Membrane filtration integrity testing

Filters for use in sterile gas filtration must conform to standards similar to those mandated for sterile hquid filtration. Nondestmctive integrity tests may be apphed. The tests are performed by wetting the filter with an appropriate solvent, commonly 60/40 isopropyl alcohol/water for hydrophobic membranes, and applying air or nitrogen gas at a preset pressure. 

Standard Practice for Integrity Testing of Water Filtration Membrane Systems," ASTM D6908-03, American Society of Testing and Materials International, West Conshohocken, PA, 2007. 

Retention is a function of the pore size distribution of the membrane, solution properties, and operating conditions. For critical applications such as sterile or virus filtration, retention should be tested with the actual solution under different operating conditions. Typically, membrane filters are tested for integrity before use to ensure the required retention is obtained during operation. Integrity tests are based on bubble point or diffusion [6]. 

Although it might seem reasonable to consider microfiltration and tighter filtration as sterile and an alternative to conventional sterile filtration, this temptation should be avoided. Because it is not presently possible to test the membrane s integrity, it is impossible to know if larger pores or microscopic tears are present. In fact, we have observed the use of ultrafilters in a sterile filtration application where Zygosaccharomyces present in the permeate at >10 GFU/mL. 

Up to now, only hydrophilic filters have been discussed, which are used for the filtration of aqueous solutirais. Filters that are used for gas filtration such as ventilation filters on tanks and boilers are lipophilic filters. Some hydrophobic filter membranes are used to filter oils and other lipophilic solutions. A physical integrity test with water cannot be performed with this type of filter. For moistening, isopropyl alcohol has been used in the past, but the disadvantage of this substance is that it is highly flammable. Therefore, an alternative method has been developed, which is called the water intrusion test [15, 16]. 

The seeond type of filter is the membrane filter, in which depth is not eonsideied momentous. The membrane filter uses a relatively thin material with a well-defined maximum pore size and the particle retaining effect takes place almost entirely at the surface. Membranes offer the advantage of having well-defined effeetive pore sizes, can be integrity tested more easily than depth filters, and ean achieve more filtration of much smaller particles. They tend to be more expensive than depth filters and usually cannot achieve the throughput of a depth filter. Filtration technology has developed a well-defined temtinology that has been well addressed by commercial supphers. 

Diversity of Membrane Eiements and Configurations Currently, all membrane manufacturers offer their own design, size, and configuration of membrane elements and systems. The membrane systems differ by the type of filtration driving force (pressure versus vacuum), the size of the individual membrane elements, the size of the membrane vessels, the configuration of the membrane modules, the type of membrane element backwash, and the type of membrane integrity testing method and other factors. 

Direct Integrity Tests Direct integrity tests directly measure a breach in a membrane or membrane system. They monitor gas passing through a breach, filtrate flows, pressure changes, or sound that can be detected. 

American Society for Testing and Materials (ASTM). (2006). Standard practice for integrity testing of water filtration membrane systems, revision 06. ASTM D 6908. ASTM, West Conshohocken, PA. 

A bubble test to check the integrity for the membrane filter immediately after filtration but prior to release of the product. (This is done by applying air pressure through the filter until the validated bubble point is reached. Filter integrity is indicated by the absence of a steady stream of bubbles.)... 

Every day, before the filtration system starts operating, two tests (diffusion test and bubble point test) should be carried out to check the integrity of the damp membranes and to inspect the watertight seals, once the filter has cooled down. Details are given in the membrane manufacturer s instructions. These inspections are indispensable to ensure that the filter media operate at optimum efficiency (Table 11.10). 

---