RODI: Second Membrane Failing Early

[FirstContact]

So this year I added a second membrane and booster pump to my RODI. The booster pump from BRS has worked great. The add on membrane kit not so much. Basically, I think the waste water from the first membrane fouls the second membrane very quickly and then you just wasted $45 on the membrane and throw more money away on DI resin. Okay. Booster pump has worked great though.

I’m only feeding my first membrane about 125 ppm TDS. The waste water output from the first membrane is 185 ppm TDS. A second membrane should be able to handle that right? My water pressure before the first membrane is 85 psi. No issues with the first membrane at all; so, it can’t be a problem related to the prefilters or water quality. This is the second membrane I have tried as a dual stage with waste water feeding the second membrane from the first. It definitely just doesn’t work for any reasonable amount of time. I put a new membrane in the second stage in April. It now runs at 35 ppm TDS with the flush valve open and 50 ppm with the flush valve closed. I always get several years out of my membranes except this dual stage set up where the waste water goes to the intake on the second membrane. Starts out great measuring at 2 or 4 ppm but that doesn’t last and the membrane is quickly fouled. My first membrane puts out 4 ppm TDS. Based on advice from BRS, I tried reseating the second membrane. Nope, doesn’t work. Swapping out the RO tubing making the second membrane the first and the first membrane the second doesn’t work either. It runs 28 ppm TDS with the flush valve open. That just proves the membrane is fouled. My HM Digital TDS triple in line meter is new and installed correctly. Also have a Hanna hand held. So there’s really no way for the first membrane to continue to work great and the second membrane to fail so soon except for the fact that your feeding it highly concentrated waste water from the first membrane? Right?

BRS says this works for them long term and that their TDS from their second membrane runs about 7 ppm. I spoke with them Friday regarding troubleshooting. I’d love for this to work, but I can’t see trying this with a 3rd membrane unless someone can tell me what I’m doing wrong.

Anyone have any thoughts? Does this work for you? What’s different about my set up? Does this not work for you also?

 

[Flippers4pups]

Let's see if anyone on the #reefsquad has any experience with multiple membrane RO/DI units. Unfortunately I don't.

 

[Retro Reefer]

Never used a dual membrane RO system but I wonder if there is a significant enough loss in pressure after first membrane that it might be causing premature failure of the second.

 

[saltyfilmfolks]

Is your rodi set up like this ?
It should be the same as the BRS minus the pump.

 

[DLHDesign]

I've got a dual membrane setup and it works like a charm for me. My input TDS is around 320ppm and my pressure is around 65PSI.

A few suggestions (please forgive if these have been tried or seem obvious; trying to come up with anything I can to help);
- Are you using 100gpd membranes in both? (versus the 75gpd or 150gpd)?
- Have you run the second membrane for at least an hour?
- Do you know if you have unusual water temperature (not ~77F)?
- You said you've tried reseating the membrane; have you checked to be sure it's oriented correctly?
- What size flow restrictor are you using?

Interesting (and unfortunate) scenario; I hope we can get to the bottom of it for you.

 

[chipmunkofdoom2]

So there’s really no way for the first membrane to continue to work great and the second membrane to fail so soon except for the fact that your feeding it highly concentrated waste water from the first membrane? Right?

TDS is only part of the equation. You don't know how much pressure you're giving the second membrane. It's also almost impossible in reject staged setups setups to make sure that both membranes are getting the brine flow they need. Under very poor pressure and flow conditions, low TDS water can foul a membrane quickly.

I've never liked dual membrane setups for these reasons, along with a few others. I personally would take the second membrane offline.

 

[BZOFIQ]

You're just better off running these in parallel.

 

[KStatefan]

This seems like this topic is coming up regularly lately.

I do not care for the dual membrane set ups either.

 

[DLHDesign]

You don't know how much pressure you're giving the second membrane.

This is certainly true and could very well be the key factor of why the OP setup is having issues. But it's also something that is easily measured by moving the existing pressure gauge.
I know when I've measured mine, there was not a significant drop in pressure into the second unit (I don't recall the numbers, sorry). But that is anecdotal and may just be my good fortune; certainly every system is unique.

 

[FirstContact]

Hello Everyone,

Thank you for your responses.

Sometime between tomorrow night and this weekend, I will double check the pressure on the second membrane and check the water temp.

Flow restrictor: rated 75-100 gpd, fits inside the RO tubing (calling this capillary?)

My set up is identical to the BRS set up with the following exceptions: my flow restrictor is internal or capillary, my auto shut off valve is made by Kent Marine (different style), and I have and use a flush valve kit installed after the second membrane.

Both membranes are 100 gpd (will verify the first membrane also). The membranes are seated correctly and not reverse oriented.

