Field Effectiveness of Ultraviolet Disinfection in Onsite Wastewater Treatment

Thesis (Master's)--University of Washington, 2018Ultraviolet disinfection (UVD) units provide advanced onsite wastewater treatment, which is essential in regions where soils are unsuitable for conventional onsite sewage systems (OSSs). Although UVD treatment systems provide high-quality effluent under testing conditions, field effectiveness of UVD units is poorly understood. This study aimed to determine the field performance of UVD units installed in Western Washington, their ability to provide high-quality wastewater effluent, and risk factors for UVD unit malfunction. The hypothesis, based on a previous report from Kitsap County, was that 25-50% of UVD units would have malfunctioning UV bulbs and that malfunctioning units would have higher fecal coliform effluent levels than those performing optimally. This study used a mixed-methods approach with original data collection and complimentary analysis of retrospective quarterly compliance samples. 97 UVD units were inspected for indicators of unit performance, and wastewater samples from 53 units were collected and analyzed for physical wastewater quality, fecal coliform concentrations, and, in 21 units, presence of male-specific (F+) coliphage. Data from inspection and sampling reports provided additional insights into provided maintenance and retrospective UVD unit performance. The study results indicated that properly functioning and well-maintained UVD units reduce fecal coliform concentrations to low levels, but that 25% of the UVD units in the study area have malfunctioning UV bulbs that are inhibiting disinfection (95% CI: 18-34%). The geometric mean (GM) fecal coliform concentrations of field samples collected after functioning UVD units (GM=18 ± 4 CFU/100 mL) was well-below the lowest Washington State Treatment Level standard (200 CFU/100 mL). Based on a multivariate Tobit regression model of retrospective compliance data, the GM fecal coliform concentration in UVD units with deficient bulbs was 122% higher than in UVD units with bulbs that were not deficient, after adjusting for other OSS deficiencies (95% CI: 36-428%, p<0.001) and the odds of effluent exceeding fecal coliform standards were 7.5 times greater among UVD units with deficient bulbs than among UVD units with glowing bulbs, after adjusting for other OSS deficiencies (95% CI: 4.0-13.9, p