Sewers are artificial conduits that transport sewage and provide drainage. Sewage consists mainly of liquid waste produced by humans, including washing soda, laundry waste, urine, and kitchen waste. There are two main types of sewer systems: storm sewers that drain surface water runoff from areas with heavy rainfall, and sanitary sewers that transport domestic wastewater from homes and industries to treatment plants. Where possible, storm sewers should be separate from sanitary sewers to reduce treatment costs for storm water runoff. Separate sewer systems use one pipe for wastewater and another for stormwater, while combined sewer systems transport both through a single pipe.