Understanding the Basic Principles of Filtration

Membrane filtration is a rapidly developing science characterized by intrinsic terminology and proprietary information. To better serve our customers’ diverse and expanding needs and establish a mutual understanding, Sterlitech has compiled this accessible reference of laboratory-scale filtration’s basic concepts and vocabulary.

Here are some fundamental definitions and principles associated with filtration technology and their relationships to each other and discuss relevant applications. Also provided are criteria for selecting filter media and devices that suit your needs.

Depth vs. Membrane Filtration

Despite differing filter materials and production techniques, all filters can be classified under depth and membrane (screen).

Membrane Filters

A Membrane Filter (or Screen Filter) performs separations by retaining particles larger than the 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).

Advantages:

Absolute sub-micron pore size ratings are possible
Can be bacteria and particle-retentive (pore size dependent)
Generally lower extractables
Generally integrity testable

Potential Disadvantages:

Lower flow rates than depth media
More costly than depth media

Depth Filters

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)

Advantages:

Lower cost
High throughputs
High dirt-holding capacity
Protects final filters
Removes a variety of particle sizes

Potential Disadvantages:

Media migration (shedding)
Normal pore size
Particulate unloading at increased differential pressure

Combination Filters

A combination filter is a self-contained, successive filter unit that utilizes the specific properties provided by a sequence of membranes and offers an economical alternative to using individual prefilters with final filters. (Examples: Capsule Filters, Syringe Filters with GF Prefilters).

Chemical Compatibility

Chemical compatibility indicates the ability of the filter media to maintain its structural integrity and function with exposure to specific chemical(s). It 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 crucial to consider compatibility as a function of temperature, concentration, applied pressure, and the length of exposure time. Though all of Sterlitech’s filtration products are constructed with materials carefully selected to accommodate a wide range of chemical solutions, it is essential to understand the relationship between the properties of the sample fluid and the filter elements under actual operating conditions.

Hydrophilic vs. Hydrophobic

Hydrophilic Membrane Filters

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 membrane's pores.

Hydrophobic Membrane Filters

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.

These filters are excellent for gas filtration, low surface tension solvents, and venting applications. They can also overcome compatibility limitations associated with specific aqueous solutions and allow water/aqueous solutions to pass through when the water breakthrough pressure is reached.

Pore Size Ratings

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 determined by the diameter of the particles captured by the media matrix and is usually stated in Micrometers (µm). Ratings can be stated as either nominal or absolute.

Nominal Filter Ratings

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 to compare 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

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 particular size can 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

Binding measures a substance’s propensity for “sticking” to the filter medium or other components. A high binding capacity for a specific 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 losing valuable products.

Extractables

Extractables are substances that may leach or otherwise migrate from a filtration system into the filtrate. Potential contaminants include wetting agents, manufacturing debris, sterilization residue, adhesives, or other system components. The type and concentration of extractables will vary with liquid sample properties.

Filters can be flushed with water (or another process-compatible solvent) before use or purchased as “pre-flushed” packs to minimize the effect of extractables. However, the necessity of flushing can also be mitigated through careful manufacturing procedures.

Examples of the effects of extractables include:

Adding extraneous peaks in HPLC analysis
Inducing cytotoxicity (kill cells) in cell cultures
Inhibiting growth and affecting recovery of microorganisms in microbiological analysis
Appearing as additional contaminants in environmental analysis

Thermal Stability

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 unsuited for high-temperature sterilization processes such as autoclaving. It should also be noted that thermal stability is related to chemical compatibility, meaning that specific 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 affected by several other properties. The primary determinants of these values are:

Water Flow Rate

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

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

Throughput is the amount of a sample that passes through a filter.

Differential Pressure (ΔP)

Differential pressure (ΔP) is the difference between the upstream and downstream pressure in the system. It is the difference in pressure measurements taken before the fluid reaches the filter and after the fluid flows through the filter. Differential pressure increases as the filter begins to clog in continuous flow applications.

Viscosity

Viscosity measures a fluid’s resistance to flow. High viscosity (at a constant temperature and pressure) lowers the flow rate through a filter (assuming viscosity remains constant).

Porosity

Porosity (“open area” or “void volume”) measures open spaces (pores) in the membrane as a percentage of the total membrane area. Generally, membranes have 50-90% open space, and flow varies in direct proportion to membrane porosity.

Effective Filtration Area (EFA)

Effective filtration area (EFA) is the filter area available for filtration; for a specific membrane, flow rates are higher (at a given differential pressure) for larger EFAs.

Filter Media and Device Configurations

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

Disposable filter devices are intended for single-use applications and provide a convenient means of filtering various 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 outlet of the device.

Disc Filters

Disc filters are economical, pre-cut filters that the end user can integrate 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 before use.

Whether you’re a beginner or an expert, this reference guide makes filtration easier to understand and apply in real-world situations. It is a valuable resource for learning about the core filtration principles, vocabulary, and concepts, from knowing the criteria for choosing lab filtration products, such as membrane discs, cellulose filter papers, and syringe filters, to mastering how filtration works in various applications.

Information provided by Sterlitech