The sewer map shows the location of the main trunk sewer lines, as well as the smaller sewer lines down to the block level. Three sizes of sewage pipes are represented by different colors, line weights, and line styles.

Philadelphia, like most older cities of the eastern United States, has a combined sanitary and storm sewer system. After rainstorms, Philadelphia's sewers pour untreated sewage directly into the Schuylkill and Delaware Rivers. Combined sewer overflows (CSOs) are a major problem in Philadelphia and many other older cities. Most of West Philadelphia is served by the Mill Creek sewer which runs from Montgomery County to the north, through the middle of West Philadelphia, south toward the Schuylkil River. Many parts of this sewer line have caved in and been replaced since the original construction.