Dual Effects of In Situ Free Nitrous Acid Accumulation on Membrane Fouling in Acidic Membrane Bioreactors

Acid-tolerant ammonia-oxidizing bacteria (AOB)-based technologies show promise for sustainable wastewater treatment. Membrane bioreactors (MBRs) are suitable for retaining acid-tolerant AOB, but membrane fouling under acidic conditions remains poorly understood. To address this knowledge gap, an acidic MBR and a neutral MBR were operated in parallel for one year. The acidic MBR sustained ∼2 mg N/L in situ free nitrous acid (FNA) and an average nitrite accumulation ratio of 92.7%. Results showed comparable membrane fouling propensity between acidic and neutral MBRs under pristine membrane, NaClO chemical cleaning, and physical cleaning. Mechanistic experiments further indicated dual effects of in situ FNA on membrane fouling. Specifically, FNA facilitated soluble protein and humic substance production in acidic sludge, increasing pore blockage potential. However, due to the sludge concentration reduction, the volumetric concentrations of soluble polysaccharides and bound extracellular polymeric substances in acidic MBR were lower than in the neutral MBR. Moreover, in situ FNA altered sludge surface functional groups, reduced membrane adhesion propensity, and enriched genera with hydrolytic capabilities and putative quorum-quenching activity, resulting in reduced fouling layer accumulation compared to the neutral system. Overall, these findings extend understanding of FNA-mediated membrane fouling and support the application of acidic wastewater treatment.