UV disinfection of chlorinated water: impact on chlorine concentration and UV dose delivery

Prechlorination of water upstream of an ultraviolet (UV) disinfection process may possibly influence the effectiveness of UV inactivation through the absorbance of UV radiation. Alternatively, UV may act to destroy chlorine, reducing the residual in water. Monochromatic UV light was more effective in degrading monochloramine whereas polychromatic UV light was more effective in degrading free chlorine. The extent of degradation varied with the type of chlorine species, water quality and type of UV irradiation (mono- or polychromatic). However, decay of chlorine and monochloramine at typical UV disinfection doses (<100 mJ/cm²) was negligible. The transmission of UV irradiation was affected slightly by the presence of chlorine and monochloramine, and these effects on inactivation of MS2 coliphage were modeled for LP− and MP-UV reactors. The presence of 1 mg/L free chlorine or monochloramine decreased the inactivation of MS2 coliphage by less than 0.1 log for a LP-UV reactor. For the MP-UV reactor, MS2 inactivation decreased by between 0.1 and 0.3 log.