Performance of three bench-scale diffusive exchange systems during treatment of acid mine drainage with high copper concentration

An alternative passive biochemical treatment has been developed for acid mine drainage (AMD), called the Sulphidogenic Diffusive Exchange System (SDES). It differs from conventional designs in that, it includes vertical drainage tubes for preferential transport of the AMD through the reactor bed. Thus, the microorganisms in the reactor bed is never exposed directly to the AMD, but only indirectly in diffusive transport or exchange processes which allow generalised chemical gradients to be established which protect the communities. An additional advantage of the new configuration is the possibility of recovering valuable minerals from the bottom of the tubes if they form there and are then sedimented out. We assessed the performance of three prototype SDES at bench scale to treat first water with sulphate and then AMD with high copper concentration (528 mg/L), at two flow rates (Q1 = 3.75 L/d and Q2 = 7.5 L/d). The prototypes differed in the volume occupied by the drainage tubes and their diameter (SDES1: 10%, φ = 5.5 cm; SDES2: 20%, φ = 5.5 cm; SDES3: 10%, φ = 4 cm). When the AMD was fed at the lower flow rate the three prototypes were able to effectively eliminate all the metals (>99%), at a volumetric rate of 0.27 mol/m3/d. The sulphate removal rates also presented no significant variation (between 0.30 mol/m3/d (SDES1 and SDES2) and 0.33 mol/m3/d (SDES3), indicating that diffusive transport does not limit reactor performance. When the flow rate was increased, differences were observed in metals removal. The reactors suffered overloading and only partial metals removal was achieved at rates which varied between 0.2 (SDES1) and 0.33 mol/m3/d (SDES2 and SDES3). Precipitates with a high copper concentration were recovered at the base of the drainage tubes.