Inflow & infiltration and why it matters.

We asked Water Quality Program Administrator Shannon Sypolt to tell us a little about Inflow and Infiltration (I&I).

I&I occurs when groundwater (infiltration) and stormwater (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 stormwater seeps through manholes or compromised pipes that need repairs or low lying areas near flooded creeks.

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 yet mixes in with wastewater that does need to be treated. Treatment plants operate more efficiently and effectively when they have a consistent flow. The additional flow requires more electricity and chemicals to treat, which ultimately increases the cost. During heavy rain events, the increased flow can be directed to equalization basins for temporary storage. However, this flow will still need to be treated at a later date.

Our field operations, engineering, and wastewater treatment staff work together to combat I&I by checking collection lines for cracked pipes, seals, and other compromised areas that can cause stormwater or groundwater to seep in. This work is done through 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 restoration and replacement projects happening near you.

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