Sanitary sewers evolved from combined sewers built where water was plentiful. Animal feces accumulated on city streets while animal-powered transport moved people and goods. Accumulations of animal feces encouraged dumping chamber pots into streets where night soil collection was impractical. Combined sewers were built to use surface runoff to flush waste off streets and move it underground to places distant from populated areas. Sewage treatment became necessary as population expanded, but increased volumes and pumping capacity required for treatment of diluted waste from combined sewers is more expensive than treating undiluted sewage.
Communities that have urbanized in the mid-20th century or later generally have built separate systems for sewage (sanitary sewers) and stormwater, because precipitation causes widely varying flows, reducing sewage treatment plant efficiency.
In the UK, the term "foul sewer" was also in use for a sanitary sewer.