Anaerobic/oxic/anoxic-membrane bioreactor process coupled with polyaluminum chloride for simultaneous nitrogen and F^- removal in semiconductor wastewater, and membrane fouling mitigation: Performance and mechanism

To address the stringent discharge standards of fluoride (F^-) and nitrogen (N) in semiconductor wastewater treatment, a novel anaerobic/oxic/anoxic membrane bioreactor (A/O/A-MBR) coupled with polyaluminum chloride (PAC) was developed. This study investigated the system performance, membrane fouling mitigation, microbial community dynamics, and the synergistic removal mechanisms. Results indicated that an optimal PAC dosage of 800 mg/L (at pH 7.0) reduced the effluent F^- concentration to below 4 mg/L (81.5% removal efficiency) and maintained the total inorganic nitrogen (TIN) removal efficiency above 81% (effluent TIN = 5.7 mg/L). PAC addition significantly enhanced the conversion of influent COD into intracellular carbon sources, increasing the proportion of endogenous carbon storage from 53.2% to 71.5%, which subsequently enhanced the contribution of endogenous denitrification (END) to N removal (from 13.1% to 24.3%). Furthermore, PAC dosing effectively mitigated membrane fouling, extending the fouling cycle from 9 to 15 days. This improvement was attributed to the increase in sludge floc size (26 to 42 μm) and the formation of a more porous cake layer. Mechanistic analysis via physicochemical characterization confirmed that F^- removal was enhanced through the formation of stable Al-F-OH complexes. 16S rRNA sequencing analysis revealed that Dechloromonas and Candidatus_Competibacter maintained a high endogenous denitrification contribution rate under PAC-induced stress. This study provides a high-efficiency, energy-efficient, and land-saving strategy for the integrated treatment of industrial F^--containing wastewater.