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View Full Version : What is the best RO membrane?


bassking16
07/02/2010, 03:17 PM
I'm having trouble keeping my TDS down from my RO/DI filter. I use a Water General 100gpd unit. I just recently replaced all my filters including the membrane, but I think I got a junk membrane. I bought a Dow Filmtech membrane. I have run at least 40 gallons through the membrane, and my TDS out of the membrane is right at about 95ppm. My dual stage DI resin system is wearing out too quick since the TDS out of the membrane is so high.

Can anyone recommend a good membrane for my system. From what I have read, the TDS out of a membran should be less the 20ppm at least. By the way, my tap TDS is about 460ppm.

Thanks for any help you can offer.

phil519
07/02/2010, 04:07 PM
what is the TDS with just RO? do you have a DI bypass so you can measure that?

phenom5
07/02/2010, 04:13 PM
I had a similar issue, and I switched up to a 75gpd membrane. The Dow 75gpd membranes have a higher rejection rate (~98%) versus the 100gpd (~93%). You'll have to get a 75gpd flow restrictor along with the membrane.

Even at that, those numbers seem a little high. How's your water pressure?

bassking16
07/02/2010, 05:36 PM
what is the TDS with just RO? do you have a DI bypass so you can measure that?

TDS for just the RO is between 85 and 95 ppm. I do have a bypass for the DI. I have two spouts. One for the DI and one for just RO drinking water. I have a 3 gallon pressurized storage tank that fills with RO water only. If I want DI, the water runs from the tank through the DI canisters.

bassking16
07/02/2010, 05:39 PM
I had a similar issue, and I switched up to a 75gpd membrane. The Dow 75gpd membranes have a higher rejection rate (~98%) versus the 100gpd (~93%). You'll have to get a 75gpd flow restrictor along with the membrane.

Even at that, those numbers seem a little high. How's your water pressure?

Water pressure is deffinetly good to go. It's approxamately 60psi give or take a little. I'll look into the 75 gpd membrane and restricter. Maybe that's all I need. I hope. At this rate, I'll be changing my DI resin every couple weeks.

sreefs
07/02/2010, 08:03 PM
Get a SpectraPureŽ High Rejection TFC Membranes, they are a sponsor on RC and have there membranes on sale for only $35. Get the 90gallon membrane it is 98% rejection, also get there 90 gallon flow restrictor I think they are about $5.00 It gets my tds before DI down to 1.5 my resin lasts a long time.

bassking16
07/03/2010, 07:40 AM
Thank you very much for the suggestion. I really appreciate that. I'll give that a shot.

PurdueWaterGuy
07/03/2010, 01:49 PM
I prefer Filmtec. Keep in mind that mine are the 1440 gallon per day size (4" x 40") :-)

Tim

@ home, 90 gallon tank, dry, waiting for the remodelling to be done
@ work, 19,000 gallon per day RO

kylesmoney
07/03/2010, 05:04 PM
Even though most 100gpd membranes have lower rejection rates your TDS should be way lower then that. If you have 500tds with the 96% rejection rate you should be getting around 20TDS, at that rate your DI would last 5 times longer then it is. A 98% would get you down to 10 and di would last even twice the previous number. Most likely your issue is water pressure. 100GPD needs 65 for sure, a dow filmtec 75 would be 50 and likely be the best route to go.

Always make sure you change your flow restrictor when you change membranes

Buckeye Hydro
07/05/2010, 06:39 AM
TDS for just the RO is between 85 and 95 ppm. I do have a bypass for the DI. I have two spouts. One for the DI and one for just RO drinking water. I have a 3 gallon pressurized storage tank that fills with RO water only. If I want DI, the water runs from the tank through the DI canisters.

That's only an 80 % rejection rate. Something is wrong. The most common cause we run across, especially if the problem occurs right after someone changes their membrane, is that the new membrane isn't fully seated.

Do you have a pressure gauge or are you guessing at what the pressure is that reaches the membrane?

Russ

bassking16
07/05/2010, 08:27 AM
That's only an 80 % rejection rate. Something is wrong. The most common cause we run across, especially if the problem occurs right after someone changes their membrane, is that the new membrane isn't fully seated.

Do you have a pressure gauge or are you guessing at what the pressure is that reaches the membrane?

Russ

Actually I am just guessing on the pressure. It seems like it should be high enough for the membrane. I did re-seat the membrane with no change. I also ordered a Spectra-Pure 90gpd membrane with matching restricter. I'm going to try that first, and if I get the same results, I'll look into boosting my pressure a little with a pump. I'm hoping I don't have to spend the money on a pump, but I will if need be. I'm also going to look into getting a pressure guage as well. Anyone know if I can get a guage at lowes / home depot, or will I have to special order?

ANyway, thanks again for all the info and suggestions. If anyone can think of anything else, I'm all ears.

Fish guy 91
04/27/2014, 05:17 AM
Yea you can make one at home depot, just be sure to make it with the john guest fittings in the store( just to make sure you get it done the first trip), and I had to use one brass union. About $13 I think. And yea I have the same issue. About 700ppm from the softened well water. And I'm getting about 50 ppm out of the cheap eBay to unit.... But my peak pressure is only 45psi going into the membrane. Soo as soon as all the parts for my 6800 pump get here I'll see if that does the job. But it pokes holes in my understanding of RO membranes all together. How does anything at all get through?

Buckeye Hydro
04/27/2014, 05:56 AM
You can get a pressure gauge at hardware stores. But you'll also need a fitting to screw on the back of the gauge, some tubing, and a fitting to tap into the tube feeding the RO membrane.

We have all this as a kit, or you can work through it at the store.

SpectraPure
05/06/2014, 12:37 PM
Please measure your ML/PER minute out of the product water tube, and then repeat for the waste water. The waste water should be 3 times greater than the product water. If you if you are below 180 on hardness(not TDS) you can operate at a lower waste ratio. Your flow restrictor might need to be adjusted, and if it is a barrel flow restrictor it might need to be replaced.

Jeremy

Sugar Magnolia
05/06/2014, 02:42 PM
Replying to a nearly 4 year old bumped thread. Hmmm.

tkeracer619
05/06/2014, 08:58 PM
I skipped right to the end and would have replied as well if I didn't see your post :lol:

Can't blame the new guy for finding the search feature :D

Fish Guy, its cool to start your own thread about stuff. Don't be shy.

Also gotta give the sponsers credit, they didn't just reply "the one I sell". They actually read the new post and answered it. If only in politi...