Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Conventional chlorination disinfection suffers from pathogen reactivation and the formation of toxic disinfection by-products (DBPs). Ultraviolet (UV) irradiation alone provides physical inactivation but lacks residual effect. The UV/Cl2 process synergistically combines UV photolysis of chlorine to generate reactive radicals (e.g., HO• and Cl•), offering multiple inactivation mechanisms. However, the operational parameter of UV intensity has been inconsistently optimized, with prior studies focusing on UV dose rather than intensity, leading to suboptimal reactor design and energy inefficiency.
This investigation systematically varies UV intensity (0.25–2.0 mW·cm−2) at a fixed UV dose to decouple intensity effects from total energy input. By quantifying chlorine photolysis kinetics, radical steady-state concentrations, E. coli inactivation, DBP formation, and bacterial reactivation, the study identifies 1.0 mW·cm−2 as the optimal intensity that maximizes disinfection while minimizing DBP risk. These findings provide a mechanistic basis for optimizing UV/Cl2 reactors in water treatment, addressing the critical bottleneck of balancing disinfection efficacy with by-product control.
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ZHANG Huaicheng, QIN Yiwen, ZHOU Hao, ZUO Jinhu, LIU Haolin, ZUO Yanting, HUANG Shouqiang, CHENG Shi, LI Wentao (2026). Influence of UV Intensity on Escherichia coli Inactivation Efficiency in the UV/Cl2 Process. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025102002
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Frequently Asked Questions
What is the mechanistic explanation for the non-monotonic inactivation trend with increasing UV intensity?
Higher UV intensity accelerates chlorine photolysis, increasing radical production and initial inactivation rates. However, it also depletes chlorine more rapidly, reducing the cumulative CT value (chlorine exposure) to 14.3%–55.7% of baseline. Since chlorine provides sustained disinfection, the overall inactivation efficiency initially rises then falls, with an optimum at 1.0 mW·cm−2.
How does the presence of natural organic matter (NOM) affect the UV/Cl2 process, and why is 1.0 mW·cm−2 more robust?
NOM scavenges radicals and consumes chlorine, reducing inactivation efficiency by up to 2.7-log. At 1.0 mW·cm−2, the process exhibits the highest resistance to interference, likely due to an optimal balance between radical generation and chlorine persistence, minimizing scavenging impacts.
What is the impact of UV intensity on disinfection by-product (DBP) formation, and how does this influence operational decisions?
Increasing UV intensity from 0.25 to 2.0 mW·cm−2 reduces total organic halogen (TOX) formation from 33.7 to 19.0 μg·L−1. This reduction is attributed to faster chlorine photolysis and radical oxidation of DBP precursors. Thus, higher intensity can mitigate DBP risk, but must be balanced against reduced CT and potential energy costs.
What are the energy and cost implications of operating at the optimal UV intensity of 1.0 mW·cm−2 compared to higher intensities?
At a fixed UV dose of 150 mJ·cm−2, operating at 1.0 mW·cm−2 requires longer irradiation time than at 2.0 mW·cm−2, but achieves superior disinfection (6.5-log vs. lower) and lower reactivation (0.07%). Energy consumption scales with dose, not intensity, so total UV energy is constant; however, higher intensity may reduce reactor footprint but compromise performance. Cost-benefit analysis favors 1.0 mW·cm−2 for optimal microbial safety and DBP control.
How does the UV/Cl2 process perform in real water matrices containing bicarbonate and chloride ions?
Bicarbonate (HCO3−) and chloride (Cl−) inhibit inactivation by scavenging radicals. At 1.0 mW·cm−2, the process shows the greatest resistance to these interferences, maintaining higher inactivation compared to other intensities. This robustness is critical for treating sand-filtered water with varying inorganic content.
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