Atmospheric air used to generate compressed air can contain up to 100 million microorganisms (bacteria, viruses, fungi) per cubic meter. Drawn in through the compressor intake, they ultimately end up inside the system. Warm and humid, the resulting compressed air provides an environment that fosters their proliferation.
Yet many applications requiring a sterile environment rely on compressed air—not only within the core processes themselves, but also as conveying or control air.
What are the consequences in the event of contamination?
Sterile air is widely used in the food, cosmetics, and pharmaceutical industries. If unfiltered compressed air comes into direct or indirect contact with products, production machinery, or packaging and filling equipment, it can become a source of contamination.
Such contamination can have severe consequences: beyond the risks it poses to consumer health by rendering the product unfit for consumption or use, it can also compromise product quality. This can lead to major issues: loss of raw materials, inability to market part of the production, recalls of products already on the market, loss of trust in the company, and legal action against the business…
How to produce sterile air?
Sterile filters are used to intercept microorganisms. These filtration devices consist of materials suitable for high-temperature steam sterilization, namely a stainless-steel housing or filter body and an appropriate filter media. They must be robust enough to withstand frequent and repeated sterilization cycles. Internal surface finishes of the housing must be carefully controlled, with the lowest possible roughness, to minimize potential bacterial retention zones.
In practice, the compressed air leaving the compressor first passes through a series of filters to remove liquid water, aerosols, solid particles, and oil in liquid or vapor form.
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Once this pre-filtered air reaches the high-quality, industrial-grade stainless-steel housing, it is purified by the sterile cartridge. The piping connecting the outlet of the sterile filter to the device using sterile air must also be cleaned and sterilized regularly. It is furthermore strongly recommended to install the sterile filter as close as possible to the compressed air point of use, in order to limit any post-filtration contamination.
Expert advice
Sterile filter housings are available in various standard sizes, covering a wide flow-rate range (typically from a few m³/h to several hundred m³/h for single-element units, and up to several thousand m³/h for multi-element configurations).
Beyond the high requirements for biosafety, it is also essential for the selected filtration devices to achieve high flow rates with low pressure drops.
Which filter media should be used?
Bacteria such as E. coli or streptococci typically range in size from 0.3 to 9 µm. Sterile filters must therefore be capable of retaining all particles larger than 0.2 µm.
In practice, pleated PTFE (polytetrafluoroethylene) membrane filters can be used, suitable for temperatures from –20 to 80 °C, with filtration ratings selected according to requirements (0.02 µm, 0.1 µm, 0.2 µm, or 0.45 µm).
Expert advice
Note that these filters can also be used to filter other gases. They offer chemical compatibility with highly aggressive substances, such as strong acids and bases, most solvents, and etching reagents used in metalworking processes.
Depth filters using three-dimensional hydrophobic media can also be used. Depending on the application, they may be made of metal, polymers, or inorganic materials such as borosilicates. This type of filter media provides retention above 99.9999% at a 0.2 μm rating. They can also be used to treat gases at high temperatures (200 °C).
Expert advice
In the food industry, depth filters are generally preferred. For pharmaceutical applications, membrane filters are typically selected.
How to sterilize the filters?
The sterile filter and associated piping must be regularly sterilized, at least twice per month, using steam at temperatures between 110 and 140 °C. This operation can be carried out in-line or in an autoclave. In the first case, a steam flow is circulated through the filter and the various sections of the installation without disassembly. In the second, the filter element or the entire filter assembly is removed from the installation and sterilized in an autoclave.
Food Industry: what specific challenges?
In the food sector, sterilizing compressed air often involves additional constraints, particularly in industries that use non-pathogenic microorganisms in their processes. This is the case, for example, in cheesemaking (penicillium), beer production facilities (yeasts), or other products derived from enzymatic processes. In these industries, the objective is to produce sterile compressed air without destroying the specific microbial cultures essential to production, and to avoid any potential interaction between residual microorganisms present in the compressed air and the useful ones, which could otherwise eliminate them.
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