News Flash Home
The original item was published from 7/25/2023 8:15:43 AM to 7/28/2023 12:00:00 AM.

News Flash

Wastewater Department

Posted on: July 25, 2023

[ARCHIVED] Probable CSO Discharges

Probable CSO Discharges

The following locations had probable discharges during the July 24, 2023 rain event.


CSO Location Discharge

002 2 Sawmill Rd NONE

003 Farmington Rd NONE

004 Burke St NONE

005 East Hollis St 07/24/2023

006 Jackson St 07/24/2023

007 Tampa St NONE

008 Broad St NONE

009 Lock St NONE

014 Bancroft St 07/24/2023


The discharge is a combined wastewater which consists mainly of storm water runoff with some portion being sanitary wastewater. Portions of the City, particularly the older downtown area and neighborhoods, are served by a wastewater collection system that has one pipe that carries both the wastewater and storm water runoff together and is identified as a combined sewer. About 1960, when newer areas of the City were developed, a two pipe system with one pipe carrying wastewater and one pipe for storm water runoff, hence the name of a separated sewer system. All wastewater is transported to the Nashua Wastewater Treatment Facility (NWTF) for treatment on a dry day. However, during large rain events, there exist nine locations, four on the Nashua River and five on the Merrimack River, where combined wastewater can overflow to the rivers, providing relief to an overwhelmed collection system, which otherwise would back into residential and commercial buildings. These nine locations are identified as Combined Sewer Overflows (CSOs), see above table. These discharges are permitted by the Environmental Protection Agency (EPA) and the New Hampshire Department of Environmental Services (NHDES).

Since 1998, the City has had a program to mitigate the volume and frequency of CSOs entering into the rivers. The CSO program was designed based on the philosophy that urban runoff contains pollutants that are harmful to the waterways. The City moved from a complete separation program to a hold and treat philosophy. Up to a two year storm event, particularly the most polluted first flush, will be collected/stored and conveyed to combined sewage treatment facilities. The CSO Program has greatly reduced the volume of combined sewage entering the rivers.

All buildings that are connected to a combined sewer should have a backflow preventer on their building sewer that connects to the sewer main to prevent combined wastewater from entering their homes.

Facebook Twitter Email