Manganese is typically found in all water supplies in the United States. Manganese concentrations are usually higher in water obtained from groundwater sources or resei:voir hypolinutlons. This is because manganese is more soluble in the reducing conditions normally found in these waters. Although manganese is not known to cause any health related problems, the secondary drinking water MCL for manganese is 0.05 mg/L. This standard was set to eliminate aesthetic problems associated with manganese bearing waters. In this study continuous-flow filters were operated in both pre-oxidative (oxidized Mn applied to filters) and auto-oxidative (soluble Mn applied) modes. The oxidants used were dllorine (HOCl/OCl⁻) , potassium permanganate (KMnO₄), chlorine dioxide (ClO₂), and ozone (O₃). Other experimental parameters included: filter media type - manganese coated or non-coated, filter loading rate --2 to 5 gpm/f², operating pH -- pH 6 to pH 9, and temperature --5 to 25℃. The most important experimental parameter was whether or not the filter media had a prior oxidized coating of manganese. If this was the case the filter produced an effluent concentration of manganese below the MCL under all pre-oxidative conditions and under auto-oxidative conditions when the pH was above neutral. Increased flow rate through the filter caused deeper penetration of manganese into the filter bed. This should not prove to cause an effluent breakthrough problem for filter depths typically used in water treatment plants. Temperature and pH effected the reaction rate of manganese oxidation in both the pre- and auto-oxidative modes. In most cases th.is did not effect the effluent quality from manganese coated filter media. However, when non-coated media was used and no oxidant was added, a decrease in pH or temperature usually adversely effected effluent quality. / M.S.