Enhancing Elemental Sulfur Selectivity in Sequential Membrane Biofilm Reactors via Decoupled pH and ORP Control

A sequential membrane biofilm reactor (MBfR) system─comprising a hydrogen-based reducing module (H 2 -MBfR) for sulfate (SO 4 2– ) reduction and an oxygen-based oxidizing module (O 2 -MBfR) for elemental sulfur (S 0 ) recovery─was operated for 269 days to evaluate decoupled oxidation-reduction potential (ORP) and pH control strategies for treating mining-influenced waters. ORP control (−360 to −400 mV) at mildly alkaline pH (∼8) achieved moderate S 0 selectivity (67.7 ± 11.4%) but promoted significant thiosulfate (S 2 O 3 2– ) accumulation (−5.00 ± 1.58 g S m –2 d –1 ), revealing the limitations of redox-only strategies. Shifting to mildly acidic conditions (pH ∼ 6) suppressed S 2 O 3 2– formation by 92%, attributable to the pH-mediated reduction of reactive bisulfide (HS – ) concentration─which governs abiotic oxidation kinetics─and the enhanced direct uptake of undissociated H 2 S by sulfur-oxidizing bacteria (SOB). This suppression remained robust under high-rate stress testing (average SO 4 2– surface loading of 25.8 g S m –2 d –1 ) despite severe hydraulic transients. The system achieved a peak SO 4 2– removal flux of 27.81 g S m –2 d –1, volumetric S 0 production of 4.16 g S L –1 d –1, and a peak S 0 recovery efficiency of 94.1%. These results demonstrate that decoupled pH-ORP control effectively minimizes undesirable byproducts, widening the operational window for selective biological sulfur recovery in MBfR systems.