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Old 11/06/2009, 11:56 AM   #1
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Join Date: Jun 2003
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Is my RO membrane working properly...and How do I know when to change it??

Is my RO membrane working properly…and how do I know when to change it?

Short version:
When your rejection drops below 95% as indicated with your TDS meter.

Long Version:
Although RO (reverse osmosis) membranes are capable of maintaining high water quality over extended periods of time they eventually will begin to deteriorate. Normally, the conductivity of the permeate (output or “clean” water) will increase as the membranes age. By comparing the difference in total dissolved solids (TDS) readings between the Tap water (inlet water presented to the membrane for treatment) conductivity and the permeate water conductivity, the percentage of rejection of the RO membrane may be calculated and the resultant number may then be used to determine the condition of the membrane and thus the operator will know when the membrane needs to be replaced. Membrane failure will be indicated by a reduction of the percentage of rejection which will be determined by calculating the differential between the input and output numbers.
In order to accurately determine the condition of the RO Membrane, a decent quality conductivity tester (TDS meter) capable of reading the tap water conductivity and the permeate water conductivity should be used. With the assistance of the TDS meter you will be able to easily determine the RO membrane’s condition.
Before performing the diagnostic test on an RO membrane, make sure that the RO system has been “ON” and producing pure water for a minimum of 10 minutes. Also check the brine (waste) line (usually yellow) to make sure that water is flowing and that the flow ratio between the permeate water and the brine water is at a ratio that is > 4 to 1. NOTE: The pressure gauge should indicate a pressure reading of > 40 PSI during this 15-20 minute test period. Rejection rates less than 95% may indicate that the membrane should be replaced.

As a general rule; the RO membrane would be considered in good condition when the rejection rate is = to or > 95%.

Why is rejection so important?
The RO membrane is the workhorse of the water purification system. It uses a thin film membrane technology to reject impurities. RO membranes are especially good at rejecting large atoms and molecules and those species that exhibit relatively strong electron charge. It is less effective in rejecting small atoms and molecules (smaller than water) and those with relatively weak electron charge. RO membranes will allow all dissolved gases (such as ammonia and carbon dioxide) to pass through. The % rejection rating for a membrane is an indication of how much of these impurities will pass through the membrane and on to the DI cartridge. A membrane that has a 98% rejection rating will pass only half as many of these impurities as one with a rating of 96%. The direct result of using a 98% rejection membrane as opposed to a 96% membrane is that you will double your DI cartridge lifetime. It is important to note that all membrane manufacturers adhere to a lower % rejection limit for small membranes (2" x 10") of 96% at 60 psi and that the only way to ensure that you are using a higher % rejection membrane is to purchase it from a source which screens for higher % rejection membranes. Spectrapure membranes are treated with a proprietary process to enhance % rejection and production characteristics. We have been testing our RO membranes for over 20 years to the industry standard of 60psi. There is a significant amount of data for us to draw upon when we share performance characteristics with our customers, help them troubleshoot production problems, or back up our ratings claims, for that matter.
Production rates are another source of confusion. For example, the Filmtec 75 GPD membrane that is so popular is rated 75 GPD at 50psi. When we test it at the industry standard of 60 psi, the rating is at 90 GPD. The production rate of RO membranes is directly proportional to the applied pressure, thus the rating difference; this is why we sell our membranes and systems as 90 GPD (at 60 psi and 77F). Unfortunately, most customers in the USA have less than 60 psi (or even 50 psi) available, let alone the 77F water temp that is used in the rating process (average water temp is more like 50F across most parts of the country). In these cases a pressure-boosting pump can obtain higher production rates. If you live in Hades (Phoenix in the summer) the 77F is not much of a limiting factor as far as production goes. But for our customers in upstate New York in January, it is a huge factor. Perhaps to be more realistic we should all rate membranes production on systems at 40 psi and 50F, although I doubt our competitors would drop their rating convention in this highly competitive market seemingly driven by perceived cost per gallon production rate.
That said, with the membranes that we hand-select and test to insure rejection greater than 98%, we typically see production rates exceeding 100 GPD at 60 psi (and 77 F)!! If you look back at their literature, Filmtec may have started the 50 psi rating as a sales gimmick wherein they stated that FILMTEC membranes have the same production at 50 psi as the competitors membranes do at 60psi. Great way to snag customers, but it requires customers to look at the production rate charts and how they are derived to really compare membrane production rates. With our hand-selected and tested 75GPD membranes, we can make sure that you are purchasing greater than 98% rejection (look at their spec on the DOW website, 96% is the bottom range of the guarantee. They are not all 98% like many think). Also, even more astounding, is that with the membranes that we hand-select and test to insure rejection greater than 98%, we typically see production rates exceeding 100 GPD at 60 psi (and 77 F) !! Why is 1 or 2% so important?? A 2 % increase in rejection from 96% to 98% can double your DI cartridge(s) life. Over the life of the membrane that can save you big $$$ in DI cartridges! That is why our hand selection and proprietary testing (we sometimes reject more than half that do not meet our SPEC of >98.0% rejection) is worth the extra cost. Who else labels their membranes with actual rejection rate on the so called "the same" membrane?

Our experience has been that RO membranes are not all the same. That’s why we back up with science the performance of our membranes which are a critical element in the performance of your system, albeit removal of harmful species or your cost to operate your system. Rejected membranes are sold to another, far less demanding market. The difference in price reflects some of our investment to perform the testing. The price difference between an untested and a tested membrane is lost in the savings if you obtain 1 or 2% improvement in rejection as your DI stage(s) life is drastically improved. Even our untested membranes are subjected to a proprietary process which increases their rejection ratio over the units purchased by other vendors from the same supplier. So, those who claim that all membranes from a particular vendor are the same may not be aware of our process by which we routinely see our average rejection increase over non treated units. DI cartridges are one of the highest operation cost factors for your system per gallon of water produced, thus our customers tell us the extra price paid is well worth the return from savings on Di. Others just want the best to take water production quality out of the equation as much as possible from their aquatic environment maintenance equation.


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