Using a two-stage hydrogen-based membrane biofilm reactor (MBfR) to achieve complete perchlorate reduction in the presence of nitrate and sulfate

We evaluated a strategy for achieving complete reduction of perchlorate (ClO₄^-) in the presence of much higher concentrations of sulfate (SO₄^2-) and nitrate (NO₃^-) in a hydrogen-based membrane biofilm reactor (MBfR). Full ClO₄ ^- reduction was achieved by using a two-stage MBfR with controlled NO₃^- surface loadings to each stage. With an equivalent NO₃^- surface loading larger than 0.65 ± 0.04 g N/m²-day, the lead MBfR removed about 87 ± 4% of NO ₃^- and 30 ± 8% of ClO₄^-. This decreased the equivalent surface loading of NO₃^- to 0.34 ± 0.04-0.53 ± 0.03 g N/m²-day for the lag MBfR, in which ClO₄^- was reduced to nondetectable. SO ₄^2- reduction was eliminated without compromising full ClO₄^- reduction using a higher flow rate that gave an equivalent NO₃^- surface loading of 0.94 ± 0.05 g N/m²-day in the lead MBfR and 0.53 ± 0.03 g N/m ²-day in the lag MBfR. Results from qPCR and pyrosequencing showed that the lead and lag MBfRs had distinctly different microbial communities when SO₄^2- reduction took place. Denitrifying bacteria (DB), quantified using the nirS and nirK genes, dominated the biofilm in the lead MBfR, but perchlorate-reducing bacteria (PRB), quantified using the pcrA gene, became more important in the lag MBfR. The facultative anaerobic bacteria Dechloromonas, Rubrivivax, and Enterobacter were dominant genera in the lead MBfR, where their main function was to reduce NO₃^-. With a small NO₃^- surface loading and full ClO₄ ^- reduction, the dominant genera shifted to ClO₄ ^--reducing bacteria Sphaerotilus, Rhodocyclaceae, and Rhodobacter in the lag MBfR.