Rain water is the purest form of water known, although maybe not so much in a city where smoke and industrial waste clouds the issue, although after it rains for a while, it is a lot cleaner. Similarly, when collecting rainwater from downspouts, it's best to let the rain wash them for an hour or so before collecting water.
More elaborate systems can be plumbed in with large storage tanks, proper runoff and drainage which can then be plumbed into the house for any non-potable water source. If one branch is diverted to have an additional UV filter as well as a carbon filter then it would indeed be potable.
