Presence of unculturable bacteria in the permeate of microfiltration membranes with various pore-sizes in wastewater treatment

Abstract

Microfiltration (MF) membranes are not completely effective at removing bacteria due to the presence of ultrasmall bacteria and the deformability of bacterial cells, both of which compromise filtration selectivity and raise concerns about the adequacy of MF for bacterial removal. Understanding the bacterial community in the permeate is crucial for enhancing filtration technologies. This study investigated the bacterial communities present in the permeate of a membrane bioreactor (MBR) system to better understand the functional potential of microbial communities in MBR effluent. Next-generation sequencing revealed that bacterial community composition varied with membrane pore size. Unculturable bacteria affiliated with Candidatus Parcubacteria (OD1) and Candidatus Omnitrophica (OP3) were enriched in the permeate, indicating strong permeability, whereas Planctomycetes were largely retained. Betaproteobacteria constituted a smaller proportion in the permeate from the 0.4 μm filtration compared to that from the 0.8 μm filtration. Functional prediction suggested that permeable bacteria prioritize core metabolic functions, with reduced emphasis on secondary pathways. While membranes with smaller pore sizes reduced ARG-related bacteria, they might simultaneously enrich for bacteria with higher biofilm-forming potential in the permeate. Additionally, several chlorine-resistant bacteria (CRB), including pathogenic genera were enriched after MF. These findings demonstrate that membrane pore size may play a pivotal role in shaping microbial composition and function in the permeate, with potential implications for optimizing wastewater treatment strategies.