It is well‐recognized that changes in ultraviolet (UV) absorbance or significant concentrations of amorphous biological wastewater solids in a water matrix can affect UV disinfection performance. However, the effects of low concentrations of drinking water particulates (turbidity) and algae on UV disinfection effectiveness are more poorly understood. This article presents the results of continuing studies at the University of New Hampshire (Durham) on the effects of particles on UV disinfection. The research was conducted in two phases of medium‐pressure UV collimated beam tests. The first phase compared the dose–response of seeded MS2 bacteriophage in waters with montmorillonite clay turbidity up to 12 ntu and algal content up to 42 μg/L as chlorophyll a (~42,000 cells/mL) to the dose–response of seeded MS2 in deionized water. The second phase compared phase 1 results with the inactivation of MS2 in the presence of naturally occurring turbidity in reservoir waters. Results indicated UV light was effective for inactivating MS2 in the presence of montmorillonite clay, algae, and naturally occurring turbidity in collimated‐beam testing.
