Combined Sewer Overflows

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Combined sewer overflows

In many older coastal cities, especially in the northeastern United States, storm sewers in the street that collect stormwater runoff from rainfall are connected to municipal sewage treatment plants that process household sewage and industrial wastewater . Under normal, relatively dry conditions runoff and municipal waste go to a sewage treatment plant where they are treated. However, when it rains, in some cases less than an inch, the capacity of a sewage treatment plant can be exceeded; the system is overloaded. The mixed urban stormwater runoff and raw municipal sewage is released to nearby creeks, rivers, bays, estuaries or other coastal waters, completely untreated. This is a combined sewer overflow (CSO) event.

Combined sewer overflow events are not rare. In Boston Harbor, for example, there are 88 pipes or outfalls that discharge combined stormwater runoff and sewage. It has been estimated that CSO events occur approximately 60 times per year, discharging billions of gallons of untreated runoff and wastewater to Boston Harbor.

Materials released during these CSO events can result in serious water quality problems that can be detrimental to both humans and wildlife . Toxic chemicals from households and industries are released during CSO events. In addition, toxic chemicals found in rainwater runoff, such as oil and antifreeze that have dripped onto roads from cars, will wash into coastal waters during these events.

Harmful bacteria and pathogens in the water are another major problem that can result after a CSO event. Some of these bacteria (coliform), live naturally in the intestinal tracts of humans and other warm blooded animals. After heavy rainfalls, scientists have measured increased levels of coliform bacteria in coastal waters near CSO outfalls. These bacteria, which indicate that there are other bacteria and pathogens that can make people sick if they swim in the water or eat contaminated shellfish, come from both animal and human wastes washed in from the streets. The bacteria are not removed or killed because the waters have not been treated in a sewage treatment plant. Because levels of these indicator bacteria are often high after CSO events, many productive shellfish beds are closed to protect human health. This can be a serious economic hardship to the fishing industry.

Combined sewer overflow events also result in increased quantities of trash and floatable debris entering coastal waters. When people litter, the trash is washed into storm sewers with rainwater. Since sewage treatment plants cannot handle the volume of water during these rainfall events, this trash is discharged along with the stormwater and sewage directly into open waters. This floatable debris is unsightly, and can be dangerous to marine animals and birds, which eat it and choke or become entangled within it. This often results in death.

Raw sewage, animal wastes, and runoff from lawns and other fertilized areas contain very high levels of nitrogen and phosphorus , which are nutrients used by marine and aquatic plants for growth. Therefore, CSO events are major contributors of extra nutrients and organic matter to nearshore waters. These nutrients act as fertilizers for many marine and aquatic algae and plants, promoting extreme growth called blooms. When they eventually die, the bacteria decomposing the plants and algae use up vast quantities of oxygen. This results in a condition known as hypoxia or low dissolved oxygen (DO). If DO levels are too low, marine animals will not have enough oxygen to survive and they will either die or move out of the area. Hypoxia has been the cause of some major fish kills , and can result in permanent changes in the ecological community if it is persistent.

There are a number of options available to reduce the frequency and impacts of CSO events. Upgrading sewage treatment plants to handle greater flow or constructing new facilities are two of the best, although most costly options. Another possibility is to separate storm sewers and municipal sewage treatment plants. While this would not prevent discharges of stormwater runoff during rainfall events, untreated household and industrial wastewater (i.e., raw sewage) would not be released. In addition, the resulting stormwater could be minimally treated by screening out trash and disinfecting it to kill bacteria. Use of wetlands to filter this stormwater has also been considered as an effective alternative and is currently being used in some areas. Another option is to build large storage facilities (often underground) to hold materials that would normally be discharged during CSO events. When dry conditions return, the combined runoff and wastewater are pumped to a nearby sewage treatment plant where they are properly treated. This option is being used in several areas, including some locations in New York City. At a minimum, screening of CSO discharges would reduce the quantity of floatable debris in nearshore waters, even if it did not solve all of the other problems associated with CSOs. Of course water conservation is another control that reduces that volume of water treated by sewage treatment plants, and therefore the volume that would be discharged during a CSO event.

[Max Strieb ]



National Research Council. "Managing Wastewater In Coastal Urban Areas." Washington, D.C.: National Academy Press, 1993.