As long as you leave expansion room there should be no problem. Leave some room & go back after the rest of the jug has frozen & top it off with tap water.

As far as additional treatment goes I wouldn't worry about it since it's going to be frozen. If you were going to set the jugs in a closet they would need treatment even using tap water. Municipal supplies here in the States are supposed to have a residual amount of chlorine at the tap farthest from the treatment plant. My water relatively reeks of chlorine. This breaks down over time.