Ultra Filtration

Ultra Filtration

Ultra filtration (UF) is a separation process by using membranes with pore sizes in the range of 0.1 to 0.001 micron. Typically, ultra filtration will remove high molecular-weight substances, colloidal materials, and organic and inorganic polymeric molecules. Low molecular-weight organics and ions such as sodium, calcium, magnesium chloride, and sulfate are not removed. Because only high-molecular weight species are removed, the osmotic pressure differential across the membrane surface is negligible. Low applied pressures are therefore sufficient to achieve high flux rates from an ultra filtration membrane.


Ultrafilter vs Conventional Filter

Ultra filtration, like reverse osmosis, is a cross-flow separation process. Here liquid stream feed flows tangentially along the membrane surface, thereby producing two streams. The stream of liquid that comes through the membrane is called permeate. The type and amount of species left in th e permeate will depend on the characteristics of the membrane, operating conditions, and quality of feed water. The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.


Conventional filters such as media filters or cartridge filters, on the other hand, only remove suspended solids by trapping these in the pores of the filter-media. These filters therefore act as depositories of suspended solids and have to be cleaned or replaced frequently. Conventional filters are used upstream from the membrane system to remove relatively large suspended solids and to let the membrane do the job of removing fine particles and dissolved solids. In ultra filtration, for many applications, no pre-filters are used and ultra filtration could remove almost all of the suspended and emulsified materials.


Ultrafiltration Membranes

Ultra filtration Membrane modules come in capillary and spiral-wound configurations. All configurations have been used successfully in different process applications. Each configuration is specially suited for some specific applications and there are many applications where more than one configuration is appropriate. For high purity water, spiral-wound and capillary configurations are generally used. The configuration selected depends on the type and concentration of colloidal material or emulsion. In all configurations the optimum system design must take into consideration the flow velocity, pressure drop, power consumption, membrane fouling and module cost.


Membrane Materials

A variety of materials have been used for commercial ultra filtration membranes, but poly sulfone (PS), PAN, PP and PVC are the most common. Recently thin-film composite ultra filtration membranes have been marketed. For high purity water applications the membrane module materials must be compatible with chemicals such as hydrogen peroxide used in sanitizing the membranes on a periodic basis.


Molecular-Weight Cutoff

Pore sizes for ultra filtration membranes range between 0.001 and 0.1 micron. However, it is more customary to categorize membranes by molecular-weight cutoff. For instance, a membrane that removes dissolved solids with molecular weights of 10,000 and higher has a molecular weight cutoff of 10,000. Obviously, different membranes even with the same molecular-weight cutoff will have different pore size distribution. In other words, different membranes may remove species of different molecular weights to different degrees. Nevertheless, molecular-weight cutoff serves as a useful guide when selecting a membrane for a particular application.


Factors Affecting the Performance of Ultrafiltration


There are several factors that can affect the performance of an ultrafiltration system. A brief discussion of these is given here.

Flow Across the Membrane Surface.
The permeate rate increases with the flow velocity of the liquid across the membrane surface. Flow velocity is critical especially for liquids containing emulsions or suspensions. Higher flow also means higher energy consumption and larger pumps. Increasing the flow velocity also reduces the fouling of the membrane surface. Generally, an optimum flow velocity is achieve by compromise between pump horsepower and permeate rate.

Operating Pressure.
Permeate rate is directly proportional to the applied pressure across the membrane surface. However, due to increased fouling and compaction, the operating pressures rarely exceed 100 psig and are generally around 50 psig. In some of the capillary-type ultra filtration membrane modules the operating pressures are even lower due to the physical strength limitation imposed by the membrane module.

Operating Temperature.
Permeate rates increase with increasing temperature. However, temperature generally is not a controlled variable. It is important to know the effect of temperature on membrane flux in order to distinguish between a drop in permeate due to a drop in temperature and the effect of other parameters.

Operating Pressure.
Permeate rate is directly proportional to the applied pressure across the membrane surface. However, due to increased fouling and compaction, the operating pressures rarely exceed 100 psig and are generally around 50 psig. In some of the capillary-type ultra filtration membrane modules the operating pressures are even lower due to the physical strength limitation imposed by the membrane module.

Operating Temperature.
Permeate rates increase with increasing temperature. However, temperature generally is not a controlled variable. It is important to know the effect of temperature on membrane flux in order to distinguish between a drop in permeate due to a drop in temperature and the effect of other parameters.

Performance of Ultrafiltration Systems



In high purity water systems, ultrafiltration is slowly replacing the traditional 0.2-micron cartridge filters. In Japan, practically all of the semiconductor industry follows this practice. An ultrafiltration membrane with a molecular-weight cutoff of 10,000 has a nominal pore size of 0.003 micron. When an ultrafiltration membrane is used instead of a 0.2-micron cartridge filter, particle removal efficiency is greatly improved. In addition, ultrafiltration membranes are not susceptible to the problem of bacteria growing through them, as is the case with 0.2-micron filters.


As the requirements for the quality of high purity water become more stringent, we can expect to see an increasing use of ultra filration as a final filter.

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