How particles affect UV light in the UV Disinfection of Unfiltered Drinking Water

Abstract

This study assessed the impact of suspended particles on both ultraviolet (UV) dose transmission and spectrophotometric absorbance measurement for unfiltered raw water with turbidity up to 10 ntu. Results from bench‐scale collimated beam experiments indicated that a low percentage of total incident UV light was attenuated by particles at 5 ntu turbidity. However, at a set applied UV dose, when particles were added to achieve turbidity from 1 to 10 ntu, the average dose in the collimated beam reactor decreased from 5 to 33%, respectively. This suggests that spikes in particles entering a UV reactor, if not adjusted and accounted for, may compromise lethal delivery of UV energy. UV absorbance coefficients, measured using direct and integrating sphere spectroscopy (which accounts for scattering effects by particles) were significantly different, suggesting that suspended particles in unfiltered water both absorb and scatter UV light. When absorbance, measured using the direct method, was used to model the average irradiance in a UV reactor, the average UV irradiance was underestimated by 3 to 20% at turbidity of 3 to 10 ntu, respectively, compared with irradiance calculations made using integrating sphere absorbance measurements. This analysis suggests that UV systems designed based on direct absorbance measurements of unfiltered waters with turbidity above 3 ntu may be overly conservative.