Sterile filtration: Microscopic pores against contamination.
Last updated: 17 December 2025
Sterile filtration is a sterilization substitute method that is used for the separation of microorganisms from liquids or gases. The sterility in this process is achieved through physical-mechanical separation. The medium to be sterilized is passed through filters with microscopically small pores. Typical pore diameters are about 0.2 µm (200 nanometers or 0.0002 millimeters), which represents the standard for sterile filtration in the pharmaceutical industry. With these fine pore diameters, microorganisms such as bacteria and yeasts are retained due to their cell size, and a sterile medium remains after filtration. A major advantage of this method is that it does not require chemical agents or heat exposure. With depth filters and membrane filters, two complementary filtration technologies are available that are often used in combination.
Sterile filtration: Depth filters and membrane filters
When designing a microfiltration for germ reduction, there are two basic techniques. Which one is used depends on the medium, the presumed microbial load and the technical feasibility.
Depth filters are filled with a porous matrix, for example cellulose, diatomaceous earth or synthetic fibers. The name describes how the filters are constructed: they are volumetric filter matrices with a certain depth. Two separation processes essentially take place in them: on the one hand mechanically by retaining particles of a certain size, and on the other hand by adsorption (i.e. attachment of the particles to the surface of the filter filling material). Well-known depth filters are, for example, HEPA filters, which are also used in households.
HEPA filters are used for air purification and are not liquid depth filters. In pharmaceutical liquid filtration, on the other hand, depth filters made of cellulose or polymer fibers are usually used as prefilters.
Membrane filters, on the other hand, consist of a semi-permeable membrane with a porous surface. The diameters of the pores are usually around 0.2 micrometers. This makes it possible to effectively retain microorganisms. Membrane filters are typically used immediately before aseptic filling to filter solutions sterile before they are transferred into the sterile container.
Use of microfilters
When used, there is a difference between the two methods, i.e. sterile filtration with a membrane filter and with a depth filter. The latter usually represents a prefiltration in pharmaceutical production in order to reduce the particle load of media. The membrane filters as final filters, in turn, have the task of ensuring the sterility of the product. This serial and combined structure has the great advantage that prefiltration protects the fine membrane filter from blockage.
Application of sterile filtration
A sterile filtration is used when products are thermally or chemically unstable – for example in vaccines, biologics or other sensitive active ingredient solutions. It enables sterile filling without exposure to heat or chemicals and can ideally be combined with the Blow-Fill-Seal (BFS) process to ensure high process reliability. However, microfiltration is generally not suitable and not validated for the reliable removal of viruses, endotoxins and dissolved substances – for this purpose, additional methods such as ultrafiltration, nanofiltration or adsorptive processes (e.g. ion exchange) are required.