Changing rodi membrane

[Gtinnel]

So the poll of the day got me thinking about my rodi system. I've suspected for a few weeks now that I need to replace my membrane because my tds after the membrane has climbed to 8, which used to always stay around 1. So unless my supply water tds skyrocketed (idk I don't check it) then it's time to replace the membrane. It's been 2 years so I suspect it's the membrane.

Does everyone just get the DOW membrane or are there any other decent options out there? I see some on amazon for as cheap as $20 (the DOW is only $40) so I'm willing to go cheaper, but I don't care to pay the extra for DOW if it's worth it. I believe DOW is kinda the industry standard and I'm guessing there is some reason for that.

Also, are there any disadvantages to going up to a 150 gpd membrane? I currently use a 100gpd but the ability to make water quicker would be nice. They both have a 98% rejection rate so it's supposedly not any less efficient. The only disadvantages I see are the membrane is slightly more expensive and I'd have to change my restrictor. I don't see a 150 gpd DOW membrane, do they not make one and if not what brand does everyone use for 150gpd?

 

[Mike konesky]

99% spectrapure 75gpd for best quality. They are just dow membranes that they test to ensure the 99% instead of 98. The Dow 100s CAN also be 99%, but it's a 50/50 chance ime. Spend the extra and get the spectrapure to avoid ending up with 98% or even 97% membranes.

 

[KStatefan]

I would not run anything but the Spectrapure or a Filmtec membrane.

 

[Gtinnel]

99% spectrapure 75gpd for best quality. They are just dow membranes that they test to ensure the 99% instead of 98. The Dow 100s CAN also be 99%, but it's a 50/50 chance ime. Spend the extra and get the spectrapure to avoid ending up with 98% or even 97% membranes.

I had never looked at spectrapure. On their website they have one that is 99% rejection at 90gpd, would I need to replace the restrictor going from 100gpd to 90 gpd. I doubt I would but honestly I never knew someone made 90gpd membranes, so what do I know. It looks like all of their other membranes on their site are still between 96-98%.

 

[Woodyman]

So the poll of the day got me thinking about my rodi system. I've suspected for a few weeks now that I need to replace my membrane because my tds after the membrane has climbed to 8, which used to always stay around 1. So unless my supply water tds skyrocketed (idk I don't check it) then it's time to replace the membrane. It's been 2 years so I suspect it's the membrane.

Does everyone just get the DOW membrane or are there any other decent options out there? I see some on amazon for as cheap as $20 (the DOW is only $40) so I'm willing to go cheaper, but I don't care to pay the extra for DOW if it's worth it. I believe DOW is kinda the industry standard and I'm guessing there is some reason for that.

Also, are there any disadvantages to going up to a 150 gpd membrane? I currently use a 100gpd but the ability to make water quicker would be nice. They both have a 98% rejection rate so it's supposedly not any less efficient. The only disadvantages I see are the membrane is slightly more expensive and I'd have to change my restrictor. I don't see a 150 gpd DOW membrane, do they not make one and if not what brand does everyone use for 150gpd?

The above comment gets my vote, I run 2 of the 90gpd 99% membranes from Spectrapure in my system. They are worth the extra expense.

this is the exact one (HERE)

As you said though check your source water, you should check it fairly regularly because your incoming TDS can change based on the weather/season even on municipal water sources. To determine if your membrane is operating below its targeted efficiency you can use the calculation below.

((SOURCE WATER TDS - RO WATER TDS) / SOURCE WATER TDS) * 100 = Rejection % of membrane

example calculation using a source water TDS of 348 and an RO TDS of 8

((348 - 8) / 348) * 100 = x%
((340) / 348) *100 = x%
(0.9770) *100 = x%

97.7% rejection ratio if your membrane is rated for 98% then this wouldn't be to bad, and you likely still have a few more months of use from the membrane more than likely longer.

 

[LeftyReefer]

I would not run anything but the Spectrapure or a Filmtec membrane.

Which are both Dow Filmtec membranes.

 

[KStatefan]

I do not think the Spectrapure are as good of a value as the used to be. Dow has improved their membranes quite a bit in the last few years. The TW30-1812-50HR has a typical stabilized rejection rate of 99% that you can get for $35

 

[Gtinnel]

The above comment gets my vote, I run 2 of the 90gpd 99% membranes from Spectrapure in my system. They are worth the extra expense.

this is the exact one (HERE)

As you said though check your source water, you should check it fairly regularly because your incoming TDS can change based on the weather/season even on municipal water sources. To determine if your membrane is operating below its targeted efficiency you can use the calculation below.

((SOURCE WATER TDS - RO WATER TDS) / SOURCE WATER TDS) * 100 = Rejection % of membrane

example calculation using a source water TDS of 348 and an RO TDS of 8

((348 - 8) / 348) * 100 = x%
((340) / 348) *100 = x%
(0.9770) *100 = x%

97.7% rejection ratio if your membrane is rated for 98% then this wouldn't be to bad, and you likely still have a few more months of use from the membrane more than likely longer.

