Assistance with RO Membrane

[CFish4812]

Hello,

I noticed the TDS creeping up on my old membranes so I ordered replacements. I have a dual 75 GPD water saver unit. I took the time to go ahead and replace the sediment filter and carbon blocks as well.

I installed everything and the unit functioned as usual. I flushed everything for one hour and made my fresh water. Water came out of DI at 0 TDS as expected.

The issue is the TDS before the DI. It read between 16-22 between several tests over several days. I decided to flush the system for an additional hour, but this made no difference. I ordered a TDS meter to double check the water.

My tap water tested in the 170-190 range. The TDS meter read in the 15-25 range when I tested the same water after the membranes so pretty close to the meter on the unit. Previous test was tap water at 190 and 18 TDS after both membranes for a rejection rate of 90.6%. This is not close to the advertised 98-99%.

I took the membranes out to make sure they were in the canisters correctly and firmly inserted and they were. I thought maybe some water was bypassing the o rings and mixing with the purified water. No luck finding a loose membrane. I checked all my hose connections and everything is good with no leaks. The pressure was 78 PSI when the unit is operating with only cold water being used.

I decided to lubricate the o rings with food grade silicone grease to ensure a proper seal and then flushed for another hour. The TDS actually increased to 26. Maybe I used too much? I tried to use a minimal amount and wiped off excess.

The last thing I could think of was to test each membrane individually to see if one had much higher TDS water than the other. Both were flushed and tested and the results were the same.

Membrane 1:
Tap Water: 165 TDS
TDS Meter before DI: 26
RO Unit TDS Meter: 22
PSI: 82
Rejection Rate: 86.7%

Membrane 2:
Tap Water: 174 TDS
TDS Meter before DI: 29
RO Unit TDS Meter: 24
PSI: 84
Rejection Rate: 86.3%

Does anyone have any suggestions or ideas? I’m concerned I’ll burn through DI rapidly with these numbers. My previous membranes resulted in 3-5 TDS before DI. Those membranes lasted 4 years so I haven’t purchased recently. Maybe a change with the product or did I get bad membranes?

Thank you to anyone who contributes to this discussion.

 

[Saltyreef]

What brand membrane?
Whats your product to waste ratio?

 

[CFish4812]

What brand membrane?
Whats your product to waste ratio?

It’s the 75 GPD DuPont Filmtec membrane. I have two of them on the unit.

I haven’t checked the product to waste water ratio, but could do so later this afternoon if that will give clues to what the issue is.

 

[AstroMelly]

How long did you break them in for? In my experience new RO membranes can take much longer than you would think to start delivering reasonable filtration results. I have had to run for several hours (4+) to get near my previous membrane performance on multiple occasions - and sometimes I just find a bad one that never quite cuts it.

I’ve got a divert tap after the RO membrane and before the DI and I just throw that early wash away.

Sorry OP I noticed you gave your timings but still relevant ^^^

 

[CFish4812]

How long did you break them in for? In my experience new RO membranes can take much longer than you would think to start delivering reasonable filtration results. I have had to run for several hours (4+) to get near my previous membrane performance on multiple occasions - and sometimes I just find a bad one that never quite cuts it.

I’ve got a divert tap after the RO membrane and before the DI and I just throw that early wash away.

Sorry OP I noticed you gave your timings but still relevant ^^^

Thanks for the information. I’ve had the membrane making water for around two weeks now. I’ve probably made 50 gallons of water using them. I always flush for 5 minutes before and after use. I’ve had this system for 6-7 years and this hasn’t happened before. It’s not ever taken this long to break in a membrane.

I believe the company I bought them from is going to send replacements. They said 90% isn’t too far off for a dual membrane system but that is also not correct in my experience.

This same setup with the previous membranes had a 98% rejection rate before DI.

 

[AstroMelly]

Yes I re-read you first post and those numbers are very high. I only run a single RO membrane - I used to run a 50god but upgraded to a 100gpd when my original pump died. We have ~300 TDS from the tap and if my RO output is <=6 then I am happy. That’s a 98% rejection rate. The best RO membranes I have had settled at 3 - sometimes running at 2. A bad one (50gpd) would not drop below about 10-12 iirc and I got a replacement sent out for that foc.

