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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 (H 2 -MBfR) for sulfate reduction to an oxidizing MBfR (O 2 -MBfR) for oxidation of sulfide to elemental sulfur. A key process improvement of the H 2 -MBfR was online pH control, which led to stable high-rate sulfate removal not limited by biomass accumulation and with H 2  supply that was on demand. The H 2 -MBfR easily adapted to increasing sulfate loads, but the O 2 -MBfR was difficult to adjust to the varying H 2 -MBfR outputs, requiring better coupling control. The H 2 -MBfR achieved high average volumetric sulfate reduction performances of 1.7–3.74 g S/m 3 -d at 92–97% efficiencies, comparable to current high-rate technologies, but without requiring gas recycling and recompression and by minimizing the H 2  off-gassing risk. On the other hand, the O 2 -MBfR reached average volumetric sulfur production rates of 0.7–2.66 g S/m 3 -d at efficiencies of 48–78%. The O 2 -MBfR needs further optimization by automatizing the gas feed, evaluating the controlled removal of excess biomass and S 0  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 H 2  and O 2  at operating costs which compare favorably with membrane filtration, without generating residual streams, and with the recovery of valuable elemental sulfur.

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