RO Membrane Rejection Rate

TheEngineer · Sep 9, 2015

I just replaced my RO membrane with a new 75gpd Filmtec. I'm running it at 80PSI, as recommended, but I'm only getting about a 94% rejection rate (64 in, 4 out) if I'm calculating it right. I ran it for an hour when I first set it up and I was expecting to get closer to the 98+% range this membrane is supposed to get. Any suggestions for what I'm doing wrong?

I confirmed I have the correct flow restrictor.

Randy Holmes-Farley · Sep 9, 2015

Is this an inline measurement? If not, the output is easily contaminated, even with CO2 from the air.

How long did you let the water run before collecting for a rejection measurement? The initial permeate after being stopped for a while will be a lot higher in TDS. If you did not toss that water before taking a measurement, that may be part of it.

The rejection rate you get is a function of a lot of things, including what is in the water. Some things in tap water get through fairly readily, including ammonia (if your water company uses chloramine, for example) and carbon dioxide.

TheEngineer · Sep 9, 2015

This an inline measurement system. I'm on a well, so no chemicals. I have high lead and high CO2 levels. I took my measurement 30 minutes after starting it.

Randy Holmes-Farley · Sep 9, 2015

So the high CO2 may be the explanation. You might get an improvement if you degas the water first to remove excess CO2, but its a pain to do that. Some of the CO2 won't be ionized and so won't be detected by the TDS, so the problem may be substantially worse than it seems because it will deplete the DI faster.

Randy Holmes-Farley · Sep 9, 2015

You may find this useful:

http://spectrapure.com/frequently-asked-questions#calcc02

TheEngineer · Sep 9, 2015

Randy Holmes-Farley said:
So the high CO2 may be the explanation. You might get an improvement if you degas the water first to remove excess CO2, but its a pain to do that. Some of the CO2 won't be ionized and so won't be detected by the TDS, so the problem may be substantially worse than it seems because it will deplete the DI faster.

I built a CO2 degassing chamber, but it is after the RO stage.

Randy Holmes-Farley · Sep 9, 2015

OK, that's certainly fine. It may be hard to see a decline in TDS on degassing as contaminants (like salt spray dust) may get into it and boost the TDS (not a concern), but have you ever measured the TDS of the degassed water? It might drop a bit, although home air has enough CO2 for 1-2 ppm TDS to show up.

TheEngineer · Sep 9, 2015

No, I haven't tested the degassed water. It runs out into my DI resin so it comes out at zero. I'll have to test the degassed directly before the DI.

Kungpaoshizi · Sep 11, 2015

Ime only 50gpd will get 98%+ rejection rates.. 75-100-150 seem to get 91% +/-.

The biggest change I've seen as far as trying to not use so much di resin, was adding a 2nd carbon block. So now I'm using a .5 micron sediment and (2) .5 micron carbon blocks. By the time the water leaves the ro stage, it's at 3ppm from the 15ppm it was. (I suspect chlorine/chloramine was ruining the carbon)