Manually manipulating water flow within a reverse osmosis system

[Sailor]

I was quite surprised to see such a dramatic display of ion activity (on the TDS meter) between deionization stages after I installed two additional DI canisters in my RODI system and separated the cation, anion, and mixed-bed resins. Nearly pure water from the membrane going into the cation resin measures as high as 500 ppm coming out, before decreasing and leveling off in the mid 20s. This phenomena occurs during the firsts 3 minutes every time I run the system after it had been idle for a day or more.

I described this in detail previously, simply seeking a technical explanation, but instead got some interesting but conflicting advice. I’m still in the dark about what exactly happens (chemically) to the water passing through the cation resin, but now I’m even more confused about what to do about it… if anything, since after all, the water exits the final stage at zero TDS.

Essentially, I was advised to employ one of three different variations of the same tactic, and that is, to install a valve to divert the first gallon or so directly to drain to avoid unnecessarily flooding the downstream filter elements with excessive contaminants, thereby shortening their useful lifespan. But here’s the kicker… I was advised by three different people to tap the plumbing in three different locations: 1) prior to the membrane; 2) after the membrane, prior to the cation resin; and 3) after the cation resin prior to the anion resin.

Has anyone here had a similar experience, and applied one of these solutions? Which one? Have you ever heard of anyone else doing it? Granted, it’d be an inexpensive and simple mod, but would it even make a measurable difference in the long run?

 

[blaxsun]

Well, while I don't have the same RODI setup - I manually flush my pre-sediment/carbon canisters (so that would be the before membrane) and I have a flush/bypass (after membrane). I just have the single DI resin so i can't offer any insight there.

 

[Battlecorals]

I was quite surprised to see such a dramatic display of ion activity (on the TDS meter) between deionization stages after I installed two additional DI canisters in my RODI system and separated the cation, anion, and mixed-bed resins. Nearly pure water from the membrane going into the cation resin measures as high as 500 ppm coming out, before decreasing and leveling off in the mid 20s. This phenomena occurs during the firsts 3 minutes every time I run the system after it had been idle for a day or more.

I described this in detail previously, simply seeking a technical explanation, but instead got some interesting but conflicting advice. I’m still in the dark about what exactly happens (chemically) to the water passing through the cation resin, but now I’m even more confused about what to do about it… if anything, since after all, the water exits the final stage at zero TDS.

Essentially, I was advised to employ one of three different variations of the same tactic, and that is, to install a valve to divert the first gallon or so directly to drain to avoid unnecessarily flooding the downstream filter elements with excessive contaminants, thereby shortening their useful lifespan. But here’s the kicker… I was advised by three different people to tap the plumbing in three different locations: 1) prior to the membrane; 2) after the membrane, prior to the cation resin; and 3) after the cation resin prior to the anion resin.

Has anyone here had a similar experience, and applied one of these solutions? Which one? Have you ever heard of anyone else doing it? Granted, it’d be an inexpensive and simple mod, but would it even make a measurable difference in the long run?

honestly I think its just what they call "Tds creep", even between di stages I experience this every time i make water as well. where is it coming from in already deionized water? I have no idea lol. And I am often asking myself the same thing but it only lasts a few seconds.

just to be Sure though, are you saying you see 500 tds out of the first DI stage before it settles back down? because that does seem a little odd to me.

 

[DCR]

I do not think there is any point in measuring the TDS of the effluent from the cation exchanger by itself. It is exchanging cations (Na, Ca, Mg, K etc) for H+ ions (acid) which are going to give a high TDS because H+ are very conductive. The TDS meter only measures conductivity which is then correlated to TDS. You can accurately measure TDS after the anion exchanger which produces OH- ions that are neutralized with the H+. You can also measure TDS after the mixed bed and the RO membrane, but the effluent from the cation is pretty meaningless.

 

[Sailor]

just to be Sure though, are you saying you see 500 tds out of the first DI stage before it settles back down? because that does seem a little odd to me.