There seems to be a lot of focus on pressure and psi. I’m wondering if anyone knows the minimum pressure the 2nd membrane needs? I’m getting 85 psi before the first membrane, but that’s with the flush valve kit closed. It drops significantly when opened to around 50ish psi, but I’ll have to look at the psi into the second membrane when the flush valve is open sometime this weekend. Never had issues with a single membrane without the booster pump. In fact, I purchased the booster pump specifically for the dual membrane set up after the 2nd membrane failed the first time. I run the system with the valve open before and after running water through my DI resin. The pre and post flush is redirected to a bucket and doesn’t go through the resin. Also, I wasn’t sure that the pressure was supposed to drop drastically between membranes, but I can test this theory and move the pressure gauge to the front of the 2nd membrane.

Does anyone with a working dual membrane set up use a flush valve kit?

 

[DLHDesign]

I’m wondering if anyone knows the minimum pressure the 2nd membrane needs?

The 100gpd membranes are rated at 50psi @ 77F - which is the optimal values for both the primary and secondary while in use (e.g.; not during a flush-out). Most residential water is actually lower temp than 77F, which means it's "thicker". This difference will result in lower throughput, which is often attempted to be offset by increasing the pressure a little bit.
For RO membranes, too much pressure is just as bad as too little - both result in the membrane being less efficient than it's designed to be. If your pressure is too high, you could be forcing particulates through the membrane that it would normally catch. If your pressure is too low, the membrane will clog up and reject more than it should.

Does anyone with a working dual membrane set up use a flush valve kit?

Yes; I use an auto-flush valve from BRS; to 500ml/min model, specifically.

One other question;
What is your approx. product::waste ratio?

 

[BZOFIQ]

@Buckeye Hydro

Perhaps Russ can chime in here.

 

[Buckeye Hydro]

I've read this thread... and it reminds me of the many many calls we get on these low pressure residential systems plumbed in series.

To address some points above:
concentrate pressure out of first membrane is similar to feed pressure into first membrane.

Yes; I use an auto-flush valve from BRS; to 500ml/min model, specifically.

that flow restrictor is for a 50 gpd membrane - you have 200 gpd.

when you string two 100 gpd membranes together and use a 75 gpd flow restrictor... yes, chances are you are going to have problems.

OPer - if you want to call us when you are in front of your system we can troubleshoot it with you.

Russ

 

[Buckeye Hydro]

For RO membranes, too much pressure is just as bad as too little - both result in the membrane being less efficient than it's designed to be.

Not so much. Residential scale (1812) Filmtec membranes can handle much more pressure than many other components in these systems. Higher pressure yields improved rejection (RO water that is more pure), although the improvement curve begins to flatten out after you exceed factory spec pressure. See data below from a Filmtec 75.

 

[Buckeye Hydro]

Does anyone with a working dual membrane set up use a flush valve kit?

Are you talking about a FLUSH KIT (bypass around the flow restrictor on the concentrate line; or
a DI BYPASS (valve that temporarily routes RO water to drain rather than to DI?

These are two very different things

 

[FirstContact]

Are you talking about a FLUSH KIT (bypass around the flow restrictor on the concentrate line; or
a DI BYPASS (valve that temporarily routes RO water to drain rather than to DI?

These are two very different things

Hi Russ!

I have both a flush kit that bypasses around the flow restrictor (my flow restrictor is rated at 75-100 gpd internal/capillary style from Marine Depot; I understand you write above that this is incorrect); and also have a DI bypass valve.

What rated flow restrictor should I be using with 2 100 gpd membranes? Should I be using the flush kit that bypasses the flow restrictor with this setup?

I would definitely enjoy trouble shooting my system on the phone with you while I am in front of my system. I can call anytime on the weekends and between 7-8 pm Mon-Fri if I’m not working late. Saturday might be better, but not sure what your schedule is. I just call the number on the website and ask for you?

 

[Buckeye Hydro]

Based upon some assumptions about your feedwater, I'd go with the flow restrictor for a 150 gpd membrane. Using a flush valve with this flow restrictor is a good idea.

Feel free to call in between 8 and 6, seven days a week. 513-312-2343 and ask for me.

Thanks,
Russ

 

[FirstContact]

Thanks for all your help today Russ!

Hi Everyone,

Yes, I used 2 100gpd membranes.
Water temperature is 74 degrees.
Water pressure into 2nd membrane with booster pump on and flush valve closed is 70 psi.
Values are lower with the flush valve open and even lower with the booster pump off. I was advised that flushing the membranes and thereby having lower psi values is not a big concern due to how infrequent the unit runs under these conditions and that this would in no way cause the membrane to fail so prematurely.

Even though my TDS out of the tap is extremely low comparatively speaking at 124 ppm TDS, I was advised that other factors in the water could cause the 2nd membrane to be failing: hardness, iron, and a couple other things were mentioned. Not entirely sure why the same would not effect the first membrane.

End result, I’m going to switch to running both membranes in parallel instead of in line.

 

[FirstContact]

Duplicate

 

[Buckeye Hydro]

Water quality issues are exaggerated @ the second membrane when you plumb membranes in series. ~20% of the pure water has been removed and contaminants are concentrated to a greater degree.

If you'd like, send us your second membrane (the bad one) and we can autopsy it. No promises but we can often figure out what contaminant is causing the issue.

Russ