Understanding City Stormwater & Sewage Systems

The clouds, dark and chilling, hid the light from the night except for the whipping static of bolts of lightning. The city sounds of police sirens and car alarms were muffled by the powerful thunder. Standing underneath the Phillips Collection balcony looking out to Dupont Circle, the rain drops, and occasional penny-sized frozen pellets, fell with ferocity onto the city streets, and the winds, reaching 60 miles per hour, whipped and slashed through the air. As the rain fell and the winds blew, the worst of it was not up above, but rather right by my toes. Everywhere and everything was trounced in rainwater. The streams that formed on the edges of Massachusetts Ave. flowed faster than the few cars braving the storm. The even fewer pedestrians waded through ankle-deep puddles to seek temporary shelter. A “flash flooding” message alert continued to flash on my cell phone screen…

Flash flooding, the most extreme result of heavy rainfall in a short time period can easily overwhelm a city’s sewer system and cause untreated sewage to flow into nearby rivers and streams. Many older cities including New York, Philadelphia and Washington, DC, rely on a combined sewage system (CSS) to manage and transport water. These CSSs carry both sanitary sewage and stormwater. During dry weather, the sewage is channeled to the sewage treatment plant, and the stormwater to nearby surface waters; however, when a city such as DC gets as little as 1/3 inch of rain, pipes in a CSS quickly overflow resulting in combined sewer overflows (CSOs), where the untreated sewage and stormwater is diverted away from the sewage plant and discharged into designated outfall points, usually into local waterways. As many are unaware of, CSOs are one of the greatest threats to water quality in urban settings with high precipitation.


Infograph from SwimGuide.org

Sewage overflows pose a substantial risk to waterways because of the toxic materials and debris that collect from the storm that then mixes with residential and industrial wastewater, which is ultimately discharged into rivers and streams via these outfall points. These toxic materials include untreated human waste, pesticides, toxic metals and petroleum products. Untreated pathogenic bacteria can lead to diseases such as E. coli, while metals and petroleum products can contain chemicals linked to brain damage and cancer. Specific chemicals from pesticides, prescription drugs and, even, caffeine, while relatively less harmful to humans, can have stronger negative effects on aquatic species. For example, prescription drugs that contain endocrine disruptors have been found to affect changes on fish species’ genetic makeup leading to the development of female characteristics found in adult males such as immature eggs in testes.

Like a vein from the heart, major cities worldwide such as the Thames River in London, the Seine River in Paris, the Delaware River in Philadelphia and the Potomac River in Washington are taking steps to mitigate water pollution from CSOs. Traditionally in the United States, major cities developed advanced piping systems or CSSs, which would aim to capture as much overflow as possible, and then send the water to a treatment facility. These piping systems are called grey infrastructure. However, with advancements in green technology and improved city planning, along with the overall reduced costs, one method that has become increasingly popular is green infrastructure (GI). Cities around the world have begun to adopt GI technologies into their plans.

Green infrastructure in the form of bioretention gardens, permeable pavements, green roofs and constructed wetlands attempts to mimic the natural filtration of water to both decelerate rainwater entering sewage and filter debris and toxic substances. Philadelphia’s constructed stormwater wetland at Saylor Grove treats 70 million gallons of urban stormwater per year and under Chicago’s Green Alley Program, Chicago plans to retrofit 3,500 acres of permeable pavement. Similar projects worldwide are on the rise.

Here along the Potomac River in Washington, DC, the city, from a court ordered consent decree from 2005, legally must address water pollution from CSOs in its Long Term Control Plan (LTCP). DC Water, the city’s water and sewer authority, is responsible for the process and completion, and has already started to construct one of its planned underground storage tunnels to handle excess water from overflows. In 2013, DC Water proposed modifications to their original approved plan to include $90 million in green infrastructure that would replace portions of grey infrastructure. Local environmental groups, including Potomac Riverkeeper Network, pushed back against the plan, fearful that the changes were mainly intended to save money and would not lead to the same reductions in CSO pollution as an entirely grey solution. The latest revised plan replaces the limited financial commitment to GI with an overall commitment to use both a combination of green and grey infrastructure to meet the goal of 96 percent in reductions in CSO pollution by 2030. The plan relies on building porous pavement, rain gardens and green roofs, along with extended underground storage tunnels.

With last June as the fourth wettest on record, it is imperative that the city continues to strengthen efforts to reduce CSOs and does not compromise the health of the river. While GI projects bring significant environmental benefits for water quality, energy efficiency and climate adaptation, a concern among environmental groups is determining the projects’ effectiveness. In contrast, storage tunnels have conclusive tangible results to combat CSOs.

Here, at the Potomac Riverkeeper Network (PRKN), our goal is a strong commitment from DC Water to reduce CSOs and improve water quality through proven technologies such as grey infrastructure, coupled with GI, which will dramatically reduce and nearly eliminate CSO pollution altogether. PRKN has been involved in this process since the beginning and we’re committed to watchdogging DC Water’s progress as it implements its 15 year plan. The final LTCP is currently under a public comment period, where PRKN will push for further clean water protections. We will keep you updated with our progress.

In the meantime, there are two easy things you can do to minimize risk and pollution to our waterways. First, during or just after a storm, it is best not to swim or paddle in the river because of potential bacterial contamination in the water. Second, do your best not to use or consume too much household water because the sewer pipes are already at excessive limits. For example, don’t run your dishwasher or take a shower until hours after a storm when the water from the pipes has discharged.

… As the clouds retreated and the rain slowed, a rainbow appeared just above the White House.

Read PRKN’s Comments on DC’s LTCP submitted by Earthjustice