Manganese became an issue for the Philadelphia Water Department (PWD) as a result of pilot studiesaimed at limiting disinfection by-products (DBPs). Pilot tests proved that moving the point ofchlorination to post-filtration improved DBPs, but also led to elevated levels of manganese in the filtereffluent. As a result, further pilot testing focused on exploring a range of chlorine doses, with respect tobreakpoint, to determine the minimum filter applied chlorine dose required to maintain manganese controlthrough filtration. Manganese control was defined as filter effluent containing less than 0.015 mg/L (agoal set well below the secondary MCL of 0.05 mg/L) total manganese. Two source waters were testedonce per season across four seasons with varied pH conditions. Results indicated that at pH 6.5,manganese control was not obtained until chlorine was dosed at 0.5 mg/L above breakpoint. The sametrend was observed at pH 5.8, though overall manganese residuals were higher. Increasing the filtrationpH to 7.2 dramatically reduced Mn residuals, and allowed for manganese control at doses as low as 0.5mg/L below breakpoint.When considering the promulgated and proposed DBP regulations, it is imperative for a utility to considerall of the implications of modifying the disinfection process. If a utility treats water with Mn naturallypresent, or introduces manganese into the treatment process, removal of chlorine prior to filtration wouldlikely result in potentially unacceptable manganese levels in the treated water. However, if breakpoint isachieved prior to filtration, maintaining relatively low levels of free chlorine across filtration promotesmanganese removal to very low levels by adsorption onto the media and subsequent contact oxidation.Determining how much of a chlorine residual should be maintained-- performing a breakpointbreakdown-- can establish the delicate balance between DBP formation and manganese control.
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