I agree that I should periodically check my rejection rate, but currently since my after membrane tds went up 8 times, I strongly suspect it's my 2 year old membrane. Assuming I was originally getting a 98% rejection rate (a complete assumption since I never checked my supply tds) it means my municipal water supply would have had to jump from about 50ppm to 400ppm, which seems unlikely.

 

[Woodyman]

I agree that I should periodically check my rejection rate, but currently since my after membrane tds went up 8 times, I strongly suspect it's my 2 year old membrane. Assuming I was originally getting a 98% rejection rate (a complete assumption since I never checked my supply tds) it means my municipal water supply would have had to jump from about 50ppm to 400ppm, which seems unlikely.

You are correct!

As stated above the Spectrapures are rebranded DOWs. I have used Spectapure for years and even used them for multiple water related projects in an industrial/manufacturing setting. I have a good relationship with them, and have always had great customer service and products. So while the DOW is the same I still buy from and support Spectrapure.

 

[Gtinnel]

I do not think the Spectrapure are has good of a value as the used to be. Dow has improved their membranes quite a bit in the last few years. The TW30-1812-50HR has a typical stabilized rejection rate of 99% that you can get for $35

I am strongly leaning toward what appears to be the 100 gpd version of that filter. It's is only rated at a 98% rejection which I am sure it's equal to or better than whatever brand membrane came in my cheap AquaFx rodi.

Does anyone have any opinion on increasing to a 150gpd membrane.

 

[Woodyman]

I am strongly leaning toward what appears to be the 100 gpd version of that filter. It's is only rated at a 98% rejection which I am sure it's equal to or better than whatever brand membrane came in my cheap AquaFx rodi.

Does anyone have any opinion on increasing to a 150gpd membrane.

If you change out your GPD rating you will need to make sure your flow restrictor is rated for the new GPD rating.

 

[Mike konesky]

I had never looked at spectrapure. On their website they have one that is 99% rejection at 90gpd, would I need to replace the restrictor going from 100gpd to 90 gpd. I doubt I would but honestly I never knew someone made 90gpd membranes, so what do I know. It looks like all of their other membranes on their site are still between 96-98%.

You can use the same restrictor as a 75gpd, a 550. Their 90gpd is just a Dow 75 at higher pressure. Kinda false advertising, but whatever.

 

[Gtinnel]

If you change out your GPD rating you will need to make sure your flow restrictor is rated for the new GPD rating.

I knew that they had to match. I just don't see a down side to upgrading other than a slight cost difference in the membrane and having to swap the restrictor. If there is no other down side then I'm considering increasing it so I can produce water quicker. Although I don't see a DOW FilmTec 150gpd membrane. If I can't find one that size from DOW I will probably just stick with the 100gpd FilmTec.

 

[Woodyman]

I knew that they had to match. I just don't see a down side to upgrading other than a slight cost difference in the membrane and having to swap the restrictor. If there is no other down side then I'm considering increasing it so I can produce water quicker. Although I don't see a DOW FilmTec 150gpd membrane. If I can't find one that size from DOW I will probably just stick with the 100gpd FilmTec.

You can always run 2 of the 75s to hit the 150 or even 2 of the 100s to hit 200gpd.

No real downside to upgrading that comes to mind.

 

[Gtinnel]

You can use the same restrictor as a 75gpd, a 550. Their 90gpd is just a Dow 75 at higher pressure. Kinda false advertising, but whatever.

Makes sense. I currently use a 100gpd membrane and I get significantly more than 100gpd out of it, but my pressure is probably higher than it should be honestly. Last time I measured it I was getting closer to 150gpd from it.

 

[KStatefan]

Makes sense. I currently use a 100gpd membrane and I get significantly more than 100gpd out of it, but my pressure is probably higher than it should be honestly. Last time I measured it I was getting closer to 150gpd from it.

Pressure and water temp will make a big difference in production rate. When I installed the 50HR membrane the first of the month at 74 psi and 72° F I was getting 90 gpd out of it. I expect half that this winter when the water temp drops 30° F.

 

[Gtinnel]

Pressure and water temp will make a big difference in production rate. When I installed the 50HR membrane the first of the month at 74 psi and 72° F I was getting 90 gpd out of it. I expect half that this winter when the water temp drops 30° F.

Yes, I assume I get almost 50% more production from it because my pressure is running around 100psi, I've never checked the water temp of my cold supply line, but obviously it fluctuates significantly by season. I'm assuming the higher pressure doesn't hurt anything since I've been running it that way for 2 years, and I had another rodi unit that was run at that pressure for many many years before that.

 

[KStatefan]

Higher pressure is good rejection and production both go up.