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The closed underground fluorspar Burg mine (Rio Tinto France) produces a near-neutral mine drainage (pH 6.3) with high concentrations of iron and manganese (14 mg.L-1 and 12 mg.L-1, respectively). Average flow rate is 27 m3.h-1. A passive water treatment is being developed by BRGM, the French geosurvey to replace the current lime treatment by a more environmentally friendly, economical and lower sludge producing technology. Different set-up and operating conditions were investigated at laboratory scale and are still currently being optimised at pilot-scale. The pilot consists of a 1 m3 settling tank, in which iron (Fe2+) is oxidised and iron hydroxides are settled, followed by an upflow 1 m3 fixed-bed bioreactor filled with a mixture of limestone and pyrolusite (MnO2) and supplied with air, in which manganese precipitates. The fixed-bed was inoculated with sludges coming from two passive mine drainage treatment plants in which some manganese removal occurs. Residence time ranged from 50 h to 20 h. Results are promising: maximum removal rates were 99% for both iron and manganese. Iron and manganese concentrations were decreased under the 1 mg.L-1 standard. Iron removal rate in the settling tank varied from 80 μg.L-1.h-1 to 160 μg.L-1.h-1. Manganese removal rate in the bioreactor ranged from 130 μ.L-1.h-1 to 350 μ.L-1.h-1. Surprisingly, up to 38% of the manganese was removed in the settling tank at low residence time. Residence time and aeration rate are still being optimised and clogging is being assessed. Currently, the pilot has been operating for 6 months and will continue to operate for another 6 months.

Biological manganese removal from mine drainage in a fixed-bed bioreactor at pilot-scale