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Principles of Filtration
Filtration is a science of growing information, distinctive terminology, and proprietary knowledge. These basic concepts have been complied so that we at Sterlitech Corporation can establish a common ground with you, our customer, on the basic language of filtration. As always, if you have questions about any of these concepts or how they apply to your specific applications, contact our Technical Service Department.
We will explain some of the fundamental aspects of filtration technology and how they relate to each other and to your application. Then, we will guide you through the logic of selecting the proper filter media and device.
Filter media have many different properties that affect the performance of the filter in certain applications. When selecting the best filter media or device for your application, consider the important properties described on the following pages.
Depth vs. Membrane Filtration
A Depth Media is a filter consisting of either multiple layers or a single layer of a medium having depth, which captures contaminants within its structure, as opposed to on the surface. (Example: Glass Fiber Media)
- Lower cost
- High throughputs
- High dirt-holding capacity
- Protects final filters
- Removes variety of particle sizes
- Media migration (shedding)
- Normal pore size
- Particulate unloading at increased differential pressure
A Membrane Filter typically traps contaminants larger than the pore size on the surface of the membrane. Contaminants smaller than the rated pore size may pass through the membrane or may be captured within the membrane by other mechanisms. Membrane filters are typically used for critical applications such as sterilizing and final filtration. (Example: Supor and GN-6 Metricel membranes).
- Absolute sub-micron pore size ratings are possible
- Can be bacteria and particle retentive (pore size dependent)
- Generally lower extractables
- Generally integrity testable
- Lower flow rates than depth media
- More costly than depth media
A Combination Filter combines different membrane pore sizes, or combines depth media and a membrane filter to create self-contained serial filter units. They can offer an economical alternative to using individual prefilters and final filters. (Examples: Acrodisc syringe filters with GxF glass fiber/.45µm GHP membrane, Acrodisc PF syringe filters, VacuCap PF devices, and AcroPak 500 capsules).
Chemical Compatibility is defined as the ability of a filter’s materials of construction to resist chemicals so that the filter’s function is not adversely affected, and the filter material does not shed particles or fibers, or add extractables to the sample being filtered. It is important to remember that compatibility is specific for a particular chemical or combination of chemicals, at a particular temperature. To select the proper filter device, you must determine the compatibility of the filter components with the fluid. Temperature, concentration, applied pressure, and length of exposure time affect compatibility. The materials used in the manufacture of filtration products are carefully chosen for their resistance to a wide range of chemical solutions. Still, understanding the compatibility between the fluid to be filtered and the filter elements under actual conditions of use is essential.
Hydrophilic vs. Hydrophobic
Hydrophilic filters are easily wet with water. Hydrophilic filters can be wetted with virtually any liquid and are the preferred filers for aqueous solutions as appropriate by compatibility. Note that in the filtration industry, “hydrophilic” is used somewhat differently than in some other fields where it refers to a material to which water clings.
Once wetted, hydrophilic filters do not allow the free passage of gasses until the applied pressure exceeds the bubble point and the liquid is expelled from the pores of the membrane.
Hydrophobic filters will not wet in water but will wet in low surface tension liquids, for example, organic solvents such as alcohols. Once a hydrophobic filter has been wetted, aqueous solutions may also pass through.
Hydrophobic filers are best suited for gas filtration, low surface tension solvents, and venting. In certain applications, hydrophobic filters are used to filter aqueous solutions because of compatibility requirements.
Water or aqueous solutions can also pass through a hydrophobic filter once the water breakthrough pressure is reached.
Pore Size Rating is the pore size of the filter determined by the diameter of the particle that it can be expected to retain with a defined, high degree efficiency under defined conditions. Pore size ratings are usually stated in Micrometers (µm). Ratings can be stated as either nominal or absolute.
Nominal filter ratings are an arbitrary value, indicating a particulate size range at which the filter manufacturer claims the filter removes a certain percentage of particulate load. Nominal ratings vary from manufacturer to manufacturer and cannot be used to compare filters among manufacturers. Processing conditions such as operating pressure and concentration of contaminant have a significant effect on the retention efficiency of the nominally-rated filters.
Absolute filter ratings are a value associated with a filter that represents the size of the smallest particle completely retained. Complete retention is within the experimental uncertainty of a standard test method consistent with the intended filter usage. Among the test conditions that must be specified are test organism (or particle size), challenge pressure, concentration, and detection method used to identify the contaminant.
