[untamed]

Originally Posted by Bryan

Untamed, do you notice a drop in dkh from passing through the second stage?

Sorry, I've never attempted to measure that difference. In theory, the dkh should only be HIGHER after the 2nd stage.

The idea, of course, is that the CO2 reacts with the CaCO3. The more the CO2 successfully reacts, the higher the pH goes.

If you stopped the water flow through the reactor...added CO2 until you got to pH6.5...and then stopped CO2 and all flow...then the pH in the reactor would slowly rise back up as the reaction takes place. It wouldn't get all the way to pH 8.0 because the solution in the reactor would become saturated with CaCO3 and higher pH would start to recalcify the CaCO3. (which is what happens when you add too much Kalk too fast).

Since we maintain the first stage at pH of 6.5, any water leaving that stage will be at pH6.5 and therefore contains a LOT of residual CO2. That is unavoidable. That residual CO2 is the problem and drives down the pH of your tank. This is also why you can't flow water too fast through the reactor. Too high a flow rate just blows water and CO2 through the reactor and there isn't enough time for the reaction to take place. That's why slowing the flow rate of your reactor can actually make it deliver MORE Ca/Alk and why larger reactors can deliver more than smaller reactors.

With a multi-stage reactor, that residual CO2 reacts in the 2nd stage and causes the 2nd stage to be higher in pH. CO2 is delivered based on the pH in the first stage, so the 2nd stage is allowed to move higher in pH without causing additional CO2 to be added.

Sorry for the long-winded response!