Interrelation between extracellular polymer substances (EPSs) and MPs in an MBR

Abstract

Nowadays, microplastics (MPs), as ubiquitous pollutants in aquatic ecosystems, present a major emerging threat. Wastewater treatment plants (WWTPs) are considered a hotspot of MP contamination. In this context, membrane bioreactors (MBRs), which combine conventional activated sludge processes with filtration, serve as an effective technology to remove MPs from wastewater. One of the main factors that affect MBR performance is the amount and composition of extracellular polymeric substances (EPSs) in the mixed liquor (ML). In previous studies carried out at lab-scale with model MPs, it has been found that the presence of MPs in wastewater treatment processes affect the EPS characteristics. In this work, the inter-relationship between EPSs and MPs has been investigated in a MBR at an industrial scale. Samples of the ML were taken from a WWTP during a six-month period and EPSs and MPs were analysed. It was observed that the higher the concentration of MPs, the higher the amount of EPSs were secreted by microorganisms, which seems to indicate that MPs act as an environmental stressor. Additionally, ML samples were centrifuged and the retention of MPs in the centrifuged solid (CS), which mainly depends on the size and morphology of MPs, was evaluated in relation to the amount and composition of EPSs. Remarkably, the concentration of humic acids showed a linear correlation with respect to the percentage of MPs retained in the CS. According to this, a possible mechanism based on hydrophobic interactions has been proposed as being responsible for the retention of MPs in sludge flocs.