This diagram show the water suplly system in Australia. It is compose by three types of water: the pure water (which is find for example from the river), the storm water (the water which come from storm) and watse water (obviously the renewable one).
Pure water is come from dam or storage to households, shops, factories etc. Like the pure one, watse water can also come from the dam or storage, or as we know, from the households, cities, factories etc; it reachs the water treatment plant where it can be renew. I think watse water is the after-using of mainly pure water.
But water from watse is not the only one which is from the city, there is the liquid collected after storms too. This last type is the only one which is not use (or not just after the collecting), because it will be rejecting to the river where it may be renew (it is just my hypothese). The reason why it cannot come from the dam or storage is because we don’t know when storms will arrive.