Treatment Facility

Step 1
Primary Treatment
The main goal of this section is to screen out large objects or particles of debris.
Grit Building: Wastewater is screened for debris such as sticks, rags, & chunks of grease and disposed at landfill.
Main Pump Station: (2) 100 hp pumps are used to force gravity sewer to the lagoons following screening. Each pump is capable of pumping over 5 million gallons per day.
 
Step 2
Secondary Treatment
The main goal of this section is to reduce pollutants and restore oxygen.
Lagoons:  Derry uses 3 "Facultative Aerated Lagoons" to treat wastewater. These lagoons cover over 30 acres of land and at full capacity can hold over 850 million cubic feet (6.3 billion gallons!). The wastewater is retained within the lagoon system for a period of approx. 60 days to properly remove harmful bacteria and other microorganisms.
Blower Building: This building houses all the blower units used to add oxygen back into the wastewater. These 200 hp units are capable of providing over 15,000 cubic feet of air per minute if required. The system is fully automated and is operated on variable frequency drives to save electricity. (this operation alone costs between $15,000 & $20,000 per month in electricity!)
 
Step 3
Final Discharge
The main goal of this section is to test the quality of wastewater and discharge it back into the environment.
Treatment Plant: (3) 75 hp effluent pumps are designed to pump approx. 2,800 gallons per minute through a 9 mile force main to the Merrimack river. Prior to leaving the plant, chlorine is injected to remove any remaining bacteria or microorganisms. This pipeline crosses the municipalities of Londonderry, Manchester, and Litchfield prior to discharge into the river. Current flows to the Merrimack river are approx. 2.5 million gallons per day. Two sewer operators perform various tests in the treatment plant's lab that  measure the quality of incoming raw sewer and out going treated sewer to confirm compliance to federal and state regulations.