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.
Authors: Wayne Lem, P.Eng., Calgon Carbon
Corporation, Pittsburgh, Pennsylvania, USA.
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