Introduction: The Growing Challenge of Combined Sewer Overflows

Combined sewer systems, designed in the late 19th and early 20th centuries, collect stormwater runoff, domestic sewage, and industrial wastewater in a single pipe. During dry weather, all flow goes to a treatment plant. But when heavy rain or snowmelt overwhelms the system, the untreated mixture is discharged directly into nearby rivers, lakes, and coastal waters. These events are called combined sewer overflows (CSOs). More than 700 U.S. cities rely on combined sewers, and each year tens of billions of gallons of untreated wastewater enter the environment. CSOs pose serious threats to public health, aquatic ecosystems, and recreational water quality.

Conventional solutions, such as increasing pipe capacity or building massive storage tunnels (gray infrastructure), are expensive and often disruptive. Over the past two decades, a complementary approach has gained traction: green infrastructure. By using natural processes to manage stormwater at its source, green infrastructure reduces the volume and velocity of runoff entering sewer systems, making CSOs less frequent and less severe. This article explores how green infrastructure works, its benefits beyond CSO mitigation, and the practical considerations for widespread adoption.

What Is Green Infrastructure?

Green infrastructure refers to a network of decentralized, vegetated systems that mimic natural hydrology. Instead of conveying stormwater away as fast as possible, these features capture, absorb, filter, and slowly release water back into the ground or the atmosphere. The U.S. Environmental Protection Agency (EPA) defines green infrastructure as “a cost-effective, resilient approach to managing wet weather impacts.” Learn more about EPA’s green infrastructure program. Key types include:

Green Roofs

Green roofs are vegetated layers on building rooftops that retain rainfall, provide insulation, and reduce stormwater runoff. They can be extensive (shallow soil, low-maintenance plants) or intensive (deeper soil, more diverse vegetation). In a moderate storm, a green roof can retain 50-80% of precipitation.

Rain Gardens and Bioretention Cells

Rain gardens are shallow, planted depressions designed to capture runoff from roofs, driveways, and streets. Water infiltrates into the soil, where plants and microorganisms remove pollutants. Bioretention cells are engineered versions with underdrains and specialized soil mixes.

Permeable Pavements

These surfaces allow water to pass through concrete, asphalt, or pavers into an underlying stone reservoir. They are used for parking lots, walkways, and low-traffic roads, dramatically reducing runoff and recharging groundwater.

Urban Tree Canopy and Green Spaces

Mature trees intercept rainfall, reduce erosion, and promote infiltration. Parks and natural areas provide additional storage and treatment capacity. Expanding the tree canopy is one of the most cost-effective green infrastructure strategies.

Rainwater Harvesting Systems

Barrels, cisterns, and tanks capture roof runoff for later use in irrigation or non-potable applications. This directly reduces the volume entering sewers.

Constructed Wetlands

Engineered wetland systems mimic natural marshes, treating stormwater through sedimentation, plant uptake, and microbial activity. They can be integrated into the urban landscape or located at the edges of drainage areas.

How Green Infrastructure Mitigates Combined Sewer Overflows

The core mechanism is simple: source control. Instead of allowing stormwater to rush into combined sewers, green infrastructure holds it back. The indirect benefits multiply across the watershed. Here is a detailed breakdown:

Reducing Runoff Volume

By intercepting rainfall on roofs, pavements, and lawns, green infrastructure retains water that would otherwise go directly to drains. For a typical 1-inch rain event, a well-designed green roof can retain more than 0.6 inches of water. Permeable pavements eliminate runoff from that surface entirely. The cumulative effect across hundreds or thousands of installations substantially reduces peak flows.

Slowing the Flow (Detention)

Even when water does eventually leave a green infrastructure feature, it releases much more slowly than from a hard surface. This extended detention reduces the hydraulic load on sewer pipes and allows treatment plants to keep up. In many systems, the difference between an overflow event and no overflow is just a few hours of peak flow reduction.

Enhancing Infiltration and Groundwater Recharge

Rain gardens, bioswales, and permeable pavements allow water to percolate into the soil. This not only removes it from the sewer network but also replenishes aquifers, mitigating baseflow reductions caused by urban development. Infiltration also supports soil moisture for urban trees and vegetation.

