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.