Probable CSO Discharges
The following locations had probable discharges during the June 30, 2021 rain event.
CSO Location Discharge
002 2 Sawmill Rd NONE
003 Farmington Rd NONE
004 Burke St NONE
005 East Hollis St 07/01/2021
006 Jackson St NONE
007 Tampa St 07/01/2021
008 Broad St 06/30/2021 and 07/01/2021
009 Lock St NONE
014 Bancroft St NONE
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.