when to replace ro membrane

roons

In Memoriam
im going to test ro output b4 di, at what point do you say , ok, this tds is too much for the di to handle? and replace ro membrane...................im on my second di but first ro mem in 2 years, i ordered a new ro but dont want to replace it if the old one is still working so im going to test first..............
 

moumda

Reefer
IMO if a ro membrane is doing a 10 to 1 reduction in tds it's still good. If using color changing di this may need to be changed more often with this way of doing things but a person can just see what costs more.
 

BeanAnimal

Premium Member
A DOW filmtec 75 GPD membrane (that is what you purchased isn't it?) has a 98% rejection rejection ratio. I would replace it when it drops below 92% or so, depending on the incoming water TDS.

If you incoming TDS is 150, then you can go a lot longer, as the DI does not have to work very hard. If your incoming TDS is 400, then you need to pay close attention the the membrane life.

400 TDS input:
At 98% rejection your RO TDS is about 8, at 90% rejection it is 40.

200 TDS input:
at 98% rejection RO TDS is about 4, at 90% rejection it is 20.

You can see that at 4 or 8 TDS things are not that much different, yes the RESIN will still have to do DOUBLE the work at the lower TDS, but the "TIME" involved is long. At 20 TDS life will be somewhat short for the resin and it is likely less costly just to replace the MEMBRANE to reduce resin usage. However, at 40 TDS the resin is going to get chewed up in no time.

I hope this give you an idea. There is no "real answer" without doing a lot of math to determine the optimum replacement window based on RESIN cost, INPUT TDS, and MEMBRANE cost.

Bean
 

BeanAnimal

Premium Member
If it is not a dow filmtec 75 GPD membrane, don't buy it. That is not a DOWN filmtec membrane, it is an import knock off that uses filmtec material.... not the same. it is also a 100 GPD unit and therefore has a worrse rejection (not really a problem for you) but also a much higher clean to waste ration due to the real worl operating pressure requirement (around 70 PSI, as opposed to the dows 50 psi)

WIth input of 28, your output should be near zero. Yur DI should last a LONG time, and yes I would still use it to take out whatever the RO missed.
 

bbragg

New member
Wow, 28ppm on the input. That's like "mountain springwater" clean. Coors would be proud to use that stuff.
 

roons

In Memoriam
<a href=showthread.php?s=&postid=6987559#post6987559 target=_blank>Originally posted</a> by BeanAnimal
If it is not a dow filmtec 75 GPD membrane, don't buy it. That is not a DOWN filmtec membrane, it is an import knock off that uses filmtec material.... not the same. it is also a 100 GPD unit and therefore has a worrse rejection (not really a problem for you) but also a much higher clean to waste ration due to the real worl operating pressure requirement (around 70 PSI, as opposed to the dows 50 psi)

WIth input of 28, your output should be near zero. Yur DI should last a LONG time, and yes I would still use it to take out whatever the RO missed.


so your saying the dow would produce more clean water with the same waste water amount? like i said with 28 in this is still reading 0 after 2 years.......................so when it reads anything but 0 i should change it ?
 

BeanAnimal

Premium Member
YOur scenario is slightly different than most people. Your input water is very clean. Even it the TDS is 2 or 3 on the output, you are then in the same boat as people with a DOW 75GPD membrane a 300 input!

Your resin usage will increase at that point, but still be rather low. I would likely replace the membrane every 3 years or so no matter how good the ouput (but thats just me and I am sure others with a lot of knowledge may tell you otherwise).

Sorry I don't have a concrete answer for you! DO however replace your pre filters every 6-8 monts or so. Bacteria can be a problem long term in filters.
 

roons

In Memoriam
yeah i was replacing pre filters every 6 months but am going to go 12 months unless that would cause a problem...........so i guess this membrane is ok for me since i have rather clean water to begin with, is the dow much more expensive?
 

BeanAnimal

Premium Member
No, thats the funny part, the dow membrane is a few buck more.

To answer your other question, yes, for the same amount of waste water a dow filmtec 75GPD membrane will produce less waste water than the GE DESAL or other 90% rejection membranes. The other membranes are rated at 70 PSI or so. The Dow is rated at 50 or so. Your house is likely 40 PSI or so, much closer to the optimum operating pressure of the DOW.

Bean
 

roons

In Memoriam
sorry if i dont follow , for the same amount of ro water you mean? the dow will produce less waste?
 

BeanAnimal

Premium Member
Yes for the same amount of CLEAN WATER, the DOW will produce less waste. A membrane has an optimum operating pressure. Below that pressure it sheds water instead of passing it through the membrane. The GE DESAL membranes specs are listed at 60, but the literature states that the optimum pressure is 70+.

The DOW FILMTEC is rated at a 50 PSI optimum pressure.

So lets say your house is at 35 PSI. that is 15 PSI from the down, and a whopping 35 PSI from the GE. You will likely never get anywhere near 4:1 waste to product ration even if all other things (temp, back pressure) are perfect.

It's no wonder you find the "My waste hose shoots water out like a garden hose, but my product water does not even trickle, it drips, like a drip ever 5 seconds. It takes me 2 days to make 20 gallons of water Even worse, my output TDS is 10". You usally find out that the person has an off brand unit, with an off brand membrane, undersized horizontal resin and a water temperature of 55 degress with an input pressure of 30 PSI.


Bean
 

roons

In Memoriam
ah, your di should be vertical? mines laying horizontal..............i find it takes about 110 gals of waste to make 44 gals clean.........so thats not too bad, but if im understanding you, if i used the dow id pay less on my water bill yes?
 

BeanAnimal

Premium Member
Something does not add up correctly. Your waste/product ratio is 2.5:1 (to good to be true) and still have a membrane that is doing 95% or better.
 

tkeracer619

Premium Member
Reading this post makes me think somethings wrong with my ro/di. Hah kewl I get to take something else apart.
 
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