How much TDS is too much?

Siberwulf · Jun 9, 2023

Hey all!

I just installed a second stage DI unit (I was running a general DI, and based on the amount of Si in my RODI, I opted to add an Anion stage before the general stage) to my RODI setup. I let it flush out this and this morning I noticed it's still registering 5 TDS coming out of the final stage.

Is that too much? I know we all want to be at 0 TDS, but is 5 enough to be concerned with?

Thanks in advance!

Randy Holmes-Farley · Jun 9, 2023

The answer depends on what that 5 ppm TDS is. More than likely it is fine, but 5 ppm copper is obviously a huge issue.

Two DI is series should get TDS to zero.

How high is your incoming tap water, and how high is the TDS post RO?

exnisstech · Jun 9, 2023

I aim for zero and have multiple canisters of resin in series so it's always 0. The first chamber is really the only one that gets changed much. Having more than 1 canister allows me to still get 0 tds output when the first is depleted so I can finish what I'm doing and change the resin at my leisure.

EDIT: I was running cation and anion for a while but find its easier for me to just run mixed bed since the first chamber is all the gets changed often.

Reefer Matt · Jun 9, 2023

Siberwulf said:
Hey all!

I just installed a second stage DI unit (I was running a general DI, and based on the amount of Si in my RODI, I opted to add an Anion stage before the general stage) to my RODI setup. I let it flush out this and this morning I noticed it's still registering 5 TDS coming out of the final stage.

Is that too much? I know we all want to be at 0 TDS, but is 5 enough to be concerned with?

Thanks in advance!

I'm not sure that you can add just an anion stage before the mixed bed. Usually with separate beds, it goes from cation to anion, then mixed bed. The tds between the cation and anion will usually register whatever is left after your membrane, until after the anion bed. The two resins work together to produce 0 tds. BRS has a video on this in their "Pro Series RODI" video series.

Randy Holmes-Farley · Jun 9, 2023

You can use an anion alone before a mixed bed, it just depletes the mixed bed unevenly.

Randy Holmes-Farley · Jun 9, 2023

Reefer Matt said:
I'm not sure that you can add just an anion stage before the mixed bed. Usually with separate beds, it goes from cation to anion, then mixed bed. The tds between the cation and anion will usually register whatever is left after your membrane, until after the anion bed. The two resins work together to produce 0 tds. BRS has a video on this in their "Pro Series RODI" video series.

TDS will actually rise after a single type of bed, whether anion or cation, becuase the ions swapped in (H+ or OH-) are more conductive than any other ions. pH is also super out of whack, high or low.

Reefer Matt · Jun 9, 2023

Randy Holmes-Farley said:
TDS will actually rise after a single type of bed, whether anion or cation, becuase the ions swapped in (H+ or OH-) are more conductive than any other ions. pH is also super out of whack, high or low.

Thank you. That is about what I remember from the video now that you mention it. It was also a reason why I did the two separate beds, followed by a mixed bed. Plus the main reason is I only have to change out the resin that actually gets used the most (anion), rather than wasting resin with a mixed bed only.

Randy Holmes-Farley · Jun 9, 2023

Reefer Matt said:
Thank you. That is about what I remember from the video now that you mention it. It was also a reason why I did the two separate beds, followed by a mixed bed. Plus the main reason is I only have to change out the resin that actually gets used the most (anion), rather than wasting resin with a mixed bed only.

Yes, that seems reasonable.

KrisReef · Jun 9, 2023

Randy Holmes-Farley said:
You can use an anion alone before a mixed bed, it just depletes the mixed bed unevenly.

I’m convinced that my understanding of chemistry is wrong. How can elements have uneven atomic weights and be “elements” that fit the definition of unchangeable, basic atomic units? Same with nuclear particles, the basic elements are not the smallest units with standard mass and charges?
My mind is a mixed bed of faith and degredated & depleted molecules that cannot be resolved through a mass spectrometry process. Sorry for the rant.
“Let there be light.”

Siberwulf · Jun 9, 2023

Randy Holmes-Farley said:
The answer depends on what that 5 ppm TDS is. More than likely it is fine, but 5 ppm copper is obviously a huge issue.

Two DI is series should get TDS to zero.

How high is your incoming tap water, and how high is the TDS post RO?

sigh.

So I went to swap my TDS meter to "In" from "Out" and realized it was already set to "In" and when I swapped it...it said 0. Good grief I need a weekend.

"In" in this context is for post-RO checking.

KrisReef · Jun 9, 2023

Siberwulf said:
sigh.

So I went to swap my TDS meter to "In" from "Out" and realized it was already set to "In" and when I swapped it...it said 0. Good grief I need a weekend.

"In" in this context is for post-RO checking.

Time to replace your resin? That’s what I always do

Randy Holmes-Farley · Jun 9, 2023

KrisReef said:
I’m convinced that my understanding of chemistry is wrong. How can elements have uneven atomic weights and be “elements” that fit the definition of unchangeable, basic atomic units? Same with nuclear particles, the basic elements are not the smallest units with standard mass and charges?
My mind is a mixed bed of faith and degredated & depleted molecules that cannot be resolved through a mass spectrometry process. Sorry for the rant.
“Let there be light.”

I'm not sure what you are asking, but TDS is a measure of conductivity, with an attempt to extrapolate it roughly to the amount of sodium chloride or other salts that would have that conductivity.

When passing through a DI, if the solution starts as sodium and chloride, the sodium is taken out and H+ is added in its place. H+ is much more conductive than is an equal number of Na+ ions, and so theh conductivity rises (and hence TDS rises, regardless of the actual weight of solids in the water).

Same happens for chloride being removed and OH- added back. OH- is much more conductive.

When BOTH are swapped out, the H+ and OH- combine to form nonconductive H2O, and the TDS drops again. When just one half of the pair is swapped out, that cannot happen.

Tidal Gardens · Jun 9, 2023

This is what I, (Luke), have been using at my home system for almost 9 years. I run a container of Cation (Purple Resin) and then a container of Anion (Blue Resin) then the final chamber is a Mixed Bed in which I mix myself at 90% Anion to 10% Cation. I have been producing ZERO TDS since i switched to this method. Your upfront cost would be another DI Chamber and a bag of Ciaton. Hope this helps.

PS... I never trust any TDS in my water because as Randy mentioned, you don't know what that 5ppm is and there is no reason to risk it.

Randy Holmes-Farley · Jun 9, 2023

Siberwulf said:
sigh.

So I went to swap my TDS meter to "In" from "Out" and realized it was already set to "In" and when I swapped it...it said 0. Good grief I need a weekend.

"In" in this context is for post-RO checking.

Zero is good.

Reefer Matt · Jun 9, 2023

Siberwulf said:
sigh.

So I went to swap my TDS meter to "In" from "Out" and realized it was already set to "In" and when I swapped it...it said 0. Good grief I need a weekend.

"In" in this context is for post-RO checking.

Problem solved! But you also provided a post for rodi discussion, so thank you! Happy Reefing!

Salty_Northerner · Jun 9, 2023

my local water RO shop says 4 TDS and when I called he said there was actually 10 TDS but he just hummed and haawd and really wouldn't give me an answer. So when I asked about buying an RO/DI system he said OH It's really expensive and the add on DI was even more $$. So I thought heck with you and bought a BSR 4 stage Plus RO/DI system.

I'm thinking I've been having issues over to much copper and silicates in the water I buy from this shop.

Problem for me ~solved~