Reverse Osmosis, Ultra- and Diafiltration and Microfiltration

Reverse osmosis (RO) is a membrane separation process, driven by a pressure gradient, in which the membrane separates the solvent (generally water) from other components of a solution. The membrane configuration is usually cross-flow. With reverse osmosis, the membrane pore size is very small allowing only water and perhaps very small amounts of very low molecular weight solutes to pass through the membranes. It is a concentration process using a 100 MW cutoff, 700 psig, temperatures less than 40°C with cellulose acetate membranes and 70-80°C with composite membranes. Hyperfiltration is the same as RO.

Ultrafiltration (UF) is a membrane separation process, driven by a pressure gradient, in which the membrane fractionates dissolved and dispersed components of a liquid as a function of their solvated size and structure. The membrane configuration is usually cross-flow. In UF, the membrane pore size is larger than RO, thus allowing some components to pass through the pores with the water. It is a separation/ fractionation process using a 10,000 MW cutoff, 40 psig, and temperatures of 50-60°C with polysulfone membranes. In UF of skim milk, lactose and minerals are not fractionated; for example, in the retentate would be 100% of the protein but the same % of lactose and free minerals in solution (in the water phase) as existed in the skim.

Diagram of reverse osmosis and ultrafiltration

This can be visualized with another schematic, as follows, which may be more informative:

Diagram of reverse osmosis, ultrafiltration and microfiltration

Diafiltration is a specialized type of ultrafiltration process in which the retentate is diluted with water and re-ultrafiltered, to reduce the concentration of soluble permeate components and increase further the concentration of retained components. This schematic shows the process of diaflitration, as a step in ultrafiltration.

Diagram of diafiltration

Microfiltration (MF) (see diagram above) is a membrane separation process similar to UF but with even larger membrane pore size allowing particles in the range of 0.2 to 2 micrometers to pass through. The pressure used is generally lower than that of UF process. The membrane configuration is usually cross-flow. MF is used in the dairy industry for making low-heat sterile milk as proteins may pass through but bacteria do not. The permeate of skim milk is used as "bacteria-free" skim (although thee is no fail-safe guarntee as there could be pin-holes in the membrane) since all of the milk components will pass through the membrane. In that case, the retentate, skim enriched in bacteria, is high-heat treated. MF skim can then be stadardized for fat with high heat-treated cream.