AWWA ACE61672

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 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 shave off some of the arsenic peaks. This system may cease operation in the future so that all aspects of treatment can be centralized and managed at LAAFP. 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. Phase one included 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 was 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 involved a four-month pilot-scale testing program 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 involved a one-month demonstration-scale testing program conducted to confirm the pilot-scale results using a commercially available high-rate Actiflo® clarification process. This paper focuses on the findings of bench- and pilot-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 at the bench and pilot scale. Includes tables, figures.