Very low RO rejection?

cds333 · Jun 13, 2021

This RO system is driving me crazy. I just replaced the membrane, carbon filter, and added a booster pump. It is still only putting out 10 tds even with the pressure increase and new membrane. With the old membrane & without the pump, it was putting around 14. Changing the membrane did not seem to have much effect.

The input water is from a well, and is coming in at 65F / 4.5pH / 30psi / 75tds

I am using the BRS 5-stage 75gpd system (without the DI resin) with a 550 flow limiter and the pump brings it up to just over 60psi.

(I am not using the DI resin because at such a high TDS it gets used up way too quickly and is too expensive to replace)

So with a rated minimum 95% rejection it should still be much lower. It is closer to 80% now.

I am using genuine membranes btw.

So it is not the membrane, it is not the pressure, and it is not the filters...

_

I have heard in a BRS video that you can increase the flow limiter size to increase the waste water, which will lower the tds. However other sources state that doing so may do the opposite.

Would going up to an 800 from the current 550 do anything in a system with such terrible rejection?

Or what is the next step to achieve the membrane's rated rejection?

Thanks!

laverda · Jun 14, 2021

Something is clearly wrong if your TDS at the source is only 75 and you getting 10TDS out of your membrane. I get less then that when my TDS going in is over 335.
Make sure you membrane is seated properly and the seals are in good condition.

cds333 · Jun 14, 2021

It is a new membrane. The old membrane had the same issue. They are genuine Filmtec membranes from Dow. I also tried a completely different membrane housing.

KStatefan · Jun 14, 2021

Changing the flow restrictor would not change the rejection rate much if any. Can you adjust the pressure up more?

Mike konesky · Jun 14, 2021

The pH makes me think your well water has co2. Can you test your kitchen sink water pH, then wait 20min and do it again?

cds333 · Jun 15, 2021

The aquatec 6800 24vac pump will not go any higher. Its max seems to be around 63psi. However the increase from 30psi to 60psi only dropped it by about 4 tds.

---

The pH readings are as follows:

T+0
T+20m
T+100m

4.61 @ 68F
4.65 @ 68F
4.68 @ 71F

laverda · Jun 15, 2021

I have a brand new filmtec membrane that is giving me very high numbers as well. I have been meaning to call them about it. Perhaps they had a bad batch.

cds333 · Jun 15, 2021

It's not the membrane. I have been through 4 membranes many years apart, both TW30 and BW60, and never get close to 96%

KStatefan · Jun 16, 2021

Have you had your water tested to determine what is in it? The membrane is rated for salt rejection. I know not all Ions have the same rejection rate. Is it possible your water is high in something that the membrane has a low rejection rate on?

I do not remember if the different Ions are listed in the manual or not.

https://www.dupont.com/content/dam/dupont/amer/us/en/water-solutions/public/documents/en/45-D01504-en.pdf

Randy Holmes-Farley · Jun 16, 2021

FWIW, I had about 90% rejection based on TDS for 20 years using two different Spectrapure systems. Those systems were "supposed" to have higher rejection, but whether it was due to high silicate and ammonia in the water or some other issue, I do not know.

laverda · Jun 16, 2021

I replaced my old membrane as the TDS coming out of it was getting to high. The new membrane produced the same number from the start. I did some playing around and disconnected it and hooked it up as the only membrane and the numbers were still to high. My original primary membrane at 314 TDS in is only 4 out. The new filmtec TW30-1812-100 is much higher when installed as the only membrane. I do not recall the specific numbers.

JAC- · Jul 10, 2021

Hey friend, I just happened across your post and saw that it wasn't resolved. Many membranes are not good at removing nitrate and nitrite. Check your ro water (without it going through the DI resin) for both those compounds and see if you are detecting either. If you are, running through a bed or two of only anion resin is a much more cost effective way to remove it and then you can scrub with a bed of mixed to get you down to 0 TDS.

There are also whole house options that can remove those for you and regenerate the resin if that's a concern too as they can feed bacteria in your plumbing. However they are $$$ and will put sodium in your water as it's basically a water softener. Your RO would be easily able to remove the sodium though.

I hope that helps.