White Papers

The San Francisco Public Utility Commission (SFPUC) is an innovative leader in potable water distribution system research and operation. The SFPUC continually strives to improve water quality by conducting research and implementing operational improvements to surpass increasingly stringent regulatory standards. In 2002, the SFPUC planned a 2004 transition to chloramines as a secondary disinfectant to reduce disinfection byproduct concentrations. The SFPUC anticipated nitrification problems when chloramine decayed, releasing free ammonia to the water. High ammonia concentrations occur in unmixed dead zones due to natural chloramine decay, and in high disinfectant-demand areas such as biofilm-coated surfaces. Ammonia is reduced to toxic nitrite and then nitrate by autotrophic bacteria. Nitrate increases bacterial growth, furthering residual loss, free ammonia production and nitrification. This positive feedback loop may result in unsafe levels of bacteria and nitrites, and necessitate removal of the reservoir from service or breakpoint chlorination to kill the bacteria and oxidize all nitrogen species to gas.

The SFPUC selected their second largest enclosed reservoir, Sunset Reservoir South Basin, as a test site for evaluating methods to thoroughly mix the water. Circulating water throughout the reservoir was targeted as a solution to long detention times and dead zones that result from stratification, short-circuiting and physical impediments. A series of studies assessed the ability of submersible pump systems (Boozarpour et al., 2007) and a SolarBee hydraulic circulation unit (Water Quality Bureau, SFPUC, 2004) to mix the water. Tests 1 and 2, described in the 2004 report, and additional analyses indicated that previous inlet/outlet modifications were unlikely to prevent stratification during warm seasons. Test 3 assessed the ability of a single SolarBee unit to mix, destratify and disperse chlorine in Sunset Reservoir. This report describes Test 3 methods and results.

Download the SolarBee summary white paper (PDF)

Download the San Francisco Public Utility Commission full report (PDF)

Download the San Francisco Public Utility Commission appendix (PDF)