Soluble microbial products in membrane bioreactors in the presence of ZnO nanoparticles

In membrane bioreactors (MBRs), soluble microbial products (SMPs) are a key factor in membrane fouling. In this study, the production, characteristics, and fouling potential of SMPs in the presence of zinc oxide (ZnO) nanoparticles (NPs) were investigated in order to clarify the impact of environmental nanoparticle pollutants (ENPs) on SMP behavior in MBRs. Batch tests showed that the SMP production was elevated as both NP concentration and exposure time increased. It was also found that the increase of SMPs was mainly due to the enhanced production of biomass-associated products (BAPs). Further examination of bound extracellular polymeric substances (EPS) demonstrated that the release of intracellular polymeric substances (IPS) occurred, which subsequently caused the production of BAPs in the presence of NPs. Chemical analysis and Fourier transform infrared (FTIR) spectroscopy confirmed the increased production of polysaccharides and proteins of SMPs while no significant changes were observed in EPS concentration and composition. Quartz crystal microbalance with dissipation monitoring (QCM-D) analysis indicated that the increase of adsorption abilities was attributed to the elevation of SMP concentration in presence of ZnO NPs rather than the change of fluidity. Filtration tests showed that membrane fouling worsened with elevated SMP and NP concentrations. Scanning electron microscopy (SEM) demonstrated that a slime gel layer with NPs scattered within was formed on membrane surfaces in high NP concentrations. Energy dispersive X-ray (EDX) microanalysis and X-ray mapping (XRM) indicated that more NPs were present during membrane surface fouling when ZnO concentrations were increased. The escalation of membrane fouling may not only be due to elevated SMP concentration under the presence of ZnO NPs, but also be due to the complex interactions with SMPs and subsequent NP aggregation in organic matters to form a dense fouling layer on membrane surfaces.