See my response to tober below. Water that goes into your tub then down the drain is equally used and consumed in the eyes of the Water Department, since I doubt they check to see if the water is still potable (i.e. unused) after it goes into your drain. I think the inequalities are things where you draw water from your home and NOT use the drain: drinking water yet peeing in someone else's toilet, giving water to pets who pee outside, filling a pool yet emptying it uncontrolled into the city sewer system, watering plants, etc. On the other hand, your sewer system might have more outgoing than incoming if you have a dinner party and they pee in your toilet, or more significantly if it rains and somehow (a leak? a wet basement's sump pump?) the water from the rainstorm ends up leaving via your sewer system.