Membrane filtration and concentration Membrane filtration and concentration are pressure-driven, physical separation technologies that use semi-permeable membranes with precisely controlled pore sizes (ranging from nanometers to micrometers) to separate, purify, or concentrate liquid mixtures without phase change. They work on a simple principle: the membrane acts as a selective barrier, allowing small molecules (water, monovalent ions, solvents) to pass through while retaining larger components (proteins, polysaccharides, colloids, bacteria, macromolecular impurities).