Public Works explains … What is infiltration & inflow?

The City of Bonney Lake, like many municipalities, must constantly monitor Inflow and Infiltration into the sanitary sewer and stormwater collection systems to ensure that the public health and environment are protected. If a cross-connection exists between these two systems, this can lead to wastewater contaminating our local freshwater bodies putting public health in jeopardy and eliminating opportunities for recreation. Conversely, if stormwater/groundwater enters the sanitary sewer collection system, it can be overwhelmed leading to sanitary sewer overflows and increased costs for treatment at the wastewater treatment facility.

What is Inflow and Infiltration?

Water sources that flow into a dedicated sanitary sewer system from groundwater and stormwater is called inflow and infiltration (I/I).  Sanitary sewers are strictly designed to transport wastewater from sanitary plumbing fixtures, such as toilets, sinks, bathtubs and showers.


Inflow occurs from surface water sources that flow directly into the sanitary sewer via a defined route from cross-connections with storm drains, downspouts, and through holes in manhole covers. Inflow occurs as a result of storm events such as rainfall, snowfall, or snow melt that contribute to excessive sewer flows. Peak inflow can occur during heavy storm events.


Infiltration is groundwater that inadvertently enters into the sanitary sewer system through holes, breaks, joint failures, connection failures and other openings. Infiltration quantities often exhibit seasonal variation in response to groundwater levels. Storm events can trigger a rise in groundwater levels and increase infiltration flows.

Take a look at the drawing in this post. The brown house on the left shows many common sources of I/I. The blue house on the right shows proper connections to the sanitary sewer and stormwater collection systems that eliminates these sources of I/I.

City of Bonney Lake Sanitary Sewer staff will be visiting neighborhoods throughout the City to inspect for many of these issues. If you suspect that you may have one of these types of connections at your residence, please contact the City of Bonney Lake Public Works-Operations at 253-862-8602 and ask to speak to Todd Bright, Assistant Superintendent of Public Works – Sewer.

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