12-14 September 2023
Pharmaceutical water systems (PW and WFI) require a ventilating filter on the storage tank. It prevents dirty ambient air from being sucked into the system when water is withdrawn from the tank. This is often referred to as a sterile filter. However, there are numerous requirements for "real" sterile filtration according to the current EU GMP Annex 1. But is sterile filtration necessary at all for this application?
First of all, it is important to understand that pharmaceutical water systems, neither PW nor WFI systems, are or have to be sterile. The maximum permissible bacterial counts are specified in the pharmacopoeias (max. 100 cfu / ml for PW and max. 10 cfu /100 ml for WFI). PW and WFI bulk is therefore not "sterile" water. Therefore, no "sterile filter" is required as a vent filter. Even though tank vent filters are usually 0.2 µm filters, which are also used similarly for sterile filtration, they should not be called sterile filters for this application.
However, testing of the integrity of the WFI tank filter is necessary - the current EU GMP Annex 1 requires it for WFI systems:
6.11 Where WFI storage tanks are equipped with hydrophobic bacteria retentive vent filters, the filters should not be a source of contamination and the integrity of the filter tested before installation and after use. Controls should be in place to prevent condensation formation on the filter (e.g. by heating).
The requirement to test such WFI tank filters "in place" for integrity and to sterilise them has - after long discussion - been deliberately deleted from the now valid Annex 1. The majority of the authors could be convinced that such a strict requirement would have meant unnecessary additional costs and additional GMP risks for 99% of the WFI systems - and would thus ultimately harm rather than benefit the patient.