Membrane fouling by nano/microplastics: origin, mechanisms and mitigation strategies

The detection of nano/microplastics throughout water treatment plants constitutes a potential threat on the performance of water purification technologies, particularly the impact of these particles on the membrane processes remains unclear. This thesis investigates for the first time the fouling of ultrafiltration membranes by nano/microplastic fragments and fibres, and mitigation solutions based on surface chemistry control and periodic cleaning strategies.Nano/microplastic poly(ethylene) fragments extracted from a facial scrub and custom-made nano/microplastic poly(ethylene terephthalate) fibres were used to represent the hydrophobic plastic particles released in household wastewater via rinse-off personal care products and textile fibres from laundry washing.Microplastics subjected to turbulences, similar to those applied in water treatment facilities, were found to be fragmented into hazardous nanoplastics, which increased the total number of particles by one order of magnitude. The filtration of such fragments across commercial ultrafiltration poly(sulfone) membranes reduced the water flux due to the adsorption of the particles onto the membranes. Membrane fouling was reduced by either enhancing the hydrophilicity of the membrane surface or applying periodic gas scouring, which limited the adsorption of the particles thanks to the shear forces generated at the membrane surface. The mechanism of membrane fouling by nano/microplastics was sequentially modelled into intermediate and complete pore blockage, followed by cake layer formation. Using the extended Derjaguin–Landau–Verwey–Overbeek theory, polar forces was identified as the predominant intermolecular interactions contributing to membrane fouling, as opposed to electrostatic and Van der Waals forces. The mechanical forces applied to the particles during gas scouring were qualitatively compared to the chemical interaction forces generated between the particles and the membranes. Establishing this force balance unravelled for the first time the preferential interactions between nano/microplastics and filtration membranes, which could be applied to larger scale filtration process to predict the loss of performance of such processes due to nano/microplastics. In a more realistic system consisting of nano/microplastic fibres and organic water contaminants, the fibres accumulating onto the membrane did not affect the water flux and were easily removed by gas scouring, unlike organic contaminants which plugged the pores of the membranes.This work opens the understanding of nano/microplastic interactions with water filtration processes and offers advanced chemical and physical solutions to reduce and control membrane fouling by nano/microplastics. In a near future, adapting this work for more complex water matrices, and plastic particles has the potential to contribute to the development of in-line fouling monitoring tools for membrane units, constituting a great asset for water companies