Wastewater Treatment Facility Information


The Wastewater Treatment Department is dedicated to collecting and treating wastewater that flows into our facility from Nashua and Hudson and delivering a clean, clear effluent into the Merrimack River. The Wastewater Treatment Facility is located at 2 Sawmill Road in the City of Nashua. The Department is under the umbrella of the Division of Public Works.


The first wastewater treatment tanks were installed at the Nashua Wastewater Treatment Facility in 1959. At that time only a small portion of Nashua's flow came into the facility. In 1974, the facility was expanded. New interceptor lines in the city assured that all wastewater flow came into the facility. The facility was upgraded to secondary treatment in 1989 with a design flow of 16 million gallons per day. In 1998, the city started construction of a $10 million dollar anaerobic digester complex, which went on line in December 2000. The facility began construction of a new Wet Weather Treatment Facility in 2007 that was an alternative response to the EPA’s consent decree to the City of Nashua to reduce combined sewer overflows. This $27.5 million dollar facility was a huge savings to the City, when compared to the $250 million dollars it was projected to cost to separate all the sewers. This Wet Weather Treatment Facility went online in 2009 and took the plant’s peak capacity from 50 million gallons per day, up to 110 million gallons per day. In December 2014, a $12 million underground combined sewage treatment facility opened along Bancroft Street. This new facility helped to reduce combined sewer overflows into the Nashua and Merrimack rivers.

The Present:

The city is working to eliminate the combined sewer overflows (CSOs) that are discharged into the Nashua and Merrimack rivers. CSOs occur when storms overpower the collection system and cause wastewater and storm water to be dumped into the rivers without treatment. The program will cost over $100 million dollars.

Plant Flows:

          Design Flow = 16,000,000 gallons per day

          Peak Design Flow = 110,000,000 gallons per day

          Average Daily Flow = 11,000,000 gallons per day

          Total Flow (One Year) = 4,150,400,000 gallons

          Total Septage (One Year) = 390,150 gallons

Plant Efficiency:

Biochemical Oxygen Demand (BOD) = 92% removal on average
Total Suspended Solids = 95% removal on average