Adding a second rodi membrane

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Gtinnel

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A response to another thread just made me realize I really should add a second membrane to my rodi filter. I currently use a dow filmtec 100gpd membrane, if I add a second membrane do I just make it the same membrane so they have matching gpd or does it matter? Also I have an old membrane housing (it's around 20 years old) sitting around, would there by any issue with using it assuming the seals don't leak from age or would I just be better off buying the water saver kit from brs? Does the size of the restrictor change when adding a second membrane?

Just to make sure I do it correctly the waste water line from membrane 1 goes to the input side of membrane 2. The waste from membrane 2 goes through the restrictor to the drain. The filtered water from both membranes get tied together with a push fitting wye and then go to the di.

My pressure is plenty high enough to support 2 membranes so is there any reason not to add the second one? If not I should have done this years ago.
 
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A response to another thread just made me realize I really should add a second membrane to my rodi filter. I currently use a dow filmtec 100gpd membrane, if I add a second membrane do I just make it the same membrane so they have matching gpd or does it matter? Also I have an old membrane housing (it's around 20 years old) sitting around, would there by any issue with using it assuming the seals don't leak from age or would I just be better off buying the water saver kit from brs? Does the size of the restrictor change when adding a second membrane?

Just to make sure I do it correctly the waste water line from membrane 1 goes to the input side of membrane 2. The waste from membrane 2 goes through the restrictor to the drain. The filtered water from both membranes get tied together with a push fitting wye and then go to the di.

My pressure is plenty high enough to support 2 membranes so is there any reason not to add the second one? If not I should have done this years ago.

That sounds correct. How high of TDS is your source water?


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Mrtakeoff53

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Just wondering why you think you need 2 membranes? Are you looking for more flow through the system.

In the set-up you described, the 2nd RO membrane will get dirty very quickly due to the excess ‘stuff’ coming out of the first membrane waste water. You have described operating them in series, to filter the waste of RO #1 but tied them back as if operating them in parallel. I don’t think you will get good results this way.

If you are really wanting cleaner water, you need to tie the clean output side of RO #1 into the input of RO #2. Then tie the clean water from RO #2 to your DI. in this set-up, you are cleaning clean water to drop the TDS even more before it hits the DI stage. You will essentially have 1 clean water output from the system and 2 dirty water outputs to the drain.
 
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That sounds correct. How high of TDS is your source water?


1634222908898.png
I am aware that I should have checked that at some point to know my rejection rate, but I honestly never have. I can say that I get 1ppm tds coming out of my membrane, so assuming I get 99% rejection rate then that would calculate out to around 100ppm. So I would assume that it is plenty low enough to keep it under the recommended tds of 300ppm for adding a second membrane. I probably should eventually add a 3 stage tds meter too.
 

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Just wondering why you think you need 2 membranes? Are you looking for more flow through the system.

In the set-up you described, the 2nd RO membrane will get dirty very quickly due to the excess ‘stuff’ coming out of the first membrane waste water. You have described operating them in series, to filter the waste of RO #1 but tied them back as if operating them in parallel. I don’t think you will get good results this way.

If you are really wanting cleaner water, you need to tie the clean output side of RO #1 into the input of RO #2. Then tie the clean water from RO #2 to your DI. in this set-up, you are cleaning clean water to drop the TDS even more before it hits the DI stage. You will essentially have 1 clean water output from the system and 2 dirty water outputs to the draiI

I am not sure the goal of the OP here, but 2 ro membranes are commonly ran in this manner in order to reduce the amount of wastewater. It is not really about getting cleaner water. As you indicate if the goal is cleaner water prior to the DI then you run the clean water to the second RO. Gives very clean water... and massive amount of wastewater.
 
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Gtinnel

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Just wondering why you think you need 2 membranes? Are you looking for more flow through the system.

In the set-up you described, the 2nd RO membrane will get dirty very quickly due to the excess ‘stuff’ coming out of the first membrane waste water. You have described operating them in series, to filter the waste of RO #1 but tied them back as if operating them in parallel. I don’t think you will get good results this way.

If you are really wanting cleaner water, you need to tie the clean output side of RO #1 into the input of RO #2. Then tie the clean water from RO #2 to your DI. in this set-up, you are cleaning clean water to drop the TDS even more before it hits the DI stage. You will essentially have 1 clean water output from the system and 2 dirty water outputs to the drain.
Yes I am wanting to add a second membrane to both increase flow and decrease waste water.
I believe the way I'm planning on hooking them up is the traditional way to adding a second membrane, which isn't uncommon. It allows for the second membrane to filter the waste water from the first membrane. BRS actually sells a kit to add a second membrane this way.
I am not wanting to add a second membrane to make lower tds water. The water coming out of my membrane now is only 1ppm, which is easily reduced to 0 with my di, with allowing my di to last for a reasonable amount of time.

As for the second membrane getting dirty quicker I have no idea. Does the waste side of the first membrane have an incredibly high tds. If the ratio for the first membrane is 4:1 then the tds of the water entering the second membrane is only 25% higher, right? It's taking the stuff that is in 5 parts of water and reducing the same stuff into 4 parts of water. Actually it would be slightly less than 25% because 1ppm still makes it through my first membrane. Even if the life expectancy of the 2nd membrane is slightly less I don't see that as a big deal. Although it does make me wonder for people running 2 membranes do you check the tds after each separately or after they combine? If not done after combining how do you know which membrane needs replaced or do you just replace them at the same time?
 
