Bloomberg BNA – Environmental Protection Agency guidance released March 7 describes how communities can incorporate green infrastructure—such as porous pavement, green roofs and strategically placed grassy swales—to manage sewer overflows during heavy rains.
These techniques can be incorporated into the long-term control plans as another means for reducing combined sewer overflows, the agency said in “Greening CSO Plans: Planning and Modeling Green Infrastructure for Combined Sewer Overflow (CSO) Control”. Many older U.S. cites have sewer systems in which both domestic sewage and stormwater are conveyed in a single pipe. These systems are designed to overflow during heavy rains to keep excess flows from overwhelming the treatment plant.
EPA warned that the frequency and severity of CSOs which in turn are dependent on the form, quantity, and intensity of precipitation, are expected to increase with climate change.
The guidance reflects the updated national green infrastructure strategy that EPA released in October 2013. In that strategy, the agency reiterated the use of green infrastructure in enforcement actions such as long term control plans to address combined sewer overflows.
The guidance uses case studies to explain how green infrastructure can be installed, maintained, monitored, and managed alongside often more costly measures such as traditional pipes, storm drains, and deep tunnels used to store excess flows until they can be treated. The document also provides a step-by-step guide to using software to quantify how much of the overflow is reduced by using green infrastructure.
The Clean Water Act requires cities to implement long-term control plans, which often involve costly replacement or upgrades of aging combined sewer systems. Green infrastructure is a low-cost approach that mimics natural processes to reduce the volume and rate of stormwater flows into the combined sewer systems.
The guidance is important because more than 700 cities in the Northeast, the Great Lakes region, and the Pacific Northwest use combined sewer systems to collect and convey both sanitary sewage and stormwater to wastewater treatment plants. During heavy rains, the wet weather flows exceed the capacity of the sewer systems and treatment plants, resulting in diversion of stormwater containing untreated human, commercial and industrial waste, toxic materials and debris into surface waters. These direct discharges in turn can lead to beach closures, contamination of drinking water supplies and other environmental and human impacts.
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