This study investigated the impact of coagulated/flocculated particulate matter on low-pressure (LP) UV inactivation of spiked Escherichia coli under drinking water conditions using a standard bench scale collimated beam apparatus. The effect of floc particles in both coagulated river water and coagulated process (treatment plant) water was assessed. Laboratory grade water and uncoagulated river water were used as controls. The dose-response curves of spiked E. coli were determined at UV doses of 5, 10, 15, 25, and 40 mJ/cm2. The results indicated that surface water particles (turbidity from 12 to 32 NTU essentially have no influence on UV inactivation of spiked E. coli if they are appropriately accounted for in the UV transmittance determination. However, the presence of floc particles formed by coagulation and flocculation led to significantly lower inactivation of E. coli by LP UV. A limited investigation of medium pressure UV inactivation undertaken as part of the study indicated that shielding effects for medium pressure UV warrant further investigation. The support for the inactivation results provided by the particle size analysis carried out suggests that criteria related to particle size may be very useful in addition to turbidity for UV regulation and operation. The results emphasize the need for UV disinfection/inactivation to be placed following filtration, because a sedimentation upset (i.e. where some floc particles may reach a downstream UV unit) could potentially compromise disinfection/inactivation.

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