Influence of UV Intensity on Escherichia coli Inactivation Efficiency in the UV/Cl2 Process
The ultraviolet/chlorine (UV/Cl2) advanced oxidation process generates multiple radical species, enabling synergistic disinfection. However, the systematic influence of UV intensity on process performance remains inadequately characterized. This study investigated UV intensities from 0.25 to 2.0 mW·cm−2, assessing chlorine photolysis kinetics, bacterial inactivation, and disinfection by-product (DBP) formation. Results demonstrate that inactivation efficiency is not solely governed by total UV energy but is co-regulated by reaction kinetics and mass transfer. Increasing UV intensity accelerated chlorine photolysis by 33.7%–277.8%, elevating steady-state concentrations of hydroxyl radicals and chlorine radicals by factors of 1.6–3.8 and 1.3–3.2, respectively, thereby enhancing initial inactivation rates. However, higher intensities reduced cumulative chlorine exposure (CT value) to 14.3%–55.7% of baseline, causing overall inactivation to first increase then decrease. At a fixed UV dose of 150 mJ·cm−2, an intensity of 1.0 mW·cm−2 achieved optimal 6.5-log inactivation of Escherichia coli and the lowest bacterial reactivation rate (0.07%). Common water constituents (HCO3−, Cl−, natural organic matter) inhibited disinfection, with natural organic matter exerting the strongest suppression (2.7-log reduction). Notably, 1.0 mW·cm−2 exhibited the greatest resistance to interference. Elevated intensity reduced total organic halogen formation from 33.7 μg·L−1 to 19.0 μg·L−1. Balancing disinfection efficacy and DBP risk, 1.0 mW·cm−2 is identified as the optimal UV intensity for the UV/Cl2 process in sand-filtered water treatment.