Ultrafiltration membranes find widespread use in water treatment but suffer from performance losses due to
fouling. This study investigates the fouling behaviors of commercial polyethersulfone (PES), regenerated cellulose
(RC), and recycled PES membranes during the filtration of wastewater containing polystyrene (PS) nanoparticles
and a mixture of PS and bovine serum albumin (BSA). Optical coherence tomography (OCT) is used for
real-time, in-situ examination of fouling, providing unique insights into nanoplastics-induced fouling formation.
Membrane performance was evaluated through flux decline and permeability recovery measurements. Results
indicate that nanoparticles form a uniform cake layer, with membrane characteristics such as zeta potential and
hydrophobicity influencing fouling reversibility. The RC membrane showed complete permeability recovery
after physical cleaning, while hydrophobic interactions hindered recovery for PES membranes. Notably, flux
reduction was greater for PES membranes (particularly with a 150 kDa MWCO) during PS filtration and worsened
with the combined PS and BSA solution. Using the OCT technique, we observed two distinct types of fouling
and their resulting structures: (i) a loose or fluffy-like formation; and (ii) flocs or particle agglomerates. This
study also combined OCT with conventional techniques like SEM and CLSM. This knowledge offers guidance for
choosing the proper membrane material in water treatment applications.