Assessment of the fate of manganese in oxide-coated filtration systems

"This study examined the fate of manganese in manganese oxide (MnO_x(s)) coated filter media. Specific objectives of the project included the following:

1. Determination of the effect of influent pH upon Mn(II) sorption and oxidation and upon the physical characteristics of the coating on the media.

2. Determination of the effect of backwash rate upon MnO_x(s) coatings.

3. Examination of the effect of air scour upon MnO_x(s) coatings.

4. Observation of the effect of an increasing MnO_x(s) coating on the physical characteristics of anthracite coal filter media.

5. Development of an overall mass balance on manganese loading and accumulation on the filter media.

Resolution of the stated objectives involved construction, optimization, and continuous operation of a pilot-scale filtration system for the purpose of removing manganese from filter-applied water. The pilot-scale filter system functioned like a typical water treatment plant filtration system with similar hydraulic loading rates, influent manganese concentrations, free chlorine dosage, filter media bed depths, filter run times, and backwash rates.

With regard to the fate of manganese in MnO_x(s)-coated filter media, it was determined that as long as free chlorine was present to oxidize sorbed manganese, manganese continued to accumulate and remained on the media in sufficient concentrations to promote continual removal of soluble Mn(II). While oxide coating that was susceptible to breakage was removed in backwash, some portion of coating remained on the media. The combination of MnO_x(s) accumulation during filtration and its partial removal during backwash maintained a net amount of MnO_x(s) coating optimal for catalyzing further manganese removal and, yet, did not hinder filtration for turbidity by significantly altering the size of the media.

The results of the pilot-scale study also indicated the following pertinent conclusions:

1. Neutral or slightly acidic pH conditions (7 ≥ pH ≥ 6) inhibited Mn(11) oxidation before filtration and, instead, promoted sorption and oxidation of Mn(II) on MnO_x(s)-coated media. Alkaline filter influent pH (pH > 7) allowed some Mn(II) oxidation before filtration, resulting in significant manganese removal by MnO_x(s) particle filtration.

2. Although the intent of MnO_x(s)-coating on the filter media was to remove influent Mn(II) from filter-applied water, MnO_x(s) that was removed by particle filtration also provided MnO_x(s) surface area within the filter and, thus, additional sorption sites for Mn(II) removal.

3. Increases in fluid backwashing rate tended to produce greater amounts of MnO_x(s) release from filter media for the duration of these backwash operations. However, backwashing did not result in complete MnO_x(s) release from the media surface; rather, there was always sufficient Mn0O_x(s) retained to permit efficient soluble Mn(II) removal after the filtration operations were restarted.

Removal of soluble Mn(II) by sorption and oxidation proved to be a dependable, low-maintenance Mn(II) removal technique that worked well within a wide range of raw water influent conditions. Because the process is cost-effective and easily integrated into new or existing water treatment facilities, it is an economical and competitive alternative for removal of soluble Mn(II)."