Combined Sewer Systems: Down, Dirty, and Out of Date

Abstract

When combined sewer systems were introduced in 1855, they were hailed as a vast improvement over urban cesspool ditches that ran along city streets and spilled over when it rained. These networks of underground pipes were designed to dry out streets by collecting rainwater runoff, domestic sewage from newly invented flush toilets, and industrial waste-water all in the same pipe. Waste- and stormwater was then discharged directly into waterways; in the early twentieth century, sewage treatment plants were added to clean the wastewater before it hit streams. Combined sewer systems were—and still are—a great idea, with one catch: when too much stormwater is added to the flow of raw sewage, the result is frequently an overflow. These combined sewer overflows (CSOs) have become the focus of a debate regarding the best techniques to manage growing volumes of sewage and stormwater runoff in many older U.S. communities. In dry weather, a combined sewer system sends a town’s entire volume of waste-water to a sewage plant, which treats and discharges it into a waterway. Rain and snowmelt, however, can fill up a combined sewer. The sewers have been specifically designed with escape overflow pipes so that the mixture of sewage and stormwater doesn’t back up into buildings, including homes. The resulting CSO dumps raw sewage into lakes, rivers, and coastal waters, potentially harming public health and the environment. In April 1994, the U.S. Environmental Protection Agency (EPA) issued the CSO Control Policy, the national framework for control of CSOs, through the National Pollutant Discharge Elimination System permitting program. This policy mandated that communities dramatically reduce or eliminate their CSOs, and the agency began working with municipalities to improve antiquated sewage systems so they could reach Clean Water Act goals. Under this policy, communities with combined sewer systems must establish a short-term plan to control these discharges as well as a long-term control plan. The EPA’s mandate on CSOs leaves communities with two basic options, according to Joan B. Rose, a public health microbiologist at Michigan State University. Communities with CSOs can build separate underground pipes for sewage and stormwater. Or they can keep their combined pipes and somehow build more capacity. “But if they shut down [combined sewer systems],” she says, “communities must find a way to store or treat peak flows when it rains.”