In some water filtration setups, there are two carbon stages before the reverse osmosis (RO) stage, and then another carbon filter after it. What exactly does that final stage do?
(The usual note given by RO sellers is that the carbon stage after RO "improves taste". This is not thorough enough of an explanation for me, yet it's the only answer I can find on the internet.)
I can’t refer you to an authoritative source for this answer, but I remember reading it somewhere online when I was researching RO prior to installing my own system.
Water that has passed through an RO membrane becomes slightly acidic; the pH drops below 7.
Acidic water slowly attacks the rubber liner inside the RO storage tank, which releases a slightly rubbery, “off” flavor into the water. This can be prevented by passing the RO water through a remineralizer cartridge before it reaches the storage tank.
A remineralizer contains minerals that react with the acidic water and release calcium and magnesium ions. This makes the water taste more natural, and most importantly it makes it slightly alkaline with a pH above 7.
An RO system that includes a remineralizer does not need a final polishing carbon filter between the tank and the point-of-use faucet because the rubber liner in the tank does not degrade with alkaline water.
When no remineralizer is used, a final polishing carbon filter between the tank and the point-of-use faucet is recommended to adsorb and eliminate the rubbery “off” flavor of stored acidic water. It “improves taste”, as they say.
