Optimum/recommended time to change RO TFC membrane?

[Rst]

QUESTION: Should I change the RO TFC membrane?

Background:

I make approximately 1oo gallons of fresh RODI water on average every month to meet the needs of my 170 gal reef system filled with mostly LPS corals & tangs. I use two separate TDS meters, one after the RO system(consists of 2 TFC membranes & 1 carbon cartridge), and another meter after separate stages of cation & then Anion & finally mixed bed filters. (if interested, see BRS video on "Does mixed bed DI resin really need to be mixed?"). The very large TFC membranes are rated at 2000 gals combined, but cost $300 to replace.

I use 2 TDS meters, first after the RO system and second after the DI stages (since the color changing resins are not in visible housings). I monitor the second TDS resin meter, ensuring it is always at zero.

I have just changed the carbon cartridge part in my RO system. During the making of my last batch of water, my RO tds meter reads 4 ppm, but the DI cartridges are still keeping the final output at zero.

 

[Randy Holmes-Farley Verified Community Expert]

Why do you think it needs to be replaced?

Replace it if flow gets so low that it isn't useful (i.e., too much water is being rejected), or if the TDS drop across the RO membrane is not adequate (meaning there might be a hole).

 

[chipmunkofdoom2]

RO membranes can last for many years. My last one lasted 10 years before rejection rates started to suffer significantly.

The most important factor you can control is changing your prefilters regularly. By and large, the sediment filter and carbon blocks protect the RO membrane from large particulate matter and chlorine/chloramines. If you're changing these filters every 6 months (or as indicated by testing) then you're doing all you can do. The other thing that impacts membrane life is the quality of your source water. Someone with municipal tap water of 200 ppm TDS will likely have much longer membrane life than someone with 800 ppm TDS well water. But, you can't really control the quality of your source water.

Dow's typical salt rejection rate for their 75GPD membranes is 96% at 50PSI and 70*F. If you wanted to replace your membrane as soon as it drops below 96% rejection rate, you could. Keep in mind that due to feed water temperature and pressure, your actual rejection rate may be lower. A better bet might be to measure your rejection rate now and change the membrane when that starts to drop.

To summarize, RO membranes will last for years if properly protected. The only way to tell if they need to be replaced is to measure the rejection rate.

 

[Fumanchu]

Talking about OVERKILL is your source water that bad that you need 2 membranes an 3 DI canisters . 4 ppm tds after a dual membrane sounds about normal 2 after a single membrane. IMO 3 canisters is great market ploy from BRS excluding those with truly horrible tap water . I'm sure your L.P.S are thankful but I think it's overkill and that you don't have a problem .IMO the monies would be better spent on premium salts and New corals .
Best of luck and happy reefing

 

[chipmunkofdoom2]

Talking about OVERKILL is your source water that bad that you need 2 membranes an 3 DI canisters . 4 ppm tds after a dual membrane sounds about normal 2 after a single membrane. IMO 3 canisters is great market ploy from BRS excluding those with truly horrible tap water . I'm sure your L.P.S are thankful but I think it's overkill and that you don't have a problem .IMO the monies would be better spent on premium salts and New corals .
Best of luck and happy reefing

I don't think it's fair to say that using single bed resins is only necessary in situations with terrible tap water. You can likely always see a benefit regardless of how dirty your water is. It might be most beneficial with dirty tap water, but that doesn't mean others can't benefit from running single beds followed by a mixed bed.

Here's a picture of my single bed resins (cation is on the left, anion is on the right). I started using them in October of 2017. As you can see, the anion resin has changed colors about 70% of the way up, but the cation anion hasn't changed colors in the slightest. Only a few beads at the bottom of the canister have turned yellow. The final mixed bed hasn't changed color at all:



I expect I'll get another 5 months or so out of the anion resin. I don't know how long I'll get out of the cation resin, because it looks like it hasn't even started depleting yet. I will say that my water is relatively clean, about 5 - 6 ppm TDS going into the resin. But I disagree with the notion that there is no benefit to single beds unless you have exceptionally dirty water.

 

[Fumanchu]

Ok ....... I said in my opinion only and at the end of the day 0 tds is 0tds and if you feel better with your ions separated good for you .. but what I said was 3 chambers isn't necessary and I'm sure your going to disagree but I'm not gonna argue your right more power to 3 DI CANISTER I'm sure they grow and make corals nicer ...
Happy Reefing

 

[Rst]

Why do you think it needs to be replaced?

Replace it if flow gets so low that it isn't useful (i.e., too much water is being rejected), or if the TDS drop across the RO membrane is not adequate (meaning there might be a hole).

Randy,
I have never really known when is the right time to replace my membranes, and since I now have run over 5500 gallons thru the RO system and my TDS output reading has risen from zero when new to now reading 4ppm, I thought that I should ask. I also thought this question answered from experience, might also be helpful to others.

I don’t have a device to tell me how many gallons has been rejected, I can’t use that as a gauge. I do have a cheap gallon meter on my source water and it’s now reads 5500 gallons since I first installed the RO system. I can tell you that currently, my city water on input to the RO system is 158 ppm and output is 4 ppm.

So I thought this was a good time to make this enquiry and want to know when should I expect to change the membrane and if appropriate, at what approximate ppm should I wait until changing.

Thanks for your input.

 

[Rst]

RO membranes can last for many years. My last one lasted 10 years before rejection rates started to suffer significantly.

