Membrane Filtration

Membrane Filtration Systems

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Membrane filtration is a method of filtering particles from liquids or gases using a membrane. This technique is used for various applications, including dairy processing and wastewater treatment. The semi-permeable membrane keeps large particles out of the permeate while allowing smaller molecules to pass through.

Molecules move from high-concentration to low-concentration areas in a natural way. Using external pressure, molecules can flow from low-concentration to high-concentration areas. The permeate will travel across the membrane at a constant rate due to the difference in stress on both sides of the membrane. This allows for higher overall yields in the permeate or retentate end product.

The four primary methods of pressure-driven membrane filtration are microfiltration, ultrafiltration, nanofiltration and reverse osmosis, in decreasing pore size order.

Microfiltration (MF)

Microfiltration membranes have pore diameters ranging from 0.1 to 10 micrometers. These filters are typically used to remove large particles, colloids, and microorganisms from feed streams. This method is prevalent in the food and beverage industries for treating wastewater before it is discharged into a municipal sewer.

Ultrafiltration (UF)

Ultrafiltration is similar to microfiltration except that the pore diameters are smaller–ranging from 0.01 to 0.1 m. UF membranes are commonly used in protein concentration and wastewater treatment, and they are used to reject viruses and polypeptides.

Nanofiltration (NF)

Nanofiltration membranes, like reverse osmosis membranes, have a thin-film composite layer (1 m) on top of a porous layer (50 to 150 m) for tiny ion selectivity. NF membranes reject multivalent salts and uncharged solutes but allow monovalent salts to pass through. They can also operate at lower pressures than reverse osmosis membranes, making them ideal for achieving the best flow-to-rejection ratio.

Reverse Osmosis (RO)

Reverse osmosis membranes are even thinner than nanofiltration membranes, allowing water molecules to pass but rejecting all monovalent ions in aqueous solutions. They are also capable of removing viruses and bacteria from feed solutions. For seawater desalination and industrial water treatment, reverse osmosis filtering is commonly used. However, because the operating pressure for RO and NF membranes is significantly higher than for MF and UF membranes, the total yield of RO and NF membranes is lower.