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SFPUC June 10, 2020 Page 6 <br /> i <br /> yr � <br /> S A <br /> r�5• 1 <br /> Figure 2.Typical liquid mercury bead in a new UV lamp <br /> Some portion(depending on the temperature) of Hg0 is vaporized when the lamp is energized. <br /> Occasionally, a UV lamp will break for unknown reasons—this is an area of intensive <br /> investigation. <br /> Water is flowing through a UV reactor at 3-5 ft/s, so it only takes a few seconds for the water <br /> passing a broken lamp to reach the downstream valve. Smaller particles from the vapor phase <br /> mercury may be transported farther or be more readily dissolved in water than liquid HgO. <br /> SFPUC assumes that little mercury is transported downstream with the water pipeline,based on <br /> the following. <br /> • Mercury's highly hydrophobic and dense character would cause it to settle in the pipeline <br /> and attach indefinitely to the pipe wall or in crevices. However, visual monitoring of the <br /> main 10.5-ft diameter pipeline downstream of the UV building(downstream of where the <br /> 12 reactor pipelines merge back into one pipeline) in 2019 did not identify any <br /> accumulated elemental mercury. <br /> • Samples about 25 miles downstream in the Alameda East Portal following four typical <br /> UV lamp breaks in 2018 (on 6/10/18, 6/15/18, 7/2/18 and 7/17/18) nine times over the <br /> two-hour window when water from the UV treatment system should have arrived were <br /> consistently non-detected at 0.2 ug/L(one-tenth the drinking water standard) for all <br /> samples. <br /> Nonetheless, the data at Alameda are uncertain because hydraulic calculations (Taylor, 1954; <br /> Metcalf& Eddy, 199 1) indicate that if mercury were completely entrained in the water and <br /> transported downstream, it would pass by the Alameda facility within 7-15 hours over a duration <br />