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PERFORMANCE TESTING OF A UV DISINFECTION AND OXIDATION SYSTEM TO ACHIEVE REGULATORY APPROVAL FOR WASTEWATER REINJECTION

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Abstract:

Orange County Water District (OCWD) detected N-nitrosodimethylamine (NDMA) at levels exceeding 100ppt in the reclaimed water produced by their Water Factory 21 facility [treated by lime treatment, recarbonation, and filtration followed by either granular activated carbon (GAC) or reverse osmosis (RO)]. Using a medium pressure-lamp based UV system and hydrogen peroxide, this carcinogenic contaminant was subsequently treated successfully in the Water Factory 21 facility beginning in 2000. In 2004, portions of the Water Factory 21 facility were demolished and replaced with a new Interim Water Purification Facility (IWPF).

The new Interim Water Purification Facility sends five million gallons per day (MGD) of purified water to the county's seawater intrusion barrier. This water is purified using a multiple barrier water purification system that includes microfiltration, reverse osmosis and ultraviolet light with hydrogen peroxide. Whereas NDMA can be degraded by UV photolysis alone, the combination of UV and hydrogen peroxide, which is an advanced oxidation process (AOP), provides an additional barrier together with the upstream membrane processes to other chemical contaminants that may be present in the water supply. The facility uses the same technologies as the planned Groundwater Replenishment (GWR) System, which is a larger advanced water purification system that is scheduled to be operational in 2007, producing 70 million gallons of water each day. When completed, the GWR System will be the largest indirect potable reuse project of its kind in the world. The IWPF and the GWR System will take purified wastewater from the neighboring Orange County Sanitation District (OCSD) plant and purify it to surpass all state and federal drinking water standards. The wastewater is currently discharged into the ocean. Water produced by the IWPF will mimic water produced by the permanent GWR System when it is complete, and therefore allows the IWPF to operate as a validating pilot for the GWR System.

The new UV system at the IWPF is low-pressure high-output (LPHO) amalgam lamp-based system that is more energy efficient than the medium-pressure system previously installed at the Water Factory 21 facility. The new Demonstration Ultraviolet Disinfection and Oxidation System consists of one complete train of the nine trains that will comprise the larger, permanent GWR System. This single train is comprised of three chambers connected in series. Each chamber comprises two reactors while each reactor houses 72 LPHO amalgam lamps. The lamps have a turndown capability from 100% to 60% of their nominal input power.

Operation of the IWPF is required to maintain water supply to the seawater intrusion barrier. A prerequisite to this operation is an operating permit from the California Department of Health Services for the IWPF process. The approach to the performance validation testing of the IWPF UV System was to conduct an intensive series of performance tests to demonstrate both NDMA destruction and MS-2 bacteriophage inactivation in order to obtain this operating permit.

The GWR System design specifications and the 2003 NWRI/AWWARF UV Guidelines require validation of > 50 mJ/cm2 delivered MS-2 germicidal dose, > 4 log inactivation of MS-2 and >1.2 log reduction of NDMA. The disinfection capability of the UV System is determined by measuring the log inactivation of MS-2 bacteriophage seeded into the influent stream upstream of the UV System. Because of the much greater sensitivity of MS-2 to UV exposure compared with NDMA, it was expected that all effluent MS-2 bioassay results would have non-detect values. Therefore, if the influent MS-2 level were 105 pfu/mL, one could conclude that > 4-log inactivation had been achieved. This log reduction corresponds to a reduction-equivalent-dose (RED) of greater than 80 mJ/cm2, which meets the design criteria of > 50 mJ/cm2. In a similar fashion as the disinfection performance demonstration, the ability of the UV System to destroy NDMA was determined by measuring the log reduction of NDMA concentration after passing through the UV System. The variables studied include flow rate, hydrogen peroxide concentration, and the number of reactors powered on. The results from this demonstration study are being used to provide a confident prediction that the permanent GWR UV System will meet the target disinfection and oxidation criteria at a flow rate of 70 MGD.

The Demonstration UV Disinfection and Oxidation System installed in the IWPF at the OCWD has demonstrated the capability of meeting the GWR UV System design specifications regarding the reduction of NDMA, specifically achieving 1.2 log reduction of NDMA, in 5 MGD while utilizing only 4 of the 6 reactors installed. The NDMA results demonstrate that each of the six UV reactors performs equally. As expected, the reduction of NDMA is relatively independent of the influent concentration of hydrogen peroxide over the range from 0 to 5-ppm.

While the range of the influent water UV transmittance (UVT) was small during the testing (i.e., 93.6% to 95.3%), a direct correlation was observed between treatment efficiency and UVT. Furthermore, the chloramine present in the influent was responsible for most of the UV absorbance measured at 254nm. Chloramines come from chlorine added to the feed of the upstream microfiltration process for biofouling control. Therefore, a reduction of the chloramine dose could result in increased efficiency of the UV system.

The trials in which MS-2 bacteriophage was seeded into the UV System influent stream clearly demonstrated a minimum log reduction of 4.4 log and a minimum reduction equivalent dose (RED) of 115 mJ/cm2. These results were achieved with one of the six reactors in operation at the maximum available flow rate of 5 MGD. The NDMA results indicate that the performance of each individual reactor is additive and, therefore, it is reasonable to conclude that the same additive result is true for virus inactivation. Thus, if the 4.4-log inactivation were demonstrated with one reactor on, then this would translate to 17.6-log inactivation with 4 reactors on.

These performance levels for disinfection and NDMA destruction were achieved while consuming less than one-third the energy of the original medium-pressure UV system at Water Factory 21. This performance efficiency is sufficient for the permanent UV system to meet the design specifications for the permanent GWR System.

In the autumn of 2004 the California Regional Water Quality Control Board, Santa Ana Region, approved OCWD's permit for water from the Interim Water Purification Facility to be used for the seawater intrusion barrier. At about the same time, the California Department of Health Services granted a conditional certification for the UV system as an appropriate technology for disinfection and to provide an additional barrier to unwanted contaminants.

Document Type: Research Article

DOI: http://dx.doi.org/10.2175/193864705783858972

Publication date: January 1, 2005

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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