New RO Membrane Putting Out Higher TDS Water - What Could Cause This?

[george9]

Hi all,
I have kind of an interesting issue that I cannot figure out. Recently I measured 12 TDS coming out of my RO chamber with tap water of 140 TDS, and since the membrane was about 1.5 years old, I decided it was time to replace it given the higher rejection rate. Ordered the new membrane, installed it last night and was getting 14 TDS out of the RO chamber initially, and this actually increased to 20 TDS after about an hour. After about another hour, it dropped to 16 TDS but hasn't dropped below there and I cannot figure out what is wrong here. While after DI, it is coming out 0 TDS, it should at the very LEAST be no more than 12TDS after RO like the old filter was immediately before changing it, so I am not sure where the error is coming from.

Here's the stats as it currently stands:
BRS 75 GPD 4-stage system - https://www.bulkreefsupply.com/75gpd-4-stage-value-ro-di-system-bulk-reef-supply.html
Pre-filters are 1.5 months old, so I know they are good still
Tap - 140TDS
RO - 16 TDS (previously 12 with old filter immediately before changing)
RODI - 0 TDS

Things I do not know and wish I did:
1. Water pressure (but this shouldn't matter here - main issue is why am I getting higher TDS immediately after changing to a new filter?)
2. TDS of RO water when the original membrane was new back in late 2021 (I only knew about measuring final TDS after DI, which was always 0 so it is hard to compare to the RO TDS values I am getting today). Maybe the original membrane also took a while to drop in TDS and my DI was simply eating the excess without me knowing at this time - I'll never know this because I didn't measure :/

So there are a couple key things I do not know. Despite this, the way I see it is this: the TDS of the new membrane needs to be at equal to the old one to know it is working properly since no other variable changed - and in this case it clearly is not. I let the new membrane run about 2 hours with the DI disconnected to see if the TDS slowly creeps down, but I wasn't able to get it lower than 16. I tried removing the filter and re-seating it but everything looks okay inside visually. Could it be the filter just needs to run and re-hydrate for over 2 hours since it was dry? Maybe this filter is defective? I may try re-installing the old one and seeing if I still get 12TDS although just a day ago I know I was. I am running out of options!

Any input/anecdotes appreciated!

 

[MnFish1]

Have you double checked your TDS meter? Also - sometimes the membrane is not seated correctly in the holder - and water is leaking around it rather than going through it.

 

[george9]

I did double check that the membrane was seated properly and really pressed it into the chamber. I ran some more water through it today and no TDS drop, it fluctuated between 13-16. So then I put the old membrane back and tested it and it was actually doing the same thing.

I may have been premature with creating this thread because all signs are pointing to this being a pressure issue now. Since I didn’t know the post-RO TDS of my old membrane when it was new, it probably was always throwing out more TDS than I would like, but I never tested it before hitting the DI so never knew. I only tested after the DI to make sure it was 0. I live in a high rise so lack of pressure makes complete sense. I only make about 5g of water a week, I bet I would’ve noticed this sooner if I had a large tank or made more water. Since the new and old RO match in TDS now, I probably didn’t even need a new RO membrane, but rather more pressure!

For anyone in the future with the same issue reading this - make sure you have enough pressure in your system!! I am going to survive with 15 tds hitting my DI chamber, since my last DI resin lasted over a year despite this and even then it was still putting out 0 TDS when I changed it.

 

[MnFish1]

I did double check that the membrane was seated properly and really pressed it into the chamber. I ran some more water through it today and no TDS drop, it fluctuated between 13-16. So then I put the old membrane back and tested it and it was actually doing the same thing.

I may have been premature with creating this thread because all signs are pointing to this being a pressure issue now. Since I didn’t know the post-RO TDS of my old membrane when it was new, it probably was always throwing out more TDS than I would like, but I never tested it before hitting the DI so never knew. I only tested after the DI to make sure it was 0. I live in a high rise so lack of pressure makes complete sense. I only make about 5g of water a week, I bet I would’ve noticed this sooner if I had a large tank or made more water. Since the new and old RO match in TDS now, I probably didn’t even need a new RO membrane, but rather more pressure!

For anyone in the future with the same issue reading this - make sure you have enough pressure in your system!! I am going to survive with 15 tds hitting my DI chamber, since my last DI resin lasted over a year despite this and even then it was still putting out 0 TDS when I changed it.

BTW- I kind of mis-read your post - why are you concerned about your TDS being 16 if in the end its zero? The membrane doesn't remove all TDS.

 

[george9]

BTW- I kind of mis-read your post - why are you concerned about your TDS being 16 if in the end its zero? The membrane doesn't remove all TDS.

I was using that as a reference to tell when my RO membrane was shot. A good membrane should have a 96-99% rejection rate and I was getting 88-92%, but didn’t realize I likely was always getting a low rejection rate due to crappy pressure and my old membrane was probably still good. I wish I would’ve tested in-line TDS when the system was new so I had a point of reference for today

my tap TDS is 140 and it’s 16 after RO, it should be like 4-6 after RO with a good rejection rate

 

[Buckeye Hydro]

Yes - you need a pressure gauge and a tds meter to run one of these systems. Both are inexpensive. You can find two different pressure gauge add on kits here:

Pressure Gauges

At Buckeye Hydro we specialize in providing the highest quality water filtration systems and accessories for every potential need.

www.buckeyehydro.com

The effect of pressure on membrane rejection is interesting. Rejection doesn't go up much when increased in excess of the factory spec (50 PSI for Filmtec residential membranes). But yes it does drop off when pressure is less than factory spec.

 

[t5Nitro]

Bumping this thread as I'm still searching for answers on a similar problem. Burned through two sets of brand new RO membranes with early increased rejection rates. Mine is not due to low pressure. The RODI unit is connected directly to the main water line of the house and also is running on an RO buddy. Is it possible there's too much pressure? Doesn't seem that would be much an issue with the above graph.

 

[george9]

Bumping this thread as I'm still searching for answers on a similar problem. Burned through two sets of brand new RO membranes with early increased rejection rates. Mine is not due to low pressure. The RODI unit is connected directly to the main water line of the house and also is running on an RO buddy. Is it possible there's too much pressure? Doesn't seem that would be much an issue with the above graph.

What is your rejection rate? Have you tried removing the filters and putting them back in?

Speculating here because I am no expert - but I would think too much pressure could cause the water to move too quickly through the membrane and not be as efficient at filtering but I will wait for someone with more experience to chime in and confirm that.

I wasn’t able to figure mine out, just stopped testing TDS after RO lol. I still get 0TDS water at the end and I don’t burn through filters. My issue is likely water pressure since I live in a high rise although I still have yet to get a pressure gauge out of laziness.

 

[t5Nitro]

Rejection rate is pretty high. New the membranes post were about 4 to 5. Over time there's creep after start up. Example being from today starting at 7, increasing to 15 after making about 30 gallons.

 

[george9]

Rejection rate is pretty high. New the membranes post were about 4 to 5. Over time there's creep after start up. Example being from today starting at 7, increasing to 15 after making about 30 gallons.

Do you know how your water pressure compares to the manufacturer recommendation?

 

[t5Nitro]

I don't because I also don't have a pressure gauge. Maybe I should pick up one or two.

 

[Buckeye Hydro]

Excessive pressure adversely affecting membrane performance, in this situation, is not the problem. Membranes will perform better with higher pressure. Is there an upper pressure limit? Yes - but it is well above anything you'd find in a residential setting and well in excess of any pressure coming from typical booster pumps.

Russ