High-Rate Sulfate Removal Coupled to Elemental Sulfur Production in Mining Process Waters Based on Membrane-Biofilm Technology

It is anticipated that copper mining output will significantly increase over the next 20 years because of the more intensive use of copper in electricity-related technologies such as for transport and clean power generation, leading to a significant increase in the impacts on water resources if stricter regulations and as a result cleaner mining and processing technologies are not implemented. A key concern of discarded copper production process water is sulfate. In this study we aim to transform sulfate into sulfur in real mining process water. For that, we operate a sequential 2-step membrane biofilm reactor (MBfR) system. We coupled a hydrogenotrophic MBfR (H₂-MBfR) for sulfate reduction to an oxidizing MBfR (O₂-MBfR) for oxidation of sulfide to elemental sulfur. A key process improvement of the H₂-MBfR was online pH control, which led to stable high-rate sulfate removal not limited by biomass accumulation and with H₂ supply that was on demand. The H₂-MBfR easily adapted to increasing sulfate loads, but the O₂-MBfR was difficult to adjust to the varying H₂-MBfR outputs, requiring better coupling control. The H₂-MBfR achieved high average volumetric sulfate reduction performances of 1.7–3.74 g S/m³-d at 92–97% efficiencies, comparable to current high-rate technologies, but without requiring gas recycling and recompression and by minimizing the H₂ off-gassing risk. On the other hand, the O₂-MBfR reached average volumetric sulfur production rates of 0.7–2.66 g S/m³-d at efficiencies of 48–78%. The O₂-MBfR needs further optimization by automatizing the gas feed, evaluating the controlled removal of excess biomass and S⁰ particles accumulating in the biofilm, and achieving better coupling control between both reactors. Finally, an economic/sustainability evaluation shows that MBfR technology can benefit from the green production of H₂ and O₂ at operating costs which compare favorably with membrane filtration, without generating residual streams, and with the recovery of valuable elemental sulfur.