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The Drainage System After we use water, it becomes wastewater. It goes down the drain. The drainage system takes this dirty water away. These pipes are usually larger than supply pipes. They use gravity to work. The pipes are sloped slightly downwards. This makes the water flow down and out of the building. The wastewater goes into the city sewer system or into a septic tank on the property. To stop bad smells from coming back up, pipes have a U-shaped bend called a trap. Water sits in the trap