In the United States, we flush more than 5 billion gallons of water down the toilet every day. Wastewater from your toilets, sinks, bathtub, shower, dishwasher, and washing machine flows into a small pipe under your home. In areas with public sewers, this pipe connects to a system of larger underground pipes that carries the dirty water (sewage) to a treatment plant.
Gravity does most of the work, but sometimes pumps help move the water along. In the treatment plant, wastewater is cleaned in stages. Screens catch the big stuff. Solids separate from the liquid in settling tanks. Skimmers remove the stuff that floats. Scrapers remove the stuff that sinks.