Begin Main Content Area

Combined Sewer Overflows

Combined Sewer Overflows (CSOs) are intermittent overflows or other untreated discharges from a combined sewer system (CSS) to surface waters prior to reaching a sewage treatment facility. CSSs collect both sewage and stormwater and were commonly constructed prior to modern public health and environmental laws and regulations. CSOs occur when the flow in a CSS exceeds the carrying capacity of the system. The U.S. Environmental Protection Agency (EPA) and DEP have taken measures over the past few decades to reduce or eliminate these discharges.

Historically, Pennsylvania had over 150 permitted CSO communities with over 1,900 outfalls throughout the Commonwealth. To date, 30 of these communities have either ceased operation, closed their CSO outfalls, or separated their storm and sanitary sewer systems. Many others have reduced their number of outfalls by rerouting a portion of their collection system or by enhancing the capacity of their collection system. DEP maintains a list of existing and former CSO communities in Pennsylvania.

Requirements for CSO Communities

Communities with CSO discharges are required to take action to control these discharges and reduce their impact to waterways. This begins with the implementation of technology-based Nine Minimum Controls (NMCs), which include the following:

  • Proper operation and regular maintenance programs for the sewer system and CSO outfalls;
  • Maximization of storage in the collection system;
  • Review and modification of pretreatment requirements to ensure that CSO impacts are minimized;
  • Maximization of flow to the POTW for treatment;
  • Elimination of CSOs during dry weather;
  • Control of solid and floatable materials in CSOs;
  • Pollution prevention programs to reduce contaminants in CSOs;
  • Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts; and
  • Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls.

CSO communities are also required to develop a Long-Term Control Plan (LTCP) that details specific controls to be constructed, a schedule for implementation, and a Post-Construction Compliance Monitoring (PCCM) Plan. The ultimate goal for the LTCP is compliance with state water quality standards. The NMCs, LTCP, and PCCM Plan requirements and schedules are incorporated into the DEP-issued NPDES permit for the system’s CSO discharges.

NPDES Permitting Program

The Pennsylvania Combined Sewer Overflow (CSO) Policy (385-2000-011) was originally published on March 1, 2002, and was replaced on September 6, 2008 to support compliance with CSO and NPDES permit requirements. Under the Policy, CSO communities must continue implementing the NMCs and their LTCPs and DEP will continue its efforts to ensure the communities remain on track to complete their LTCPs. CSO implementation requirements are established in NPDES permits, which are issued and enforced by DEP. CSO communities that operate their own sewage treatment facility must obtain an individual NPDES permit for major sewage facilities or minor sewage facilities. Communities with satellite CSSs (i.e., do not operate their own sewage treatment facility) may be eligible for coverage under the PAG-06 NPDES General Permit if the CSS serves less than 75,000 people.

For more information on CSOs and answers to common questions, please refer to DEP’s CSO Frequently Asked Questions (FAQ) document or contact DEP’s Division of NPDES Permitting at