Impact of long-term salinity exposure in anaerobic membrane bioreactors treating phenolic wastewater: Performance robustness and endured microbial community

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.