The system within the system

As everyone may know by now through other posts or general knowledge that the Romans build the aqueducts. At the high point of Rome's population the aqueducts were bring in around 150 million gallons per day. So the real question is where is that water go after it got to Rome. Some went into storage tanks, but the rest had to be taken care of. So the Romans built the first under ground sewer system. The system would take the water and deposit it in the river down stream of the city.  Then after, the first public bathrooms were built over the sewer system. Which was better than going in pots and taking to the sewer.