RODI: Membrane flushing, TDS creep, and best practices questions

[Salt_Creep]

Hello,
I have recently moved to a larger tank and larger holding containers and I have found that I need to replace my DI resin just about every week or else my TDS starts creeping up, which raises my ALK in my tank via ATO. I do have well water, so its gross. At the RODI unit, it goes in at about 850ppm, into the DI at around 15ppm and out at 0 ppm with fresh resin, but I will start to see 1-3 TDS after about 20 gallons. I use a booster pump at make at about ~75psi. I just ordered an auto-flush valve, but I also learned more about TDS creep, and I think this is where I am shooting myself in the foot.

I have always set up my RODI to pretty much automatically fill up the storage tank based on a float valve. If the level drops, it fills back up. My ATO is in my fresh storage tank, so every time my ATO runs, the small amount of water is replaced. Seemed nice, but I am now wondering if it is more efficient to only fill the tank once so that I can first flush all of the TDS creep out from a bypass valve.

I guess the question I am trying to answer is how long does it take for TDS creep to really form? If my RODI runs for a few minutes every hour, am I just pumping high TDS water into my resin? If not, what else could I do to possibly extend the life of the resin, because replacing it every week is not practical.

 

[mfinn]

If it were me, I wouldn't allow it to run like that.
I would let it run down until the storage is in the bottom 1/4. Not sure how big it is. That should improve the amount you get out of a cartridge.
With TDS that high, I would experiment with BRS's 3 separate resin mixes.
See if you get more life out of the di resin.

 

[Rick.45cal]

I’m in a similar boat as you and you likely have high CO2 in your well water on top of the high TDS which makes you go through DI resins faster. I definitely wouldn’t operate your system the way you are, I personally use a substantial portion of my DI water out of my reservoir then manually turn on my RODI unit. I still have the float valve so it shuts itself off and I don’t have to worry about forgetting about it and then having to clean up water off of the floor. (It happens to all of us).

My schedule is largely based on how much water I drink in a week because I pull off the RO for drinking water into containers and store it in my fridge, when it’s time for me to make water for myself, my reservoir is usually ready for more water too. So I turn on the unit, by passing the Di stage and flush the product water down the drain until the TDS drops to the normal level (post membrane) then I switch the valve to produce DI water for the reservoir. Somewhere in the process I’ll fill my containers that I drink from the RO using the bypass and then switch it back and let it finish filling the reservoir.

There are several folks here that store and aerate their RO water before they pass it through the DI unit to eliminate the excessive CO2 and extend their DI resin use substantially. I however haven’t done that. I use more DI resin than I probably should, but not enough to justify another large reservoir in my living space.

 

[MTBake]

Aerating the well water should help the resin last longer.

Should let the ato reservoir run near empty before filling it. Its best to flush any tds creep out of the membrane before sending water to the di stage. I do this with a T fitting and shut off valve.

 

[Albertan22]

As others have stated or implied, yes you are shooting yourself in the foot running your RODI like that. Even with 15tds water coming in your cartridge should last a lot longer than 20 gallons. Turn off the RODI and let the reservoir drain down then run the RODI for a day to refill the reservoir. I bet you get 10-50x more gallons out of the same cartridge running it that way.

 

[Salt_Creep]

Thanks for the replies. I will start letting my storage tank run down and then fill it all up at once.

 

[Mikeltee]

Wow that's insane. It sounds like you could use a better setup for starters... check out airicewater

I love the idea of making as large of a batch as you can, shutting off your DI, and then flush the RI for a couple minutes before you make a new batch. You may be fine once you get a good DI setup though.

 

[h2so4hurts]

Another much easier but more expensive method to remove CO2 is vacuum degassing in line after RO but before your DI stages. This is very common on lab DI water units. A filter and a vacuum pump will probably run about $500 but could pay for itself quickly depending on how much water you make.