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CONSTRUCTED WETLANDS FOR THE REMEDIATION OF BAUXITE RESIDUE DISPOSAL AREA LEACHATE AND RUN-OFF

Courtney, R. Curtin, T. Higgins, D. Fleming, L. Clune, L.

Bauxite residue from the Bayer process contains residual caustic soda with alkaline leachate of pH > 10.5. Bauxite Residue Disposal Area (BRDA) closure and aftercare plans must address the timeframes required for leachate to reduce to ≤ pH 9.0 so it can be discharged to the receiving environment. A novel approach to ensure that BRDA leachate can be passively treated and made suitable for discharge within a short period (months) of BRDA closure is to incorporate constructed wetland(s) into the Closure Plan. Constructed wetlands are gaining global acceptance by regulators in mine closure. However, little research has been conducted into the using wetlands to treat alkaline leachate.

Aughinish Alumina has received funding from its owner Rusal and the International Aluminium Association to research the use of a constructed wetland to treat BRDA leachate. The 44 m2 plasticlined reed bed is located within the Aughinish BRDA and contains three species of reeds typically found in wetlands. An automatic leachate dilution and dosing system feeds the wetland inlet at the target pH and flow-rate. Within the wetland, microbial activity generates CO2 which carbonates the water thereby reducing the pH. Alumina and some trace metals in the leachate precipitate within the wetland. A discharge pH of < 9.0 has been achieved and most metal impurities are reduced simultaneously. This paper reports the wetland performance over the first 18 months of operation in terms of water chemistry and precipitate, examines mechanisms that drive remediation and discusses implications for larger scale use.