RO/DI system question

[Ace25]

On mine, the first canister gets "used up" 100% before the second color changing canister even starts to get used. Not sure what the science behind that is, all I know is when I use 1 canister and replace when TDS hits 1, the new canister last just as long as it should, but when I daisy chain them together and leave the bad DI canister inline as the first canister, the second "new" DI resin will be all used up within just a couple days. Only way to fix that issue is have an inline TDS meter BETWEEN the 2 canisters so you know when the first one goes bad, but that really isn't saving anything doing it that way, otherwise you really don't know when the first one is spent and spitting out 1+ TDS into the second canister. Of course, 1 TDS into a DI cansiter isn't really an issue, but usually once you start to see 1 TDS, 10-25 TDS is usually just a few more gallons away. At my work we use giant 4' DI bottles and they get used up in less than a month and we have to run the water for 5 minutes every day to "flush" the DI because it is usually 2x as high as tap water TDS (tap=700+ TDS, DI=1500+ TDS unless flushed) if you let DI resin sit in water for any period of time.

[Garf]

To me that makes no sense, but I guess I haven't tried it before. Theoretically, using 2 DI canisters would be exactly the same as one larger one. In mine, the resin changes color from blue to brown when it gets used up. The brown separates into 2 distinctly different colors and one sinks to the bottom, the other stays in the middle, and the blue unused resin stays at the top. So the water passes through the used resin first, which would 'release' the weaker ions and those get picked up by the unused resin at the top. When this is used up, the released ions would just go to the next DI canister like it was more media in the same canister. So what am I missing?

I wouldn't rely on colour changing resins. A TDS meter in the us range is a much better than a ppm.

[Floyd R Turbo]

Interesting Ace. I guess I find that result to be the opposite of what I would have expected, I would not think that passing the water through used resin would make any difference on the end result as long as there was unused resin after it. After all, this is exactly what is happening in my one canister setup.

I don't rely on the color changing resin for TDS, I rely on it to tell me when I should start paying more attention to my TDS.

[Ace25]

Believe me, I thought EXACTLY like you are thinking as well, which is why I did put 2 DI canisters back to back. It took me a couple refills to figure out it was not working like I expected it to. Thinking about it more, and with my experience at work, I think I know why it doesn't work.

If water sits in a DI canister, it leaches bad stuff back into the water. On a normal home RO/DI system you are supposed to back flush the membrane before and after every use and you're supposed to let the clean water run for a minute or 2 before collecting. If you let the filter sit with water in it (like you connect/disconnect or use a float valve but only have water running though every couple days) then all the bad stuff in the DI canisters get released. That 1500 TDS that is sitting in the first canister gets pushed right into the second canister, in turn severely shortening the lifespan of the resin. When my resin was completely exhausted, getting 1 TDS, my first canister looked near 100% used up/color changed, but the second canister looked 90% good going by color, but TDS told a different story.

[Floyd R Turbo]

yeah your 1500 TDS part is where you lose me though, my tap TDS is 500-650, and out of my RO membrane it's under 30. So running tap water through DI, of course that's going to burn that out like nuts, but we're not doing that. I know what you're saying happens when the RO membrane sits there, which is why I tap off the RO outlet and drain about a gallon of water out before engaging my DI, and that water will fire out at >80 then slowly ramp down to below 20. But then when I flip the valve to go through the DI bed and sit there and watch the TDS out of the DI, even after a week of sitting there and no water running, it rarely jumps up above 5 or 6 and then ramps down to zero within less than 1 changeover of the DI chamber. I never see the TDS going as high as double the DI input

[Ace25]

That is opposite of what I experience with DI that has been sitting for a few days, it is always way higher than tap water TDS until the water that is in the DI canister gets flushed out. On a home unit, it isn't that much, but it is a short term spike, but still I think that really reduces a secondary DI filter if placed in line, just a waste of DI from my experience. I get much more life just using a single canister than using dual.

On a DI only canister that is 4' tall, there is A LOT of water sitting in there stagnant which is why I see 1500 TDS for up to 5 minutes running the faucet on full open. If the DI canister didn't spill bad stuff into the water, then I shouldn't see much higher than tap TDS, but I see way higher and the company that replaces the DI bottles even said that is how DI works and needs to be flushed daily or else the water coming out will be much worse than tap water. I was told the only way to avoid the issue is to drain the water out of the DI after each use, but that is not a realistic solution IMO.

quote I found:

DI resin can and does release stored up TDS even before it is exhausted. Weakly ionized substances don't have a strong electrical charge or bond so start sluffing off and get released back into the treated water. These include silicates, phosphates and nitrates just to name a few and none are what you want in your reef.

[Floyd R Turbo]

Well since I am the king of valves, I would say that what I should do to solve this is flush the RO membrane like I have been doing (and I need to truly flush it, with a flush kit, I'm just running the bad water out of it right now) then when that's done, I can open another valve between the 2 DI chambers and flush out the first, then when that's clear flush out the second, and then start making water. Since I make water 40 gallons in one shot without shutting it off, I figure this will work OK. Still trying to wrap my head around the whole problem with 2 DI chambers but maybe it's the flushing part that was missing, because let's say you run them 100% all the time, it really should be no different running 2 separate chambers than it would be running one larger one as long as you never let the water sit stagnant or as long as you flush them as described. What do you think, does that make sense?

[Ace25]

I agree, if you put a flush valve in between the 2 DI canisters that should solve all the issues I am speaking of.

[Garf]

Ace25 - I suggest you tell your boss that the people supplying your resins are a bunch of jokers and get some proper suppliers in. They are either telling you lies, supplying spent resin, regenerating resins with sub standard chemicals or a combination of all three. A fully charged mixed bed should never leach dissolved solids. Separate anion and cation chambers do leach some solids, but these are recircularory systems and continually reprocess the same water,in effect replicating a mixed bed by by repetition. These are also regenerated as previously described.
Floyd - you are right, two connected mixed beds is the same as one longer mixed bed.

[Ace25]

Good to know Culligan can't be trusted. I will inform the state of California, my employer.

BTW, just got home and did a brand new test. Tested TDS of my tap water, then I took the DI canister off my RO/DI unit, pulled the DI container out of the canister, and let the water drain into a cup that was sitting in the DI container. I last used my RO/DI filter on Saturday, 4 days ago. Results:

Tap water = 560 TDS
DI canister = 1330 TDS

Soooo.. maybe I won't be talking to my employer after all.

After running water through RO/DI filter (7 stage Seachem w/ BRS filters, Spectrapure membrane) for 30 seconds post membrane flush, TDS = 0. It is that 1330 number that gives me the greatest concern. It proves to me without a doubt that bad stuff does get released from the DI resin back into the water if it sits. A 30 second flush of the DI chambers seems to solve those issues though, so it isn't a big deal as long as you do that 1 step.

Here is the thing I am a little confused about. When you put 2 DI canisters back to back, my thinking is canister 1 does ALL the work until the DI is all used up, then canister 2 takes over, because in theory it should be 0 TDS once it comes out of the first canister. So why did BOTH of my DI canisters get used up within days of each other, multiple times? Went through 8 bags of DI resin in a year because of that, now I go through 4 in a year. I would think having 2 separate canisters would not = 1 larger chamber because 1 canister can get the water to 0 TDS all by itself.