Ultrafiltration is a method of removing very small particles from liquid. A membrane used with this method usually has pores that are 0.01 to 0.001 microns in size. This is small enough to remove most bacteria, viruses, high molecular weight substances, and polymer-type molecules. During ultrafiltration the flow of liquid gets split into two streams, which is known as cross-flow separation. One stream goes through the porous membrane and is then called permeate, and the other is concentrated in the particles and other matter that have been unable to penetrate the membrane.
The process is commonly used for treating drinking water in compliance with strict standards. Many organisms have become resistant to other methods of disinfecting water, but an ultrafiltration system removes pathogens by moving water through a membrane, physically removing each organism. The membranes themselves are often formed into small, hollow fibers that are less than a millimeter (about 0.04 inches) in diameter, which are bundled by the thousands in a filter housing. Modules that contain the membranes come in a variety of configurations, including spiral-wound, plate-and-frame, and tubular. The type and concentration of material that needs to be removed helps determine what kind of configuration is used.
Efficiency of ultrafiltration systems is affected by the flow of the liquid over the surface of the filter membrane. The higher the velocity of the liquid, the higher the rate of permeation, but higher flow rates can increase the energy consumption of the system. This leaves designers to consider acceptable energy usage versus an optimal rate for the application. Pressure is also directly correlated with how much fluid permeates the membrane, and is limited by how strong both the module and membrane is.
More fluid crosses the membrane as the temperatures go higher, but other factors need to be considered to know if a temperature change has affected the permeation rate. Consistent monitoring is needed to see if an ultrafiltration system is working properly, so it is necessary to record the use and operation of the system every day. If the rate of permeation drops by 10%, this is an indication that the membrane should be cleaned. Anti-bacterial agents can also be run through to kill any pathogens that can build up, especially if the system is shut down for a couple of days. Ultrafiltration is beneficial for large-scale water systems as well as small private and municipal water sources.