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SFPUC June 10, 2020 Page 10 • Use high-quality UV lamps: High-quality design and manufacturing material is purchased and installed. Protecting Potable Supplies The Tesla Facility has a slow velocity/in-line baffle/quiescent zone downstream of the UV reactors on the main outlet pipe on the 120" effluent line to help mitigate the risk from a potential mercury release and settle out glass fragments and solid-state mercury. The zone has a blowoff for removal of settled materials, although the it is located on the side—rather than on the bottom—of the pipe. Modifying the Closure Time of the UV Isolation Valves As observed through trend data on SCADA, it takes 5-10 minutes to complete full closure of the upstream and downstream isolation valves in response to a burst UV reactor. To decrease the total water discharged, staff plans to modify the Calgon UV programmable logic controllers from initially fully opening the pipe outlet valve to immediately begin closing when a break is detected. Sealing Existing Floor Drains on the Existing Trench Floor Contract drawings from the construction of the Tesla Facility indicate existing floor drains along the trench floor. A sheet note on Contract DB-116, drawing E-19934 states the floor drains/sanitary drain is for washdown water during normal operations, whereas the 42" emergency overflow culvert(downstream of the trench) is for routine draining. The four drains in the trench are plugged to prevent contaminated water from inadvertently leaving the designated containment area and pathway. Discharging to Grit Pond Discharged water flows by gravity from the trench, past the sump, and into Grit Pond#1 through a culvert below the pavement. Pumping and storing discharges in rented baker tanks, and off-site disposal of that water as a waste for off-site disposal, is not warranted. The baker tanks can be drained into Grit Pond#1, and future discharged can flow directly into the pond through the existing culvert. Reducing Exposure Facility staff follow Mercury Spill Response and Clean-up Standard Operating Procedures (SOP, Attachment A) so that operators may limit exposure to mercury if a UV lamp break should occur while the water treatment system is on-line or off-line. In particular: • The UV Building is well ventilated until elemental mercury vapor concentrations are measured by the Jerome mercury vapor analyzer to be less than the Tier 1 ESL of 0.31 ug/m3 for mercury in indoor air or within 25% of the outdoor ambient air concentration. • Condensed mercury and quartz fragments could collect in the bottom of a shut-down reactor. Staff will attempt to install a U-shaped mercury trap on the reactor outlet piping and attempt to capture any lamp glass and Hg prior to discharge of the drainage water into the trench drain.