Removal of multiple electron acceptors by pilot-scale, two-stage membrane biofilm reactors

We studied the performance of a pilot-scale membrane biofilm reactor (MBfR) treating groundwater containing four electron acceptors: nitrate (NO3-), perchlorate (ClO4-), sulfate (SO42-), and oxygen (O₂). The treatment goal was to remove ClO4- from ~200μg/L to less than 6μg/L. The pilot system was operated as two MBfRs in series, and the positions of the lead and lag MBfRs were switched regularly. The lead MBfR removed at least 99% of the O₂ and 63-88% of NO3-, depending on loading conditions. The lag MBfR was where most of the ClO4- reduction occurred, and the effluent ClO4- concentration was driven to as low as 4μg/L, with most concentrations ≤10μg/L. However, SO42- reduction occurred in the lag MBfR when its NO3-+O₂ flux was smaller than ~0.18gH₂/m²-d, and this was accompanied by a lower ClO4- flux. We were able to suppress SO42- reduction by lowering the H₂ pressure and increasing the NO3-+O₂ flux. We also monitored the microbial community using the quantitative polymerase chain reaction targeting characteristic reductase genes. Due to regular position switching, the lead and lag MBfRs had similar microbial communities. Denitrifying bacteria dominated the biofilm when the NO3-+O₂ fluxes were highest, but sulfate-reducing bacteria became more important when SO42- reduction was enhanced in the lag MBfR due to low NO3-+O₂ flux. The practical two-stage strategy to achieve complete ClO4- and NO3- reduction while suppressing SO42- reduction involved controlling the NO3-+O₂ surface loading between 0.18 and 0.34g H₂/m²-d and using a low H₂ pressure in the lag MBfR.