Membrane separation processes selectively separate substances based on size or properties.

Membrane separation is a process in which a membrane is used to separate the components in a solution by rejecting unwanted substances and allowing the others to pass through. The driving force is the difference in concentration or pressure between the two sides of the membrane. Membranes act as semi-permeable barriers that divide two phases — permeate and retentate — on the basis of particle size and electric charge, which is what makes them useful wherever a food or beverage plant needs to concentrate, clarify or fractionate a stream without heat.

Four pressure-driven methods cover most food applications. Reverse osmosis (RO) concentrates a solution by removing water, separating the solvent (generally water) from other components; it is applied to water purification, recovery of sugar from candy, recovery of oil-seed protein and concentration of dilute solutions such as fruit and vegetable juices and milk, with a particle size range of 5×10−3 μm to 10−4 μm. Ultrafiltration (UF) concentrates large and macromolecules — suspended solids and solutes of high molecular weight are retained while water and low molecular weight solutes pass through — and is used in the dairy industry mainly for production of protein concentrates from whey and cheese from milk, with a particle size range of 0.15 μm to 5×10−2 μm.

Nanofiltration (NF) concentrates organic components by removing part of the monovalent ions such as sodium and chlorine (partial demineralization) and is mainly used to remove divalent ions and larger mono-valued ions such as heavy metals. It is applied to pre-concentration of thin juices in the production of sugar from sugarcane and beet, desalting of lactose, production of protein concentrates, and the demineralization and de-acidification of whey obtained from ultrafiltration, with a particle size range of 5×10−2 μm to 5×10−3 μm. Microfiltration (MF) removes bacteria and separates macromolecules, colloids and suspended particles from solution, and is used for clarification of beer and wine (removal of yeasts and microorganisms), pretreatment of cheese whey, removal of oil droplets and fat globules, and wastewater treatment.

For a plant owner, the appeal is that membranes recover minor but valuable components from a main stream without substantial energy costs, on a very simple flowsheet, with extremely high selectivities, and are potentially better for the environment. The trade-offs are equally real: membrane stages cannot be easily staged compared with processes such as distillation, there can be chemical incompatibilities with process solutions, fouling of the membranes can become a major problem, and there is a temperature limitation. These constraints govern the choice of method, materials and cleaning regime as much as the separation duty does.