h experiments, while UV/VUV degraded 71.7% of urea without introducing substantial total dissolved solids (TDS). Therefore, UV/VUV was considered as a promising technology for further exploration. In comparison, although UV/K2S2O8 exhibited higher urea removal than UV/VUV, it caused dramatic increases of TDS, which made the regulatory threshold for the TDS increment difficult to maintain. Within UV/VUV studies, some common components in SPW (e.g., cyanuric acid, humic acid, nitrate, and bicarbonate) inhibited the removal process, whereas chloride and sulfate facilitated it, while free chlorine at doses ≤ 3 mg-Cl2/L and pH levels from 6.8 to 8.0 imposed little impact on urea degradation. Overall, UV/VUV degraded 40.0% and 22.2% of urea from tap water and SPW, respectively; both were lower than the efficiency observed in ultrapure water. As for reaction byproducts, urea phototransformation via UV/VUV yielded nitrate and ammonia as the key products with the mass balance of nitrogen element being met. However, the contents of organic carbon decreased at a rate slightly lower than urea degradation, suggesting that urea was mostly mineralized and slightly converted to unknown organic compounds. The results hence demonstrate that UV/VUV is an effective alternative for urea removal from SPW. [Display omitted] •Seven (V)UV-based processes were compared to control urea.•VUV exhibited higher urea removal yet lower TDS increment than others.•Direct VUV irradiation and indirect oxidation via HO· and SO4·- degraded urea.•SO42− and Cl− enhanced yet organic matter inhibited removal of urea.•The key pathway for urea photolysis was mineralization, leading to NH4+ & NO3−.