AWWA WQTC63976

With the recent Federal Register publication of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2), improved protection against resistant pathogens is mandated and focus is shifting onto meeting the requirements and practical constraints associated with solution options. One technology well-suited for meeting LT2 guidelines is disinfection by ultraviolet (UV) irradiation, which has proved effective against a range of microbial pathogens. A concern with UV treatment is the potential for performance reductions associated with fouling of the quartz sleeves that encase UV lamps. As mineral and particulate materials accumulate on sleeve surfaces, the effective lamp sleeve transmittance decreases, reducing the efficacy of the disinfection process. Chemical and mechanical sleeve wiping systems aid in removal of lamp sleeve fouling materials, but there can be uncertainty in determining to what extent these potentially costly systems may be necessary to ensure adequate UV exposure, and how often they should be activated to maintain adequate reactor function. Information is also limited with respect to the fouling effects that could be anticipated if sleeve wiping measures were interrupted. Experiments utilizing a commercial medium pressure (MP) UV disinfection reactor installed at the City of Albany, New York's Loudonville UV Treatment Facility were conducted to assess the effects of an interruption to sleeve wiping. The surface water source used had relatively low concentrations of the dissolved minerals sometimes associated with problematic sleeve fouling. Effects of sleeve wiping interruption were assessed by monitoring the temporal distribution of UV dose, and by post-experimental analysis of lamp sleeves. The UV transmittance of both clean and fouled sleeves was characterized over the germicidal UV wavelength range, and chemical analysis of lamp sleeves was performed to quantify metals content on sleeve surfaces. An analysis of spatial distribution for sleeve transmittance demonstrated that in this perpendicular-flow MP reactor, fouling was relatively uniform along the length of quartz sleeves. Sleeve transmittance at a wavelength of 254 nm decreased from approximately 92 % to 61% (relative to air) after four weeks. Transmittance data were also collected at other wavelengths relevant to reactors utilizing MP lamps. Flow rate and reported dose data were used to estimate reductions in UV irradiance, and over the experiment duration of 4 weeks, the average reported dose decreased by 9.5%. Iron was present at the highest surface concentrations in sleeve foulant, which was notable due to relatively low iron concentrations in the source water. Modeling of mineral speciation and solubility based on source water composition allowed identification of potential contributors to the foulant that was observed. Comparisons to previous groundwater fouling experiments highlighted differences in the potential driving forces that influence fouling rate. Includes 11 references, tables, figures.