Some US cities have undertaken sewer separation projects — building a second piping system for all or part of the community. In many of these projects, cities have been able to separate only portions of their combined systems. High costs or physical limitations may preclude building a completely separate system. In 2011, Washington, D.C., separated its sewers in four small neighborhoods at a cost of $11 million. (The project cost also included improvements to the drinking water piping system.)