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

Selenate (SeO) and sulfate (SO) are frequently present together with chromate (CrO) in certain industrial wastewaters. SeO and CrO are required to be reduced while SO 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 CrO, SeO and SO bioreduction in a methane (CH)-based membrane biofilm reactor (MBfR). The model was calibrated using steady-state experimental data of two reported CH-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 SeO or SO 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 CrO and SeO removal. A two-stage MBfR system with optimized operational conditions showed promise in retaining high-purity (>98%) selenium nanoparticles when treating both CrO and SeO impacted wastewaters. Moreover, the model indicated that efficient CrO removal (>90%) along with minor SO 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 CH-based technology for remediating CrO contaminated industrial wastewaters.