Sterile or Final Compressed Air and Gas Filtration
Selecting the right filters for sterile air and gas filtration
Final or sterile air filtration removes microorganisms from air or other gases, to protect contacted food and beverage products, ingredients, packaging and equipment surfaces from airborne microbial contamination. The selection criteria and sizing for these filters depends on filter type, required removal performance, gas type, and process parameters including gas temperature, flow rate, and supply pressure.
Key Attributes of Sterile Air Cartridge Filters Are:
- Validated microbial removal performance
- Hydrophobicity, or the filter’s ability to remain dry under normal conditions in humid air, resulting in unrestricted airflow during filtration
- Low resistance to airflow, to enable compact sizing and improved energy savings
- Robustness in use, including during exposure to repeated in situ steam sterilization cycling, durability in forward and/or reverse operation, resistance to steaming and normal operating temperatures, and oxidative stability over time
- Compliance with national and international food contact compliance regulations
A sterile air cartridge filter is defined as one which, when challenged with a minimum of 107 colony-forming units (cfu) of Brevundimonas diminuta/cm2 of effective filtration area, will reproducibly produce a sterile effluent, free from viable microorganisms in the filtered air. Microbial validation is the historical method used for validating removal performance of sterile air cartridge filters; it is the most sensitive validation method. Filter media pore size rating (e.g. 0.2 micron) is immaterial and should not be used as a basis to claim expected microbial removal performance.
Users must therefore focus on the type of validation carried out by filter manufacturers to demonstrate microbial removal performance. The following are typical types of challenge test conditions used:
- Liquid bacterial challenge – the most rigorous form of microbial challenge; demonstrates bacterial removal performance even under worst case conditions due to process upsets
- Aerosol bacterial challenge – demonstrates bacterial removal performance in normal operation in dry air
- Aerosol spore challenge – demonstrates spore removal performance in normal operation in dry air
- Bacteriophage challenge - demonstrates the ability of the filter to remove aerosolized bacteriophage
In addition to microbial challenge, some air filters are validated by particle challenge testing in dry air, such as HEPA/ULPA final filters.
There are two main types of air filters offered in the food and beverage industry for sterile air filtration: membrane filters and depth filters. Important performance differences between these types are achievable microbial retention and achievable airflow.
Today’s sterile air membrane cartridge filters are typically composed of PTFE media, due to this material’s high degree of natural hydrophobicity. They feature a fixed pore structure, with no chance for filter media flexing or unloading of contaminants, often a concern with depth filters. Membrane filters’ very fine pores and narrow pore size distribution results in excellent retention capability under both liquid and aerosol bacterial challenge conditions. Membrane filters are almost always the preferred choice for sterile air filtration, especially in critical applications. Using validated membrane filters provides the lowest risk, even under worst-case, process upset conditions.
Depth filters are also found in the food and beverage industry in dry air applications to create sterile air. Examples used are depth cartridge filters, typically composed of borosilicate micro-fibers and HEPA/ULPA filters, typically composed of microglass fibers or membrane composites. While depth cartridge filters may be treated to render them hydrophobic, HEPA/ULPA filters exhibit low hydrophobicity. Depth cartridge filters are typically validated for aerosol bacterial challenge, while HEPA/ULPA filters are classified according to particle removal capability. Depth filters cannot pass microbial liquid challenge validation, which is why they must be used on dry air to deliver the expected retention performance. Generally, depth filters provide higher airflows than membrane cartridge filters, with HEPA/ULPA filters providing the highest airflows.
In summary, since hydrophobic membrane filters provide excellent microorganism retention under both liquid and aerosol bacterial challenge conditions, they are the preferred choice for sterile air filtration, especially in critical applications. Sterile air cartridge filters are typically used on compressed air supplies, and at atmospheric pressure as tank vent filters. They can also be used on low pressure, low to mid-volume airflow applications as far as is economically feasible. It is important to install sterile air filters as close as possible to the point of use.
The Pall Emflon® membrane filter range offers the following options for sterile air filtration, with specific features and benefits.
Pall provides the Emflon® family of validated membrane cartridge filters to address the critical air/gas sterilizing filtration needs in the food and beverage industry. Table 1 provides an overview.
All filters in the Pall Emflon family are comprised of hydrophobic PTFE membranes with polypropylene hardware. They are validated for microbial removal, repeated steam cycling capability, and operating temperature compatibility with typical air/gas filtration applications. They meet national and international standards for food contact compliance.
The filters are used to sterilize air/gases or provide microbial bioburden reduction in applications throughout the food and beverage industry, including in compressed air/gas filtration and tank venting.
® is a registered trademark of Pall Corporation.
Pall also offers a variety of housing options designed specifically for air and gas applications. The AdvantaTM AGT housings series are sanitary housings designed with a low pressure drop. Features of this advanced sanitary housing such as the surface finish, steam sterilizability, and ease of filter integrity testing have been tuned to match today's exacting requirements both technically and economically. VFK and VFS Series filter housings are specifically designed for venting applications in the food and beverage industry. The in-line flow pattern of these housings ensures minimized pressure drop and user-friendly filter cartridge change-outs. They can be equipped with a variety of air/gas filter cartridges to address the various needs of the industry, efficiently protecting product and processes from airborne contamination. See the table below for Pall’s air, gas and vent housing.