Sagamore Avenue Low Pressure Sewer Extension, Portsmouth, NH
Several properties in the Sagamore Avenue neighborhood of Portsmouth, New Hampshire (City) were identified as having failed septic systems, holding tanks, poor soil conditions, and/or lack of space to address wastewater flows with on-site systems. The New Hampshire Department of Environmental Services (NHDES) performed a study of the non-point sources to Great Bay and identified septic system discharges, specifically systems within 200 meters of a waterway, as the largest in state non-point source of nitrogen pollution. Sagamore Creek, which runs through the project area, was identified as an impaired waterway and is a tributary to the Great Bay. The City’s Consent Decree mandated the construction of the Sagamore Avenue sewer extension.
The project consists of approximately 10,000 linear feet of low pressure sewer for 80+ properties adjacent to Sagamore Creek. The low-pressure sewer system was equipped with small grinder pump stations installed in each property with common force mains located in the right of way. The use of grinder pumps and low pressure sewers is one aspect that makes this project unique. Grinder pumps are most commonly used when a gravity sewer service cannot be used due to topographic challenges This type of system reduces construction costs by using small diameter pipes that are installed at shallower depth to avoid many municipal pump stations, deep excavations, and significant bedrock removal.