Association of State Wetland Managers - Protecting the Nation's Wetlands.

The Wetland Wanderer: Combined Sewer Overflows: An Historical Holdover and its Relationship with Wetlands

by Brenda Zollitsch

Can municipalities really still be dumping untreated sewage directly from a pipe into local bays, rivers, streams and wetlands?  Shouldn’t that be impossible in today’s regulatory environment?  Unfortunately, it is not.  Combined sewer systems that dump sewage-containing wastewater along with stormwater into local water bodies are found in 32 states, located in nine EPA regions.  Most are found in Northeast, Great Lakes Region and Pacific Northwest states.   While these combined systems are regulated under the Clean Water Act, the nearly 900 billion gallons of often-polluted overflow released from them have a documented adverse impact on the nation’s waters, including wetlands.


A Combined Sewer System (CSS) is a wastewater collection system owned by a state or municipality that is specifically designed to collect and convey both sanitary wastewater (including residential, industrial and commercial wastewater that may include sewage) and stormwater through a single piped system.  These pipes discharge directly to surface nydep715161waters (streams, rivers, wetlands, lakes, ponds) when collection capacity is exceeded during high level precipitation events (EPA, 2002).  These overflows, when they occur, are called Combined Sewer Overflows (CSOs).   Combined systems are an artifact of 19th Century planning, which focused on getting rain water and sewage out of population centers as quickly and cost effectively as possible, with little consideration for downstream impacts.

There are significant public health and environmental impacts for CSO receiving waters.  Common pollutants transported when sewer systems overflow include:

  • Bacteria, viruses and protozoa
  • Organic compounds, metals, oil and grease and toxic pollutants
  • Biochemical oxygen demands (BOD)
  • Sediment
  • Trash and floating debris, and
  • Nutrients (nitrogen and phosphorus, for example)

Compared to surface water runoffs in separated sewer systems, CSOs are highly polluted with carbon, nitrogen and phosphorus.  These pollutants can lead to a variety of adverse public health effects, as well as beach and shellfish bed closures, contamination of drinking water sources, devaluation of property, aquatic habitat impairment, fishing restrictions, reduced oxygen levels and fish kills, unpleasant odors, aesthetic impairment, and algal blooms.

Although wetlands alone cannot solve the CSO problem (See a list of actions to address combined sewer overflows in EPA Region 2’s publication Keeping Raw Sewage & Contaminated Stormwater Out of the Public’s Water, page. 7), there are important roles that wetlands can play in addressing it.

First, CSO communities need to stop the filling of natural wetlands.  Filling in wetlands reduces opportunities for rainfall and snowmelt infiltration, adding to the volume of water entering the storm sewer system and leading to more overflow.  Working to protect and restore natural wetlands in critical CSO  watersheds will reduce the amount of surface flow entering the CSO system, especially when wetland protection is combined with green infrastructure efforts that reduce impervious cover simultaneously.

wetlandrestoration71615Second, practitioners in the U.S. and internationally have successfully used the restoration of filled wetlands to contain and treat CSO effluent.  These restored natural biofilters are used to treat pollutants like heavy metals and polycyclic aromatic hydrocarbons (PAHs), which are often contributed by high traffic street surfaces present in most CSO communities.

Success of these efforts is generally dictated by the proximity/location of the restored wetland to the nonpoint source pollution, the wetland basin and its hydrology.  When restored properly, restored wetlands can provide a favorable chemical, biological, and physical environment for pollutant removal, including the bio-transformation of agricultural nonpoint source (surface runoff) nitrogen pollutants into Nitrogen gasses.  To address specific CSO needs, the restored wetland needs to be designed to retain “the contaminant of greatest local concern that requires the longest retention time for degradation and the percentage reduction of the contaminant required by law and/or regulation” (Environmental Finance Center, 2006).  For these projects, many types of wetlands are eliminated — those that are natural and undisturbed, those with a watershed to wetland water surface area of 10:1 or greater, those with retention times less than a day and many others. EPA’s Urban and Rural Treatment Wetlands Manual provides basic guidance on sizing and siting restored wetlands for water quality treatment, while a range of agency and industry documents provide extensive design guidance and case studies. 

harborbrook071615Third, constructed wetlands can be developed to mimic the water pollution filtration functions of a natural wetland.  Constructed wetlands are “planned systems designed and constructed to employ wetland vegetation to assist in treating wastewater in a more controlled environment than occurs in natural wetlands.”  These can be engineered to have surface flow or to only have subsurface flow.  Unlike natural wetland restoration efforts, constructed wetlands provide flexibility in site location, can be optimized to accommodate the specific amount of planned wastewater, and (due to their function-focused design) can often treat more wastewater in a smaller area than natural wetlands.  Perhaps more importantly, the use of constructed wetlands enable avoidance of impacts to natural wetlands.

In the stormwater world, constructed wetlands are referred to as “structural best practices” and have been tested extensively by researchers and industry experts.  For example, the University of New Hampshire Stormwater Center has tested various treatment options and found gravel wetlands to perform at a much higher level than other alternatives (such as bioretention ponds and manufactured structural treatment devices).  While much of the work of wetland managers is disconnected from efforts related to constructed wetlands, clearly wetland experts can provide guidance on wetland structure and functions that have potential to improve treatment success.

statenisland71615Fourth, while it may seem that this is an either-or decision, the reality is that an increasing number of communities are selecting to marry their gray and green infrastructure and design creative (and highly effective) CSO and stormwater management plans that include all three wetland elements — wetlands protection, wetland restoration, AND constructed wetlands in one integrated system.  Examples of this work can be found in the Staten Island Bluebelt, Philadelphia’s Green City Clean Waters Initiative, Chicago’s Stormwater Green Infrastucture Strategy and smaller CSO communities like Washington, Indiana.

juni71615Finally, it is important to remember that CSOs are intensely weather-dependent, with increased releases of polluted waters from CSOs accompanying larger rain events.  Recent trends in rainfall often attributed to climate change, create an additional challenge for the management of CSOs.  With the frequency of high volume precipitation events growing in many regions of the United States, the need to find solutions that reduce the impacts of CSOs is increasing as well.  Investments in CSO control are already long-term and capital intensive.  Evidence is mounting that the integration of creative wetland-related elements may provide one of many cost-effective approaches to addressing the CSO issue. 

For more information about Combined Sewer Overflows, go here.

To review the findings of the University of New Hampshire Stormwater Center, go here.

For one set of guidance on decision making for urban and rural treatment wetlands go to the Environmental Finance Center’s document here.

For information on the impacts of climate change on CSOs download the following report here.

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