Yes, that's exactly what I'm saying. What I'm seeing on the meter is real. It's not a false reading. But it is perfectly normal, and the effect is reversed after the water passes through the anion resin. I'm just wondering if installing a bypass valve to mitigate the effects of the TDS spike that occurs only during the first few minutes is a worthwhile endeavor. In other words, does this spike in ion activity significantly impact the resin's service life?

 

[Sisterlimonpot]

Yes, that's exactly what I'm saying. What I'm seeing on the meter is real. It's not a false reading. But it is perfectly normal, and the effect is reversed after the water passes through the anion resin. I'm just wondering if installing a bypass valve to mitigate the effects of the TDS spike that occurs only during the first few minutes is a worthwhile endeavor. In other words, does this spike in ion activity significantly impact the resin's service life?

If you missed it, I think below explains it quite well.

I do not think there is any point in measuring the TDS of the effluent from the cation exchanger by itself. It is exchanging cations (Na, Ca, Mg, K etc) for H+ ions (acid) which are going to give a high TDS because H+ are very conductive. The TDS meter only measures conductivity which is then correlated to TDS. You can accurately measure TDS after the anion exchanger which produces OH- ions that are neutralized with the H+. You can also measure TDS after the mixed bed and the RO membrane, but the effluent from the cation is pretty meaningless.

Conductivity of the ionic charge of the H+ ion will give you a high reading on the TDS readout. Essentially, not reading TDS rather its reading the ability to pass current across the 2 prongs on the probe.

Only after water passes through the anion will it be neutralized and provide a more accurate reading of actual TDS.

The meter can't discriminate between what's allowing current to pass through the sensor, it's just telling you the value of conductivity (displayed as TDS) between the 2 prongs on the probe.

 

[Sailor]

I do not think there is any point in measuring the TDS of the effluent from the cation exchanger by itself... the effluent from the cation is pretty meaningless.

I agree 100%. The TDS of the effluent from the cation exchanger by itself is meaningless. It’s just by chance that I became aware of this phenomena in the first place. I added a dual-canister DI assembly to my existing system. It came with a dual TDS meter and sensors installed in the inlet and output ports.

Prior to the upgrade, I saw about 600 TDS going in and zero coming out of my 4-state RODI system. Now, with 5 TDS sensors at various points in the system, I can actually see ‘how the sausage is made’ so to speak. Armed with this information, I’m wondering if it’s worthwhile to install a bypass to extend the service life of the resin (as I had been advised to do by several others). Does it really have that much of an impact on the resin? And if so, where should I insert the bypass?

I understand that a TDS meter senses conductivity based on the presence of ions in the water. But I don’t understand why there is Na, Ca, Mg, etc. in the water at all after exiting the membrane. Granted, the tap water here in Phoenix is about 600 TDS, but after passing through the pre-filters and the membrane, the TDS reads less than 10. That’s essentially pure H2O with a few renegade minerals squeaking through.

So, again, nearly pure water goes into the cation resin. Within seconds after starting up, the post-cation TDS reads steady at 28 for about a minute. Then, over the next two minutes, it rises to around 500 before falling back to 28, where it stays all day long. Why does it do that? And why only between the second and third minute of system operation?

More importantly (to help me understand the chemistry), if the local tap water was closer to 100 TDS than the actual 600 TDS, would I still be seeing a 440-500 spike coming out of the cation resin?

 

[Sisterlimonpot]

Where in Phoenix? I'm in west valley (Litchfield).

Most of my knowledge is from spectrapure. I've spent quote a few days lingering in the warehouse talking with the resident scientists there.

 

[Sailor]

Conductivity of the ionic charge of the H+ ion will give you a high reading on the TDS readout. Essentially, not reading TDS rather its reading the ability to pass current across the 2 prongs on the probe.

Each response to my query is like getting one more piece of the puzzle that helps me see “the big picture”. Thank you for that.

