Treatment of spent lithium-ion battery discharging wastewater by MBR under increasing salinity: Performance, membrane fouling, and microbial community succession

The rapid expansion of spent lithium-ion battery (LIB) recycling has led to the generation of saline, organic-rich discharging wastewater, raising urgent demands for effective treatment. This study investigated the performance of the membrane bioreactor (MBR) in treating such wastewater, focusing on the effects of salinity stress on pollutant removal, membrane fouling, and microbial community dynamics. Results showed that increased salinity initially inhibited COD and NH 4 + -N removal, but the system recovered through acclimation and achieved high removal efficiencies of 90.74 % and 99.23 % at 1.3 % salinity, respectively. At low salinity levels (0.1 %–0.6 %), transmembrane pressure (TMP) gradually increased, leading to severe membrane fouling. In contrast, at higher salinity (0.9 %–1.3 %), TMP remained stable with negligible fouling observed. Sludge characterization revealed that smaller flocs under low salinity contributed to pore clogging. Microbial analysis showed that salinity stress reduced species richness and promoted the enrichment of salt-tolerant taxa. Methyloversatilis , capable of degrading methylated compounds under saline conditions, became the dominant functional genus in later stages. Additionally, Pseudonocardia , a filamentous bacterium, gradually accumulated and likely enhanced sludge granulation, which may have contributed to fouling mitigation. These findings clarify the adaptive mechanisms of MBRs under high salinity and demonstrate their potential for treating complex LIB discharging wastewater. • MBR application for LIBs discharging wastewater is systematically evaluated • Microbial acclimatization enables stable COD and NH 4 + -N removal at 1.3 % salinity • High salinity reduces membrane fouling via floc structure change • Methyloversatilis and Pseudonocardia linked to organic degradation and flocculation