A Large-Scale Uv Pilot-Plant Study: Tertiary Effluent Disinfection and Effect on Ndma and Cyanide

Abstract

The Sanitation Districts of Los Angeles County (Sanitation Districts) are currently facing new regulatory issues related to NDMA and cyanide. To prevent NDMA formation from chloramination, the Sanitation Districts are considering the use of ultraviolet (UV) disinfection to replace chloramination. UV disinfection may also result in incidental removal of NDMA. However, a recent study indicated that low levels of cyanide might be formed as a result of UV irradiation. To fully evaluate the effect of UV on tertiary effluent disinfection, NDMA removal, and potential cyanide generation, the Sanitation Districts recently conducted a large-scale pilotplant UV testing. The testing was conducted following the procedures outlined in Ultraviolet Disinfection Guidelines for Drinking Water and Wastewater Reclamation (NWRI Guidelines). A UV transmittance (UVT) range of 63% to 79% was tested. Flow rates ranged from 1.5 to 3.5 million gallons per day (MGD). The pilot plant was run with varying number of UV banks and lamp power setting. Influent and effluent NDMA, total cyanide, total coliform, and MS-2 coliphage concentrations were measured. This paper presents the results of this study and discusses several technical issues related to UV testing and full-scale design.The study indicated that inert media filtration had little effect on the fate of NDMA, while chloramination significantly increased the NDMA concentration. Both collimated beam and pilot testing results showed a linear relationship existed between log-reduction of NDMA and UV dose. At a UV disinfection dose of 100 mJ/cm 2 , the removal of NDMA in the filtered effluent by the pilot-plant was approximately 40%. UV irradiation of filtered secondary effluent did not result in cyanide formation for doses up to 500 mJ/cm 2 in collimated beam testing, and for delivered UV doses up to approximately 140 mJ/cm 2 in pilot testing. At a delivered UV dose of 100 mJ/cm 2 derived from MS-2 testing, the total coliform concentration in the UV disinfected effluent meets the California Title 22 Water Recycling Criteria for unrestricted reuse. Several technical issues relevant to UV testing and design, such as single-bank vs. multiple-bank testing and adequacy of using collimated beam testing to derive delivered dose, were identified and discussed.

Formation, Detection and Removal of NDMA in Water Treatment Process

“…The major products of the reaction are DMA and nitrite, while minor products include nitrate, formaldehyde, formate, and perhaps methylamine. Because UV photolysis may not destroy NDMA precursors [12], some have suggested that reformation of NDMA may occur within drinking water distribution systems if chlorination is performed after the UV treatment [13] And UV treatmen for NDMA will be feasible but more expensive [14 ].…”