With an overhead sewer: the house waste water from above grade (usually from first floor and above) drains to the city or village sewer main by gravity, and all waste water from below grade (usually all the basement plumbing fixtures) drain into a basin in the basement from which it is discharged into the gravity overhead sewer line by a pump.
In addition, in Chicago the basement pump basin can collect some ground water from around the house, which is then pumped into the gravity overhead sewer line as well. In the suburbs the clean ground water must be collected by a separate pit and pump and the water must discharge onto the exterior lawn or into a storm sewer.This plumbing design changes the house's original drainage system. The original lowest drain (usually the basement floor drain) is raised to a higher level (usually first floor bathtub drain).