Bacteria associated with particles may be sheltered from chlorine and ultraviolet (UV) disinfection. The objective of this study was to compare the disinfection effectiveness of UV irradiation and free chlorine for naturally occurring particle-associated coliform (PAC) and non-particle associated coliform (NPAC) in wastewater using a single wastewater source, under identical laboratory protocols. Samples containing NPAC were obtained using three different methods: EGTA extraction, filtration, and blending followed by filtration. Unaltered wastewater was used for samples containing PAC. Wastewater samples were inactivated with UV irradiation at doses between 0-100 mJ/cm2, and with chlorine at CT values between 0-525 mg min/L. The dose-survival relationships and inactivation rates between PAC, NPAC, and chlorine and UV irradiation were then compared. As expected, both UV and chlorine were effective for inactivation of NPAC. However, under prolonged contact times, chlorine appears to be more effective for inactivation of PAC than UV irradiation. Contact time appears to be the most important factor in determining the effectiveness of chlorine disinfection of PAC, and chlorine CT alone was not a good indicator of disinfection effectiveness in wastewater. PAC was found to survive at UV and chlorine disinfection doses typically applied in a wastewater treatment plant.

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