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Combined Sewer Overflow

Flooding from Manhole OverflowCSO

In the City, the majority of the sewer lines collect both rainwater from streets and sewage from homes and businesses. These lines are called combined sewers. When it is not raining, the wastewater flows through these lines to the Wastewater Treatment Plant for treatment before being discharged to the river. When it rains, the system is quickly overwhelmed and the mixture of rainwater and sewage is relieved through a series of outfalls called combined sewer overflows. This is necessary to prevent the mixture from backing up into the streets and basements throughout the City. It is estimated that about 220 million gallons of combined sewage is discharged annually to the Merrimack River. 

Tackling Merrimack River Water Quality Through Combined Sewer Overflow Abatement

Construction in Manchester NHThe City of Manchester is investing over $338 million over the next 20 years to address Combined Sewer Overflows (CSOs) to improve Merrimack River water quality. The city has already invested $100 million over the past 20 years to mitigate CSO activations during Phase I of this program. Phase II of this program is one of the largest public works projects in the history of the city. Work has already started on a dozen different projects. Construction is ongoing at the wastewater treatment plant, will start this spring on Christian Brook on North Street, and this fall adjacent to Fisher Cats Stadium. The center piece of the Phase II program is the Cemetery Brook Tunnel Project. 

Installation of new drainage systemCemetery Brook is the main drainage basin for the city starting at Stevens Pond and discharging into the Merrimack River near Fisher Cats Stadium. A new drainage system will be constructed along the former railroad corridor from Mammoth Road southwest to Queen City Avenue to “separate” Cemetery Brook. The new drainage system will consist of an underground tunnel two miles long, 12-feet in diameter, and 30 to 80 feet deep. The tunnel will eliminate construction disruptions to residents, businesses, and traffic while significantly decreasing construction time.

Installation of new drainage systemIn addition to the water quality improvements in the Merrimack River, these construction projects will be leveraged to provide urban revitalization to many areas of our inner city. In addition to the CSO work, other utilities such as Manchester Water Works and Liberty Gas will upgrade their utilities to serve future generations. Restoration efforts are fully leveraged as roads will be rebuilt along with new curbing, sidewalks, crosswalks, bike lanes and other amenities. The successful themes of water quality improvements, infrastructure upgrades, urban revitalization, and environmental justice to our inner city that were so successful during Phase I will continue throughout Phase II of the city’s CSO mitigation program. 


Public Involvement

Stay Informed – CSOs have an impact that is important for a community to understand. The City will keep you updated on the progress of the program.

Watch River Water Quality – If you spot sewage or observe odd discharges to the river during dry weather, notify the Manchester Environmental Protection Division at (603) 624-6341.

Construction Activity Discharge - If you notice sediment, sand, or mud leaving active construction sites and going into marshes, ponds, streams or other water bodies or to catch basins in the streets call the Storm Water Hotline at (603) 665-6899. Sediment, which carries bacteria is the largest source of pollution to the Merrimack River.

Need More Information – More detail is available by contacting Manchester EPD at (603) 624-6341.