A sanitary sewer is an underground pipe or tunnel system that is used for transporting sewage from houses and commercial buildings (but unlike other sewers, not storm water) to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of an overall system that is referred to as a “sewage system” or sewerage. Sanitary sewers serving industrial areas may also carry industrial wastewater. In municipalities served by sanitary sewers, separate storm drains may carry surface runoff directly to surface waters. An advantage of sanitary sewer systems is that they avoid combined sewer overflows. Sanitary sewers are often much smaller in diameter than combined sewers which also transport urban runoff. Backups of raw sewage can still occur if excessive storm water inflow or groundwater infiltration occurs due to leaking joints, defective pipes etc. in aging infrastructure.
The reason for sanitary sewers is that sewage treatment is less effective when sanitary waste is diluted with storm water, and combined sewer overflows occur when runoff from heavy rainfall or snowmelt exceeds the hydraulic capacity of sewage treatment plants. To overcome these disadvantages, some cities created separate sanitary sewers to collect only municipal wastewater and exclude storm water runoff, which is collected in separate storm drains. The decision to build a combined sewer system or two separate systems is mainly based on the need for sewage treatment and the cost of providing treatment during heavy rain events. Many cities with combined sewer systems built their systems prior to installing sewage treatment plants, and have not subsequently replaced those sewer systems.