Ultrafiltration

There are four filtration ranges within membrane filtration. These enable targeted particles to be removed and targeted particles to be withheld in liquid. The filtration ranges are microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. It is the membrane selectivity, also called the rejection rate, that sets the filtration ranges apart. UF separates suspended solids and solutes of high molecular weight within the range of 0.01 to 0.1 microns. One micron is equal to one-millionth of a meter. UF can, among others, be used to remove bacteria, viruses, colloids, proteins, pyrogens, and pathogens. The technology is used in multiple industries to treat various kinds of water and wastewater.

Ultrafiltration is a simple pressure-driven technology, which can filter water and wastewater without the use of chemicals. Chemicals are only needed for membrane cleaning. Thus, it is a safe and sound solution, enabling greener filtration processes.

Simply put, pressure forces the water purification process by forcing the liquid through the semipermeable membrane surface. A feed stream enters the membrane, and a feed pump generates pressure. This will separate the feed stream into two new streams called permeate and retentate. The permeate consists of water molecules and dissolved solutes, which can pass through the semipermeable membrane barrier. Meanwhile, suspended solids and solutes of high molecular weight are retained by the membrane barrier. It is the membrane pore size that controls the selectivity, meaning the size of the particles that are retained.

The membranes can operate at low pressure, which makes them an energy-efficient, low-cost solution.

Ultrafilters can be produced of organic and inorganic materials. While organic materials are the most inexpensive solution, they suffer from a low degree of thermal, chemical, and mechanical strength, making them inadequate for harsh environments, such as corrosive and high-temperature environments. Inorganic ultrafilters possess all of these superior attributes and can withstand aggressive and frequent chemical cleaning, which is essential in many industries to maintain excellent hygiene standards. Therefore, there is a limitation to which industries and water and wastewater treatment tasks organic membranes can withstand. As a result, more and more industries start to switch to inorganic membranes.

Based on more than 20 years within water treatment, LiqTech has developed a superior inorganic ultrafiltration membrane based on silicon carbide, which is the second hardest material in the world, only beaten by diamonds. The extremely hard material delivers a consistent product quality regardless of the feed water.

Ultrafilters are used for many industrial large-scale ultrafiltration systems to treat various kinds of water and wastewater. Ultrafiltration systems are, among others, used within dairy, biotech, pharmaceutical, food and beverage, drinking water treatment, water reclamation, and pool filtration.

UF can also be used as a pretreatment before nanofiltration (NF) and reverse osmosis (RO). Pretreatment filtration processes protect the more sensitive NF or RO membranes to extend their lifetime and reduce the risk of fouling issues. As pretreatment separates the larger objects, it may also reduce the need for chemical cleaning to lower the operating expenses and operational downtime. Whether pretreatment is required depends on the feedwater and the desired product quality.

Ultrafiltration and nanofiltration are both used in many industries to treat various kinds of water and wastewater. The main difference between the two filtration methods lies in the membrane selectivity, which controls the size of the rejected particles. The membrane pore size structure defines the selectivity. UF has a pore size in the range of 0.01 to 0.1 microns, and NF has a pore size of 0.001 to 0.01 microns. As UF membranes have a larger pore size structure than NF membranes, they generally require a lower pressure, meaning lower energy.

Moreover, ultrafilters are less likely to experience fouling issues. Both UF and NF can be used as a pretreatment before reverse osmosis to protect the sensitive RO membranes and lower the risk of fouling and the need for chemical cleaning. Yet, as nanofilters’ pore size is limited to a range of few nanometers, UF is frequently chosen over NF.