Regrowth Potential of Bacteria after Ultraviolet Disinfection in the Absence of Light and Dark Repair

The objective of this study was to investigate the regrowth potential of E. coli and indigenous wastewater bacteria after ultraviolet (UV) disinfection in the absence of light and dark repair and under different nutrient conditions. Light repair was prevented by keeping the samples in the dark following UV exposure (15 and 40mJ/cm2). Dark repair was controlled for by varying the initial and surviving bacterial concentrations, and achieving complete inactivation and no growth in control samples. Net growth and die-off in samples were determined 6, 18, 24, 30, and 48h after UV disinfection. The results of this study showed that the percent regrowth of E. coli and indigenous wastewater bacteria were higher after UV disinfection at 40mJ/cm2 than at 15mJ/cm2, and the regrowth was also sustained for a longer time. Even in the absence of any nutrients, significant regrowth of E. coli was observed in phosphate-buffered saline (PBS) indicating that the lysis of UV-damaged cells may provide a source of nutrients for the surviving bacteria. In spite of the availability of nutrients in wastewater, the presence of inhibiting and toxic substances appeared to slow down the regrowth of E. coli. Indigenous coliform bacteria in wastewater could achieve much higher regrowth compared to laboratory cultured E. coli even after a UV dose of 40mJ/cm2.