Mitigation of membrane particulate fouling by nano/microplastics via physical cleaning strategies

Membrane fouling by nano/microplastics (NP/MPs) is an emerging concern threatening the performance of water and wastewater treatment facilities. The NP/MPs can lead to surface adsorption, fouling and potential mechanical abrasion of the membranes. In this work, periodic gas scouring was applied during the filtration of nano/microplastics across ultrafiltration membranes to investigate the impact of shear forces on the adsorption of nano/microplastics. A series of surface energy and chemistry-modified membranes were also used including acrylic acid, cyclopropylamine and hexamethyldisiloxane plasma-modified membranes, allowing for a set of materials with controlled hydrophilicity, roughness and surface charge. Bubbling gas within the system at a gas flow rate of 0.5 to 1 L·min-1 and a water flow rate of 2 L·min-1 was found to limit the water flux decline across the pristine and hydrophobic membranes compared to the filtration experiments performed without cleaning from 38 to 22 and 23%, respectively. The adsorption of nano/microplastics onto the surface of the membranes was also simultaneously decreased from 40 to 25 and 19%, respectively. Interestingly, for the hydrophilised membranes no enhancement in permeance was observed when performing gas scouring due to the already low tendency for selective adsorption of the nano/microplastics onto their surface. The correlation of a dimensionless fouling number to the shear stress number suggested that the shear forces induced by gas scouring reduced nano/microplastics adsorption up to a gas injection ratio (volume fraction of gas) of 0.3, where the wall shear stress at the surface of the membrane was limited. This work offers an advanced physical strategy to reduce and control membrane fouling by nano/microplastics, with potential for this strategy to be adapted for more complex water matrices and plastic particles.