Watch alarming TDS level as water passes through a deionization canister

[Sailor]

I just witnessed a phenomena that totally wrecked my understanding of how dissolved solids exist and are measured in water. I’m hoping that someone here can explain exactly what, how, and why I’m seeing what I’m seeing, and that is, the TDS value of water coming out of a deionization canister rising as much as 50 times the TDS value of water going into it (as shown in the attached video).

My reverse osmosis system uses 3 separate deionization canisters, each with its own purpose. Water exits the membrane, passes through the cation resin, then the anion resin, and finally a mixed resin bed. There’s a TDS sensor downstream from the membrane and another downstream from the cation resin, so you can see the direct effect the cation resin has on the water chemistry.

Before we go further, let’s stipulate what a TDS meter measures, and what it does not. A TDS meter measures the quantity, not the quality or type of dissolved solids in water. A TDS meter senses the conductivity of the water and returns a value based on the sum of all the ions in the water. That’s it. It measures ions. It does not necessarily measure pesticides, chemicals, residues, or any other non-ionic pollutants that might be contaminating the water.

Note that the tap water here in the Phoenix area runs at about 500-600 TDS. Note also that upon startup, it takes about a minute or two to purge stale water from the system after it had been sitting idle for a day or so and begin to display stable TDS values at various points in the plumbing. Water passes through the sediment filter and dual carbon block elements and enters the membrane still north of 500 TDS, then exits the membrane initially at about 200 TDS, but drops to and stabilizes at about 6 to 8 TDS within the first minute.

Okay, here’s the crazy part. Water exits the membrane at about 6-8 TDS and flows into the cation resin. For the first minute or so, water exiting the cation resin holds steady at about 28 ppm on the TDS meter, but then rises precipitously to as much as 450 TDS before retuning to a stable 28 TDS about a minute later.

Question #1: Water enters the membrane at a very high TDS, but comes out at about 6 to 8 TDS. It appears that the membrane is physically removing minerals, separating dissolved solids and/or suspended solids from the product water and disposing them via the waste water. Is that what’s happening at this stage in the RO system?

Question #2: Water leaving the membrane and entering the cation resin is virtually free of dissolved solids. There is only a relatively small number of ions in the water. Then, suddenly, the TDS meter reads triple digits before stabilizing at a value still 4 times higher than the incoming value. It appears that the cation resin is creating new ions out of thin air! How does this happen?

I can see how the cation resin might affect the pH of the water, rapidly altering the H+ ion concentration, which would indeed impact the TDS reading. Is it really that simple? So when I see the TDS value climbing through 400 it’s not because the cation resin is creating calcium or magnesium ions from non-ionic Ca or Mg atoms that somehow got through the membrane; instead it’s flooding the system with hydrogen ions, a process that I assume would be reversed when the water passes through the anion resin canister downstream.

Questions #3 & #4: Am I on the right track here, or totally out to lunch? More importantly, is this rising TDS phenomenon something to worry about, or not? After all, at the end of the day, the product water comes out at zero TDS.

Here's the video. Note that there's more information in the description section if you click the link to watch the video on YouTube rather than the embedded clip here.

 

[blaxsun]

I have the same TDS meter, and I place the in probe on the water from my tap to the RODI and the out probe on the water that goes to my pressure tank. Mine doesn't fluctuate more than a few TDS.

 

[Sailor]

I have the same TDS meter, and I place the in probe on the water from my tap to the RODI and the out probe on the water that goes to my pressure tank. Mine doesn't fluctuate more than a few TDS.

Thanks for the response, but I don't have an issue with any of my TDS meters. They are all pretty accurate. The rising (and subsequent falling) of the TDS value coming out of the cation resin is real. It is NOT a meter malfunction. My post is simply asking why does this phenomena occur.

 

[Buckeye Hydro]

It is good practice to allow the effluent from the cation stage to go to drain for about a minute before you send the effluent to the anion resin. You can do this with a three way valve.

3-Way Valve

At Buckeye Hydro we specialize in providing the highest quality water filtration systems and accessories for every potential need.

www.buckeyehydro.com

 

[KStatefan]

You will not get valid measurement if you are measuring between the cation and anion DI.
@Randy Holmes-Farley has explained why here before

 

[Sailor]

You will not get valid measurement if you are measuring between the cation and anion DI.
@Randy Holmes-Farley has explained why here before

I think you’re missing my point, KState. I’m not questioning my TDS measurements, as I'm absolutely certain they're valid. I swapped meters to verify the phenomena and sure enough, get virtually the same numbers every time. My original post is asking for a technical explanation why this phenomena occurs.

I searched this forum and several other sources prior to posting, including a ton of material from @Randy Holmes-Farley, and found numerous postings that dance all around my specific question, but none that address it directly.

Nearly pure water leaves the membrane and enters the cation resin, but the TDS coming out of the resin reads rock solid at 28 ppm for about a minute, then, during the second 60 seconds, it rises to nearly 450 ppm before dropping back down to 28 where it stays all day long. I don’t have a TDS sensor after the anion resin, but there is one after the final mixed bed, and that always reads zero.

Randy and others explain that much of this is a function of CO2 in the water as well as H+ and/or OH- ions. But again, the starting point is the nearly pure water coming out of the membrane. Where do these ions come from and what are they binding to? And how do you explain the timing? That is, why is the TDS reading stable for an entire minute before rising so high and falling back down? What exactly is happening here?