Combined Sewer Overflow Systems: What They Are and Why You Should Care

photo credit: Delaware River Keeper Network

Last year, the Philadelphia Water Department released more than 12 billion gallons of raw sewage into the Delaware River through its Combined Sewer Overflow (CSO) system. To visualize how much pollution that is, imagine you started filling up Olympic sized swimming pools with all the sewage overflowing from Philadelphia’s pipes. You would need 18,000 of them to store all of that raw, untreated sewage. It points to a serious problem hiding in plain sight. CSO pollution threatens public health, harms wildlife, and affects the long-term health of the Delaware River and Bay.

​Combined sewer overflows are systems that carry both sewage and stormwater in the same pipes. They were once considered a major improvement in city sanitation, especially in older cities like Philadelphia. But when heavy rain overwhelms these systems, the mix of stormwater and untreated sewage can overflow directly into nearby rivers and streams. 

photo credit: USEPA

photo credit: USEPA

Philadelphia still relies on this older sewer design. In the late 1800s, combined systems replaced open drainage ditches and helped improve sanitation in growing cities. Over time, though, population growth and development created more pavement, rooftops, and other hard surfaces that send rainwater rushing into the sewer system. When too much water enters the pipes at once, the system cannot handle it, and untreated sewage is discharged into the river.

​These overflows are legal under current permits, but that does not make them harmless. Combined sewer overflow pollution sends harmful bacteria, viruses, and parasites into the water, making the river less safe for fishing, boating, and other recreation. It can also lead to sewage backups in homes and streets during major rain events, creating another layer of risk for nearby communities.

The damage does not stop with people. Sewage pollution can fuel harmful algal blooms and reduce oxygen levels in the water, making it harder for aquatic life to survive. Species in the Delaware River, including Atlantic sturgeon, have already faced major pressure from pollution and habitat loss. When water quality declines, the effects ripple through the food web.

Those impacts continue downstream into the Delaware Bay, where clean water supports oysters, aquaculture, and wildlife. Horseshoe crabs depend on healthy coastal ecosystems, and their eggs are a critical food source for the threatened red knot during migration. In short, what happens upstream in Philadelphia affects the entire Delaware watershed.

​This is why the American Littoral Society is pushing for stronger action. Other cities, including Chicago, Portland, and Milwaukee, have made major improvements that reduced combined sewer overflow pollution and helped restore local waterways. We believe Philadelphia can do more as well. Our work focuses on raising community voices, researching funding solutions, and exploring pilot projects that could reduce sewage pollution entering the Delaware River.

For people who live, work, and spend time along the river, this issue is not abstract. Cleaner water means healthier neighborhoods, safer recreation, and stronger habitats for fish and wildlife. It also means protecting the Delaware Bay, which supports both local livelihoods and species that depend on this ecosystem to survive.

​You can help by learning more, sharing this issue with friends and family, and speaking up for a cleaner Delaware River. If you have seen the effects of combined sewer overflow pollution firsthand, consider sharing your story. Continued support for the American Littoral Society helps make this work possible and strengthens the push for solutions that protect both people and wildlife.