Currently the membranes I am running will get to 5 but no lower.

I run dual DI chambers and I’ve still got my original setup - only the pump is broken - so I did consider running dual RO membranes but never got round to it. My 100 gpd gives me about 20 litres an hour and that’s enough.

4 years is a long time to be running a single membrane - I take it you’ve cleaned/replaced your sediment and carbon filters as well? Sorry if you already detailed that but I tend to replace those alongside every couple of membranes or at least rinse the washable sediment filter.

EDIT: I saw you had replaced the pre filters! Bad batch imo.

 

[CFish4812]

Yes I re-read you first post and those numbers are very high. I only run a single RO membrane - I used to run a 50god but upgraded to a 100gpd when my original pump died. We have ~300 TDS from the tap and if my RO output is <=6 then I am happy. That’s a 98% rejection rate. The best RO membranes I have had settled at 3 - sometimes running at 2. A bad one (50gpd) would not drop below about 10-12 iirc and I got a replacement sent out for that foc.

Currently the membranes I am running will get to 5 but no lower.

I run dual DI chambers and I’ve still got my original setup - only the pump is broken - so I did consider running dual RO membranes but never got round to it. My 100 gpd gives me about 20 litres an hour and that’s enough.

4 years is a long time to be running a single membrane - I take it you’ve cleaned/replaced your sediment and carbon filters as well? Sorry if you already detailed that but I tend to replace those alongside every couple of membranes or at least rinse the washable sediment filter.

Yes you’re correct I replaced the sediment and carbon filters at the same time I replaced the ro membranes. I think my last ones made it so long since I have a smaller system and my water needs weren’t as high as they used to be.

I’m definitely just wanting that 97-98% rejection rate. At this point I’d even be happy with 95-96%, but 86-90% is not the performance I’m looking for.

 

[AstroMelly]

Yes I re-read you first post and those numbers are very high. I only run a single RO membrane - I used to run a 50god but upgraded to a 100gpd when my original pump died. We have ~300 TDS from the tap and if my RO output is <=6 then I am happy. That’s a 98% rejection rate. The best RO membranes I have had settled at 3 - sometimes running at 2. A bad one (50gpd) would not drop below about 10-12 iirc and I got a replacement sent out for that foc.

Currently the membranes I am running will get to 5 but no lower.

I run dual DI chambers and I’ve still got my original setup - only the pump is broken - so I did consider running dual RO membranes but never got round to it. My 100 gpd gives me about 20 litres an hour and that’s enough.

4 years is a long time to be running a single membrane - I take it you’ve cleaned/replaced your sediment and carbon filters as well? Sorry if you already detailed that but I tend to replace those alongside every couple of membranes or at least rinse the washable sediment filter.

EDIT: I saw you had replaced the pre filters! Bad batch imo.

I went and found an image of the best performing filter I ever ran - I searched high and low and cannot find anyone supplying these in the UK any more.

These were giving me 99% plus rejection and seemed to last well. I produce quite a lot of water so they were lasting maybe 8-12 months.

 

[CFish4812]

I went and found an image of the best performing filter I ever ran - I searched high and low and cannot find anyone supplying these in the UK any more.

These were giving me 99% plus rejection and seemed to last well. I produce quite a lot of water so they were lasting maybe 8-12 months.

Thanks for sharing. I just hope it isn’t something I’m doing wrong. Everything looks fine though. If the replacements don’t work I figure I’ll try a different brand.

 

[AstroMelly]

I went and found an image of the best performing filter I ever ran - I searched high and low and cannot find anyone supplying these in the UK any more.

These were giving me 99% plus rejection and seemed to last well. I produce quite a lot of water so they were lasting maybe 8-12 months.

Thanks for sharing. I just hope it isn’t something I’m doing wrong. Everything looks fine though. If the replacements don’t work I figure I’ll try a different brand.

If you’ve been running a dual RO membrane system for 4 years plus then I doubt you are doing anything wrong.

I’ve found most of the budget friendly membranes seem to be white labelled Chinese stuff. If you want a quality product sometimes you have to pay a little more and go with a reputable brand name.

At least then you will hopefully have someone who will be able to stand by their product.

Good luck with your replacements!