Coupling of electricity generation and denitrification in three-phase single-chamber MFCs in high-salt conditions

High-salt conditions reduce the efficiency of electricity generation and nitrogen removal in microbial fuel cells (MFCs). In this work, we propose a three-phase single-chamber MFC (TP-MFC) by setting up a phase with immobilized cells in a conventional bipolar single-chamber MFC (common MFC). Cells from Halomonas were used as the immobilized phase, because these cells secrete the compatible solute ectoine and exhibit simultaneous nitrification and denitrification (SND). This enhanced the efficiency of SND and subsequent electricity generation under high-salt conditions. The average voltage of TP-MFC generated during the stable period in the presence of 30 g/L NaCl was 439.3 mV, which was 55.2% higher than that generated in common MFC. In addition, the N-removal rate of TP-MFC at 72 h was 63.4%, which was 38.4% higher than that of common MFC. The 16S rRNA diversity analysis showed an improved abundance of Pseudomonas, Acinetobacter, Alcaligenes, and Halomonas in TP-MFC, indicating that the ectoine secreted by immobilized Halomonas conferred substantial salt-tolerance on the electrogenic bacteria growing in a high-salt environment. This paper establishes an efficient and convenient method for improving the salt tolerance of microbial flora in MFCs, which is of great significance for the application of MFCs in high-strength wastewater treatment.