Model-based analyses of chromate, selenate and sulfate reduction in a methane-based membrane biofilm reactor

Selenate (SeO₄^2-) and sulfate (SO₄^2-) are frequently present together with chromate (CrO₄^2-) in certain industrial wastewaters. SeO₄^2- and CrO₄^2- are required to be reduced while SO₄^2- reduction should be minimized to avoid the production of toxic sulfide. In this study, a modified biofilm model was employed to investigate the interactions between CrO₄^2-, SeO₄^2- and SO₄^2- bioreduction in a methane (CH₄)-based membrane biofilm reactor (MBfR). The model was calibrated using steady-state experimental data of two reported CH₄-based MBfRs reducing these oxyanions. The modeling results suggested that the majority of methanotrophs (>80%) were located in the outer layer of the biofilm, while the oxyanions-reducing bacteria preferred to grow close to the membrane. The introduction of SeO₄^2- or SO₄^2- enriched selenate/sulfate-reducing bacteria (SeRB/SRB) but decreased the abundance of chromate-reducing bacteria (CRB). A biofilm thickness of >300 μm, an HRT of higher than 4 h and an influent dissolved oxygen concentration of 0.3 mg /L were favorable for simultaneous high-level CrO₄^2- and SeO₄^2- removal. A two-stage MBfR system with optimized operational conditions showed promise in retaining high-purity (>98%) selenium nanoparticles when treating both CrO₄^2- and SeO₄^2- impacted wastewaters. Moreover, the model indicated that efficient CrO₄^2- removal (>90%) along with minor SO₄^2- reduction (<10%) could be realized via maintaining appropriate biofilm thickness (200-250 μm) and influent dissolved oxygen (0.7-0.8 mg /L) in a single MBfR. These findings offer insights for the design and operation of CH₄-based technology for remediating CrO₄^2- contaminated industrial wastewaters.