The main method for passive removal of manganese is the "Pyrolusite" system, in which a bed of limestone is inoculated with Mn-oxidizing bacteria. Nine sites of this type, plus four limestone-lined channels, have been studied. Other information is incorporated from published work. Effective Mn removal requires oxidizing well-aerated water, as well as prior removal of essentially all dissolved Fe and Al, and pH above about 6.5. Most of the Pyrolusite systems removed Mn from influent values of 6

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... o 30 mg/L to effluent levels between 0.5 to 1.5 mg/L for a period of 2 years or more. Based on the depth distribution of dissolved O 2 and Mn oxide precipitate, most Mn removal occurs in the top 0.3 m of the bed, just below the water surface. The deeper parts of beds do not appear to contribute significantly. Most Mn removal rates range from 1.5 to 5 g/m 2 /day, with the lower values from beds with influents containing appreciable Fe and Al. Several of the systems have failed because of plugging of the inlet area with silt, leaves, Fe and/or Al precipitate, grass and other materials. Several field tests and experiments suggest that special bacterial inoculation may not be necessary. Three successful limestone-lined channels have been observed, one with an Mn removal rate of about 10 g/m 2 /day. A shallow bed or channel, lined with limestone, and containing algae to enhance O 2 , appears to be an improved design.