How does it work?
Water is filtered through Calcite, the Calcite dissolves into the water and raises the pH/ Alkalinin
Calcite works better at lower pH, if you were to drop some Calcite into an acid solution it will react with the acid start to fizz and dissolve quickly, however if you add Calcite to neutral pH water, very little reaction is seen, it will still raise the pH of the water but requires much longer times to do so.
Aren't Alkalinity and pH the same?
No they are not the same, pH is the measure of how acidic or how alkaline a solution is, a solution with a pH of 7 is neutral pH, anything below pH 7 is acidic anything above pH 7 is alkaline.
Alkalinity is the measure of how well a solution can buffer (or resist) changes to pH, so a solution that is alkaline (above pH 7) does not necessarily have high alkalinity.
OK to give you an example, if I had a solution with a ph 8.5 (an alkaline solution), say I add a drop of acid to that solution, if the pH stays the same then the solution probably has high alkalinity, the solution can resist changes to pH, but if the pH of the solution drops then the solution probably has low alkalinity, cannot resist changes in pH.