AWWA WQTC62549

The Los Angeles Department of Water and Power (LADWP) is evaluating enhanced coagulation (EC) as a technology to address several treatment goals including arsenic, turbidity, and disinfection byproduct (DBP) precursor reduction at the 600-MGD Los Angeles Aqueduct Filtration Plant (LAAFP). Currently, the plant process consists of preozonation followed by high-rate direct filtration. The average arsenic concentration in the Los Angeles Aqueduct (LAA) is 25 µg/L. A chemical addition system at Cottonwood, 180 miles upstream of LAAFP, is being used to remove the arsenic peaks. This system may cease operation in the future so that all aspects of treatment can be centralized and managed at LAAFP. Water from the LAA is typically blended with water from the State Water Project, West Branch Aqueduct (SWP-W), just upstream of LAAFP. A future water source will be water from the SWP East Branch Aqueduct. LAA water contains high arsenic levels, low TOC and bromide concentrations, whereas East or West Branch water contains low arsenic levels, but higher TOC and bromide concentrations. East Branch water can also have higher, unsettled turbidity compared to West Branch water at its proposed interconnection to the LAA. With the increased DBP precursor material from SWP water use, and with unmitigated LAA arsenic concentrations without Cottonwood operation, the treatment enhancements at LAAFP will need to be robust. The team of Metcalf & Eddy / Carollo Engineers was retained to evaluate the integration of EC at the LAAFP. A three-phase approach was undertaken as part of the project's first task: The first phase was an eight-week bench-scale testing program developed to evaluate the impact of raw water blend, coagulant type/dose, polymer, pre- and post disinfection, and sedimentation. A range of ferric chloride dosages were tested in order to reduce ambient and spiked arsenic levels. The results from the bench-scale testing program formed the basis for selecting the process train configurations for pilot-scale testing. The second phase was a four-month pilot-scale testing program developed to evaluate the compatibility of EC with the existing deep bed mono-media filters at LAAFP, to determine the maximum practical ferric chloride dosages in direct and in conventional filtration modes, to determine the quality of the filter effluent water in terms of key water quality characteristics, and to evaluate the sludge and filter backwash water quality characteristics. The third phase was a one-month demonstration-scale testing program conducted to verify micro-sand enhanced settling (Actiflo®) technology for enhanced coagulation. This paper focuses on the findings of pilot- and demonstration-scale testing of EC strategies evaluated at LAAFP. It discusses the required ferric chloride dosages to meet the treatment goals at LAAFP, the impact of raw water blend on treatment performance, and the lessons learned in clarification and filtration processes. Includes tables, figures.