UV Systems for Reclaimed Water Disinfection – From Equipment Validation to Operation

The Sanitation Districts of Los Angeles County (Districts) operate seven tertiary water reclamation plants (WRPs) with a combined treatment capacity over 200 million gallons per day (MGD). Although chloramination has provided reliable and effective disinfection, the Districts decided to change this disinfection practice at one of the WRPs to ultraviolet (UV) irradiation to minimize N-nitrosodimethylamine (NDMA) formation.During the process of designing the UV disinfection system, the Districts had an opportunity to participate in an equipment validation study that was conducted by a third party consultant. A pilot scale UV system equipped with low-pressure high-output UV lamps was validated according to Ultraviolet Disinfection Guidelines for Drinking Water and Wastewater Reclamation, and the equipment validation report was approved by the California Department of Health Services (DHS). Following the equipment validation testing, the Districts conducted additional UV disinfection studies, using the same pilot system, at two WRPs in 2004 and 2005. The objectives of these studies were to verify the UV dose regression model developed from equipment validation testing, and to determine if the UV dose regression model developed at one WRP could be used for design of a full-scale UV disinfection system at another WRP with similar treatment processes and water quality.Results showed that the delivered UV doses from the Districts' tests were different from those calculated using the dose regression model. Factors that may attribute to this apparent discrepancy include differences in collimated beam testing procedures, water quality, data analysis procedures, and assumptions made to calculate doses of multiple-bank systems. The findings from this study have implications on UV system design and operation which rely on the dose regression model. It is recommended that the factors discussed in this study be considered in UV equipment validation and full-scale UV system design and operation to ensure that the full-scale UV systems function as designed and provide adequate safety factor for protection of human health.