What are Combined Sewer Overflows?
What are Combined Sewer Overflows?
CSO is an acronym that stands for combined sewer overflow and is a term used to describe what happens when combined sewer systems become overwhelmed by excess stormwater and overflow into nearby streams and rivers.
Combined sewer systems are stormwater and wastewater conveyances that carry domestic sewage, stormwater runoff, and sometimes even industrial waste in one pipe. Most combined sewer systems in the United States were installed in the mid to late 1800s as a result of a considerable population shift from rural to urban areas. As cities experienced significant population growth, they also became overwhelmed with more wastewater, which resulted in unsanitary conditions and sometimes even disease outbreaks, ultimately leading to a need for centralized wastewater management. At the time, combined sewer systems were the preferred option for managing stormwater and wastewater in urban areas because less pipe was needed to transport both stormwater and wastewater and they were considered more affordable than systems that separated stormwater from domestic sewage. The first combined sewer system in the United States was installed in Chicago in the late 1850s, and by the end of the century, most cities with centralized wastewater management had combined systems.
When combined sewer systems overflow, these wet weather discharges release a combination of stormwater, untreated human and industrial waste, oil and grease, metals, sediment, deicing chemicals, floating debris, and other stormwater pollutants into our waterways. All wet weather discharges are known to have detrimental effects on receiving waters, and CSOs have resulted in degraded water quality, unsafe swimming conditions, and beach closings and shellfish harvesting restrictions in order to prevent spreading human pathogens and illness. According to the US Environmental Protection Agency, combined sewer overflows are a priority water pollution concern for nearly 860 municipalities across the United States. Pennsylvania is considered to have the largest CSO problem in the United States, and 152 communities within Pennsylvania, including Pittsburgh, Philadelphia, and Harrisburg experience CSOs.
Combined sewer systems were never designed to handle all of the water during wet weather flows and a number of overflow events were expected; however, growth of urban centers as well as an increase in extreme rainfall events have exacerbated CSO events. The carrying capacity of combined sewer systems is too often exceeded during periods of rainfall or snowmelt, causing a combined sewer overflow at relief points throughout these systems. These overflow points can’t simply be plugged, because they prevent wastewater backups in basements, flooding onto streets, and overloading of wastewater treatment facilities.
Widespread problems of combined sewer overflows have led to local, state, and federal efforts to control or eliminate CSOs. Major federal regulations, such as the Water Pollution Control Act of 1948 and the Clean Water Act of 1972 and its subsequent amendments aimed to restore or maintain the chemical, physical, and biological integrity of the Nation’s water by eliminating the discharge of pollutants, including CSOs. Today, CSOs and other wet weather flows are regulated under the National Pollution Discharge Elimination System program. These regulations prohibit untreated discharges from combined sewer systems during dry weather and require CSO system owners to implement minimum control measures in order to lessen the volume of water bypassing the wastewater treatment plant and reduce the total pollutant loads associated with combined sewer overflow events.
In order to meet these goals, communities employ a variety of creative approaches such as retrofitting stormwater basins, disconnecting impervious surfaces, and improving infiltration of stormwater with green infrastructure. In many areas, low-impact development design, which increases the amount of stormwater that soaks into the ground, is being used to limit runoff from new development. In addition to having a plan for addressing water quantity, communities must also develop long-term plans for dealing with water quality problems caused by CSOs, such as increasing the capacity of wastewater treatment plants, separating wastewater from stormwater, and other strategies for eliminating or minimizing CSO discharges.
Even with all minimum control measures in place, it’s not possible to eliminate all CSO events quickly. Communities in Pennsylvania have been making upgrades to improve and repair their combined sewer systems over time, but the capacity of every drainage network and wastewater treatment plant is often exceeded because of the historic increase in impervious surfaces and larger and more frequent storms that cause an increased volume of stormwater runoff. While the responsibility for addressing CSO problems falls on the communities that are operating the systems, we can all help lessen the impact of stormwater runoff by doing simple things like installing rain barrels, creating rain gardens, and disconnecting our downspouts from the stormwater systems in our communities.
