Decentralized Stormwater Controls for Urban Retrofit and CSO Reduction
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Every year in the U.S., it is estimated that combined sewer overflows (CSOs) result in about 850 billion gallons of sewage and stormwater overflowing from wastewater collection systems. CSOs occur in systems in which municipal wastewater is conveyed in the same pipes as stormwater runoff. These systems work well until runoff from a large storm exceeds the capacity of the pipes, causing the excess water to overflow, quite often causing significant damage to homes, roads, etc., as well as potential public health issues.

Eliminating these CSOs can be very costly and technically challenging. The traditional approaches include separation or construction of large, centralized end-of-pipe or interceptor solutions that can be extraordinarily expensive. The Water Environment Research Foundation (WERF) wanted to look beyond these conventional approaches and examine the effectiveness of various decentralized controls that use more natural elements to dampen the stormwater surges that can overwhelm combined sewer systems. The overall goal of the project was to provide guidance for selecting and implementing decentralized urban best management practices (BMPs) for stormwater control, and enhance these BMPs with a specific focus on reducing stormwater volume and improving stormwater quality.

Geosyntec's Scope of Services

Working cooperatively with the Low Impact Development Center, Oregon State University, and the University of Florida, Geosyntec conducted a detailed literature review and collected existing data sets on the effectiveness of eleven individual decentralized urban stormwater control techniques for mitigating and reducing CSOs. The BMPs that Geosyntec covered in the course of our research included:

  • Downspout disconnection;
  • Filter strips;
  • Infiltration practices;
  • Pocket wetlands;
  • Porous pavement;
  • Rain barrels/cisterns;
  • Rain gardens;
  • Soil amendments;
  • Tree box filters;
  • Vegetated (green) roofs; and
  • Vegetated swales

Our team categorized each BMP according to its functional usefulness related to process (e.g., volume control); effectiveness (load and percentage reduction); cost; and ancillary benefits (e.g., aesthetics, energy conservation, etc.). Geosyntec then co-authored the final technical report for WERF describing our research results. The report is available on WERF's web site.

Notable Accomplishments

Geosyntec and our team provided WERF with the first comprehensive national research study to address the usefulness and effectiveness of these decentralized controls for CSO reduction purposes. The need for research in this area has been identified as a high priority topic by many major metropolitan wastewater districts. Given the high costs of centralized controls, these agencies now consider decentralized solutions as increasingly important components of their long-term CSO reduction strategies and planning processes.

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Project Summary

  • Location: USA
  • Client: Water Environment Research Foundation
  • Project Practice Areas: Water and Natural Resources
  • Type of Facility: Combined Sewage Overflow
  • Services Provided: Literature Review; Data Compilation; Technology Evaluation
  • Type of Work: Technology Evaluation
  • Governing Regulation: Clean Water Act (CWA)