#15 | Separation operations with membranes

By Eva Mena Gil y Elena de Vicente Aguilar (EMUASA)

The ability to separate, concentrate and purify certain substances present in fluid media through membranes is a phenomenon that has been known since the 18th century. However, their fundamental development as parting agents on an industrial scale did not occur until around the last decade of the 19th century.

Separation processes with membranes applied to water treatment have recently become vitally important water reuse techniques. It is currently used in many applications, including:

• Production of drinking water from seawater
• Treatment of industrial effluents and recovery of components
• Concentration and purification of macromolecular solutions
• Elimination of toxins present in the blood through the artificial kidney, etc.

According to a market study carried out by Fortune Business Insights™, illustrated in Figure 1, in 2021, 45.8% of the membranes marketed were for water and wastewater treatment. They are followed in order by the number of membranes used in food industries, those used for gas separation and a small percentage of other applications.

To carry out these separation operations, there are different techniques. These differ fundamentally in the type of membrane used and the working pressure with which the operations are carried out. These conditions essentially depend on the type of water to be treated and the desired final quality. There are four different membrane separation techniques based on the pressure gradient, the species that are retained are illustrated in Figure 2.

• Microfiltration, MF: The membranes used in microfiltration retain solid particles found in colloidal suspension, large molecular solids, etc. they have a pore size of 0.1 to 10 µm and work at pressures below 2 bars.
• Ultrafiltration, UF. These types of operations are performed to separate macromolecules, colloidal matter, viruses, proteins, etc. The membranes used in ultrafiltration have a pore size between 0.01 and 1 µm and work with a pressure range between 2 and 10 bars.
• Nanofiltration, NF. Nanofiltration is a membrane filtration process with a pore size of less than 0.1 µm operated between 10 and 60 bar pressure. In these operations, it is possible to separate small molecules and polyvalent ions (Ca, Mg), etc.
• Reverse osmosis, RO. Unlike other techniques, reverse osmosis uses a dense semi-permeable membrane to remove ions, organic and inorganic substances, microorganisms, etc. In this technique, a pressure greater than the osmotic pressure is applied, for this reason, the membranes must be able to resist high pressures.

Figure 2. Schematic illustration of membrane filtration spectrum. Source: (Lee et al., 2016)

Bibliography

Lee, A., Elam, J. W., & Darling, S. B. (2016). Membrane materials for water purification: Design, development, and application. Environmental Science: Water Research & Technology, 2(1), 17–42. https://doi.org/10.1039/C5EW00159E

Membranes Market Size, Share, Growth | Forecast Report, 2029. (n.d.). Retrieved 1 March 2023, from https://www.fortunebusinessinsights.com/membranes-market-102982