Electrospinning-Graphene Promoted Low-Temperature Nitrogen Removal of Immobilized ANAMMOX Granules

To address the limited nitrogen removal performance of anaerobic ammonium oxidation (ANAMMOX) processes at low temperatures, this study successfully enhanced the low-temperature nitrogen removal performance of immobilized ANAMMOX granules by electrospinning (ES) and reduced graphene oxide (RGO). The lab-scale experimental results indicated that the total inorganic nitrogen (TIN) removal efficiency of novel immobilized ANAMMOX granules (M3) coated with 70-μm-thick chitosan/polyacrylonitrile electrospun membranes (CS/PAN-ES) and modified with 1.0 mg/(gVSS) RGO was significantly higher than M1 (common immobilized granules) and M2 (immobilized granules modified with 1.0 mg/(gVSS) RGO) at 4°C-13°C, especially at 4°C (60.9%-71.0%). These improvements were attributed to the stimulation of ANAMMOX activity at low temperatures by RGO and the enhanced structural stability of immobilized granules by CS/PAN-ES membranes. Furthermore, based on 16S rRNA sequencing analysis, CS/PAN-ES membranes and RGO synergistically promoted the growth of Candidatus Kuenenia (6.2% → 21.6%) and increased the abundance of functional genes nirK, hzs, and hzo at 4°C-13°C. This represents an important mechanism by which CS/PAN-ES membranes and RGO synergistically improved the extremely low-temperature activity of immobilized ANAMMOX granules. This study provides an effective strategy for the application of ANAMMOX processes in low-temperature environments.