Membrane or Depth Filtration

When is a 0.2 micon filter not a 0.2 micron filter?

It is important to understand that a membrane filter is not the same as a pleated Polypropylene filter, even though the rating may be the same. To take a membane filter rated at 0.2 micron and replace it with a pleated polypropylene filter rated at 0.2 micron may seem fine until you investigate how they are rated and tested.

0.2 micron is the rating designed to remove bacteria but a 0.2 micron pleated polypropylene filter will not stop bacteria, fact!  Please beware of the unscrupulous suppliers who will sell a filter at 0.2, 0.45 or even 0.1 micron as a replacement for a membrane filter because it really is asking for trouble. Why? You ask. OK, let me explain.

When is a 0.2 micron filter not a 0.2 micron filter?

In filtration it is important to understand how a filter is rated and how it is tested, if in fact it is tested at all. It is important to understand why it is not advisable to replace a 0.2 micron membrane filter with a pleated polypropylene filter rated at 0.2 micron. This could literally risk life and is undertaken by those with poor knowledge, or less scrupulous filter sales companies. Let me explain the science here...

Membrane Filters
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A membrane filter falls into two categories, "validated" and "non validated" or "tested" and "non tested". A membrane filter presents a physical barrier to particles and bacteria and the membrane itself is a cast sheet with small micro porous holes to stop the contaminant. A membrane filter has to be integral and the better filters are integrity tested. This ensures there are no manufacturing defects using pure water and compressed gas for the test. A membrane filter is normally "microbially" rated, a 0.2 micron usually used to stop brevundimonus diminuta a 0.3 micron bacteria, and full validation being an independent test to stop ALL bacteria at a level of dictated by the Health industry Manufacturing Association. This test is done in a dedicated laboratory and the filter provides protection to health. Please note not all membrane filters are tested from competitors and a filter is as good as the largest pore.

Pleated Polypropylene
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Pleated polypropylene filters are normally "particulate rated" not microbially. The better filters are tested to 99.98% efficiency not 100% as better membranes are. They are also made from random fibres as per the photograph and will not stop living organisms from growing, diving and passing through the media!

OK lets now go even deeper. How is a filter tested? Normally a particle rated filter is tested using a modified OSU-F2 test which uses particles of a known size suspended in a fluid and laser particle counters used to test the efficiency. This is fine with filters down to normally 2 micron but below this level particle testers are very expensive and also not seen in most filter companies, so how is say a 0.2 micron filter rated? The honest answer is often by "extrapolation” or in better terms guesswork.

Graver filters are fully tested and CFS only promotes the highest quality with true test data to support claims but we would recommend that if you want a membrane filter don’t cut costs and risk your company future with the wrong products.

The right filter for the right job

At CFS we have seen evidence of customers calling competitors, giving a part number and asking for an alternative and being offered pleated polypropylene filters when the original was a membrane. This cuts the cost for sure but can be literally fatal.  If you call CFS we will always give a direct alternative, based on over 34 years experience in filtration, and we will never, ever risk your company name, reputation or future. Take the safe option and only call CFS, so you can sleep at night knowing your staff and customers are well looked after.

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