Presence of unculturable bacteria in the permeate of microfiltration membranes with various pore-sizes in wastewater treatment
Microfiltration membranes are not completely effective barriers for bacteria due to the presence of ultrasmall bacteria and cellular deformability, both of which compromises filtration selectivity, thus raising concerns about the adequacy of microfiltration for bacterial removal. Understanding the bacterial community in the permeate is crucial for the application of membrane processes in water and wastewater treatment. This study investigated the bacterial community present in the permeate of microfiltration membranes (polycarbonate track-etched membranes) with pore-sizes of 0.4 and 0.8Âµm. Next-generation sequencing targeting the V3 and V4 regions of the 16S rRNA gene revealed that community composition was significantly dependent on pore size. The relative abundances of the phyla Parcubacteria (OD1) and Omnitrophica (OP3), which are likely members of unculturable bacteria, increased from 3.6% and <1% to 20.6% and 15.8%, respectively, after filtration, indicating their high permeability. In contrast, bacteria affiliated to phylum Planctomycetes decreased from 27.4% to 5.2% after filtration. Furthermore, the proportion of Betaproteobacteria was smaller in the 0.4Âµm (1.7% - 4.5%) permeate compared to 0.8Âµm (8.2% - 11.6%), suggesting its higher retention by smaller pore-size membranes.