Size-Exclusion Separation
Ultrafiltration membranes separate dissolved macromolecules based on size. Unlike reverse osmosis, which relies on solution-diffusion through a dense polymer, UF membranes have physical pores (2-50 nm) that allow small molecules and water to pass while retaining larger solutes. The key parameter is the molecular weight cutoff (MWCO) — the molecular weight at which 90% rejection is achieved.
The Rejection Curve
Rejection does not occur as a sharp step at the MWCO. Because real membranes have a distribution of pore sizes, the rejection curve is sigmoidal — gradually rising from low rejection for small solutes to near-complete rejection for large ones. The sharpness of this transition determines the membrane's selectivity. Track-etched membranes have narrow pore distributions and sharper cutoffs, while phase-inversion membranes show broader transitions.
Flux & Fouling
Pure water flux through UF membranes follows the Hagen-Poiseuille equation for flow through cylindrical pores: flux scales with pore radius squared and transmembrane pressure. However, in real operation, protein adsorption, cake layer formation, and pore blocking progressively reduce flux — a phenomenon called fouling. Crossflow operation, where feed flows tangentially across the membrane surface, minimizes cake buildup and maintains higher flux.
Industrial Applications
Ultrafiltration is indispensable in biopharmaceutical manufacturing for protein concentration and buffer exchange, in dairy processing for whey protein recovery, in water treatment for pathogen removal, and in industrial wastewater treatment. Ceramic UF membranes offer superior chemical and thermal stability for harsh environments, while polymeric membranes (PES, PVDF) dominate cost-sensitive applications.