Combined Sewer Overflows (CSO)

Real Time CSO Overflow Notifications

The City Wastewater Division's CSO Dashboard alerts residents of CSO events. If an overflow is detected in any of the measuring devices in the City’s three active CSO outfall locations, it will show up here in real time. 

If you would like to receive email alerts, please also sign up for our email list below. Your email will be added to a list in which our measuring devices will email you directly as events occur.

The City of Portsmouth’s original sewer system dates to the mid-1800’s. The original collection system discharged sewage directly into the North Mill Pond, South Mill Pond and the Piscataqua River. This was problematic for public health and significant modifications were made to the system in the 1960’s. This work was performed in parallel with regulatory requirements and changing regulations related to water quality. The system modifications that began in the 1960’s included, construction of a treatment facility at Peirce Island and pumping stations in the collection system to eliminate the direct discharges to the river. With these changes, there were still capacity limitations and so there were a number of relief points allowed in the collection system. These relief points allowed excess capacity in the combined collection system to discharge to a nearby water body and are now referred to as Combined Sewer Overflows. At one time there were 14 combined sewer CSO discharge sites. Today, only the three CSO discharge sites shown on the map above exist. The discharge from these outfalls is significantly lower than when there were 14 sites -- and happen only during high-volume wet weather events as authorized by permit with the Environmental Protection Agency.

Today the City owns and operates approximately 120 miles of sewers (public gravity sewers and force mains, not including laterals), which convey wastewater to the Peirce Island Wastewater Treatment Facility (WWTF) and Pease WWTF.

Sewerage systems are classified as separated or combined. The collection system at the Pease International Tradeport is a seperate sewer system. The collection system that discharges to the Peirce Island WWTF is a combined system; currently 80% separated and 20% combined. To reduce combined sewer overflow volume from the combined collection system, the City has been implementing sewer separation projects over several decades. The City has invested $55 million in overall CSO abatement activities since 1997. The figure demonstrates the areas of some of the many sewer separation projects completed in the City. It also shows planned sewer separation work that will continue as part of the City Consent Decree with the Environmental Protection Agency through a Supplemental Compliance Plan.

The sewer separation work has resulted in many positive outcomes from the City including, significant reductions in the volume of CSO discharged annually, significant reduction in street flooding, and reduced sewer backups into basements.

The City plans for CSO reduction through its Long Term Control Plan (LTCP). The LTCP is a document that the Environmental Protection Agency requires communities with combined collection system to have and allows for long term implementation of cost effective solutions for CSO reduction. The City’s first Long Term Control Plan was completed in 2002, but came after the completion of many other collection system related studies dating back to the 1950’s. The LTCP is regularly updated after the completion of major projects. The most recent version was the LTCP update in 2010. The next anticipated update is 2023 following the completion of sewer separation projects in the City’s Supplemental Compliance Plan, which are shown in the figure.

When these alerts occur, swimming in the water near these overflows is not recommended and should be avoided for 48 hours following the completion of a precipitation event. See Frequently Asked Questions for more information. If you have questions regarding these notices, please contact Public Works at 603-427-1530.

Frequently Asked Questions

What is a Combined Sewer?

Combined sewers carry sewage and stormwater runoff in the same pipe to a wastewater treatment facility (WWTF). During large precipitation events the pipes can get overwhelmed. These types of sewer systems have relief points call Combined Sewer Overflows. 

What are Combined Sewer Overflows (CSOs)?

CSOs rely on the three relief points in combined sewer systems that provide for discharge (or overflow) of combined sewage and stormwater into a nearby waterway. A CSO event can occur during a heavy rainfall or snowmelt when stormwater and sewage overload the system.The untreated sewage and stormwater may be discharged at CSO locations during an event.

Where and how many CSO locations are in the City?

There are three (3) active CSO locations in the City that are permitted by the Environmental Protection Agency.

• CSOs No. 10A and No. 10B are located adjacent to Parrott Avenue and discharge to South Mill Pond. This is a tidal pond that flows to the Piscataqua River.      
• CSO No. 013 is located at the end of Deer Street and discharges to the Piscataqua River. The City has posted signs at each CSO location.

Can I or my pet swim in the water at South Mill Pond?

Swimming is not permitted at the South Mill Pond during a rain storm and 48 hours following a rain storm. The City does not recommend swimming at other times because the water body is not routinely monitored for water quality and may not be suitable for swimming. Designated swimming areas that are part of the New Hampshire Department of Environmental Services Public Beach Program are routinely monitored and we recommend those locations for recreational swimming.

Where can I see the history of CSOs?

The City monitors the volume of discharge from every CSO event. The volume of overflow varies based on the duration, intensity and depth of the rain storm. A CSO does not occur every time it rains. There are other conditions such as frequency of rainfall and groundwater conditions (e.g. frozen, saturated, snow pack, etc.) that have impact on the probability of a CSO discharge event.

Who do I contact to report a CSO issue?

Please contact the Department of Public Works, City Engineer or Water/Sewer General Foreman, at (603) 427-1530.