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Combined Sewer Overflow - Stormwater

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. 

Phase I Program

This City, the United States Environmental Protection Agency (EPA) and the State of New Hampshire Department of Environmental Services (DES) agreed to a settlement for a phased approach to the CSO problem. In March of 1999, the EPA issued an Administrative Order requiring the City to spend approximately $58 million over a 10-year period to begin to eliminate CSOs and provide more detailed planning for the Phase II Program. As part of the agreement, the City is required to commit an additional $5.6 million for Supplemental Environmental Projects throughout the City.

Phase I Components

  1. Elimination of all CSOs discharging to the Piscataquog River by constructing new sewers and drains as required.
  2. Elimination of the Crescent Road, Victoria Street, Poor Street, and Schiller Street CSOs discharging to the Merrimack River by constructing new sewers and drains.
  3. Treatment plant modifications to increase the amount of CSO that can be handled by the plant.
  4. Additional engineering study of the Cemetery Brook, Stark Brook, and Tannery Brook CSOs to develop a plan for Phase II remediation of these CSOs.


The Phase I CSO abatement program was completed ahead of schedule and under the original budget.

The program not only fully separated 15 CSO drainage basins; it decreased CSO discharge by 99%. In addition, it consisted of construction of new or rehabilitated utilities, and new roads, sidewalks, and curbing.

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) 624-6513.

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.