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Principles of Filtration
Membrane filtration is a rapidly developing science, characterized by intrinsic terminology and proprietary information. In order to better serve the diverse and expanding needs of our customers and establish a mutual understanding, we at Sterlitech Corporation have compiled this accessible reference of the basic concepts and vocabulary of laboratory-scale filtration. If you have any questions regarding the following information or how these concepts relate to your specific applications, please contact our technical support team.
We will outline some of the fundamental definitions and principles associated with filtration technology, their relationships to each other, and discuss relevant applications. We will also provide criteria to guide you through the selection of both filter media and devices that are ideally suited for your needs.
Filter performance is greatly affected by the compatibility of the various properties of filter media with certain applications and operating conditions. Selecting the ideal filter media or device for your application is a multi-factor, highly relative process that should take into consideration the important properties discussed in the following guide.
Depth vs. Membrane Filtration
Despite ranging differences in filter material and production technique, there are two general categories that all filters can be classified under: depth and membrane (screen). Depth Filters consist of a matrix of randomly oriented, bonded fibers that capture particulates within the depth of the filter, as opposed to on the surface. (Examples: Glass Fiber, Cotton, Sintered Metals)
- 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 (or “Screen Filter”) performs separations by retaining particles larger than its pore size on the surface of the membrane. Particles with a diameter below the rated pore size may either pass through the membrane or be captured by other mechanisms within the membrane structure. Membrane filters are ideally suited for critical applications requiring maximum particle recovery. (Example: Polymeric Media 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 is a self-contained, successive filter unit that utilizes the specific properties provided by a sequence of membranes that offers an economical alternative to using individual prefilters with final filters. (Examples: Capsule Filters, Syringe Filters with GF Prefilters).
Chemical Compatibility indicates the ability of the filter media to maintain its structural integrity and function with exposure to certain chemical(s). This means that the filter will not exhibit pore-structure impairment, the media will not shed particles or fibers, and extractables will not be present in the filtered sample. In addition to filter material, it is important to consider compatibility as a function of temperature, concentration, applied pressure, and the length of exposure time. Though all of our filtration products are constructed with materials carefully selected to accommodate a wide range of chemical solutions, it is essential to understand the relationship of the properties of the sample fluid and the filter elements under actual operating conditions.
Hydrophilic vs. Hydrophobic
Hydrophilic filters exhibit an affinity for water (said to be “water-loving”) and can be wetted with virtually any liquid. They are the preferred material for filtration applications involving aqueous solutions--as appropriate by compatibility. (Note: In contrast to some other fields, the filtration industry does not define “hydrophilic” as “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 are water-averse and cannot be wetted with water. However, they will wet in low surface tension liquids, including organic solvents (alcohols), allowing aqueous solutions to pass through.
Hydrophobic filters are excellent for gas filtration, low surface tension solvents, and venting applications. They can also be used to overcome compatibility limitations associated with certain aqueous solutions and will allow water/aqueous solutions to pass through when the water breakthrough pressure is reached.
The Pore Size Rating refers to the size of the organisms or particles expected to be retained by the filter media under defined conditions. The pore size of the filter is defined by the diameter of the particles captured by the media matrix is usually stated in Micrometers (µm). Ratings can be stated as either nominal or absolute.
Nominal filter ratings are arbitrary values for filter performance that the manufacturer uses to indicate a range of particulate sizes for which a certain percentage of a specified contaminant of a given size is retained. Nominal ratings are variable between manufacturers and cannot be used as a means of comparison across manufacturers due to the substantial effect of processing conditions, such as operating pressure and particulate concentration, on the retention efficiency of the nominally-rated filters.
Absolute filter ratings are a value associated with media that exhibit precise and consistent pore sizes. It describes the cut-off point at which no particle of a certain size should be able to pass through the filter. More specifically, it indicates the diameter of the largest particle that will pass through the filter. Ratings are within the experimental uncertainty of a standard test method consistent with the intended filter usage and must specify the test organism (or particle size), challenge pressure, concentration, and detection method used to identify the contaminant.
