Carbon nano-onions acting as artificial pili enhance chloramphenicol degradation in an anaerobic membrane bioreactor

Carbon-based materials are commonly used to promote methane production and organic degradation by enhancing direct interspecies electron transfer (DIET). Carbon nano-onions (CNOs) were first applied in an anaerobic membrane bioreactor to treat chloramphenicol (CAP) wastewater in this study. Results demonstrate that CAP was almost undetectable in the effluent after 188 days of operation. In Phases 1-3 (days 1 to 152), CAP increase in the influent led to maximum concentrations of 291.1 mg/L for acetic acid and 86.3 mg/L for propionic acid. In Phase 4 (days 153 to 188), the concentrations of acetic acid and propionic acid increased to a maximum of 211.5 mg/L and 342.3 mg/L. During Phase 3, the maximum methane productivity was 0.72 L/d, and the methanogenic lag time was 4.97 days. However, with the addition of CNOs and a higher CAP load, the maximum methane productivity reached 0.71 L/d, accompanied by a reduced methanogenic lag time of 1.22 days. Moreover, it significantly enhanced the abundances of Geobacteraceae, norank o norank c Thermodesulfovibrionia, Syntrophobacteraceae, Methanosaetaceae and Methanobacteriaceae which exhibited much higher specific growth rates in the family level. CNOs with high electrical conductivity acted as artificial pili to enhance DIET between Geobacteraceae and Methanosaetaceae, as well as between Syntrophobacteraceae and Methanobacteriaceae or Methanosaetaceae. Furthermore, CNOs dosing during Phase 4 mitigated membrane contamination, and the maximum filtration resistance was 11% lower than that of Phase 3. These results suggest that CNOs can be used effectively in AnMBR for treating CAP wastewater, particularly for enhancing methanogens.