Performance and mechanism of the synergistic hexavalent chromium and nitrogen removal in a MABR system

Co-contaminant of hexavalent chromium [Cr(Ⅵ)] and nitrogen in wastewaters produce serious environmental problems. However, it remains little in the way of continuous flow biological reactor to concurrently eliminated Cr(Ⅵ) and nitrogen in wastewater. Here, a single membrane aerated biofilm reactor (MABR) system was employed to achieve synergistic Cr(Ⅵ) and nitrogen removal. Cr(Ⅵ) and nitrogen removal performances, Cr(VI) reduction products, and microorganism functional groups characteristics, as well as microbial community structure and functional genes were investigated. Moreover, we revealed the role of “secretion”, potentially secreted by the biofilm, in nitrogen and Cr(Ⅵ) degradation. Results indicated that the Cr(Ⅵ) removal efficiency and simultaneous nitrification and denitrification (SND) rate reached 90.60 ± 1.45 % and 96.03 ± 1.47 %, respectively, when the influent Cr(Ⅵ) concentration reached 50 mg/L. Cr(Ⅵ) was mainly reduced to Cr(Ⅲ), which was attached to biofilm and secretion surfaces. Microorganism functional groups, including carbonyl (C = O), carboxyl (O-C = O) and hydroxyl (OH–) played an essential role in the adsorption and chelation of Cr(III). Acinetobacter, Lentimicrobium, NS11-12 marine group and Hydrogenophaga were identified as the main Cr(Ⅵ) reducers and denitrifiers. The abundance of genes related to Cr(VI) reduction and nitrogen transformation increased notably in both the biofilm (NasA, ChrR) and secretion (NemA, NfsAB, AzoR) systems under high Cr(VI) stress. The “secretion” played an “secondary system” to enhance nitrogen and Cr(Ⅵ) removal. This study provides new insights into the concurrent Cr(Ⅵ) and nitrogen removal from MABR system by the integrated biofilm and secretion.