Impact of wavelength, exposure sequence, and organic matter on UV disinfection and DNA repair

The effects of natural organic matter (NOM) and wavelength sequence on the disinfection of MS2 and E. coli K-12 with multi-wavelength UV irradiation was investigated in this research. Disinfection kinetics were quantified from water samples prepared in phosphate buffered saline (PBS) with and without Suwanee River NOM. Samples were irradiated using a standardized UV collimated beam protocol. Combinations of UV wavelengths were achieved with UV-C (222, 265 nm) and UV-A (365 nm). Endpoint kinetics at 24 h were determined for irradiated E. coli K-12 samples subjected to photorepair and dark repair conditions. Disinfection and regrowth of E. coli was quantified using culture-based assays. DNA damage in E. coli was quantified using cyclobutane pyrimidine dimer (CPD) enzyme linked immunosorbent assay (ELISA). NOM improved disinfection of MS2 by 365 nm, but not for 222 nm or 265 nm, while NOM decreased disinfection of E. coli for 222, 265, and 365 nm. Sequential UV-C wavelength disinfection for MS2 and E. coli were unaffected by NOM. UV-A pre-treatment in samples with NOM decreased disinfection for both microorganisms. However, under the same disinfection conditions, CPD ELISA results showed less DNA repair in E. coli NOM samples compared with PBS. After disinfection by 16 mJ/ cm² using 222 or 265 nm with or without UV-A pretreatment, culturable E. coli concentrations decreased during light repair incubations even though DNA repair increased as photorepair fluence increased. Multiple wavelength combinations of KrCl, LP, and UV LED 265 offered competitive alternatives to published electrical energy requirements per 1-log inactivation of MS2 for medium pressure UV. This study demonstrates treatment conditions for which bacterial regrowth may be suppressed after UV disinfection for post-treatment environments with little or no residual, limited nutrient availability, and conditions supporting regrowth.