AWWA JAW62924

The Metropolitan Water District of Southern California carried out a study to evaluate manganese (Mn) desorption from filter media during biological filtration startup. Mn contamination of the filter media was a byproduct of prior use of ferric chloride (FeCl₃), which contains Mn, for coagulation and free chlorine exposure. During pilot- and full-scale testing at pH 6.5, FeCl₃ coagulation (without chlorine) released soluble Mn at nearly four times the level released by aluminum sulfate (alum; 105 µg/L for FeCl₃ and 27 µg/L for alum). Pilot-scale experiments demonstrated that Mn was present in the filter column in three valence states [Mn(II), Mn(III), and Mn(IV)]. Mn(II) in the filter influent was from the FeCl₃. Mn on the anthracite media was hypothesized to be Mn(II)/Mn(IV) and first physically displaced by ferric iron, aluminum, and hydrogen, and then catalytically oxidized to Mn(IV) by silica sand. Includes 39 references, tables, figures.