I have water bottles. I fill them with dirty water, take them to the firepit and can turn them into clean water.
I have canteens. I fill them with dirty water, take them to the firepit...and still have canteens full of dirty water. What am I missing here? Can you not pour water out of a canteen?
I have 4 options to make clean water. Canteen which can either require a bottle of clean or dirty water and an empty canteen, or Bottle which either requires a bottle of dirty water or a canteen of clean water and an empty bottle. No option lets you clean dirty water from a canteen. No option anywhere lets you do anything with the dirty water canteen aside from drink it.
I ended up drinking the dirty water from the canteen, using medicine and then using then using the advanced campfire to fill the canteen with clean water (4 dirty bottles I believe). I didn't see an option to dump the canteen. You can also craft a new canteen.