I just installed a reverse osmosis and deionizer water filtration system, and it is extremely slow. I calculate that it will take 3-4 hours to filter out 10 gallons of water.

I am planning on filling up two of my 5 gallon plastic buckets the night before brew day, and snapping a lid on it. So 5 gallons of water will be exposed to air for roughly 1.5 hours while it is filling, and be exposed to the air inside of the bucket after I snap the lid on.

I recall reading that water will naturally absorb carbon dioxide from the air, and this will increase the level of carbonates in the water, making it harder and increasing its pH.

My goal is to replicate the purity of the distilled water I buy from Walmart.

Is this something I should be concerned about, or would the distilled water contain similar levels of carbonates due the absorption of carbon dioxide from the air?

3 Answers 3


'[W]ater will naturally absorb carbon dioxide from the air'

Yes it will. But consider this: at atmospheric pressure and room temperature the solubility of CO2 is roughly 0.7 volumes (~1,400 ppm). Factor in that our atmosphere is currently ~0.04% CO2 and the most CO2 you could absorb from the atmosphere is ~0.6 ppm. This reaction won't happen immediately (or probably even in 10 hours), and in fact your water may already contain that much CO2 coming out of the treatment system.

But more importantly:

'...and this will increase the level of carbonates in the water, making it harder and increasing its pH.'

This actually doesn't happen. CO2 only increases carbonates (and therefore hardness) in the presence of calcium or magnesium (typically as chalk). The acidity (CO2 decreases pH of your water) helps to dissolve the Ca or Mg that would otherwise be insoluble, resulting in increased levels of carbonate/bicarbonate/hydroxide ions (depending on pH). Incidentally, this is the exact opposite of what happens when you boil water to remove bicarbonate hardness (boiling drives out dissolved CO2, decreasing the solubility of the dissolved carbonates and allowing them to precipitate).

The water you collect through your system will have a fixed amount of hardness already (in Ca and Mg ions), and probably a very small amount, depending on the nature of your deionizing system.

So unless you were to add CO2 and throw a bunch of chalk in to your water while it's sitting nothing will change the hardness.

'Is this something I should be concerned about...?'

Really, there's nothing at all to worry about here.

  • Hey Franklin. "CO2 only increases carbonates in the presence of calcium". Since I will be adding CaCL and CaSO4 back to the water, what should I expect from this, and is there anyway for me to calculate how many ppm of carbonates would be formed? On the other hand, with the distilled water I buy from Walmart, I never thought of this. Oct 26, 2015 at 17:36
  • I presume that it would help if I didn't add the CaCL and CaSO4 until right before I dough in. How fast is this reaction? Oct 26, 2015 at 17:37
  • 2
    No the addition of CaCl2 or CaSO4 will have no effect on the carbonate levels of the water. I should have been more specific in saying that it is really only calcium or magnesium carbonate which, in the presence of CO2, can dissolve as the more soluble bicarbonate. Oct 29, 2015 at 12:57

I really wouldnt worry about it. I have buckets of walmart RO water sitting around for months for my aquarium and ive never heard of these issues and aquarists are some anal peeps!


If I was super paranoid about my water, I would do the following:

  1. Scrub, Rinse, then Sanitize cornie keg and all racking equipment
  2. Fill keg with [insert your favorite inert gas]
  3. Fill keg with water from the bottom up by connecting the RO tap to the out post on the keg
  4. Close keg and use gas on the gas post to seal keg, momentarily venting with quick-release valve a few times
  5. Fashion tin foil hat and place on head

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