A demonstration study was undertaken to develop an abiotic iron oxidation process to increase the Fe(II) removal rates (IRR) at low pH (6 to 7) compared to conventional oxidation ponds and aerobic wetlands (typical IRR is 10 to 20 g/m 2 /d) while producing a high-quality sludge. An alkaline mine drainage from an underground mine pool containing 60 to 80 mg/L Fe(II) was used in the study. Batch tests were conducted in a 330 gal tank at various initial concentrations ranging from 5 to 1300 mg/l

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... ded Fe(III) solids. Results indicated test durations to obtain less than 1 mg/L of Fe(II) decreased from greater than 48 hours for low initial Fe(III) to less than 2 hours in tests with initial Fe(III) greater than 1000 mg/L. Following batch tests a flow-through reactor system was employed consisting of two-330 gal tanks, a complete-mix oxidation reactor followed by a clarifier tank. Fe(III) solids were recirculated from the clarifier to the reactor to obtain reactor concentrations ranging from 10 to 2000 mg/L. The reactor lowered Fe(II) to approximately 3 mg/L when Fe(III) was 2000 mg/L and contact time was 2.3 hours. IRR of 0.52 mg/min or 740 g/m 2 /d were achieved during this flow-through test. IRR exceeding 1 mg/min or 1300 g/m 2 /d were obtained when Fe(III) was 1800 mg/l and contact time was 1.3 hours, but with a slightly higher effluent of 14 mg/l Fe(II). The recirculated Fe(III) sludge had a specific resistance to filtration (SRF) of 410 11 m/kg, a coefficient of compressibility of 0.37 and solids concentrations greater than 20%. The SRF is similar to that observed for flocculent solids formed at high pH (>8), but the compressibility is similar to high-density sludges. This research demonstrated the effectiveness of a recirculated sludge process to increase IRR over passive treatment and to obtain similar IRR as conventional chemical (lime) treatment.