Synergistic effect of combined UV-LED and chlorine treatment on Bacillus subtilis spore inactivation.

An ultraviolet (UV)-based advanced oxidation process (AOP) for disinfecting water is introduced in this paper. This study aimed to evaluate the potential of UV light-emitting diodes (UV-LEDs)/chlorine AOP (UV/Cl2) for Bacillus subtilis spore inactivation. Chlorine was combined with 265 and 280 nm LEDs (UV265/Cl2, UV280/Cl2) and investigated. The addition of 4.0 mg/L of free chlorine at pH 7.0 in the presence of 125 mJ/cm2 of UV irradiation resulted in an additional 1.8-log reduction in UV265/Cl2 and 1.5-log reduction in UV280/Cl2. There was no observed enhancement in spore inactivation with the addition of a radical scavenger, t-BuOH, which indicated the role of •OH in the synergistic effect. To quantitatively evaluate the synergism, the primary treatment with UV/Cl2 was followed by further UV or Cl2 treatment. After UV/Cl2 pretreatment at different pH levels, the 265 and 280 nm LEDs treatment enhanced an approximate 0.4-0.5-log reduction compared to UV only, and Cl2 treatment enhanced an approximate 0.7-1.1-log reduction compared to Cl2 only. In addition, at pH 7.0, in UV265/Cl2-Cl2 and UV280/Cl2-Cl2, the inactivation rate constant k increased by approximately 2 and 1.5 times, respectively. The CT for the lag phases (CTlag) reduced to approximately 67 and 58%, respectively. Similar results were obtained at pH 7.5 and 8.0, and in the secondary effluent. The synergistic effect on spore inactivation suggested that the pathogen inactivation efficiency of sequential UV and chlorine disinfection processes, which are commonly applied, can be significantly enhanced by adding chlorine prior to UV treatment.