The Los Angeles Department of Water and Power (LADWP) is evaluating enhancedcoagulation (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 directfiltration. The average arsenic concentration in the Los Angeles Aqueduct (LAA) is 25 µg/L. Achemical addition system at Cottonwood, 180 miles upstream of LAAFP, is being used toremove the arsenic peaks. This system may cease operation in the future so that all aspects oftreatment can be centralized and managed at LAAFP.Water from the LAA is typically blended with water from the State Water Project, West BranchAqueduct (SWP-W), just upstream of LAAFP. A future water source will be water from the SWP EastBranch Aqueduct. LAA water contains high arsenic levels, low TOC and bromideconcentrations, whereas East or West Branch water contains low arsenic levels, but higher TOCand bromide concentrations. East Branch water can also have higher, unsettled turbiditycompared to West Branch water at its proposed interconnection to the LAA. With the increasedDBP precursor material from SWP water use, and with unmitigated LAA arsenic concentrationswithout 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 ECat 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 waterblend, coagulant type/dose, polymer, pre- and post disinfection, and sedimentation. A range offerric chloride dosages were tested in order to reduce ambient and spiked arsenic levels. Theresults from the bench-scale testing program formed the basis for selecting the process trainconfigurations for pilot-scale testing. The second phase wasa four-month pilot-scale testing program developed to evaluate the compatibility of ECwith the existing deep bed mono-media filters at LAAFP, to determine the maximum practicalferric chloride dosages in direct and in conventional filtration modes, to determine the quality ofthe filter effluent water in terms of key water quality characteristics, and to evaluate the sludgeand filter backwash water quality characteristics. The third phase was a one-month demonstration-scale testing program conducted to verify micro-sandenhanced settling (Actiflo®) technology for enhanced coagulation.This paper focuses on the findings of pilot- and demonstration-scale testing of EC strategiesevaluated at LAAFP. It discusses the required ferric chloride dosages to meet the treatmentgoals at LAAFP, the impact of raw water blend on treatment performance, and the lessonslearned in clarification and filtration processes. Includes tables, figures.
