Inflow & Infiltration

What is Inflow?

Inflow is rainwater or groundwater that enters the sewer system through a direct, piped connection or leaking manhole covers. It is clean water that does not need to be treated as wastewater.

What is Infiltration?

Infiltration is groundwater that enters the sewer system through broken or leaking pipes or leaking sewer manholes. It is clean water that does not need to be treated as wastewater.

What are the typical sources of inflow?

Typical inflow sources include roof downspouts, sump pumps, and driveway drains that are connected to the sewer system. Leaking sewer manhole structures or covers located in low or wet areas are also sources of inflow. 

Why is it important to remove infiltration & inflow (I/I) sources?

The City is required by the Massachusetts Department of Environmental Protection (MassDEP) to identify sources of rainwater runoff and groundwater that enter the sewer system. The City pays the Massachusetts Water Resource Authority (MWRA) for every gallon of water that enters the sewer system regardless of its source. Removal of inflow to the sewer system will reduce sewer capacity and overflow issues, protect surface water quality, and decrease the City’s sewer costs.

What work was previously done to identify I/I and why?

The City’s Wastewater Master Plan identified areas where flow metering data suggests inflow issues (i.e., flow increases in response to a rainfall event). The City is currently in a multi-phase, long-term plan to identify and remove sources of I/I in the sewer system. Previous efforts have included identification of potential inflow sources, dye testing, and closed-circuit television (CCTV) of sewer pipes.

How is I/I identified?

Both infiltration and inflow can be estimated by installing flow meters in sewer manholes. Flow meter data is correlated to rainfall and groundwater levels.

Infiltration is identified through the inspection of sewer manholes and CCTV of sewer pipes. CCTV involves placing a TV camera into the pipe and recording the condition of the pipe. Flow isolation is also used to identify infiltration by measuring the flow during off-peak hours. 

Inflow is typically identified by conducting smoke testing, dye testing, or building inspections. Smoke testing involves introducing non-toxic smoke into the sewer pipe and noting if smoke protrudes comes out at roof leaders, catch basins, yard drains, or other potential inflow sources. Dye testing involves pouring a non-toxic, non-staining dye into the potential source and seeing if it appears in the downstream sewer manhole. Building inspections are when City personnel enters a building to verify whether a sump pump is improperly connected to the sewer system.

How is I/I removed? 

Infiltration caused by broken or cracked pipes or manholes can be removed by either replacing or rehabilitating the infrastructure. Inflow caused by leaking manhole covers can be removed by replacing the cover, or installing waterproof covers in low, wet areas. Inflow caused by improper drain or sump pump connections can be removed by re-piping the discharge of the source.