Sewage is the wastewater and human waste that flows through underground pipes from toilets, sinks, and drains in homes and buildings. Every time you flush a toilet, wash dishes, or take a shower, that dirty water becomes sewage that travels through a network of pipes called sewers to treatment plants.

Before modern sewage systems existed, people threw waste into streets or nearby rivers, which spread terrible diseases and made cities dangerous places to live. The invention of sewage systems in the 1800s was one of the most important advances in public health. Suddenly, cities could safely remove waste and keep their water supplies clean. Engineers designed elaborate underground networks where gravity pulls sewage downhill toward treatment facilities.

At sewage treatment plants, workers and specialized equipment clean the water through several stages, removing solid waste and harmful bacteria. The cleaned water can then flow safely back into rivers and oceans. This invisible infrastructure, running beneath every street in modern cities, prevents disease and makes urban life possible for billions of people.

The related word sewer refers to the pipes that carry sewage, while a sewage system means the entire network of pipes, pumps, and treatment facilities working together.