Shouldn't my 75 GPD RO membrane remove 99% of TDS?

[randyBRS]

Hey guys!

We have a pretty short FAQ this week, but it's jam packed with good info about TDS and that sometimes elusive 99% rejection rate from our RO membranes. In this one, Ryan explains how the 99% number came to be and reasons why many of us struggle to achieve it. He also provides some helpful tips to get the best rejection out of your RODI unit and ways to make sure you are getting an accurate reading.

Enjoy!

 

[Chris155hp]

I'm interested in learning more on how to increase waste water to improve efficiency. I already have a booster and am still getting 6-7 out bc of my well water

 

[don_chuwish]

I'm so lucky to have low TDS tap water. 40-50ppm tops. My sediment filter is barely turning brown after years of use. DI resin lasts a really long time. However it does make it difficult to decide when I should change carbon & RO membrane.

 

[jason2459]

Great video and I've really gotten sick of having some certain other company's marketing of their special membranes and systems thrown in my face. No doubt they are nice but like the video says, look at the specs necessary to achieve the advertised rates.

No one will ever go wrong getting an RODI system from BRS.

 

[guc32]

If I have a 75gpd system with a 550ml restrictor and say I get 2:1 waste ratio, going with a larger restrictor will result in what?

Im always confused in what restrictor size will do.

 

[revhtree]

I needed this video!

 

[randyBRS]

If I have a 75gpd system with a 550ml restrictor and say I get 2:1 waste ratio, going with a larger restrictor will result in what? Im always confused in what restrictor size will do.

It confused me too for quite a while! Basically, with the flow restrictor you are trying to find a happy balance between the waste water ratio and TDS rejection. For a majority of households a 550mL restrictor paired with a 75gpd membrane will typically produce the 99% rejection rate at a 4:1 waste to product ratio (minus the variables like pressure, temp, TDS chemical makeup). This is why they are included on all of our 75gpd units.

Essentially, the 550mL restrictor puts just the right amount of "back-pressure" on the waste water line, to force water through the membrane layers and provide as close to 99% rejection as possible. If you increase the flow restrictor size to a 600mL or 800mL, you decrease the back-pressure that forces water through the membrane, thus allowing more water to escape through the waste water line. This ends up increasing your wasteroduct water ratio, by decreasing the contact time within the membrane layers. This can result in lower TDS, but that is because it is processing less water (as most of it is going down the drain). Decreasing the flow restrictor size has the opposite effect, where you put more back-pressure on the membrane layers. The increased back-pressure reduces your wasteroduct water ratio (because more is being forced into the product water), but it also increases the amount of TDS that is let through the membrane.

So to basically sum it up;
*Larger restritor = more waste water / lower TDS
*smaller restrictor = less waste water / higher TDS

-Randy

 

[jason2459]

What I used to do is use a precise needle valve to adjust to my needs in lieu of a set flow restrictor.

But that gets a bit more advanced and a set flow restrictor is easy and safe.

I will not be linking to my current rodi thread for this one as it would be rude in this case (asked in a pm where my link is as I seem to keep linking off to a thread of mine).

 

[kchapmanjr]

I'm so lucky to have low TDS tap water. 40-50ppm tops. My sediment filter is barely turning brown after years of use. DI resin lasts a really long time. However it does make it difficult to decide when I should change carbon & RO membrane.

I'm with ya Don. I will say that I had slightly higher TDS when I was still in So. Oregon compared to water up here in P-town. I get around 28 TDS straight out of the tap here.

 

[don_chuwish]

I'm with ya Don. I will say that I had slightly higher TDS when I was still in So. Oregon compared to water up here in P-town. I get around 28 TDS straight out of the tap here.

Nice!
Having read about the effect of flow restriction and considering the low TDS source water, I may just look into restricting more for less waste. I have perfect pressure (75psi) and the 150gpd upgrade.

 

[Cflip]

Nice!
Having read about the effect of flow restriction and considering the low TDS source water, I may just look into restricting more for less waste. I have perfect pressure (75psi) and the 150gpd upgrade

Nice!
Having read about the effect of flow restriction and considering the low TDS source water, I may just look into restricting more for less waste. I have perfect pressure (75psi) and the 150gpd upgrade.

.

Or go you can go with dual RO membranes for less waste.

