Clark County Water Reclamation District (CCWRD) serves the suburban area in and around the city of Las Vegas. The Las Vegas Valley is served by three treatment plants, which all discharge to the Las Vegas Wash. The wash empties into the Las Vegas Bay of Lake Mead. Lake Mead is also
the source of drinking water for the Las Vegas Valley. In 1996 USGS reported symptoms of estrogen exposure in carp captured in the Las Vegas Bay of Lake Mead. Since this initial report, additional studies of Lake Mead have shown that trace concentrations of pharmaceuticals, steroids, and personal
care products do reach the lake via the Las Vegas Wash. Ozone has been shown to be an effective means of oxidizing trace organic contaminants while providing exemplary disinfection. The Southern Nevada Water Authority (SNWA) employs ozone disinfection at 900 MGD capacity to serve the drinking
water needs of the Las Vegas valley. CCWRD partnered with SNWA to investigate the applicability of ozone to filtered secondary effluent for both disinfection and contaminant oxidation. Both static and flow-through bench scale ozone systems were used to determine the ozone demand and dose using
the filtered secondary effluent. The continuous flow system was used to evaluate disinfection and contaminant oxidation at three ozone doses ranging from 3-8 mg/L. Experiments were performed during both summer and winter conditions to determine the effect of seasonal water quality changes.
A separate engineering evaluation was conducted to compare the capital and O&M costs of ozone to its present UV disinfection system. Trace pharmaceuticals, steroids, and personal care products were measured using liquid chromatography with tandem mass spectrometric detection with reporting
limits ranging from 1–10 ng/L. Estrogenicity was measured using an in vitro bioassay using human breast cancer cells, which are extremely sensitive to both endogenous estrogens and estrogenic chemicals. Significant estrogenicity was observed in the filtered secondary effluent;
however, even the lowest ozone doses evaluated were capable of removing estrogenicity to less than detection. Likewise, several pharmaceuticals were also detectable, and most were well removed by ozone. Some chemicals were more resistant to ozone oxidation, such as the pharmaceutical meprobamate
and the chloro-phosphate flame-retardant tris-chloroethylphosphate (TCEP). Ozone was found to be effective for disinfection even at the lowest ozone doses evaluated. The engineering cost estimates for ozone for disinfection was estimated to be about the same as the present UV system. CCWRD
will make a decision on which disinfection method to use in the summer of 2006.
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