Below are typical challenge organisms for specific membrane pore sizes:
|Absolute-rated Filter Media (Pore Size)||Challenge Organism|
|0.1 um||Acholeplasma laidlawii|
|0.2 um||Brevundimonas diminuta|
|0.45 um||Serratia marcescens|
|0.8 um||Lactobacillus species|
|1 um||Candida albicans|
Binding is the tendency of certain substances to “stick” to the filter medium (or other filter components) and be removed from the fluid. This is usually based on charge. Binding can be a desirable characteristic, as in the case of nucleic acid or protein binding on transfer membranes, which allows them to be separated and identified. Binding can also be an undesirable characteristic, as in the case of protein binding during filtration, which can lead to a loss of valuable products.
Extractables are substances that may leach or otherwise come off the filtration system and into the fluid being filtered. These contaminants may include wetting agents in the filter media, manufacturing debris, chemical residue from sterilizing the filter, adhesives, or components of the filter materials of construction. The type and amount of extractables will vary with the type of liquid being filtered.
Extractables can be minimized by flushing the filter with either water of a process-compatible solvent before using it. Some filters are sold pre-flushed. Careful manufacturing procedures can eliminate the need to flush filters.
Extractables can affect filtration in almost every application:
- in HPLC analysis, they can add extraneous peaks;
- in cell culture, they can cause cytotoxicity (kill cells)’
- in microbiological analysis, they can inhibit growth and affect recovery of microorganisms;
- in environmental analysis, they can appear as addition contaminants.
Thermal stability is the ability of the filter media and device components to maintain integrity and functionality at elevated temperatures. Thermal stability is important when considering filter sterilization, such as autoclaving. Certain filters cannot be autoclaved because of insufficient thermal stability. Keep in mind that there is a relationship between chemical compatibility and thermal stability; many types of filter media may be compatible with a chemical at room temperature, but not at high temperature. Thermal stability can be characterized by determining the maximum operating temperature under specified conditions.
Flow Rate and Throughput
Flow rate and throughput are two important related measures of filter media and device performance described in this section. This performance is affected by many different variables. The most important variables are detailed in the subsequent text.
Water flow rate is a measure of the amount of water that flows through a filter at a defined pressure. It is related to the degree of contamination, differential pressure, total porosity, and effective filtration area. Commonly expressed in the membrane industry in units of milliliters/minute (mL/min) at a given pressure.
Air flow rate is a measure of the amount of air that flows through a filter. It is related to the degree of contamination, differential pressure, total porosity, and filter area. Commonly expressed in the membrane industry in liters/minute (L/min) at a given pressure.
Throughput is the amount of fluid able to pass through a filter prior to plugging.
Differential pressure (ΔP) is the difference between the pressure in the system before the fluid reaches the filter (upstream pressure) and the pressure after the fluid flows through the filter (downstream pressure). In a constant flow application, the differential pressure increases as the filter beings to clog.
Viscosity is a measurement of a fluid’s resistance to flow. For example, a slow-flowing liquid like honey has a higher viscosity than “thin” liquid like water. The higher the viscosity (at a constant temperature and pressure), the lower the flow rate through a filter (assuming that the fluid is Newtonian, that is, that the viscosity does not change as conditions change).
Porosity (also called “open area” or “void volume”) is a measurement of all of the open spaces (pores) in the membrane. Generally, membranes have 50-90% open space. Flow rate is directly proportional to the porosity of the membrane (more pores = higher flow rate, for a give pore size and thickness of filter medium).
Filter Area. Filter medium and devices are available in a wide range of sizes with different Effective Filtration Areas (EFA). EFA is the filter area that is available for filtration; for a given membrane, the larger the filter area, the higher the flow rate at a given differential pressure.
Filter Media and Device Configurations are available in a wide variety of sizes and configurations from disc membranes to small syringe filters to large capsule filters.
Disposable Filter Devices such as syringe filters and capsule filters are the most convenient means for filtering any sample volume. These devices usually consist of a membrane integrally sealed into a polymeric housing with fittings that attach easily to a syringe, tubing, or piping on the inlet and/or outlet of the device. These devices are typically pre-sterilized, ready for use, and intended primarily for one-time use.
Disc Filters are installed by the end-user into a reusable filter holder made of stainless steel, glass, or polymeric housing material. From strictly a material cost standpoint, the membrane disc is less expensive. However, this method requires the end-user to install the filter integrally (i.e. without bypass) into the filter holder and often to sterilize the filtration system prior to use.
Filter Area, Flow Rate, and Throughput Examples
|0.2 um Supor Membrane Devices||
Typical Water Flow
|25mm Acrodisc Syringe Filter||2.8||0.039
|AcroPak 200 Capsule||200||2.35||12|
|AcroPak 500 Capsule||500||8.0||25|
|AcroPak 1000 Capsule||1000||16||50|
*Estimated throughput when filtering RPMI media with 10% bovine calf serum.
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