One of the examples of a minor transform is making inedible food, edible. I would think this goes the same with non-potable water to potable water. (I'd almost say it's easier vs the food)
As for quantity per body... well that's sorta up in the air. Although unless there's a long time limit in purifying, it doesn't matter if it's a lot or a little. If you can do 1 Liter of water per phase, and 2 phases per turn, that's still 12 phases in a minute, and 720 in an hour. So unless you are requiring to purify a whole lake, or purify water constantly say for an army encampment, then the quantity in this case really probably won't matter.