Scale-up of a Medium Pressure UV System for the Treatment of N-Nitrosodimethylamine (NDMA) and its Advantages Over Low Pressure UV Systems

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Recently there has been considerable concern in California about the detection of N-nitrosodimethylamine (NDMA) in drinking water at levels as high as 900 ppt. NDMA was found to be a carcinogen in animals and assessed as a Class 1 carcinogen. It is currently listed as a priority pollutant on the US EPA National Priorities List. California has set an 'action level' of 20 ppt for NDMA.

NDMA is thermally stable in aqueous solutions. Conventional methods such as biological treatment, air stripping and activated carbon are not effective for NDMA treatment. NDMA is photochemically labile, so advanced oxidation technologies that are based on irradiation with ultraviolet (UV) light have been promoted for the removal of NDMA in contaminated waters. Direct UV photolysis readily destroys the compound and has been used commercially for over 10 years for the treatment of NDMA contaminated groundwater.

In this paper, the selection and scale-up considerations for UV systems designed to destroy NDMA are reviewed. A fundamental part of UV system selection involves an evaluation of medium pressure and low pressure UV systems. The differences between medium pressure and low pressure systems are discussed and system costs are compared.

The water evaluated for this discussion was an NDMA-contaminated drinking water at LaPuente Valley Water District treated through an ISEP® continuous ion exchange module for perchlorate removal. The stream was tested for the use of ultraviolet treatment in October 1998. The UV irradiations were carried out in a semi-batch 1 kW UV Rayox® reactor (Calgon Carbon Corporation). The treatment results from the 1 kW Rayox® reactor were used in Calgon Carbon's proprietary model to confirm the results expected in the full-scale system. The full-scale equipment, installed in December 1999, to treat a flowrate of 2,500 gpm consisted of two Calgon Carbon 12 lamp Rayox® UV Towers utilizing medium pressure UV lamps and a skid mounted peroxide dosing module. Ultimately, the full-scale system performed very closely to predicted results from lab scale efforts. For virtually all large scale treatment applications, medium pressure UV systems are advantageous over low pressure systems when comparing the overall capital, installation, and operating costs for the project. While economics is a big determinant in system selection, other factors such as footprint, number of existing installations, reliability, and ease of maintenance must be included in the selection criteria. Taking into account all these factors, the Rayox® Tower, with its high efficiency medium pressure UV lamps, was able to achieve optimal performance for NDMA destruction with the lowest overall lifecycle costs.

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