Syntrophic acetate oxidation having a key role in thermophilic phenol conversion in anaerobic membrane bioreactor under saline conditions

• Predominance of syntrophic acetate oxidation and hydrogenotrophic methanogenesis. • Enlarged hydrogenotrophic subpopulation acting as electron sink. • Conversion rates of 29 mg phenol·gVSS-1d-1 at 55 °C and 6.5 gNa+·L-1. • High substrate conversion rates of membrane-attached biomass. • Non-identified thermophilic phenol-degrading microorganism(s) Phenol conversion under saline thermophilic anaerobic conditions requires the development and sustenance of a highly specialized microbial community. In the present research, an anaerobic membrane bioreactor (AnMBR) fed with an influent containing 0.5 g·L -1 phenol and 6.5 gNa + ·L –1 was operated at 55°C for 300 days. Phenol degradation was limited when phenol was the sole substrate. However, phenol removal efficiency significantly (p < 0.001) increased to 80% corresponding to a conversion rate of 29 mgPhenol·gVSS –1 d –1 when acetate (0.5 gCOD·L -1 ) was simultaneously provided. Isotopic analysis using 1– 13 C labeled acetate and measuring 13 CH 4 revealed that acetate was first oxidized to hydrogen and CO 2 , prior to methanogenesis, resulting in an increased abundance of hydrogenotrophic methanogens. It is hypothesized that the latter is of crucial importance for achieving effective anaerobic oxidation of phenol and its metabolites. Remarkably, the phenol conversion rate in the membrane-associated biomass was three times higher than in the suspended biomass. The observed difference in conversion rate could be explained by the presence of an increased abundance of hydrogenotrophic methanogens in the membrane-associated biomass confirmed by a microbial community analysis of Archaea. Benzoate was measured in the permeate, suggesting that phenol degradation occurred via the benzoyl-CoA pathway. Results of the current study, suggest that syntrophic acetate oxidation coupled with hydrogenotrophic methanogenesis, which results in the presence of an abundant electron sink, plays a key role in enhancing thermophilic phenol degradation. The obtained insights widen the application of anaerobic digestion to treat saline phenolic-rich wastewater at high temperatures.