Ultrafiltration or UF Membranes and Some of Their Important Uses
In contrast to simpler and more conventional methods of filtration in which relatively large particles of suspended solids are removed from a liquid by passage through coarsely porous materials, ultrafiltration offers a means to filter out far smaller suspended particles and even solutes with a reasonably high molecular weight. In such case, the passage is through thin layers of material with extremely small pores and known collectively as UF or ultrafiltration membranes. Water and other solutes of low molecular weight can pass through these filters and are referred to as the permeate, while any larger molecules present in the mixture remain trapped and form the retentate.
The process is similar to that known as osmosis, the mechanism that promotes the movement of water along the roots of plants and into the cells of their stems and thus acting to retain their rigidity. This natural activity also involves passage through semipermeable material in the form of cell walls. The movement is the result of a force known as osmotic pressure, which arises from a difference in the concentrations of dissolved solids in the water on either side of those walls.
For manmade, semipermeable UF membranes to operate, also requires the application of pressure in order to force the separation of permeate from the retentate. However, in this case, the pressure source is often also of manmade origin and so needs to be applied externally. In some applications, though correspondingly slower, the separation process may be driven by a concentration gradient. The technology has numerous applications in the chemical, pharmaceutical, and the food and beverages industries.
Of particular interest is the potential role of these UF membranes in the purification of drinking water. Providing a shortcut that omits the various processes involved in the secondary and tertiary filtration and chlorination stages upon which large-scale water treatment plants depend, ultrafiltration systems can produce potable water from raw sources in a single step, following the initial screening of large objects that might obstruct the flow or cause damage.
One of the most significant benefits of this type of system is that the equipment required is compact and so installing a treatment plant will require far less space overall. Additional bonuses are that no chemicals are required by the process and, regardless of the quality of the feed water, ultrafiltration consistently results in a product that is between 90 and 100% free of pathogens and which therefore exceeds typical regulatory standards.
On the downside, UF membranes are prone to become fouled and so the need to clean them fairly frequently and, eventually to replace them, tends to result in increased running costs. This, in turn, renders their use uneconomical for large-scale water treatment operations such as those conducted by the country’s municipalities. Nevertheless, for in-house cleansing and recycling of industrial process water, and in the food-processing and pharmaceutical industries, ultrafiltration systems can provide companies with a highly efficient and thoroughly practical option.
The dairy industry of today now uses UF membranes as a means to concentrate the protein content of whey used in the production of cheese. Replacing the lengthier, previous process, in which the whey was first steam-heated and then spray-dried or dried on drums, ultrafiltration has resulted in a single-stage operation capable of delivering a whey retentate with between 10 and 30 times the protein concentration of the original source, whilst simultaneously isolating a lactose-rich filtrate. Once again, in addition to reducing production time and cost, ultrafiltration ensures a more consistent product composition.
UF membranes are equally important for other essential processes in the dairy industry, in particular as a means of removing any pathogens that may be present in milk, while the manufacturers of fruits juice employ them to concentrate and clarify their products, Perhaps, the most dramatic of their applications is in the dialysis machines that filter various toxins and excess water from the bloodstream when a patient’s renal function is compromised or in cases of acute poisoning due to certain substances such as aspirin.
In practice, the uses for this technology are manifold and it is being constantly refined to extend the life and performance of the semipermeable materials. These include polymers such as cellulose acetate and polypropylene as well as ceramics for use in high-temperature treatments.
Selecting the correct UF membranes and configuration for a given job is crucial and the help of an expert such as WaterIcon will be invaluable.