Combined Sewer Overflows: A Growing Urban Challenge

Combined sewer overflows (CSOs) represent one of the most persistent and complex water quality problems facing older urban centers worldwide. In a combined sewer system, sanitary sewage and stormwater runoff share the same underground pipes. During dry weather, all flow goes to a treatment plant. But when heavy rain or rapid snowmelt exceeds the system’s capacity, the mixture—untreated human waste, industrial effluent, street runoff, and debris—is discharged directly into rivers, lakes, or coastal waters through designated outfalls. These CSO events can introduce pathogens, nutrients, heavy metals, and floatables into the environment, forcing beach closures, harming aquatic life, and posing serious public health risks.

Regulatory agencies such as the U.S. Environmental Protection Agency (EPA) have long pressured municipalities to reduce CSO frequency and volume through long-term control plans. Traditional solutions—enlarging tunnels, building massive storage basins, or separating sewers—are prohibitively expensive and disruptive. As a result, urban planners and environmental engineers have turned to sustainable, nature-based solutions. Among the most promising is the constructed wetland, an engineered ecosystem that mimics natural marshes and swamps to treat and store CSO flows before they reach sensitive water bodies.

What Are Constructed Wetlands?

Constructed wetlands are human-made systems designed to replicate the physical, chemical, and biological processes found in natural wetlands. They consist of shallow basins filled with gravel, sand, or soil, planted with emergent vegetation such as cattails, reeds, and rushes. Water flows through the substrate and plant root zone, where a complex community of microorganisms, plants, and invertebrates work together to remove pollutants. These systems can be configured as free water surface (FWS) wetlands, where water is exposed to the atmosphere, or subsurface flow (SSF) wetlands, where water moves horizontally or vertically through a porous medium below the surface.

Constructed wetlands are not new; they have been used for decades to treat municipal and industrial wastewater, agricultural runoff, and mine drainage. Their application to CSO management, however, has gained traction only in the past two decades as cities seek cost-effective, multi-benefit infrastructure. Unlike conventional concrete tanks, constructed wetlands integrate into the urban landscape, providing green space, wildlife habitat, and flood attenuation while treating water.

Key Design Parameters for CSO Wetlands

Designing a constructed wetland for CSO control requires careful consideration of several factors:

  • Hydraulic Loading Rate: The volume of water per unit area per time must be managed to avoid short-circuiting or flooding that reduces treatment efficiency.
  • Detention Time: Water must remain in the wetland long enough for sedimentation, filtration, and biological uptake to occur—typically 24 to 72 hours for CSO applications.
  • Vegetation Selection: Native plants with deep root systems and high pollutant tolerance are preferred. Species like Phragmites australis (common reed), Typha spp. (cattails), and Scirpus spp. (bulrushes) are common choices.
  • Substrate Composition: A mix of gravel, sand, and organic matter supports microbial growth and provides filtration media.
  • Forebay and Distribution: A settling basin before the main wetland helps remove grit and large solids, preventing clogging and maintaining long-term performance.

How Constructed Wetlands Manage CSOs

When a CSO event occurs, the excess flow is diverted from the sewer system into the constructed wetland rather than discharged directly to a water body. The wetland acts as both a storage basin and a treatment reactor. The natural processes that remove pollutants include:

  • Sedimentation: Heavy particles, including sediment, organic solids, and attached contaminants, settle out as water velocity decreases in the wetland basin.
  • Filtration: As water percolates through plant stems, roots, and the porous substrate, physical straining removes suspended solids and larger pathogens.
  • Biodegradation: Aerobic and anaerobic bacteria colonize the root zone and substrate, breaking down organic matter, oils, and grease. Nitrogen is transformed through nitrification-denitrification cycles.
  • Plant Uptake: Emergent vegetation absorbs nutrients such as nitrogen and phosphorus, as well as some heavy metals, incorporating them into plant tissue.
  • Adsorption: Clay particles and organic matter in the substrate bind to dissolved pollutants, including phosphorus, metals, and some organic compounds.
  • UV Disinfection: In free water surface wetlands, exposure to sunlight helps inactivate pathogens like E. coli and enterococci.

During dry weather, the wetland may remain dry or hold a shallow base flow of treated water. The system can be designed to drain slowly between events to reaerate the substrate and maintain healthy microbial communities. Many CSO wetlands are also integrated with real-time monitoring and control systems that optimize flow diversion based on rainfall forecasts and real-time water quality data.

