Most milk jugs I've seen have small openings which might make it harder to add heapfuls of kalk with a large spoon for a messy person like me. Also, I'm not sure how leak proof they are under pressure. They might work fine for gravity drip but may not be as good when used with a dosing pump.
I currently use a Rubbermaid container as my Kalk dispenser. I added 2 air hose fittings and attached it to my Osmolater. Works great. It has a gasket, is leakproof, and has a large opening to dump kalk in. Mine is only 2.6L.
http://www.rubbermaid.com/hpd/consum...rdMenuIndex=10
This one is however is 3.7L.
http://www.rubbermaid.com/hpd/consum...rdMenuIndex=10
Until I get a real kalk reactor, this works quite good. I put 3 teaspoons in once or twice a week and give it a good shake once in a while. The kalk dispenser sold by Tunze is almost exactly the same thing but costs $130 or more.
EDIT: Oh, I read further below and saw you are gravity dripping. So then these might be overkill?