text: The ammonia doesn't stay behind; it's a gas at room temperature.\nFractional distillation will do it, just like moonshinin' does to ethanol.  Heat the ammonia water and the dissolved ammonia gas will come out of solution at a greater concentration than the water vapor if you control the heat.  But if you're trying to make the water potable you must do many fractions; it's not worth it unless in extreme circumstances.