The City of Nashua announced on April 15 that two locations experienced combined sewer overflow (CSO) discharges into local rivers during the rain event on April 14.
Combined sewer overflows occur when heavy rainfall overwhelms the wastewater collection system, causing a mixture of storm water runoff and sanitary wastewater to be released into nearby rivers. This process helps prevent sewage from backing up into residential and commercial buildings.
According to the city’s report, CSO discharges were recorded at Broad Street (CSO 008) and Locke Street (CSO 009), both releasing into the Nashua River. The discharge at Broad Street began at 6:50 p.m. and ended at 7:05 p.m., while the Locke Street discharge started at 6:50 p.m. and stopped at 7:00 p.m. No other CSO locations reported discharges during this event.
The city explained that older neighborhoods in downtown Nashua are served by a single-pipe system carrying both wastewater and storm water, known as a combined sewer. Newer areas built after about 1960 use separated systems with different pipes for each type of water flow. On dry days, all wastewater is sent to the Nashua Wastewater Treatment Facility for processing; however, during significant rain events, nine permitted CSOs—four on the Nashua River and five on the Merrimack River—can overflow directly to local waterways under permits from the Environmental Protection Agency (EPA) and New Hampshire Department of Environmental Services (NHDES).
Since starting its mitigation program in 1998, Nashua has aimed to reduce both how often and how much untreated combined sewage enters its rivers by collecting or storing polluted runoff for treatment whenever possible. The city advises that buildings connected to combined sewers should have backflow preventers installed to keep contaminated water from entering homes.
