Chemical disinfection preceding UV treatment: An assessment of microbial regrowth in a model distribution system

The goal of this study was to compare disinfection strategies for controlling microbial growth in distribution systems from a blended water source in a warm climate. This research compared the efficacy of chlorine (Cl₂) to monochloramine (NH₂Cl) with and without post-treatment with ultraviolet (UV) light for heterotrophic bacteria control. Two influent streams were pre-treated with either chlorine or monochloramine, and consisted of a blend of groundwater, surface water and desalinated water. Annular reactors (ARs) containing coupons made of PVC material were used to simulate common operating conditions in a distribution system. Two ARs acted as controls and received the chlorinated water or water treated with monochloramine. The remaining two ARs received water that was additionally treated with UV light. The data presented show that treatment with Cl₂ alone was the most effective disinfection strategy against suspended heterotrophic (HPC) bacteria in influent and effluent samples and also against attached HPC bacteria. Chlorine with or without post-UV treatment was more effective than monochloramine at removing suspended and attached HPC bacteria. Levels of free chlorine concentration were reduced following treatment with UV light, which resulted in the increased bacteria counts in the AR. UV treatment also appeared to enable nitrification in the AR treated with NH₂Cl, as ammonia was completely converted to nitrate in the NH₂Cl/UV-treated AR whereas concentrations less than 0.2 mg l⁻¹ of nitrate or nitrite were detectable in the NH₂Cl-treated AR.