I imagine you could always pull Kalk from the avast with a peristaltic pump if you really wanted to. You'd have to somehow match flow rates going in with what you take out, but you could do that in a hacky way too.
1. Put the line that's extracting Kalk water near the top, so that it's always getting mixed Kalk, and it can't actually pull a large amount out.
2. Add and extract at the same time, or extract then add.
3. Extract faster than you add, and more than you'd add
Eg set it up to extract Kalk water out at 50% faster then you add, extracting at the same time as putting in, BUT because you have the removal line near the top of the reactor, it won't actually do that.
Because you extract Kalk water faster / more than you'd ever add, the system would actually only ever extract the amount you added. If you added 1L and it tried to extract 2L, it can't; it'd extract 1L of Kalk and the rest would just be air.
I'm not sure if that's clear. In different terms it'd be a resource constrained system where it would always be limited by how much you add. Ignoring wear and tear, you could even let the Kalk extract run 24/7 and only add at night, and it'd therefore only extract at night.