Hi,
Kalkwasser - German for "limewater," it is a highly concentrated solution of calcium hydroxide in water with a high pH of 12.00+ used to supplement calcium and maintain high pH levels in reef aquariums. The popular use of calcium hydroxide or Kalkwasser, probably did more to allow us to better keep SPS corals than just about any other advancement in reef keeping. The kalk reactor is a FR that will constantly stir the kalk, because kalkwasser is very hard to dissolves in water. The common method of dosing Kalkwasser is via water top-off. This was the reason why I have installed this in my system.
As for the system you’ve planned, if you have space for the 80liter drum, then it will work fine. Note that you don’t have to use float switch (float valve will work fine and more reliable) if you are going to gravity feed, but consider to place the drum higher than the sump level. Or else you need a feed pump that is connected to the float switch in the sump (can’t use float valve). In this case you don’t need the on/off valve before the float switch because you can off the pump during w/c.
Hope you understand what I mean. If not pls advice and I will try to put it in drawing.