Stormwater inflow/infiltration identification underway

posted on 5/26/2026

The City of Dover is embarking on another phase of identifying and reducing inflow/infiltration (I/I) of stormwater into the sanitary sewer system. The work is necessary both for federal and state compliance, as well as ensuring the health and vitality of natural resources like the Great Bay Estuary.

The city’s sewer and drainage systems have evolved over the course of 100-plus years, moving from a combined sewer and drainage system that dumped directly into the Cochecho River in its early years into a separated system: one for stormwater and one for the sanitary sewer system with a complex treatment system. With thousands of homes in the city built before the separated system was introduced in the 1970s, there have been stormwater outflows from roof drains and basements that have been unwittingly connected to the sanitary sewer system. When significant rain events occur, this stormwater “inflows” into the sanitary sewer system, which overwhelms the wastewater treatment facility, reducing its treatment efficiency. “Infiltration” occurs when stormwater seeps into cracked sewer pipes and broken maintenance holes.

The greatest flow reductions have been achieved by reducing direct inflow connections into the sewer system, such as from roof drains, catch basins, and residential sump pumps. These connections are illegal under city code. The city understands that many of these illegal connections, if not all, are not done knowingly by the property owner. It just evolved with the sewer and drainage systems.

Since 2000, the city has been working to identify and correct inflow and infiltration into the system. It’s made significant progress in reducing stormwater from entering the sanitary sewer system, but a significant amount of work remains. In 2019, the city, working with a vendor, conducted flow tests at strategic locations in the sewer system. These flow tests help identify areas where high volumes of stormwater are entering the system during rain events. The Bellamy Road, Knox Marsh Road, and Mount Vernon Street areas were identified as having high flow.

The city is working with a vendor, Wright-Pierce, to identify the sources of these stormwater discharges. The scope of work authorized by the City Council included a small part of visual inspection by drone; this work will not occur. Instead of using drones, the vendor, Wright-Pierce, will contact property owners to request permission to inspect roof drains and other potential sources of I/I that may be entering the sewer system during storm events without their knowledge. Other work for the project includes maintenance hole inspections, video inspections of more than 60,000 feet of sewer pipe, dye testing, and smoke testing. All these help identify connections in buried infrastructure.

Residents who suspect their stormwater drains are connected to the sewer system are encouraged to contact City Engineer Jillian Semprini by email or at 603-516-6450. There are no punitive punishments, just the satisfaction that correcting these issues will help lower overall treatment costs and ensure the health of our natural resources.

For more information, contact Community Services at 603-516-6450.