Synergistic algicidal effects of combined UV-LED/chlorine treatments on Tetraselmis sp.: Optimization and mode-of-action

[Display omitted] •Disinfection of Tetraselmis sp. using UV-LEDs, chlorine and UV-LED/chlorine.•UV-LED/chlorine promotes algal inactivation compared to individual treatments.•UV-LED/chlorine disinfection is a wavelength-dependent process.•UV285/chlorine showed the best inactivation effect and the lowest energy-efficiency.•Radicals were found to play an important role in the synergy treatment. Unwanted proliferation of and adhesion by microalgae, as involved in fouling and species invasion, are important problems across many maritime operations and coastal industrial activities. The algicidal mechanisms of ultraviolet light-emitting diodes (UV-LEDs) and the synergistic algicidal effects between UV-LEDs and chlorine treatments in seawater were investigated in this study. Tetraselmis sp. was employed as the challenge organism. Five UV wavelengths (265, 280, 285, 300 and 315 nm) and three chlorine dosages (0.6, 0.8 and 1 mg L−1) were examined in a factorial experiment to optimize the disinfection performance achieved by combined UV-LED/chlorine treatments in seawater. Overall, the combined treatments showed a more effective inactivation of the microalgae than did the single treatments. Treatment with UV285 combined with 0.8 mg L−1 chlorine resulted in 3.12-log inactivation at 100 mJ cm−2 (i.e. 2.34 Einsteins L-1, 67 s). This optimal combination showed a higher inactivation rate constant and lower energy consumption (EE,2) than did the other treatments. The contribution of UV-LEDs, chlorine and hydroxyl radical (·OH) in the inactivation of Tetraselmis sp. by the combined UV-LED/chlorine disinfection process was investigated to illustrate the mechanism of this synergistic process, the results indicated that ·OH generated by photolysis were not the only contributors to the inactivation of the microalgae. Although a UV300/chlorine treatment demonstrated the highest chlorine photolysis rate, the inactivation constant of this treatment was lower than that of UV285/chlorine. These results demonstrated a synergistic algicidal effect of the combined disinfection system caused by exposure to the 285 nm UV-LED and low dosages of chlorine, which could provide an important basis for further developments and possibly industrial applications in the future.