Insights into the membrane fouling aggravation under polyethylene terephthalate microplastics contamination: From a biochemical point of view

Abstract

This study evaluated the potential effects of polyethylene terephthalate microplastics (PET-MPs) on a membrane bioreactor. The results showed that 10–30 mg/L PET-MPs exposure showed a minor effect on reactor performances, while 50 mg/L PET-MPs significantly decreased the chemical oxygen demand and NH4+-N removal efficiencies. Long-term PET-MPs exposure resulted in sludge foaming, and significantly increased extracellular polymeric substances (EPS) contents from 463.08 ± 62.42 to 952.54 ± 23.67 mg/m2, which was the main factor in triggering severe membrane fouling. Excitation emission matrix and Fourier transform infrared spectroscopic analysis revealed significant changes in fluorescence properties and functional groups of EPS. Particularly, protein secondary structure analysis indicated that high abundant β-sheet facilitated the exposure of inner hydrophobic groups of bacterial cells and EPS, and consequently enhanced the interfacial interactions of foulants-foulants and membrane-foulants. Long-term PET-MPs exposure significantly shifted the microbial community structures. In particular, the abundances of foam-forming filamentous bacteria and quorum-sensing (QS) bacteria, such as Thiothrix, Kouleothrix and Aeromonas, were constantly increasing with the increasing PET-MPs concentration. Furthermore, PET-MPs exposure significantly up-regulated the abundances of genes encoding QS signaling molecules and exopolysaccharides, such as lasI, lsrA and algZ. Collectively, these findings suggested that PET-MPs exposure stimulated the excessive growth of fouling-causing bacteria, which contained more biofilm formation related genes, that consequently accelerated membrane fouling.