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  Review of Sewer Design Criteria and RDII Prediction Methods (EPA/600/R-08/010) January 2008

Rainfall-derived infiltration and inflow (RDII) into sanitary sewer systems has long been recognized as a source of operating problems in sewerage systems. RDII is the main cause of sanitary sewer overflows (SSOs) to basements, streets, or nearby streams; it can also cause serious operating problems at wastewater treatment facilities. SSOs usually contain high levels of pathogenic microorganisms, suspended solids, toxic pollutants, floatables, nutrients, oxygen-demanding organic components, and oil and grease. There are serious potential health and environmental risks associated with SSOs.

The nation’s sanitary sewer infrastructure is aging, with some sewers over 100 years old. Nationwide, there are more than 19,500 municipal sanitary sewer collection systems serving an estimated 150 million people and about 40,000 SSO events per year. In 2002, EPA signed a cooperative research and development agreement (CRADA) with Camp Dresser and McKee, Inc. to develop a public-domain Sanitary Sewer Overflow Analysis and Planning (SSOAP) Toolbox. The purpose of the toolbox is to assist municipalities in developing plans to mitigate SSO problems. The toolbox will contain a suite of computer software tools to facilitate the analysis of RDII and performance of sanitary sewer systems. In addition, the CRADA includes a recently published technical guide and an SSOAP user manual for the application of the toolbox. A beta version of SSOAP is planned for release to the public in 2008.

This report provides a literature review of the RDII quantification methods to support the development of the SSOAP toolbox. The literature review is centered on the 1999 Water Environment Research Foundation report in which eight methods are thoroughly assessed using real data from three sewerage agencies. While there is no single RDII method that is universally applicable, the RTK method was chosen for implementation because it is most widely accepted. The method has long been an option in the EPA Storm Water Management Model and is extensively used.

Since RDII is closely associated with the structural conditions of sewers and the hydrologic/hydraulic criteria used to design them, background information is provided. This report also includes a summary of the nation’s wastewater infrastructure condition, origins of and problems caused by infiltration and inflow, and EPA regulatory approaches to address aging systems and SSOs. This report also presents the components of wastewater flows that form the basis for sanitary sewer design. Historical and current sewer design practices and flow design standards of selected states and local sewerage agencies are described.

See Also

Computer Tools for Sanitary Sewer System Capacity Analysis and Planning (PDF) (104 pp, 3.53 MB) (EPA/600/R-07/111) October 2007

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