Case Study: The Green Bay Metropolitan Sewerage District

The Green Bay Metropolitan Sewerage District (GBMSD) in Wisconsin operates one of the largest constructed wetlands for CSO management in the United States. The 120-acre system, completed in 2004, treats combined sewer overflows from the City of Green Bay and surrounding communities. During wet weather, flows are diverted into a series of four wetland cells with a total storage capacity of 160 million gallons. The wetland incorporates a forebay for solids removal, deep zones for storage, and shallow marsh areas for treatment. Since its commissioning, the facility has reduced CSO discharge volume by over 90% and has significantly lowered pollutant loads to the Fox River and Green Bay. The project also created valuable wildlife habitat and a public education area with walking trails.

Environmental and Economic Benefits

Constructed wetlands offer a suite of benefits that extend well beyond CSO control, making them a cornerstone of sustainable urban water management.

Water Quality Improvement

By intercepting and treating the first flush of polluted runoff, constructed wetlands significantly reduce the mass of pollutants entering natural water bodies. Typical removal efficiencies for CSO wetlands include 70–90% for total suspended solids (TSS), 40–60% for biochemical oxygen demand (BOD), 30–50% for total nitrogen, and 20–40% for total phosphorus. Pathogen reduction can exceed 90% under optimal conditions. These reductions help water bodies meet regulatory standards for recreation, fishing, and aquatic life.

Flood Attenuation and Climate Resilience

Constructed wetlands act as stormwater detention basins, storing large volumes of water during extreme rainfall events and releasing it slowly. This reduces peak flow rates in downstream sewers and receiving waters, mitigating flood risks. As climate change intensifies precipitation patterns, the storage capacity of wetlands becomes increasingly valuable. They also provide cooling through evapotranspiration, countering urban heat island effects.

Habitat Creation and Biodiversity

Well-designed constructed wetlands create diverse habitats for birds, amphibians, insects, and aquatic organisms. Native wetland plants attract pollinators and provide food and shelter for wildlife. In urban settings, these green corridors connect fragmented natural areas and promote regional biodiversity. Some CSO wetlands have become important stopover sites for migratory waterfowl.

Community and Aesthetic Value

Converted from underutilized land such as vacant lots, brownfields, or flood-prone areas, constructed wetlands can become community assets. They offer opportunities for environmental education, passive recreation, and nature appreciation. Walking paths, observation decks, and interpretive signage can transform a wastewater infrastructure project into a beloved public park. The integration of green space also increases adjacent property values and improves quality of life in urban neighborhoods.

Cost-Effectiveness

Compared to conventional grey infrastructure such as deep tunnels, storage tanks, or sewer separation, constructed wetlands typically have lower capital costs and much lower operational and energy expenses. A 2012 study by the EPA found that nature-based solutions for CSO control can save 30–60% over traditional approaches. Maintenance costs are generally limited to periodic vegetation management, sediment removal from forebays, and monitoring. Furthermore, wetlands can be phased in over time, allowing municipalities to spread investment and adapt to changing conditions.

Challenges and Technical Considerations

Despite their advantages, constructed wetlands for CSOs face several challenges that must be addressed during planning and operation.

Land Area Requirements

Constructed wetlands require significant land area relative to their treatment capacity. A typical CSO wetland may need 1–5% of the contributing drainage area for effective treatment. In dense urban centers where land is scarce and expensive, this can be a major barrier. Solutions include using multiple small wetlands distributed across the watershed, stacking wetlands vertically (e.g., terraced designs), or integrating them into linear parks along river corridors.

Performance Variability

Treatment efficiency can vary with temperature, season, rainfall intensity, and antecedent dry periods. Cold winter conditions slow microbial activity and plant uptake, reducing removal rates for nitrogen and pathogens. Heavy, short-duration storms may overwhelm the wetland’s hydraulic capacity, causing untreated bypass. To mitigate these risks, designers can incorporate multiple cells in series, deeper zones for storage, and bypass channels for extreme events. Real-time controls and early warning systems that divert flow only when treatment capacity is available can optimize performance.

