Green Approach to Treating Combined Sewer Overflows Saves Streams & Money

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CSO During a Heavy Rain Event

In the late 19th and early 20th century, rapid urbanization prompted communities throughout the nation to install sewer systems. Since building one system was cheaper than two, they designed “combined” sewer systems that would carry both sanitary sewage from homes and businesses and stormwater run-off from streets and roofs. These lines were sized to handle a light rain, but when prolonged or heavy rainfall occurred, excess water backed up into to relief structures then overflowed from the “combined system" and discharged into a nearby body of water.

While this system was efficient especially during dry weather, it made consistent treatment difficult because of the large variation in flows between wet and dry weather. Moreover, in time the discharges became increasingly polluted, degrading water quality and threatening public health and the environment.

Today the US EPA estimates that there are over 770 communities in 31 states and the District of Columbia with combined sewer systems serving 40 million people. These systems discharge around 850 billion gallons of untreated wastes into rivers, streams, lakes, and estuaries each year, polluting waterways with bacteria and other pathogens, toxic chemicals, pesticides, and debris.

In 1994, the US EPA published the Combined Sewer Overflow (CSO) Control Policy requiring municipalities to develop a plan to reduce or eliminate CSO-related pollution. Because of the massive size of the problem, the EPA recognized communities could not afford to address
this problem immediately, so a two-phase program was developed. The first phase involved “nine minimum controls” communities could implement easily without much expense. These included things such as ensuring “proper operation and regular maintenance programs for the sewer system and the CSOs.”

The second phase required for communities to develop Long-Term Control Plans (LCTPs) to address this problem over a 5- to 20-year timeframe. Plans also take into account the community’s ability to pay for the improvements based upon median household income. Despite these accommodations, communities continue to struggle to find ways to adequately meet this mandate, as they face mounting costs in other areas, such as fuel and insurance.

As a result, communities are looking for alternative approaches to dealing with CSOs. Many, like porous pavements or rain gardens, are aimed at reducing the amount of stormwater run-off. One cost-effective solution for treatment, however, is a constructed wetland. A wetland is an area, similar to a bog or marsh, where water moves slowly. They are often referred to as “nature’s kidneys” due to the way wetland plants and microorganisms ingest and breakdown waste and pollutants. In fact, organic pollutants, which make up the bulk of sanitary sewerage, include many of the very nutrients plants need to grow and thrive, such as nitrogen and phosphorous. By using the right plants and microorganisms in the right amounts and right location, BLA designers mimic and enhance natural systems to address this critical issue.

BLA experts in this area have designed over 100 wetland treatment systems to process waste from sewage and agricultural wastes. Now the firm is working with communities to help them take advantage of the cost savings and other benefits wetlands offer to meet their LTCP needs. In the case of Washington, Indiana, BLA provided planning and design services to update the current LTCP to include an equalization basin to capture the first flush rainfall event and a constructed wetland and ultraviolet disinfection system to treat the remaining overflows up to a 10-year/1-hour storm event. The wetland treatment system not only offers a sustainable solution, but could save the city up to $33 million over the traditional technologies recommended in the original plan. The updated plan, incorporating the wetlands, was recently approved by the Indiana Department of Environmental Management.

Rendering of Proposed Bee Slough Wetland

Similarly, with 23 CSOs in its system, Evansville, Indiana was looking at estimates using conventional methods as high as $65 million for a portion of just one drainage basin. Like many of Evansville’s CSO discharge points, Bee Slough is located behind the Ohio River flood levee. As a result, it becomes a holding area for stormwater and combined sewage when the river level is high. The resulting “ponds” serve as a breeding grounds for mosquitoes as well as posing an odor problem for nearby residents and riverfront attractions. It seemed natural, therefore, to transform this “ponding” area into a treatment system with wetland plants doing the work¾an alternative that could save the city up to $50 million!

Furthermore, if approved as part of the LCTP update, a path adjacent to the wetlands will enhance the city’s trail system by linking neighborhoods to the Pigeon Creek Greenway. As currently envisioned this wetland trail could also feature opportunities for educational and public art elements to further enhance the area. In this way, this “green” solution promises to not only save money and optimize the use of the city’s levee, but also enrich the city’s greenway, one of the major gateways into the city’s revitalized downtown riverfront.

If your community would like to learn more about innovative approaches to address Combined Sewer Overflows, please contact us at: 1.800.423.7411

For more information see: EPA Combined Sewer Overflow Education Page