Water is used in a lot of recipes, with no current way to increase its purity. Here's an idea for dealing that:
Make a new buildable structure called Water Filtration system. Fill it with 100l of water, fuel it with charcoal (for filters) and let it run. When it's done, you get 10l of water at a higher purity level. If you want, you can collect 100l of higher purity water and put it back into the system, to produce even higher purity water. The amount of purity increase can be adjusted for whatever balance seems appropriate. Alternatively, set it up similar to smelters, where there are X slots, and you put in either buckets of water or charcoal. The more coal you put in the greater the purity increase, but the fewer buckets of water increased, similar to the lime / ore system.
High purity water wouldn't be self replicating in this system. The purity of water you reach is always dependent on how much effort you're willing to put into it, rather than just reaching high purity once and then maintaining. Carrying buckets of water, and making coal clamps, is the negative pull forcing players to balance work with reward, which in my opinion is a good model for purity mechanics.