Industrial wastewaters are becoming increasingly associated with extreme conditions such as the presence of refractory compounds and high salinity that adversely affect biomass retention or reduce biological activity. Hence, this study evaluated the impact of long-term salinity increase to 20 gNa⁺.L⁻¹ on the bioconversion performance and microbial community composition in anaerobic membrane bioreactors treating phenolic wastewater. Phenol removal efficiency of up to 99.9% was achieved at 14 gNa⁺.L⁻¹. Phenol conversion rates of 5.1 mgPh.gVSS⁻¹.d⁻¹, 4.7 mgPh.gVSS⁻¹.d⁻¹, and 11.7 mgPh.gVSS⁻¹.d⁻¹ were obtained at 16 gNa⁺.L⁻¹,18 gNa⁺.L⁻¹ and 20 gNa⁺.L⁻¹, respectively. The AnMBR's performance was not affected by short-term step-wise salinity fluctuations of 2 gNa⁺.L⁻¹ in the last phase of the experiment. It was also demonstrated in batch tests that the COD removal and methane production rate were higher at a K⁺:Na⁺ ratio of 0.05, indicating the importance of potassium to maintain the methanogenic activity. The salinity increase adversely affected the transmembrane pressure likely due to a particle size decrease from 185 μm at 14 gNa⁺.L⁻¹ to 16 μm at 20 gNa⁺.L⁻¹. Microbial community was dominated by bacteria belonging to the Clostridium genus and archaea by Methanobacterium and Methanosaeta genus. Syntrophic phenol degraders, such as Pelotomaculum genus were found to be increased when the maximum phenol conversion rate was attained at 20 gNa⁺.L⁻¹. Overall, the observed robustness of the AnMBR performance indicated an endured microbial community to salinity changes in the range of the sodium concentrations applied.

This study examined the temperature susceptibility of a continuous-flow lab-scale anaerobic membrane bioreactor (AnMBR) to temperature shifts from 35 °C to 55 °C and its bioconversion robustness treating synthetic phenolic wastewater at 16 gNa^+.L⁻¹. During the experiment, the mesophilic reactor was subjected to stepwise temperature increases by 5 °C. The phenol conversion rates of the AnMBR decreased from 3.16 at 35 °C to 2.10 mgPh^.gVSS^−1.d⁻¹ at 45 °C, and further decreased to 1.63 mgPh^.gVSS^−1.d⁻¹ at 50 °C. At 55 °C, phenol conversion rate stabilized at 1.53 mgPh^.gVSS^−1.d⁻¹ whereas COD removal efficiency was 38% compared to 95.5% at 45 °C and 99.8% at 35 °C. Interestingly, it was found that the phenol degradation process was less susceptible for the upward temperature shifts than the methanogenic process. The temperature increase implied twenty-one operational taxonomic units from the reactor's microbial community with significant differential abundance between mesophilic and thermophilic operation, and eleven of them are known to be involved in aromatic compounds degradation. Reaching the upper-temperature limits for mesophilic operation was associated with the decrease in microbial abundance of the phyla Firmicutes and Proteobacteria, which are linked to syntrophic phenol degradation. It was also found that the particle size decreased from 89.4 μm at 35 °C to 21.0 μm at 55 °C. The accumulation of small particles and higher content of soluble microbial protein-like substances led to increased transmembrane pressure which negatively affected the filtration performance. Our findings indicated that at high salinity a mesophilic AnMBR can tolerate a temperature up to 45 °C without being limited in the phenol conversion capacity.