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How we find and fix I/I

Learn how King County finds and identifies sources of I/I and the methods and technologies we use to reduce or fix it.

We use sewer system evaluation surveys, flow monitoring, and flow modeling to examine the conditions of the sewer system, identify areas of high flows during wet weather, and help to identify likely sources of I/I. Once an I/I problem has been identified, there are many methods and technologies available to reduce it. Read about our methods to find and fix I/I below.

Finding I/I

Sewer system evaluation surveys:

  • Closed-circuit television (CCTV) cameras can be robotically sent down sanitary sewer lines and along each side sewer to record sewer conditions. CCTV inspections can identify breaks, root intrusions, leaking water, and deteriorating conditions.
  • Smoke testing involves pumping smoke through sewers from maintenance holes in streets and observing where smoke exits. The exiting smoke can indicate a broken pipe or identify where roof or foundation drains are improperly connected to the sewer system.
  • Dye testing involves pouring nontoxic fluorescent colored dye down roof drains or catch basins to see if that dye makes its way into the sewer. This provides verification that the storm drainage being tested is directly connected to the sewer.

Flow and rainfall monitoring:

  • Flow monitoring establishes a direct correlation between each local sewer agency's wastewater flows and the effect of rainfall on I/I entering the system. The effects are measured and compared downstream.
  • Rainfall monitoring measures and reports rainfall quantities to provide a credible basis for the local agency flow data. 

Flow modeling:

King County and local agencies use flow modeling data from the Wastewater Treatment Division’s Flow Monitoring Program to model existing flows and to project future flows in the wastewater system. For detailed information about the County's flow modeling methods, refer to Chapter 3 of the Executive's Recommended Regional Infiltration/Inflow Control Program.


Fixing I/I

Pipe repair:

Broken pipes, maintenance holes, and joint connections can be repaired in a variety of ways. Trenchless repair methods require less digging than traditional “dig and replace” repair methods and minimize damage to yards and landscaping.

  • Pipe bursting is a technique that pulls a hardened steel breaker head through the old pipe, breaking it up, and replacing it with a new pipe all in one process.
  • “Cured-in-place pipe” repair involves pulling a resin-saturated liner through a damaged pipe, which is then cured with steam or hot water to form a tight-fitting, jointless replacement pipe.

Stormwater disconnections:

  • Stormwater connections such as roof drain downspouts, yard drains, and sump pumps can be disconnected from the sewer system. 
  • Property owners can redirect the downspouts onto lawn and garden beds, hook up a rain barrel or cistern to their downspouts, or redirect the connections to a separate stormwater system. 


Disconnecting a downspout consists of cutting the downspout, capping the drainage line, and redirecting the flow.