 

[jeff williams]

I'm using the 100 per day membrane with 60 psi water pressure and around 350 tds tap water I checked my tds after the Ro and it was 49 but I didn't let it run an HR first. And I burn through Di I get about 50-75 gallons per Di cartridge. This is on new Rodi unit less than 2 months old

 

[don_chuwish]

Or go you can go with dual RO membranes for less waste.

Already have dual - thus the 150gpd.

 

[Frop]

Hey guys!

We have a pretty short FAQ this week, but it's jam packed with good info about TDS and that sometimes elusive 99% rejection rate from our RO membranes. In this one, Ryan explains how the 99% number came to be and reasons why many of us struggle to achieve it. He also provides some helpful tips to get the best rejection out of your RODI unit and ways to make sure you are getting an accurate reading.

Enjoy!

I guess I've seen a few of these videos when I read the opening paragraph in the announcer's voice in my head.

 

[Reef man 89]

In NH my TDS right out of the tap is 3tds. Would it be a problem if I just took my water stright from the tap for my tank? Or should I keep running it threw my RO/DI system first?

 

[Cflip]

Already have dual - thus the 150gpd.

Sorry missed that

 

[jason2459]

I'm using the 100 per day membrane with 60 psi water pressure and around 350 tds tap water I checked my tds after the Ro and it was 49 but I didn't let it run an HR first. And I burn through Di I get about 50-75 gallons per Di cartridge. This is on new Rodi unit less than 2 months old

Membrane may be bad, not seated all the way, or you're testing right away and you're seeing what's called TDS creep.



TDS creep will kill DI quickly. Short cycling promotes this too. Best thing to do is discard the water coming from the RO membrane before it hits the DI until thr TDS goes down to normal. Should also backflush the membrane right at startup too manually or automatically.

For the TDS creep you can get an expensive 3 way valve or just get a T and a ball valve. If you use the T and ball valve make sure all valves down stream from the DI are closed.

Here's mine just after RO membrane and before DI. My DI are BRS 2.5"x20" housings with refillable cartridges. Two of them which rotate once the first one is exhausted the 2nd goes into the first and new DI goes into the 2nd again. Good insurance that you will always get ~0 TDS. Sorry, off topic...

Notice the TDS meter ported in before the T.

So, after backflush, which should be done, you make sure all valves are closed. Open the valve under the T and let that run to a drain for a bit or collect it to water plants etc.

If you have the TDS meter setup after the RO membrane and before the T you can watch the TDS creep go down and once normalized close off that valve and start up normal operations.

Left meter shows Tap water TDS. Right meter shows my normalized TDS. It varies a bit here and there and I normally hit around 4-5 TDS before DI. ~0 after

 

[don_chuwish]

+1 to what Jason said. My simple example:

I flush the membrane for about 20 seconds when first starting, then turn the valve up to send product water to the DI canisters. A quick check of the "out" TDS usually shows it quickly dropping from <10 down to zero.

 

[randyBRS]

I'm using the 100 per day membrane with 60 psi water pressure and around 350 tds tap water I checked my tds after the Ro and it was 49 but I didn't let it run an HR first. And I burn through Di I get about 50-75 gallons per Di cartridge. This is on new Rodi unit less than 2 months old

Hey there,

Another point to consider, along with checking the RO membrane itself to ensure it is fully seated or that there are no cracks on the tip of the membrane or on the housing, is that 100gpd membranes will also have less effective rejection rate than a 75gpd. If your specific unit uses DOW Filmtec membranes, they rate their 100gpd membranes at 98% rejection rate under the same laboratory conditions DOW tested them. If you have a BRS unit and changed the RO Membrane to a 100gpd, you want to make sure you change the 550mL set flow restrictor that was included to a 600mL restrictor.

It is definitely a good idea to let the unit run for 20 minutes before taking a reading as well. I would also check with the manufacturer of your 100gpd unit (if it isn't a BRS system) to see what flow restrictor is used. Some manufacturers include capillary style restrictors that can be trimmed and altered to meet your specific water source requirements. If that is the case, it is worth checking with them prior to making those permanent type of adjustments.

-Randy

 

[jason2459]

100gpd membranes usually aren't as efficient as a 75gpd membrane but getting a TDS of 49 from a Tap with a TDS of 350 is well beyond the normal drop in 100gpd efficiency. Even if say the temp was really cold and the pressure really low one should still expect over 90% rejection from a membrane with very slow production and at some point wont produce at all.