Optimizing Autotrophic Sulfide Oxidation in the Oxygen-Based Membrane Biofilm Reactor to Recover Elemental Sulfur

Biological sulfide oxidation is an efficient means to recover elemental sulfur (S⁰) as a valuable resource from sulfide-bearing wastewater. This work evaluated the autotrophic sulfide oxidation to S⁰ in the O₂-based membrane biofilm reactor (O₂-MBfR). High recovery of S⁰ (80-90% of influent S) and high sulfide oxidation (∼100%) were simultaneously achieved when the ratio of O₂-delivery capacity to sulfide-to S⁰ surface loading (SL) (O₂/S^2- → S⁰ ratio) was around 1.5 (g O₂/m²-day/g O₂/m²-day). On average, most of the produced S⁰ was recovered in the MBfR effluent, although the biofilm could be a source or sink for S⁰. Shallow metagenomic analysis of the biofilm showed that the top sulfide-oxidizing genera present in all stages were Thauera, Thiomonas, Thauera_A, and Pseudomonas. Thiomonas or Pseudomonas was the most important genus in stages that produced almost only S⁰ (i.e., the O₂/S^2- → S⁰ ratio around 1.5 g of the O₂/m²-day/g O₂/m²-day). With a lower sulfide SL, the S⁰-producing genes were sqr and fccAB in Thiomonas. With a higher sulfide SL, the S⁰-producing genes were in the soxABDXYZ system in Pseudomonas. Thus, the biofilm community of the O₂-MBfR adapted to different sulfide-to-S⁰ SLs and corresponding O₂-delivery capacities. The results illustrate the potential for S⁰ recovery using the O₂-MBfR.