Approved by the Energy, Environment, and Water Policy Committee on December 10, 2020
Approved by the Public Policy and Practice Committee on May 4, 2021
Adopted by the Board of Direction on July 16, 2021


The American Society of Civil Engineers (ASCE) supports the Environmental Protection Agency’s (EPA) national policy statement for combined sewer overflow (CSO) systems that establishes a consistent national approach for controlling existing CSO discharges to meet Clean Water Act (CWA) goals. Specifically, ASCE supports:

  • Revisions to the EPA long-term control plan guidance on CSOs and green infrastructure to incorporate the strengths of green infrastructure that reduce the volume of stormwater flow into existing combined sewers.
  • Increased funding for achieving the required level of control, such as EPA funded clean water state revolving funds and construction grants to support implementation of long-term control plans.
  • Flexibility to adapt to more stringent water quality standards at the local and regional level to reflect site-specific conditions.
  • Guidance for developing control options and strategies that include consideration of cost versus performance without sacrificing public health and safety.
  • Disallowing the construction of new combined sewers.


ASCE recognizes the problems inherent with combined sewer systems, including the environmental and health risks resulting from overflow conditions. Historically, combined sewers were an acceptable technology for collecting storm water, sanitary sewage and industrial effluents in a single conduit for discharge to water courses. This practice is no longer acceptable because of today’s understanding of the environmental and health risks imposed by this practice. 

Combined sewer systems serve hundreds of communities with millions of residents. Most communities with CSOs are located in the Northeast and Great Lakes Regions. Although large cities like New York, Philadelphia, and Atlanta have combined sewer systems, most communities with CSO problems have fewer than 10,000 people.

Industrial and residential growth has stretched these systems beyond their design limitations. This, coupled with reductions in green spaces and increased development, has accelerated the rates in which runoff contributes to sewer systems, hydraulically overloading these systems and creating more frequent overflow conditions. Separation of flows in combined sewer systems, including roof leaders and other urban conduits that carry runoff, requires significant investment. In addition, separate stormwater discharges will require treatment to remove contaminants and debris that enter the storm systems at the street level. 

Many cities have reduced CSOs by providing temporary storage within the collection system until treatment capacity is available. Storage can be provided by increasing the capacity of sewer interceptors providing in-line storage, adding underground storage, and other methods. 

Green stormwater infrastructure is considered as part of a sustainable adaptive management strategy to control CSOs. Often, large-scale traditional infrastructure approaches (tunnels, pipes, tanks) can be reduced in size and cost with judicious use of green stormwater infrastructure. The green infrastructure approach provides additional benefits to include removal of pollutants from runoff, green public space and the reduction of urban heat island effects. These measures were not available when the current regulatory structure and supporting long-term control plan guidance were developed and the language and process in federal guidance will require updating in order to encourage appropriate consideration of this approach.

The primary burden of financing and executing the remedial work required to comply with the current federal regulatory requirements fall on state and local governments. The costs for these improvements often exceed the ability of sewer system rate payers to afford. Additional federal funding is required to ensure these systems can achieve regulatory water quality goals within reasonable timeframes. 


Remedial CSO works projects designed to correct the problems of controlling and separating combined sewer discharges are often large, complex, and economically challenging. Increased funding is required to achieve environmentally safe and functional sewer systems and new approaches for sustainable, adaptive management strategies to control CSOs need to be implemented.

ASCE Policy Statement 395
First Approved in 1992