The Massachusetts towns of Franklin, Medway, Millis, and Bellingham (“Towns”) own and operate satellite sewer collection systems that convey wastewater to a wastewater treatment plant (“WWTP”) for treatment and discharge into waters of the United States. The Charles River Pollution Control District (“Charles River”) owns the WWTP, which is part of a publicly owned treatment works. Charles River was the only entity that applied to the U.S.