When you flush the toilet or wash your clothes, the used water, called waste-water, goes down the drain. This wastewater travels through a network of underground pipes known as the sewer system. The city system treats the wastewater to keep it clean. Both large objects, such as sticks, cans, rocks, and small debris, such as gravel and sand, are separated from the wastewater. It is treated with oxygen, which allows micro-organisms to grow and eat small bits of organics. The wastewater is then recycled and clarified again. The wastewater is then disinfected with chlorine to kill harmful pathogens before being released into a nearby river, lake, or sea. In towns that do not have a sewer system, each house has its own septic system. Toilet water flows into a big underground tank where bacteria helps break down the waste. Then the water flows out into the soil, where it is absorbed.