AWWA WQTC57162

Manganese became an issue for the Philadelphia Water Department (PWD) as a result of pilot studies aimed at limiting disinfection byproducts (DBPs). Pilot tests proved that moving the point of chlorination to post-filtration improved DBPs, but also led to elevated levels of manganese in the filter effluent. As a result, further pilot testing focused on exploring a range of chlorine doses, with respect to breakpoint, to determine the minimum filter applied chlorine dose required to maintain manganese control through filtration. Manganese control was defined as filter effluent containing less than 0.015 mg/L (a goal set well below the secondary MCL of 0.05 mg/L) total manganese. Two source waters were tested once 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 same trend was observed at pH 5.8, though overall manganese residuals were higher. Increasing the filtration pH to 7.2 dramatically reduced Mn residuals, and allowed for manganese control at doses as low as 0.5 mg/L below breakpoint. When considering the promulgated and proposed DBP regulations, it is imperative for a utility to consider all of the implications of modifying the disinfection process. If a utility treats water with Mn naturally present, or introduces manganese into the treatment process, removal of chlorine prior to filtration would likely result in potentially unacceptable manganese levels in the treated water. However, if breakpoint is achieved prior to filtration, maintaining relatively low levels of free chlorine across filtration promotes manganese 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 breakpoint breakdown) can establish the delicate balance between DBP formation and manganese control. Includes 7 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2002

Number of Pages:

20

File Size:

1 file , 400 KB