Unfortunately, I’m still missing a few pieces to that puzzle! You imply that there’s a difference between TDS and conductivity. Well, of course there is. One is something you can hold in you hand (theoretically) and the other is an abstract measurement. But that’s all a TDS meter does — it measures conductivity based on the concentration of ions in the water.

Understanding the chemistry behind this phenomena is one thing, and I’m still not quite there yet. But right here, right now, I’m seeking a practical solution. Does this TDS spike have a significant negative effect on the resin? Does it deteriorate the resin’s capacity so much that installing a bypass would have a noticeable or measurable service life-extending effect (on both the cation and anion resins)?

 

[Sailor]

Where in Phoenix? I'm in west valley (Litchfield).

I'm in the east valley -- Gold Canyon -- about as far east you can go and still be considered part of the greater Phoenix metropolitan area. And unfortunately, that's 20 miles away from the closest reef shop, and 30 miles or more from a few others worth traveling to for livestock and supplies.

 

[Sisterlimonpot]

Seeing that you have a bunch of tds probes scattered throughout. While you have the spike on the sensor post cation are you seeing stable readings from the sensor post membrane?

If I understand you correctly the spike is short lived upon startup. And after it settles it goes down.

I have 2 theories that might explain this.

One: this can simply be explained as TDS creep in the membrane. Which if I remember correctly is the ions freely passing across the membrane of a stagnant (no pressure, no flow) membrane. Basically a neutralizing of the separated +,- ions. And at start up those neutral ions are allowed to pass through. Edit: or maybe it wad just one type charged ion is allowed to pass through??

Or two: a similar occurrence is happening in the cation. Perhaps as water settles (no flow, no pressure) ions are allowed to neutralize until flow is reinstated and we see it as elevated conductivity.

Niether may be proven true. Either way, I think in the subject of DI longevity, I would think that wasting that high TDS before going into the anion would result in longer life of the media.

I have a similar Frankenstein RODI system with rows of separate cati and ani, then some spectrapure di like silica buster and max cap.

i just looked and i dont have a tds meter in between the cati and ani. This is very interesting and worth getting to the bottom of.

about as far east you can go and still be considered part of the greater Phoenix metropolitan area

Haha. And I'm about as far west as you can go... people say I'm closer to L.A. than Phoenix. Haha.

 

[Sailor]

I have 2 theories that might explain this.
One: this can simply be explained as TDS creep in the membrane...
Or two: a similar occurrence is happening in the cation...
This is very interesting and worth getting to the bottom of.

Yes, for two reasons… 1) to help understand the science behind all this ionic activity, and 2) to determine if modifying the system itself as well as the initial operating procedures (during startup) has a tangible benefit.

I’ve got some new information. When I upgraded my RODI system a few weeks ago, I took the opportunity to modify the plumbing to include a diverter valve after the carbon block pre-filters — not as a means to purge waste water prior to the membrane, but rather as a means to tap a sample to test for chlorine to see if the carbon elements are still filtering like they’re supposed to (and protecting the membrane).

Today I used that diverter to send about a gallon straight to the drain before redirecting the water back on its normal flow path to the membrane. That made a big difference!

Prior to today, every time I started the system to make pure water, I’d see the post-membrane TDS numbers shoot through 250 within the first 30 seconds before dropping below 10 about 30 seconds later. Then, between 60 and 180 seconds after startup, I’d see the post-cation TDS numbers rise to nearly 500 before stabilizing at less than 30.

Today, after dumping that first gallon to drain prior to the membrane, the post-membrane TDS never went higher than 100 and stabilized much sooner than usual, and the post-cation TDS never rose above 200, but seemed to decrease at a slower rate, so it still took about 3 full minutes before all numbers stabilized.

That kinda implies that TDS creep exists… in the carbon canisters! I’m going to have to do some more experimenting, in a controlled manner, changing only one parameter at a time. This is going to take some time.