New RO Membrane Higher TDS

[Compguy]

I just recently was noticing my RO Membrane was getting numbers of 5 and up pre-di, so I decided it had been along time since I changed it, so I purchased a new DOW 100gpd Membrane, I put it in and after using it for over 2 weeks now I am getting 15-20 readings pre-di, how is this possible? Is it possible that maybe I should have put in a 75gpd Membrane, would that make any difference? I replaced all other pre-filters about 2 weeks before I replaced the membrane. Of course my readings are 0 after the DI but my concern is the higher numbers pre-di. Any suggestions would be appreciated as I don't want any algae breakouts to come of this.

 

[bertoni]

The 100 gpd membranes are known to leak more contaminants than a 75 gpd membrane, at standard household pressures. If the cost of DI filtration gets to be considerable, I'd suggest replacing the membrane.

 

[HighlandReefer]

This membrane is rated at 65 psi. Is your water pressure less than this?

 

[Compguy]

This membrane is rated at 65 psi. Is your water pressure less than this?

Yes, but I run a booster pump, which I believe brings it up to around 90psi, I can't remember but I think thats what it brings it up to 90-100.

 

[HighlandReefer]

I am not familiar with your membrane. I do know that many of the 100 GPD membranes have a rejection rate around 90% whereas the lower GPD membranes operate at a 98% efficiency.

 

[Compguy]

will it hurt anything if I put in a lower gpd membrane? I have a 75gpd one laying around. I also contacted the company I purchased the unit from cause the membrane that was in there was down to around 1 or 2, and I purchased a replacement membrane that was the same that it came with.

 

[HighlandReefer]

You can take the ro membrane out if you want, but will use a lot more DI resins.

Did you check and make sure your membrane is set properly?

 

[bertoni]

I think that measuring the TDS going into the membrane and the TDS coming out would be useful, if you have a TDS meter.

 

[dwd5813]

do this

Did you check and make sure your membrane is set properly?

then do this

I think that measuring the TDS going into the membrane and the TDS coming out would be useful, if you have a TDS meter.

then post the result

 

[Compguy]

all has already been done, I keep tds meter coming in, pre-di, and out, here are my totals.

In - 370
Pre-DI - 17
Out - 0

I specifically remember double checking to make sure that the membrane seated properly.

 

[bertoni]

Hmm, that's about a 95% rejection rate. I think a good 75 gpd membrane would be more like 98%. You might want to ask the vendor what you should expect.

 

[dwd5813]

i get 97% rejection from my 75gpd dow filmtec. as said above, the 100 is supposedly a little less efficient, so i'd imagine the 95 (using Jonathan's calculation) you're seeing is about right. has there been any change in your source water tds?

 

[tmz]

I went to 75 gpd from 100gpd several years ago for the improved rejection rate. Just be sure to change the flow restrictor in the unit to match the membrane size.

 

[Buckeye Hydro]

If you have a dow/filmtec membrane, the 100 gpd membrane is spec'ed at 90% rejection rate, but that is at 50 psi. Your 95% rejection rate is very good. However you would get a higher rejection rate with for instance a 75 gpd membrane. A few percentage points difference in the rejection rate can make quite a difference in your DI life. Here are some rough estimates:

95% = 17 ppm ->294 gallons before DI exhausts
96% = 13 ppm ->385 gallons
97% = 10 ppm ->500 gallons
98% = 6 ppm ->833 gallons

Russ

 

[Compguy]

Ok, so it should be good, I just found it odd that I was getting alot better numbers with the original Membrane, but I re-ordered one that actually came with the unit. So if I wanted to move to a 75gpd Membrane I would have to change out the flow restrictor, where is that usually located and what does it look like?

 

[scottwhitson]

It is located in the drain line of your membrane housing. It is usually just a small piece that slides inside the hose.

 

[Buckeye Hydro]

They are eaither a small plastic plug with a capillary tail INSIDE the upstream end of the waste water tube, or a cartridge type - about the size of your pinky, somewhere along the waste water tube.

Russ