Pollutant Removal

Green infrastructure treats stormwater naturally. Plants uptake nutrients, soil filters sediments and heavy metals, and microbes break down organic contaminants. This water quality benefit reduces the pollutant load that would otherwise be discharged during a CSO event. Even with overflow, the less contaminated the water, the less harm to receiving waters.

Climate Change Adaptation

Urban areas already face increased flooding due to impervious surfaces and intense storms. As climate change brings more extreme precipitation, green infrastructure provides a scalable solution that can be upgraded incrementally. Gray infrastructure expansions lock cities into fixed capacity, but a distributed network of green assets can be expanded as needed.

Case Examples

Cities like Philadelphia, New York, and Washington, D.C. have committed to large-scale green infrastructure programs. Philadelphia’s Green City, Clean Waters plan aims to capture 1.5 billion gallons of stormwater per year using green tools, avoiding an estimated $4.6 billion in gray infrastructure costs. In New York, the Department of Environmental Protection has installed thousands of rain gardens and porous pavers across Staten Island, Brooklyn, and Queens. See additional EPA case studies.

Beyond CSO Mitigation: The Full Spectrum of Benefits

While the primary driver for green infrastructure in combined sewer areas is reducing overflows, the co-benefits are equally compelling. These span environmental, economic, social, and health domains.

Environmental Benefits

  • Improved Water Quality: By filtering pollutants before they reach streams, green infrastructure reduces algae blooms, fish kills, and bacterial contamination.
  • Restored Natural Hydrologic Cycles: Infiltration reduces erosion caused by high-velocity runoff and helps maintain stream baseflows during dry periods.
  • Biodiversity and Habitat: Green roofs and urban rain gardens create corridors for pollinators and birds. Native plantings support local ecology.
  • Urban Heat Island Mitigation: Vegetated surfaces and permeable materials reduce ambient temperatures, lowering energy demand and improving comfort.
  • Carbon Sequestration: Trees and soil in green infrastructure store carbon, contributing to climate change mitigation.

Economic Advantages

  • Reduced Gray Infrastructure Costs: A 2015 study by the Water Environment Research Foundation estimated that green infrastructure can achieve equivalent or better CSO control at 20-50% lower capital cost compared to tunnel storage.
  • Lower Energy Bills: Green roofs and shade trees reduce building heating and cooling loads.
  • Increased Property Values: Proximity to green spaces and stormwater management features can boost real estate values by 3-15%.
  • Job Creation: Green infrastructure construction and maintenance require skilled local labor, creating jobs in landscaping, engineering, and environmental monitoring.
  • Flood Damage Avoidance: Every cubic foot of water retained reduces the risk of basement flooding and property damage.

Social and Public Health Impacts

  • Accessible Green Space: Rain gardens and parks provide recreational opportunities and mental health benefits in dense urban areas.
  • Improved Air Quality: Vegetation filters particulate matter, ozone, and nitrogen dioxide.
  • Safer Streets: Permeable pavements reduce ponding and ice formation, lowering accident risk.
  • Community Engagement: Volunteer planting and maintenance programs foster neighborhood stewardship and pride.
  • Equity: Targeted green infrastructure in underserved neighborhoods can reduce historic disparities in flood risk and heat exposure.

Climate Resilience

Green infrastructure is inherently adaptive. As storm intensity increases, a distributed network can continue to absorb and store water. Unlike gray pipes that become undersized, vegetated systems can become more effective over time as plants mature and soil structure improves. This resilience is essential for cities facing more frequent 100-year storms. The National Oceanic and Atmospheric Administration (NOAA) and FEMA have recognized green infrastructure as a key natural defense. FEMA’s Nature-Based Solutions guidance.

Challenges and Practical Considerations

Green infrastructure is not a silver bullet. Successful implementation requires careful planning, maintenance, and community buy-in.