The most important factor you can control is changing your prefilters regularly. By and large, the sediment filter and carbon blocks protect the RO membrane from large particulate matter and chlorine/chloramines. If you're changing these filters every 6 months (or as indicated by testing) then you're doing all you can do. The other thing that impacts membrane life is the quality of your source water. Someone with municipal tap water of 200 ppm TDS will likely have much longer membrane life than someone with 800 ppm TDS well water. But, you can't really control the quality of your source water.

Dow's typical salt rejection rate for their 75GPD membranes is 96% at 50PSI and 70*F. If you wanted to replace your membrane as soon as it drops below 96% rejection rate, you could. Keep in mind that due to feed water temperature and pressure, your actual rejection rate may be lower. A better bet might be to measure your rejection rate now and change the membrane when that starts to drop.

To summarize, RO membranes will last for years if properly protected. The only way to tell if they need to be replaced is to measure the rejection rate.

Thanks chipmunkofdoom2,
I don’t know what the rejection rate of my membranes are. I do know that the psi across the membranes is approximately 50 psi and the water temperature ranges from 40 degrees in winter and maybe 60 degrees in summer. I will make an enquiry to the manufacturer (Hydrologic RO1000 System - rated 1000 gallons per day).

 

[Rst]

Talking about OVERKILL is your source water that bad that you need 2 membranes an 3 DI canisters . 4 ppm tds after a dual membrane sounds about normal 2 after a single membrane. IMO 3 canisters is great market ploy from BRS excluding those with truly horrible tap water . I'm sure your L.P.S are thankful but I think it's overkill and that you don't have a problem .IMO the monies would be better spent on premium salts and New corals .
Best of luck and happy reefing

Thanks Fumanchu,
Yes I know that this is overkill but sometimes I really can’t help myself. Nevertheless since the cost of replacing these membranes is not trivial, I like to know what method I should use to gauge.

 

[Rst]

I don't think it's fair to say that using single bed resins is only necessary in situations with terrible tap water. You can likely always see a benefit regardless of how dirty your water is. It might be most beneficial with dirty tap water, but that doesn't mean others can't benefit from running single beds followed by a mixed bed.

Here's a picture of my single bed resins (cation is on the left, anion is on the right). I started using them in October of 2017. As you can see, the anion resin has changed colors about 70% of the way up, but the cation anion hasn't changed colors in the slightest. Only a few beads at the bottom of the canister have turned yellow. The final mixed bed hasn't changed color at all:



I expect I'll get another 5 months or so out of the anion resin. I don't know how long I'll get out of the cation resin, because it looks like it hasn't even started depleting yet. I will say that my water is relatively clean, about 5 - 6 ppm TDS going into the resin. But I disagree with the notion that there is no benefit to single beds unless you have exceptionally dirty water.

Thanks chipmunkofdoom2 for your support of running 3 chambers of DI resins. Since I make a significant amount of RODI water monthly, I thought that this would save on replacing resins. Good to know that this is working for you.

Unfortunately I used non see thru resin canisters, so instead, I thought that I could just use TDS meters. Big mistake! TDS readings after the Cation and after the anion are totally screwed and don’t make any sense. Example Cation input 7 ppm and output 10 ppm, Anion input 8 ppm and output 5 ppm, mixedbed input 5 ppm and output 0 ppm. Different TDS meters would account partially for some of this but not the 7ppm input and 10 ppm output across the Cation.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks chipmunkofdoom2 for your support of running 3 chambers of DI resins. Since I make a significant amount of RODI water monthly, I thought that this would save on replacing resins. Good to know that this is working for you.

Unfortunately I used non see thru resin canisters, so instead, I thought that I could just use TDS meters. Big mistake! TDS readings after the Cation and after the anion are totally screwed and don’t make any sense. Example Cation input 7 ppm and output 10 ppm, Anion input 8 ppm and output 5 ppm, mixedbed input 5 ppm and output 0 ppm. Different TDS meters would account partially for some of this but not the 7ppm input and 10 ppm output across the Cation.

That's right, TDS after a specific anion or cation resin won't "make sense" for perfectly understood reasons (the resins swap OH- or H+ for the anions or cations, but OH- and H+ are more conductive than any other ions they are swapped for). The TDS will rise after just one of them, then hit zero after both of them are passed through.

 

[Rst]

That's right, TDS after a specific anion or cation resin won't "make sense" for perfectly understood reasons (the resins swap OH- or H+ for the anions or cations, but OH- and H+ are more conductive than any other ions they are swapped for). The TDS will rise after just one of them, then hit zero after both of them are passed through.

Chemistry has always been a challenge for me, so I am very happy you responded. In my case, are my TDS readings, after passing thru both the Cation (input 4 ppm) and then the Anion (output 5 ppm) stages sensible or should the 2nd stage Anion output actually be reading zero? At any rate, so much for using TDS meters to determine when any of my 3 stages of media are exhausted. Looks like the only alternative for me would be to change out all of the canister’s inner jackets for clear and use color changing for notifications.

With respect to this original thread subject, I have gathered that the easiest way to know when your RO tfc membrane needs to be replaced would be when the rejection ratio drops below the manufacture’s rated specs. For my example, TDS reads 156 ppm input and 4 ppm output, giving 156-4/156 = 97.3% rejection. If my rated membrane is 98% rejection, do I now change my membranes or if not, what other rule of thumb criteria would you suggest?

 

[Randy Holmes-Farley Verified Community Expert]

The second stage should be zero when the cartridges are new. As they deplete, it will rise.