Underestimation of Extracellular Lipid Contribution to Fouling in Anaerobic Membrane Bioreactors

Abstract

Extracellular lipids were mainly considered as microbial metabolites and precursors/inhibitors of methane production in anaerobic membrane bioreactors (AnMBRs). Nevertheless, the significance of lipid-related membrane fouling has rarely been explored and compared to that of common foulants such as carbohydrates and proteins in AnMBRs. This was the first work to fingerprint long-chain fatty acids (LCFAs) and investigate the underestimated lipid-related fouling in AnMBRs. The LCFAs of extracellular polymeric substances (EPSs), supernatant, and soluble microbial products (SMPs) of mixed liquor, cake layer, and permeate in AnMBRs were mainly composed of C16:0 and C18:0. The lipid content (9%) was comparable to the carbohydrate content in EPS (10%). Noticeably, lipids made up 16% in the supernatant, and it was comparable to the content of carbohydrates (14%), suggesting the importance of colloidal lipids in membrane fouling. Furthermore, SMP had a lipid content of 31%, higher than the carbohydrate content (10%). From a molecular weight cutoff point of view, similar high rejection of lipids (68.7%) compared to those of carbohydrate (64.97%, p > 0.05) and protein (58.56%, p > 0.05) suggested that complexes of lipids and other macromolecules were implicated in fouling formation. While quorum quenching can mitigate membrane fouling via reduction of proteins in EPSs and supernatants, it did not reduce lipid contents. Overall, the results suggested that lipids play a much more important role in membrane fouling formation in AnMBRs than that previously assumed.