Combined Sewer Overflows

What is a Combined Sewer?


When Nashua was built, many years ago, some sewer pipes were built to carry both wastewater (used water that comes from homes and businesses) and storm water (rain or snow that flows from streets and parking lots) to the wastewater treatment facility. These flows mix together in a single pipe, called a Combined Sewer System.

During dry weather and most storms, flow travels to the Wastewater Treatment Facility, to be treated and safely released. However, during times of heavy rain, the pipes may get overwhelmed with flow and start to release into the Merrimack and Nashua Rivers. This occurrence is called a Combined Sewer Overflow (CSO). The City of Nashua has 9 locations, listed below, where this can happen. This overflow provides a relief for the system, so there are no backups of untreated wastewater into homes, businesses, and schools, and it prevents flooding in city streets and bursting of underground pipes.

What is a Combined Sewer Overflow?


When the storm runoff overwhelms the combined sewer, it is discharged directly into the receiving water. When this occurs, there is pollution potential for the receiving waters (the Merrimack or the Nashua River, depending on what location is discharging). This pollution potential includes bacteria and viruses from untreated water as well as potential chemicals from homes and industries. 

What are the potential health risks?


Skin exposure to contaminated water has minimal health risks. The bigger concern is getting the same contaminated water into your mouth or eyes. The bacteria and viruses from untreated wastewater can make you sick. The biggest risk groups are young children, elderly people, and individuals with low immune systems.

How long will it take before the water is safe again?


Public Health recommends that individuals do not go near these locations for 48 hours after a discharge.

How will I know if there has been an overflow?


The City of Nashua’s Wastewater web page will have a banner that will inform the public any time there is a Combined Sewer Overflow occurrence, as well as the location. This notification will stay up for 48 hours after the end of the overflow, allowing the river time to recover.

Where can I see the history of Combined Sewer Overflows?


The City of Nashua’s Wastewater Treatment Facility contracts the monitoring of all of its Combined Sewers. This data will be uploaded monthly, it shows discharges by location, includes amounts discharged, and duration.

History of Combined Sewer Overflows

What is being done to minimize Combined Sewer Overflows?


The City of Nashua has spent over 69 million dollars on improvements including, but not limited to, the Wet Weather Flow Treatment Facility, The Screenings and Disinfection Facility, the sluice gate at CSO 006, the drop over structure at CSO 005, sewer separation at Harbor Ave, and a 40,000 gallon storage tank at CSO 004.

Major Wastewater Projects

Combined Sewer Overflow Locations:



CSO 002 is located at the Wastewater Treatment Facility, this overflow discharges to the Merrimack River.

CSO 003 is located parallel to Farmington Rd, this overflow discharges to the Merrimack River.

CSO 004 is located off of Burke Street, this overflow discharges to the Merrimack River.

CSO 005 is located at the pump station off of East Hollis Street, between the two lanes of the Hudson Bridge, this overflow discharges to the Merrimack River.

CSO 006 is located off of Bridge Street, this overflow discharges to the Nashua River.

CSO 007 is located off of Tampa Street, this overflow discharges to the Nashua River.

CSO 008 is located off of Broad Street, this overflow discharges to the Nashua River.

CSO 009 is located off of Locke Street, this overflow discharges to the Nashua River.

CSO 014 is located off of Bancroft Street, this overflow discharges to the Merrimack River.