Influence of UV Intensity on Escherichia coli Inactivation Efficiency in the UV/Cl2 Process

• • Optimal UV intensity of 1.0 mW·cm−2 achieves 6.5-log inactivation of E. coli at 150 mJ·cm−2, balancing disinfection and DBP control; higher intensities reduce CT value to 14.3%–55.7%, diminishing overall efficacy. • • Increasing UV intensity from 0.25 to 2.0 mW·cm−2 accelerates chlorine photolysis by 33.7%–277.8%, elevating hydroxyl and chlorine radical steady-state concentrations by 1.6–3.8 and 1.3–3.2 times, respectively, enhancing initial inactivation kinetics. • • At 1.0 mW·cm−2, bacterial reactivation rate is minimized to 0.07%, ensuring post-treatment microbial stability—critical for distribution systems. • • Higher UV intensity reduces total organic halogen formation from 33.7 to 19.0 μg·L−1, indicating that optimized intensity mitigates DBP-associated health risks while maintaining disinfection performance.