Enhancing biodegradation of C16-alkyl quaternary ammonium compounds using an oxygen-based membrane biofilm reactor

Quaternary ammonium compounds (QACs) (e.g., hexadecyltrimethyl-ammonium bromide, CTAB) are emerging contaminants with widespread use as surfactants and disinfectants. Because the initial step of QAC biodegradation is mono-oxygenation, QAC degraders require O₂, but normal aeration leads to serious foaming. Here, we developed and tested an oxygen-based membrane biofilm reactor (O₂-MBfR) that delivers O₂ by diffusion through the walls of hollow-membranes to a biofilm accumulating on the outer surface of membranes. The O₂-MBfR sustained QAC biodegradation even with high and toxic QAC input concentrations, up to 400 mg/L CTAB. Bubbleless O₂ transfer completely eliminated foaming, and biofilm accumulation helped the QAC biodegraders resist toxicity. Pseudomonas, Achromobacter, Stenotrophomonas, and members of the Xanthomonadaceae family were dominant in the biofilm communities degrading CTAB, and their proportions depended on the O₂-delivery capacity of the membranes. Bacteria capable of biodegrading QACs often harbor antibiotic resistance genes (ARGs) that help them avoid QAC toxicity. Gene copies of ARGs were detected in biofilms and liquid, but the levels of ARGs were 5- to 35-fold lower in the liquid than in the biofilm. In summary, the O₂-MBfR achieved aerobic biodegradation of CTAB with neither foaming nor toxicity, and it also minimized the spread of ARGs.