What is purpose of RO Membrane?


Team RC
Are you using is as part of an RO/DI unit or stand alone? If the membrane is no longer fuctioning correctly you will either burn through DI resin quickly (expense) if you are using a DI unit or you will have excessive TDS if you are using a RO without a DI filter. You need to measure the TDS before and after the RO membrane to determine if it is still functioning correctly.

Lou Young

New member
Yes I am using a BRS RO/DI unit. It's used and a few years old so I am not sure of the membrane is any good. Do I just measure the TDS to determine if the membrane is good or not? Thank you.


-RT * ln(k)
Measure the TDS going into the RO stage and then measure after the RO stage but before the DI stage. The number after should be about 2% or so of the number before. That would be a 98% rejection rate. If it is any higher then you probably need a new membrane.

Buckeye Hydro

.Registered Member
Even if the TDS reduction is OK, if you allowed a wet RO membrane to go unused long enough to call it "old," its likely the membrane is biofouled. Remember that membrane was partially submerged/marinating in dechlorinated water, and at least some of the membrane has dried after being wetted. I would not drink the water produced by that membrane. I would need some additional info to give you definitive advice, but in the absence of that, I'd replace it.



New member
The membrane has tiny holes that only let molecules as small as water through. This also means H+ and OH- can get through, the DI hells bring these ions and others out.


.Registered Member
I would not trust the membrane for the above reasons. Even if the membrane is producing well and rejecting will it is still suspect to bacteria. If you have not replaced any of the filters yet, it would be a very good idea to do so.

Using these filters could cause some serious damage to your tank, and also if you drink the water you could get sick. Just a friendly reminder. It is not recommended to drink DI water.