Full-scale demonstration testing of hexavalent chromium reduction via stannous chloride application

The best available technologies for hexavalent chromium [Cr(VI)] treatment can be cost prohibitive and operationally complex. Direct stannous [Sn(II)] chloride application to reduce Cr(VI) to trivalent chromium is an alternative treatment approach that could substantially decrease treatment costs. A full-scale demonstration was conducted at the Coachella Valley Water District to investigate the ability of Sn(II) to reduce Cr(VI) in a chlorinated groundwater system. An average Sn(II) dose of 0.5 mg-Sn/L, which corresponds to ~7.3 times the stoichiometric dose ratio for Cr(VI) reduction, was able to maintain Cr(VI) at <10 μg/L throughout the distribution system over a 30-day period. Tin (Sn) and chromium (Cr) concentrations decreased with increasing distance from stannous feed locations, suggesting accumulation in the distribution system. Distribution system pipe-scale analysis after the demonstration found the presence of Sn and Cr in pipe scales; however, minimal Sn and Cr release was observed during main flushing at the end of the demonstration.