Binding is a measurement of a substance’s propensity for “sticking” to the filter medium or other components. High binding capacity for a certain substance indicates that a high percentage of this substance will be removed from the filtrate. Often attributed to charge, binding can be either desirable or undesirable, depending on the application. For example, it is utilized in transfer membranes to bind nucleic acid or protein, allowing them to be easily separated and identified. However, when present during general filtration, binding may contribute to a loss of valuable products.
Extractables are substances that may leach, or otherwise migrate, from a filtration system into the filtrate. Potential contaminants can include wetting agents, manufacturing debris, sterilization residue, adhesives, or other components of the system. The type and concentration of extractables will vary with liquid sample properties.
To minimize the effect of extractables, filters can be flushed with water (or another process-compatible solvent) prior to use or purchased as “pre-flushed” packs. However, the necessity of flushing can also be mitigated through careful manufacturing procedures.
Examples of the effect of extractables include:
- Adding extraneous peaks in HPLS analysis
- Inducing cytotoxicity (kill cells) in cell cultures
- Inhibiting growth and affecting recovery of microorganisms in microbiological analysis
- Appearing as addition contaminants in environmental analysis
Thermal Stability is the ability of the filter media and device components to withstand elevated temperatures without compromising structural integrity and functionality. It is measured as the maximum operating temperature of the filter, or filter system, under specified conditions. Due to insufficient thermal stability, some filters are not suited for high-temperature sterilization processes, such as autoclaving. It should also be noted that thermal stability is related to chemical compatibility; meaning that certain filter media can be compatible with a chemical at room temperature, but incompatible at a high temperature.
Flow Rate and Throughput
Flow Rate and Throughput are related measurements of filter media and device performance that are affected by a number of other properties. The primary determinants of these values are:
Water Flow Rate measures the amount of water that flows through a filter, commonly expressed in milliliters/minute (mL/min), at a given pressure. It is influenced by the degree of contamination, differential pressure, total porosity, and the filter’s effective filtration area.
Air Flow Rate measures the amount of air that flows through a filter, commonly expressed in liters/minute (L/min) at a given pressure. It is also influenced by the degree of contamination, differential pressure, total porosity, and the filter’s effective filtration area.
Throughput is the amount of a sample that passes through a filter.
Differential Pressure (ΔP) is the difference between the upstream and downstream pressure in the system. It is the difference of pressure measurements taken before the fluid reaches the filter and after the fluid flows through the filter. Differential pressure increases as the filter beings to clog in continuous flow applications.
Viscositymeasures a fluid’s resistance to flow. High viscosity (at a constant temperature and pressure) lowers the flow rate through a filter (assuming also viscosity remains constant).
Porosity (“open area” or “void volume”) measures open spaces (pores) in the membrane as a percentage of total membrane ara. Generally, membranes have 50-90% open space and flow varies in direct proportion to membrane porosity.
Effective Filtration Area (EFA) is the area of a filter that is available for filtration; for a specific membrane, flow rates are higher (at a given differential pressure) for larger EFA’s.
Filter Media and Device Configurations include a vast array of sizes and configurations. Options range from disc membranes, to small syringe filters, to large capsule filters.
Disposable Filter Devices are intended for single-use applications and provide a convenient means of filtering a variety of sample volumes. These devices are often pre-sterilized and include “ready for use” syringe filters and capsule filters that consist of a membrane integrally sealed into a polymeric housing with fittings for easy attachment to syringes, tubing, or piping on the inlet and/or outlet of the device.
Disc Filters are economical, pre-cut filters that can be integrated by the end-user into a reusable filter holder (made of stainless steel, glass, or polymeric housing material). Note that some applications may require the end-user to sterilize the filtration system prior to use.
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