Free CO2 and DI depletion

Sounds like you have a drinking water system.
Are you using the pressure tank to store RO/DI water or just RO water or are you using it at all?
The post carbon GAC filter is for RO drinking water taste and odor, often RO has a rubbery taste so the carbon takes care of that. But, you do not want to use that GAC for your RO/DI water since it actually adds TDS back as the carbon degrades and turns to dust.
Have you read any of my threads where I talk about how to install a check valve between the RO and the DI so you can still use the pressure tank and GAC for RO only drinking water and other uses and make DI water directly from the RO membrane without the pressure tank and its stored up TDS creep?
If you are using the pressure tank and want RO only water too then follow this diagram and install the check valve between the RO and DI to isolate the DI from the tank:
http://spectrapure.com/huds/4-STAGE-DWK-RODI-NAG.pdf
If you do not want or need any RO only water then eliminate the pressure tank and the GAC carbon completely to elimiate the TDS creep issues associated with pressure tanks and short filter runs. You never ever want to store RO/DI in a pressure or bladder tank, it is agressive and starts eating the metal and rubber bladder.

 

The short answer is- only RO goes to the Storage tank/sink tap/icemaker
- if the DI is demanded/needed/utilized- the water from the tank goes thru a small carbon, and then the DI resin- the DI cannister has been mounted vertically, flowing from bottom to top-
per you suggestions years ago.

My question was- are DI resins also mounted BEFORE the Membrane? Mines not

 

No, DI always follows RO. RO acts as pretreatment for DI and does 90-98% of the treatment so the DI acts as a final polishing filter is all. Without RO, if you have much TDS at all your DI would have to hold many cubic feet of resin to be very effective and last very long.
Good quality, fresh DI resin has a removal capacity of approx 3000 to 6000 total TDS per 16 oz or pound of resin. It's easy to do the math but the difference between say 2-3 TDS going into the DI compared to 10-15 TDS going in is tremendous.

You really really do not want to use your pressure tank and GAC filter to make DI, without those the resin will probably last 8-10 or more times longer and you will have better finished water quality. Use your handheld TDS meter and take some readings on the water coming out of the pressure tank when it has been sitting for a day or two. Then take some TDS readings directly from the RO membrane without the pressure tank hooked up. The membrane will be much lower. Your DI will thank you many times over if you install the check valve like in the diagram I linked to. Your wallet will thank you even more as the resin replacements will be cut by a factor of 6-10 or more. Pressure tanks are the worst thing there is for making DI . The frequent starts and stops to satisfy the pressure in the tank leads to huge TDS spikes which get stored in the pressure tank and which contaminates the rubber bladder.
I tried two 3-5 gallon tanks and it helped slightly. I then tried a 14 gallon pressure tank and it helped a little more. I installed the check valve isolating the stored RO in the pressure tank from the DI and the problem went away completely. DI life and final water quality went up and the cost per gallon went down a ton since DI was lasting over 7-8 times longer so months and months versus a few weeks per cartridge.
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The short answer is- only RO goes to the Storage tank/sink tap/icemaker
- if the DI is demanded/needed/utilized- the water from the tank goes thru a small carbon, and then the DI resin- the DI cannister has been mounted vertically, flowing from bottom to top-
per you suggestions years ago.

My question was- are DI resins also mounted BEFORE the Membrane? Mines not

 

I always test my RO After the 'tank', as it's used many times everyday for drinkiing, cooking, coffee, etc.

For years I got 0tds on my RO also.
Last check I think it was 1 or 2 tds.
I then checked one of my auxillary lines way away in my basement, the same one that goes to my DIY ATO to keep it at the level I set; for RO/DI cleaning etc,- it was still 0

 

It is impossible to get 0 TDS out of an RO membrane since they are only a 90 to 98% efficient filter. You are probably testing at or outside the lower accuracy limits of your TDS meter. Have you calibrated the meter lately?
If nothing else and you still insist on using the pressure tank, at least bypass the carbon filter when making DI, this will make the DI last longer. GAC is only for drinking water taste and odors, it serves absolutely no purpose when making DI.

 

Spectrapure reccomends this unit as well.