AWWA WQTC69441

This paper summarizes a five-month pilot study conducted with the San Francisco Public Utilities Commission (SFPUC) Water Quality Division undertaken to confirm what was expected to be a low fouling potential of Hetch Hetchy water, given the low level of organic and inorganic constituents, and to evaluate potential operations and maintenance issues associated with the full-scale ultraviolet (UV) disinfection facility that SFPUC was concurrently planning. The scope and testing plan was not developed as a quantitative and comprehensive evaluation of the mechanisms of UV fouling. Testing was conducted at SFPUC's Tesla Portal Disinfection Facility, which serves as the primary disinfection station for the unfiltered Hetch Hetchy supply. The pilot units were supplied with unfiltered water upstream of any disinfection, but the pH had been raised to approximately 9.2 on average at an upstream lime addition facility. The project utilized two custom-modified UV fouling reactors with nominal chamber diameter of 8 inches. One was equipped with a single 3.6 KW, medium pressure (MP) UV lamp and a second equipped with four 150W low pressure high output (LPHO), and four 350W low pressure high output amalgam (LPHO-Amalgam) lamps. Due to space and water supply constraints, the reactors were connected in series, with water flowing through the LPHO/LPHO-Amalgam reactor and then into the MP reactor. The study was composed of five testing phases. Phase 1 was conducted from July 13 to August 6, 2007 to evaluate baseline fouling during summer season. Phase 2 was conducted from August 13 to September 11, 2007 to evaluate potential impacts of MP lamp ballast power output on fouling rate. Phase 3 was conducted from September 11 to October 9, 2007 to evaluate potential impacts of water flow rate on fouling rate. Phase 4 was conducted from October 22 to November 13, 2007 to evaluate potential impacts of pH adjustment (to about 8.5 using CO2) on fouling rate. Phase 5 was conducted from November 14 to December 13, 2007 to evaluate baseline fouling during late fall season. Includes 19 references, tables, figures.