Effects of large salinity fluctuations on an anaerobic membrane bioreactor treating phenolic wastewater

High salinity is becoming more common in industrial process water and final effluents, particularly when striving to close water loops. There is limited knowledge on the anaerobic treatment of chemical wastewaters characterized by distinct salinity fluctuations. This study investigates the high and fluctuating salinity effects on the conversion capacity and membrane filtration performance of an anaerobic membrane bioreactor (AnMBR) in treating phenol-containing wastewater. The AnMBR operated for 180 days with sodium concentrations between 8 and 37 gNa^+.L⁻¹. At ≤ 26 gNa^+.L⁻¹, approximately 99% COD and phenol removal efficiencies were achieved. At 37 gNa^+.L⁻¹, phenol and COD removal efficiencies decreased to 86 and 82%, respectively, while the biomass specific methanogenic activity was 0.12 ± 0.05 gCOD-CH₄.gVSS^−1.d⁻¹. Due to large salinity fluctuations, phenol and COD removal efficiencies reduced to ≤ 45% but recovered to ≥ 88%. Compared to phenol conversion, methanogenesis was more severely affected. Calculations showed a maximum in-situ phenol conversion rate of 25.5 mgPh.gVSS^−1.d⁻¹. Concomitantly, biomass integrity was compromised, and the median particle size severely dropped from 65.6 to 4.3 μm, resulting in a transmembrane pressure increase above 400 mbar. Cake layer resistance to filtration contributed to 85% of the total resistance. Nonetheless, all biomass was effectively retained in the AnMBR. A change in salinity ≥ 14 gNa^+.L⁻¹ substantially reduced the microbial richness and diversity. The microbial community was dominated by Bacteria belonging to Clostridiales and Archaea of the orders Methanosarcinales and Methanobacteriales. Our findings demonstrate AnMBRs as suitable techniques for treating chemical process water, with possible subsequent reclamation, characterized by high phenols concentrations and largely fluctuating salinity levels.