retentados

Retentados, in the context of membrane separation technologies, are the portions of a feed stream that are retained by the semipermeable membrane. They constitute the concentrate or residue that does not pass through the membrane, in contrast to the permeate, which is the portion that successfully passes through. The term is commonly used in processes such as ultrafiltration, nanofiltration, and reverse osmosis.

The composition and properties of retentados depend on the membrane’s characteristics, particularly the cutoff or pore size, and on operating conditions. They are typically enriched in solutes, colloids, or larger molecules relative to the feed, and their volume decreases as solvent passes into the permeate. Factors such as transmembrane pressure, temperature, crossflow velocity, and fouling or concentration polarization influence both the concentration and quality of the retentado. In multi-pass configurations, retentados can be recycled or subjected to additional purification steps to achieve desired product specifications.

Applications of retentados span many industries. In water treatment and desalination, the retentado from reverse osmosis or nanofiltration is a concentrated brine or solution that may require disposal or further treatment. In the food and beverage sector, retentados from ultrafiltration or nanofiltration can form protein concentrates, lactose-rich streams, or mineral concentrates, depending on the feed and membrane used. In bioprocessing and pharmaceuticals, retentados may contain high-molecular-weight biomolecules that necessitate further purification or concentration steps.

Handling retentados involves considerations of energy use, fouling management, and environmental or regulatory requirements for disposal or reuse.