Inflow & infiltration and why it matters.

We asked Water Quality Program Administrator Shannon Sypolt to tell us a little about I&I.


I&I stands for inflow and infiltration. This is when groundwater (infiltration) and storm water (inflow) enter into our closed wastewater infrastructure (sewer pipes).

  • Infiltration occurs when the groundwater table rises and water pushes into cracks, joints, seams and older parts of our infrastructure.
  • Inflow occurs when storm water seeps through manholes, or compromised pipes that need repairs or at low lying areas near creeks that are flooded by beaver dams.

So why is this bad, it’s just water, right?

Technically yes but I&I increases the flow of water to our wastewater treatment plants by millions of gallons. This water doesn’t necessarily need to be treated and yet mixes in with wastewater that does need to be treated. The additional flow requires more electricity and chemicals to treat, thus it increases the cost. In addition treatment plants operate more efficiently and effectively when they have a consistent flow. During high rain events, the increased flow can be directed to equalization basins for temporary storage but this flow will still need to be treated at a later date.

Staff in field operations, engineering, and wastewater treatment work together to combat I & I by checking collection lines for cracked pipes, seals, and other compromised areas that can cause storm water or groundwater to seep in. This can be done via smoke tests, CCTV cameras in the lines and dye testing. Once problems are identified staff performs rehabilitation by relining pipes, fixing manholes, and building relief sewers to accommodate larger flows. Check out our projects page for an updated list of rehabilitation and replacement projects happening near you.

Comments are closed.

Create a website or blog at

Up ↑