Maintenance Requirements

Unlike buried pipes, green infrastructure needs regular upkeep: weeding, mulching, plant replacement, debris removal, and sediment cleanout. Without maintenance, bioretention cells can clog, green roofs may die, and permeability decreases. Cities must budget for ongoing operations, often 2-5% of initial construction cost annually. Some municipalities partner with community groups or use stormwater utility fees to fund maintenance.

Land Availability and Constraints

In dense urban cores, space for rain gardens or large bioretention is limited. Solutions include using streetscapes (curb bump-outs), parking lanes, and vacant lots. Green roofs require structural retrofits on older buildings. Permeable pavements are not suitable for high-traffic or heavy-load areas without specialized design.

Design for Different Climates

Plants must be selected for local conditions. In cold climates, freeze-thaw cycles affect infiltration rates. In arid regions, water conservation conflicts with irrigation needs. Design standards must account for regional hydrology, soil types, and plant hardiness zones. The American Society of Civil Engineers (ASCE) publishes guidance for stormwater design standards.

Cost and Funding

Upfront costs can be higher per unit volume than piped solutions, especially in cities with existing gray infrastructure. However, lifecycle cost analyses often favor green infrastructure when all benefits are monetized. Many cities use stormwater fees, green bonds, and federal grants (e.g., EPA’s Clean Water State Revolving Fund) to finance projects. EPA water infrastructure finance resources.

Public Perception and Education

Residents may view rain gardens as unsightly or worry about mosquitoes. Proper design with drainage within 48 hours prevents mosquito breeding. Educational signage and public tours help change perceptions. Community involvement early in design increases acceptance. New York City’s “Curbside Gardens” program includes workshops and neighborhood planting days.

Regulatory and Policy Hurdles

Many municipal codes still discourage green infrastructure. Minimum setback requirements, impervious coverage limits, and parking minimums can be barriers. Updating zoning and subdivision ordinances to allow or require green infrastructure is critical. Some city councils have passed “Green Building ordinances” that mandate rainwater capture for new developments.

The Path Forward: Integrating Green Infrastructure into Urban Water Management

The most effective CSO mitigation strategies use a hybrid approach that combines green infrastructure with targeted gray infrastructure improvements. Green infrastructure handles frequent, small to medium rain events, while tunnels and tanks provide capacity for extreme storms. This synergy maximizes cost-effectiveness and system resilience.

Several trends are accelerating adoption:

  • Regulatory Drivers: EPA’s Long-Term Control Plans (LTCPs) for CSO communities increasingly require green infrastructure as part of the preferred alternative. Consent decrees in cities like Cleveland, St. Louis, and Seattle mandate green investments.
  • Climate Adaptation Plans: Cities are explicitly linking green infrastructure to climate resilience in their hazard mitigation plans and sustainability targets.
  • Technology Integration: Smart sensors and real-time controls can optimize how green infrastructure releases stored water, synchronizing with treatment plant capacity. This is called “smart green infrastructure.”
  • Green Jobs and Equity: Municipalities are tying workforce development programs to green infrastructure, training local youth and displaced workers for maintenance careers. This builds community support while addressing environmental justice.

For any city facing CSO challenges, the first step is a comprehensive inventory of existing impervious surfaces and drainage patterns. From there, a green infrastructure feasibility study identifies the highest-yield locations—often schools, public housing, and municipal parking lots. Pilot projects demonstrate effectiveness and allow for adaptive management before scaling up.

Conclusion

Green infrastructure has moved from an experimental concept to a proven, mainstream tool for mitigating combined sewer overflows. Its capacity to reduce runoff volumes, slow peak flows, and treat pollutants addresses the root causes of CSOs while delivering a host of side benefits: cleaner water, healthier communities, cost savings, and climate resilience. No single solution will eliminate every overflow, but a well-planned network of green roofs, rain gardens, permeable pavements, and urban forests can dramatically reduce the frequency and volume of discharges—often for less than the cost of traditional gray expansions.

City planners, water utility engineers, and policymakers should view green infrastructure not as an optional add-on but as an essential component of any long-term CSO control strategy. With continued innovation in design, financing, and community engagement, green infrastructure will play a central role in building the sustainable, resilient cities of the future.