COMBINED SEWER OVERFLOWS (CSOs)

 

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's. Today, only three combined sewer overflows exist. The discharge from these outfalls during wet weather events is 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.

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 are 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.



Summary of Combined Sewer Overflow Events         

Year Month CSO 10A Volume
(Million Gallons)
CSO 10B Volume
(Million Gallons)
CSO 013 Volume
(Million Gallons)
Total Volume
(Million Gallons)
2017

2017
July

June
0.0000

0.1610
0.0000

0.0000
0.0000

0.0000
0.0000

0.1610
2017 May 0.4010 0.0080 0.0000 0.4090
2017 April 0.4010 0.0150 0.0000 0.4160
2017 March 0.0000 0.0000 0.0000 0.0000
2017 February 0.0020 0.0000 0.0000 0.0020
2017 January 0.0000 0.0000 0.0000 0.0000
           
2016 December 0.0000 0.0000 0.0000 0.0000
2016 November 0.1502 0.0000 0.0000 0.1502
2016 October 0.6790 0.0090 0.1470 0.8350
2016 September 0.0000 0.0000 0.0000 0.0000
2016 August 0.0000 0.0000 0.0000 0.0000
2016 July 0.5210 0.0210 0.0350 0.5770
2016 June 0.0400 0.0000 0.0520 0.0920
2016 May 0.0000 0.0000 0.0000 0.0000
2016 April 0.0000 0.0000 0.0000 0.0000
2016 March 0.0000 0.0000 0.0000 0.0000
2016 February 0.7750 0.0120 0.0000 0.7870
2016 January 0.8440 0.0100 0.1980 1.0520
           
2015 December 0.0130 0.0000 0.0000 0.0130
2015 November 0.0000 0.0000 0.0000 0.0000
2015 October 0.0060 0.0000 0.0000 0.0060
2015 September 0.8040 0.1650 0.0090 0.9780
2015 August 0.0000 0.0000 0.0000 0.0000
2015 July 0.0000 0.0000 0.0000 0.0000
2015 June 0.0000 0.0000 0.0000 0.0000
2015 May 0.0000 0.0000 0.0000 0.0000
2015 April 1.9703 0.3078 0.23857 2.5167
2015 March 0.0000 0.0000 0.0000 0.0000
2015 February 0.0000 0.0000 0.0000 0.0000
2015 January 0.0000 0.0000 0.0000 0.0000
           
2014 December 7.7118 0.3908 1.4858 9.5884
2014 November 0.0000 0.0000 0.0000 0.0000
2014 October 0.9339 0.0000 0.0000 0.9339
2014 September 0.0000 0.0000 0.0000 0.0000
2014 August 0.2312 0.0000 0.0000 0.2312
2014 July 0.1612 0.0000 0.0000 0.1612
2014 June 0.2613 0.0000 0.0000 0.2613
2014 May 0.0000 0.0000 0.0000 0.0000
2014 April 0.0000 0.0000 0.0000 0.0000
2014 March 3.4165 0.1167 0.0000 3.5332
2014 February 0.0000 0.0000 0.0000 0.0000
2014 January 0.0000 0.0000 0.0000 0.0000
           
2013 December 0.0000 0.0000 0.0000 0.0000
2013 November 0.0000 0.0000 0.0000 0.0000
2013 October 0.0000 0.0000 0.0000 0.0000
2013 September 0.3769 0.0021 0.0046 0.3836
2013 August 0.2172 0.0000 0.0000 0.2172
2013 July 0.0922 0.0000 0.0000 0.0922
2013 June 0.7673 0.0062 0.0170 0.7906
2013 May 0.0000 0.0000 0.0000 0.0000
2013 April 0.0000 0.0000 0.0000 0.0000
2013 March 0.0000 0.0000 0.0000 0.0000
2013 February 2.2936 0.1155 0.0000 2.4091
2013 January 0.0000 0.0000 0.0000 0.0000
           
2012 December 2.5672 0.0000 0.2543 2.8215
2012 November 0.0000 0.0000 0.0000 0.0000
2012 October 0.0000 0.0000 0.0000 0.0000
2012 September 0.1360 0.0000 0.0000 0.1360
2012 August 0.7014 0.4268 0.1279 1.2561
2012 July 0.0152 0.0000 0.0000 0.0152
2012 June 1.6986 0.6230 0.3261 2.6477
2012 May 0.1438 0.0000 0.0000 0.1438
2012 April 2.4750 0.2721 0.1485 2.8956
2012 March 0.0000 0.0000 0.0000 0.0000
2012 February 0.0000 0.0000 0.0000 0.0000
2012 January 0.2651 0.0000 0.0000 0.2651
           
2011 December 0.4184 0.0003 0.0000 0.4187
2011 November 0.0385 0.0029 0.0000 0.0414
2011 October 2.8097 0.1336 0.2234 3.1667
2011 September 1.2681 0.0828 0.0280 1.3789
2011 August 0.5709 0.0354 0.0000 0.6063
2011 July 0.0036 0.0000 0.0000 0.0036
2011 June 0.0342 0.3373 0.0269 0.3984
2011 May 0.0789 0.0000 0.0000 0.0789
2011 April 0.3739 0.0000 0.0000 0.3739
2011 March 5.6200 0.0756 1.4781 7.1737
2011 February 0.0413 0.0000 0.0000 0.0413
2011 January 0.0000 0.0000 0.0000 0.0000
           
2010 December 0.0000 0.0000 0.0000 0.0000
2010 November 0.0085 0.0000 0.0742 0.0827
2010 October 0.0000 0.4511 0.5283 0.9794
2010 September 0.0000 0.0000 0.0000 0.0000
2010 August 3.1018 1.0317 0.9193 5.0528
2010 July 1.4126 0.2109 0.5869 2.2104
2010 June 1.0000 0.1898 0.0636 1.2534
2010 May 1.5751 0.0648 0.2404 1.8803
2010 April 2.8285 0.0095 0.0152 2.8532
2010 March 49.2970 13.8640 15.6310 78.7920
2010 February 8.7993 1.0963 3.3337 13.2293
2010 January 1.3990 0.2144 0.5490 2.1624