Inflow & Infiltration

Watertown DPW has a program to address inflow and infiltration (I/I) into the municipal sewer system.

Unlike our water distribution system, which uses full pipes under pressure to deliver water to homes, the sewer system is rarely full when wastewater is flowing from homes to the sewage treatment plant. When a water main breaks, it leaks water out. When a sewer pipe breaks, it allows groundwater and stormwater to leak into the pipe.

When groundwater or stormwater leaks into the sewer system, it takes up extra space that could be carrying wastewater. This can contribute to sewer backups and overflows. In addition, this additional “clean” groundwater and stormwater is treated at the MWRA Wastewater Treatment Plant, at the Town’s expense.


Inflow is stormwater that is directly piped into the sanitary sewer system to control runoff. These connections, which may include storm drains in the street, parking lots, driveways, sump pumps, foundation drains, and gutters, should not be connected into our sanitary sewer system, which is designed only to carry wastewater.


Infiltration is excess water that gets into the sanitary sewer system through open joints, cracks, root intrusion, and breaks in the pipes. These deficiencies may allow constant infiltration of groundwater. Cracked or collapsed sewer lines are generally caused by deterioration over time, poor design, installation, or maintenance.

The DPW’s I/I program consists of annual monitoring and maintenance of the sewer system. Monitoring consists of remote television inspection of the sewer system for signs of blockage, deterioration, root intrusion, and illegal stormwater connections. Manholes are similarly inspected for signs of inflow and infiltration. Flow isolation is another type of monitoring method, which helps quantify the amount of I/I in a sewer segment. During flow isolation, manhole to manhole segments of sewer are isolated and the amount of flow in the line is measured during the early morning hours (between 12 a.m. and 6 a.m.). The majority of residents are sleeping during these hours (so there is little wastewater in the system), and therefore it can be assumed that the measured flow is mostly I/I.

Maintenance activities include cleaning sewer lines and making repairs as noted during the monitoring phase. There are many methods to repair or even replace damaged sewer lines. “Trenchless” technologies now exist that permit rehabilitation without interruption of sewer service or traffic disruption. DPW prefers to use these technologies and uses “open cut” methods only when there are no other alternatives.

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