CO2 impact on DI Resin- Definate Read for Those on Well Water

[DaveS]

If you use DI resin to make 0 TDS water, you may have read about various things that impact resin longevity.  One of the things people mention is CO2.  While you might find high CO2 water anywhere, it tends to be especially prevalent for those who use well water around here.  This CO2 effect can generally be detected in the form of acidic water which goes back towards neutral after it has been left to sit out for a while in a well ventilated area.

 

The problem is, most people are unlikely to aerate their water before pushing it through an RO/DI system.  As such, many people end up going through DI resin faster than necessary.  For those on well water, you can add a whole house neutralizer system inline with the whole house water softener.  I mention more about in this thread but basically neutralizes the CO2 with calcite (think kinda like a CA reactor).

 

I am on a well and have been spending some time doing research on this issue (along with other RODI related topics) as my DI resin never seems to last very long.  Life is busy and I have a hard keeping my neutralizer filled so there are times when I push CO2 acidic water through my RODI and wanted to get a sense for how big an issue this really is.  So I ran a little test recently. 

 

1) I put in a split bed DI system where I have the cation separated from the anion.  The mixed bed DI resin that most people use is generally a 50/50 of cation/anion.  The anion is what gets expended by the CO2 and if you go through anion faster, you end up throwing away perfectly good cation resin when your TDS starts to creep up and you have to replace the resin.

 

2) with my whole house neutralizer empty (for a long time!), I made 30 gallons of 0 TDS water

 

3) I refilled the whole house neutralizer and waited a few days for all the previously stored water to be used up.  I could tell when I was on "neutralized water" as the water softener really kicks in.

 

4) I then made another 30 gallons of 0 TDS water.

 

I used the exact same container so the amount of water made was consistent between the 2 batches.  Here is a picture of anion consumption between the 2 batches based on color changing resin.

 

 

I added the red/black marks based on my eyeballing of the color delineation point to help highlight the difference.  As you can see, the same amount of filtered water would consume about twice the amount resin if it wasn't neutralized.  Granted, this isn't the most scientifically rigorous experiment but I think it does paint a pretty compelling argument that pH neutralized (CO2 removed) water can make a big difference in resin longevity.

 

If you consider the fact that a typical mixed bed resin consists of only 50% anion, in a similar 10" cartridge as the one shown, I estimate that all the anion for a typical mixed bed setup would be consumed after ~150 gallons of filtered water.  Replacing resin that frequently gets expensive!

 

I should note that while I got color changing resin in this instance, I generally don't like spending extra money for it as the color change isn't reliable.  Like my toothbrush, DI resin still has some decent life in it after the color change.  I normally use a TDS meter to tell me when the resin is really expended.

 

Hopefully some people will find this post useful as they try to optimize their RO/DI system.

[zygote2k]

Can I ask why people are obsessed with keeping 0 TDS water?

[elbowdeep88]

Nice write up.

[DaveS]

Nice write up.

 

Thanks, glad you enjoyed it. 

[ctenophore]

Great info!  Thanks Dave.  Did you have any luck with recharging all that resin?

[DaveS]

Great info!  Thanks Dave.  Did you have any luck with recharging all that resin?

 

Yup, I did a test batch and the recharged resin works just fine.  Some nasty chemicals involved but can be effective if done in large batches.  I have some ideas on modifications to the process (and equipment) which will hopefully make things go easier.  Stay tuned!

[flooddc]

Thanks for sharing Dave!

[DaveS]

Someone asked me about the various terms used for DI resin- cation, anion, mixed bed and separated.  So I figured I would write up the description here to capture the information.

 

Cation resin- a resin that absorbs positive ions

Anion resin- a resin that absorbs negative ions

Mixed-bed resin- a combination of cation and anion resin

Separate- two distinct filters of DI resin where one filter has only cation resin and the other has only anion resin

 

Mixed-bed resin is what most people get.  If you look closely at your standard resin, you can notice two different colored beads.  In general, the mixed bed is half of each type mixed together.  Collectively the two resin types in the DI cartridge handle any impurities after your RO membrane to get zero TDS.

