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Passaic Valley Water Commission has instituted a major upgrade of its Little Falls Water Treatment plant that incorporates advanced treatment concepts to achieve high quality finished water originating from a surface water source. The upgrade was necessary in order to achieve compliance with the Stage 1 Disinfectant/Disinfection Byproduct Rule and Interim Enhanced Surface Water Treatment Rule. The Little Falls Water Treatment Plant is located in northeastern New Jersey and has a design capacity of 110 MGD. The original conventional treatment process with aluminum-based coagulation followed by primary and secondary Cl2-disinfection and dual-media anthracite/sand or granular activated carbon (GAC)/sand filtration is being replaced with high-rate sand-ballasted pretreatment using iron-based coagulation, intermediate ozonation and dual media GAC/sand biological filtration. Thus the process chemistries are significantly different from the conventional route and are being implemented in a multi-phased approach. The first phase of the upgrade was initiated in February 2003. In the spring of 2003, numerous discolored water customer complaints seemed to correlate with a rise in finished water manganese levels. Although manganese is present in the source waters, the increase in finished water total manganese from 0.01 to 0.04 ppm during this upgrade is attributed to the use of ferric chloride as the primary coagulant, elimination of prechlorination and the replacement of existing anthracite/sand filters with GAC/sand media and the associated changes in plant conditions. Manganese is a known impurity in ferric chloride, where the impurity concentration is in the range of 600 to 700 mg/L for a 40% solution. After this discolored water event, a systematic study for the evaluation and control of manganese was undertaken. The study design was to monitor both insoluble and soluble manganese in the source waters, through individual unit treatment processes and in the distribution system. In addition, since the upgrade of the LFWTP consisted of phasing in the new unit treatment processes in stages while in production, provisions for effective manganese control strategies for the intermediate phases of the upgrade was undertaken. Mass balance analysis indicated the major sources of manganese in the treatment process to be those discussed above. The contribution from recycle streams consisting of filter backwash water and residuals thickening process decant, among others, is noteworthy. Process changes that affect pH, detention times and chemicals used impact manganese oxidation due to different oxidation states and the corresponding soluble/insoluble forms of manganese that can result from oxidation. This paper explores the phased process changes of the upgrade as it relates to manganese oxidation and subsequent removal of the oxide. Includes 5 references, figures.