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One especially important case of purification concerns the removal of microbial contaminants so as to produce a sterile fluid. The use of membranes of 0.2 um rating is generally regarded in critical industries as a satisfactory means of achieving
sterility, demonstrated by a bacterial challenge test (NB this is referring to sterility from bacteria, not from viruses, which are much smaller than 0.2 um). A typical challenge test uses Pseudomonas diminuta as the microbe with which to challenge
the filter.
A standard means of expressing the microbial rating of a filter is by means of the Log Reduction Value. This is defined as the logarithm (to base 10) of the ratio of the total number of bacteria in the challenge suspension to the corresponding number in the filtered fluid, when the filter is subjected to a specific challenge. Generally a filter is considered suitable for sterilizing use if its output is free of bacteria in a challenge of 1 x 10x10x10x10x10x10x10 organisms per cm2 of effective filter area (EFA). The concept of LRV can of course be applied to filtration other than at 0.2 um (or indeed to other kinds of separation), provided that the test is then undertaken with the natural bioburden of the fluid, or with a bacterium of the appropriate size.
As an example of an LRV calculation, consider a standard 293 mm diameter disc of membrane, mounted in a membrane holder, providing an EFA of 468 cm2. This is then challenged with a bacterial suspension equivalent to 1 x 10x10x10x10x10x10x10 organisms/cm2, and is assumed to produce a sterile filtrate, i.e. one free of bacteria. The total
number of organisms in the challenge is 468 x 1 x10x10x10x10x10x10x10, or 4.68 x 10x10x10x10x10x10x10x10x10.
The required ratio is: number of organisms in challenge/number in filtrate. By definition, the number in the filtrate is zero, but that would make this ratio infinitely large, so it is customary to enter a 1 for this term. The ratio then becomes 4.68 x 109, of which the logarithm is 9.67, and the LRV is then said to be greater than 9.67.