The Philadelphia Story

摘要

ONCEALED BENEATH THE STREETS OF OUR GREAT CITIES are thousands of miles of pipes and tunnels that carry away our wastewater. In big eastern cities such as Philadelphia, this system can date back to the nineteenth century. As urban populations have expanded and streets have been paved over, the volume of domestic sewage and contaminated rainwater after storms often overwhelms our treatment plants. These combined sewage overflows can't be allowed to back up into city streets, so they only have one place to go-into our waterways. Cities can spend billions maintaining and