If a home or building is on a city sewer system the draining water flows through the home's plumbing and into a single pipe that ultimately leads to the city's wastewater collection system. The water is then pumped into a treatment plant.
Once the wastewater reaches the treatment plant it is treated and cleaned so that it can be put back into the city's water supply without being wasted or harming the environment. Homes on a septic system, however, collect wastewater in a large tank buried underground. This water seeps out of the tank into a leach field for dispersion into the ground.