Infiltration describes the ingress of groundwater into a sewer system through faulty joints, damaged pipes, at manholes, etc.

Infiltration increases sewage volume and flow rate, making the sewer and treatment systems more costly.

In places where the pressure pipes are placed below the groundwater level and influenced by gravity the volume from infiltration can be very high. As a result, the pump system has to run faster than it was designed to, which causes more wear and tear and higher energy cost.

In the treatment plant, infiltration will have a negative impact. The process temperature can be lowered, which calls for higher demand of aeration and so on.
In separate systems the infiltration can cause capacity problems, increasing the risk of flooding and eventually undermine roads and structures.
As a result of the problems caused by infiltration much attention has been focused on renovation of old sewerage systems.