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I am not sure the goal of the OP here
I'm sure you probably assumed my goal was the same as everyone who adds a second membrane, increase production and reduce waste.

I would've never even considered filtering the good water through another membrane. I'm not even sure if there is a high enough pressure on the permeate side for a second membrane to work properly. Reducing your output to around 25gpd would be kind rough too, both on waiting on my mixing station to fill and paying for a ton of waste water.
 

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I'm sure you probably assumed my goal was the same as everyone who adds a second membrane, increase production and reduce waste.

I would've never even considered filtering the good water through another membrane. I'm not even sure if there is a high enough pressure on the permeate side for a second membrane to work properly. Reducing your output to around 25gpd would be kind rough too, both on waiting on my mixing station to fill and paying for a ton of waste water.
Yes I figured so just did not want to speak for you, but as you say what you are doing is pretty common. I did it this way at one time, but my incoming TDS is way too high so the waste from my first RO was unreasonable. My second membrane was getting ruined way too quickly.
 
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Running two membranes in series is fine if your tap TDS isn't too high and you have good pressure, at least 70 psi but the higher the better. A booster pump might be a good investment and a lot of the time pays for itself in less DI usage.
Pressure is not an issue. I'm using a booster pump that I've had for decades and it outputs right at 100psi. Honestly it's higher than it should be. As for the tds of my supply I'm just guessing its good based on my output of my current membrane, but I guess I should probably check it before buying another membrane.
 
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Yes I figured so just did not want to speak for you, but as you say what you are doing is pretty common. I did it this way at one time, but my incoming TDS is way too high so the waste from my first RO was unreasonable. My second membrane was getting ruined way too quickly.
How did you determine your second membrane was ruined? Did you check the tds after each membrane separately to known each one's rejection rate?
 

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How did you determine your second membrane was ruined? Did you check the tds after each membrane separately to known each one's rejection rate?

Ruined as in the tds coming out of it was very high within about 6 months(even though i regularly flush). The flow out of the second ro membrane also got to just a small trickle. This was causing me to burn though DI resin and overall was just adding a lot of cost for minimal benefit. Instead of replacing the second ro i just got rid of it and went back to the single ro.

Yes I have the pen style tds meter so disconnect a hose and fill a small cup just to measure the tds at various stages.

On a somewhat related note soon we are getting a whole house water softener and i will then try the second membrane again. As i understand it the softener does not reduce my tds, but it makes it much easier for the RO to handle and should work much more efficiently. At least that is the goal.
 
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Ruined as in the tds coming out of it was very high within about 6 months(even though i regularly flush). The flow out of the second ro membrane also got to just a small trickle. This was causing me to burn though DI resin and overall was just adding a lot of cost for minimal benefit. Instead of replacing the second ro i just got rid of it and went back to the single ro.

Yes I have the pen style tds meter so disconnect a hose and fill a small cup just to measure the tds at various stages.

On a somewhat related note soon we are getting a whole house water softener and i will then try the second membrane again. As i understand it the softener does not reduce my tds, but it makes it much easier for the RO to handle and should work much more efficiently. At least that is the goal.
Wow I'd think it has to get pretty bad before it reduces the flow considerably. It was how you knew it was the second membrane was bad that I was wanting to know. So you'd just disconnect the line and check the tds after each membrane to know which one was putting the high tds water into your di.
 
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Gtinnel

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Never guess your incoming tds, test it. But yes run as many as you want. I run 3 membranes because I have low tds to start with. DI last me years
I've already decided that before I buy the second membrane I am going to pull one of my tds probes off and add it inline before the rodi filter to see what my tds is from my supply water. If you are running 3 membranes then what is the maximum starting tds and required pressure for the third membrane to work efficiently.

I mean if one extra membrane is good, then 2 extra membranes are better right, assuming the last one gets clean enough water and enough pressure to work.
 

outhouse

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I've already decided that before I buy the second membrane I am going to pull one of my tds probes off and add it inline before the rodi filter to see what my tds is from my supply water. If you are running 3 membranes then what is the maximum starting tds and required pressure for the third membrane to work efficiently.

I mean if one extra membrane is good, then 2 extra membranes are better right, assuming the last one gets clean enough water and enough pressure to work.
Depends on incoming TDS. Mine is 37 ppm. So I can add what ever I want. At 100 you can run 2 even try 3. It's the last membranes output that determines what you can get away with, based on how much DI will you use. If you have 1 to 3 your OK. The trick is playing with different restrictors to tune your system. I also use a extra pump to keep pressure 90 ish
 
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Depends on incoming TDS. Mine is 37 ppm. So I can add what ever I want. At 100 you can run 2 even try 3. It's the last membranes output that determines what you can get away with, based on how much DI will you use. If you have 1 to 3 your OK. The trick is playing with different restrictors to tune your system. I also use a extra pump to keep pressure 90 ish
My tds meter only has two probes, but I'm going to hook it up so I can see what my supply tds is. I doubt my tds is low enough for a third membrane, even if it is low enough I'm not willing to add a second booster pump if it would be needed. I'm not likely to add a third, it's just that adding more than 1 extra membrane never even crossed my mind.
 

outhouse

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My tds meter only has two probes, but I'm going to hook it up so I can see what my supply tds is. I doubt my tds is low enough for a third membrane, even if it is low enough I'm not willing to add a second booster pump if it would be needed. I'm not likely to add a third, it's just that adding more than 1 extra membrane never even crossed my mind.
I run a probe incoming, and a probe after 3rd membrane. You only need one booster pump. Trick is restricting waste water out of last membrane to tune product.
 
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