 

However, some people have water that contains more impurities of one charge than another.  When that is the case, you can have asymmetric resin exhaustion where one of the resin types gets exhausted up before another.  This means when 1/2 of your mixed bed cartridge is spent (not top half vs. bottom half but cation vs. anion), some portion of the other half can be still good.  Unfortunately, since the resin types are mixed together, you have to throw away the contents of the entire cartridge, losing some perfectly good resin.  Spectrapure's SilicaBuster cartridge is one example of trying to address asymmetric resin exhaustion for people with high silca in their water. It basically has a 1/3-2/3 mixed bed composition where 2/3 of the resin is the type that absorbs silca.

 

In my case described above, I probably go through anion resin 10x faster than cation (that's an estimate but not totally off).  This means in a mixed bed setup, my anion would be exhausted while 90% of the cation is still good.  Put another way, when the 50% of the cartridge that is anion was exhausted, only 5% of the cartridge that is cation was used up.  I end up having to replace the entire cartridge with 45% of it was still good! 

 

Here's an indicator of my asymmetric resin exhaustion.  My DI membrane puts out 2-3 TDS water that went into the mixed bed DI cartridge.  When I had new mixed bed resin, the TDS would drop to zero (good!).  But then after making a curiously small amount of 0 TDS water, TDS would start to creep up to 1 maybe 2 (but not the full amount that was coming out of the RO membrane).  Basically everything would work as expected in the beginning.  But after my anion would get exhausted, the TDS to go up since the negative ions were no longer getting absorbed.  Since my cation was still good, it was still absorbing the positive ion which is why the TDS would not increase to the same as the RO output.

 

Lastly we get to the separate bed resin.  This is done by adding another stage to your RO/DI filter for a 2nd DI cartridge.  With 2 DI stages, one cartridge contains only anion resin and the other cartridge contains only cation resin.  When one type of resin is exhausted, you can simply replace only that type and keep running the other cartridge until it is actually exhausted.  A separated bed simply minimizes waste by allowing for more precise control and replacement of ONLY exhausted resin.  In my example, I will probably replace my anion resin 10x for everyone one time I replace my cation resin.  That's a pretty big savings on cation resin!  I guess some could also look at this as a way to generally change resin less frequently since you have 2x the amount of resin in your system.

 

 

 

 

Since I've already written a book, I'll address Rob's question of why people are "obsessed" with 0 TDS water. I'm assuming the question is about 0 TDS vs. 1-2 TDS and not the general use of DI resin as a whole.  Origami posted on this a while back but here's the quick version.  Not all positive ions are the same and not all negative ions are the same.  Some positive ions are MORE positive than others.  So when the cation is fresh, it absorbs ALL positive ions.  But once it gets exhausted, it does NOT stop absorbing positive ions.  It actually continues to absorb the more positive ions and releases back the less positive ones it previously absorbed.  That 1-2 TDS you see when the resin is exhausted are the weaker ions that were previously absorbed but now released back.  The problem is, it's the weaker ions that are more problematic for our tank inhabitants.  The same goes for the negative ions.  So when your TDS is not 0, you are filtering out the moderately undesirable stuff but getting the worst stuff put back in your filtered water. 

[DuffyGeos]

Thanks for the info.....now I have a better idea why my TDS meter creeps up to 1-2 and just stays there.

[Hoi Toider]

How do you determine which resin (cation vs anion) is being depleted?  Trial and error?

 

Thanks for your posting - well written and easy to understand!

[Origami]

How do you determine which resin (cation vs anion) is being depleted?  Trial and error?

 

Thanks for your posting - well written and easy to understand!

If it's color changing and if it's separated already (and therefore in separate canisters), as Dave's is, then it's easy to tell which is becoming depleted. Note the pictures above, which are of the same canister, but taken at different times. The resin at the bottom of the canister is changing color as it's being used up.