Maintenance and Longevity

Like any infrastructure, constructed wetlands require ongoing maintenance. Accumulated sediments in forebays must be removed every 2–5 years to maintain storage volume and prevent clogging. Vegetation may need periodic harvesting to remove nutrients and prevent dominance by aggressive species. Invasive plants must be controlled. Mosquito breeding can occur in stagnant water, but this can be managed through proper hydraulic design—ensuring continuous flow, using steep side slopes, and introducing mosquito-eating fish like Gambusia spp.

Public Perception and Safety

Some community members may harbor concerns about odors, insects, or water quality near wastewater treatment wetlands. Transparent communication, well-maintained aesthetics, and fencing where necessary can address these issues. Monitoring programs that publish water quality data help build trust. Wetlands should be designed with public safety in mind, including shallow water depths, gentle slopes, and clear signage.

Innovations and Future Directions

The field of constructed wetland design for CSO management continues to evolve. Emerging trends include:

  • Hybrid Systems: Combining constructed wetlands with other green infrastructure such as permeable pavement, rain gardens, or green roofs to create a treatment train that reduces flow and pollutants at multiple points.
  • Biochar and Engineered Substrates: Adding biochar, zeolite, or iron-rich media to enhance removal of phosphorus, metals, and emerging contaminants like pharmaceuticals and microplastics.
  • Intelligent Monitoring and Control: Integrating sensors, flow meters, and automated valves with predictive algorithms to dynamically route CSO flows to wetlands based on real-time capacity and forecasted rainfall.
  • Vertical Wetlands: Stacked or wall-mounted wetland designs that reduce land footprint, suitable for dense urban environments.
  • Renewable Energy Integration: Using constructed wetlands in conjunction with solar panels (floatovoltaics) or hydropower turbines to generate renewable energy, offsetting operational costs.

Policy and Regulatory Support

Recognizing the multiple benefits of constructed wetlands, many regulatory agencies now encourage or require green infrastructure components in CSO long-term control plans. The EPA’s Green Infrastructure program provides technical guidance and funding opportunities for municipalities pursuing nature-based solutions. Some states have also integrated constructed wetlands into their water quality trading programs, allowing communities to earn credits for pollutant reductions and offset other dischargers.

Implementing a Constructed Wetland for CSO Control: A Step-by-Step Approach

For a municipality considering a constructed wetland, the following steps are essential:

  1. Site Assessment: Evaluate potential locations based on land availability, proximity to sewer outfalls, soil permeability, topography, flood risk, and community support.
  2. Feasibility Study: Model the existing CSO system to estimate overflow volumes, frequencies, and pollutant loads. Determine the wetland size needed to achieve target reductions.
  3. Conceptual Design: Develop wetland layout, including forebay, treatment cells, outlet structure, and bypass system. Select vegetation and substrate.
  4. Permitting and Stakeholder Engagement: Obtain required environmental permits (e.g., Clean Water Act Section 404, state wetland permits). Engage with local residents, environmental groups, and regulatory agencies early in the process.
  5. Detailed Design and Construction: Prepare construction drawings, specifications, and bid documents. Ensure proper earthwork, liner installation (if needed), planting, and hydraulic connections.
  6. Startup and Monitoring: Establish baseline water quality, monitor hydrology, and adjust operations. Develop a maintenance plan covering vegetation, sediment removal, and equipment inspection.
  7. Long-Term Adaptive Management: Use monitoring data to refine operation, improve performance, and document benefits for regulatory compliance and public reporting.

Conclusion

Constructed wetlands have proven themselves as a resilient, cost-effective, and environmentally beneficial technology for managing combined sewer overflows. By harnessing natural processes of sedimentation, filtration, biodegradation, and plant uptake, these engineered ecosystems protect water quality, reduce flood risks, and create valuable green space in urban areas. While challenges such as land requirements and performance variability exist, advances in design, monitoring, and hybrid approaches continue to expand their feasibility.

As cities worldwide grapple with aging infrastructure, population growth, and climate change, constructed wetlands offer a path toward more sustainable and adaptive urban water systems. They transform a liability—overflows of untreated sewage—into an asset that cleans water, supports wildlife, and enriches communities. For planners, engineers, and policymakers seeking a triple bottom line solution, constructed wetlands represent not just a technical choice but a strategic investment in a healthier, more resilient urban future.

For further reading on constructed wetland design and CSO management, see the EPA’s CSO Control Policy and the IWA’s guide on constructed wetlands for wastewater treatment.