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Sequential Manganese Desorption and Sequestration in Anthracite Coal and Silica Sand Filter Media

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 by‐product 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.