Inactivation of indigenous coliform bacteria in unfiltered surface water by ultraviolet light

This study examined the potential for naturally occurring particles to protect indigenous coliform from ultraviolet (UV) disinfection in four surface waters. Tailing in the UV dose–response curve of the bacteria was observed in 3 of the 4 water samples after 1.3–2.6-log of log-linear inactivation, implying particle-related protection. The impact of particles was confirmed by comparing coliform UV inactivation data for parallel filtered (11 μm pore-size nylon filters) and unfiltered surface water. In samples from the Grand River (UVT: 65%/cm; 5.4 nephelometric turbidity units (NTU)) and the Rideau Canal (UVT: 60%/cm; 0.84 NTU), a limit of 2.5 log inactivation was achieved in the unfiltered samples for a UV dose of 20 mJ/cm2 while both the filtered samples exhibited >3.4-log inactivation of indigenous coliform bacteria. The results suggest that particles as small as 11 μm, naturally found in surface water with low turbidity (<3 NTU), are able to harbor indigenous coliform bacteria and offer protection from low-pressure UV light.