having a secondary water system (e.g., purple pipes / grey water), for general lawn/washing and one for drinking/cooking - strikes a good balance. You don't want your potable water system to move too slowly.
You also should have water towers at different heights (higher pressure in the potable system). any accidental interconnects (these do happen) will flow /away/ from the potable one.
finally, letting people use rainwater, runoff, and letting them recycle lightly used potable water for other uses (like flushing toilets, watering gardens) is good - try to get every liter of water used twice as a goal.
bonus: get past the ick factor and allow your sewage to be re-used (purple pipes) as the greywater after treatment at the plant.