I&I Reduction in Your Sewer Collection System: How to Buy Time Before the Next Project

I&I reduction in a sewer collection system usually gets filed under compliance — something you do because a regulator expects it, not something that changes the shape of your capital plan. That's underselling it.

Every capital improvement number gets read as a growth problem — more homes, more demand, more flow the system needs to handle. Sometimes that's the whole story. A lot of the time, it isn't. Some of that capacity gap isn't new demand at all. It's capacity you already had, and lost.

What's Actually Eating Your Capacity

I&I — inflow and infiltration — is water your system was never designed to carry: stormwater coming in through manholes and cross-connections, groundwater seeping in through cracked pipes and failed joints.

Every wet-weather event adds that flow on top of what your system is already carrying, and it competes for the same limited capacity as everything else moving through your pipes, lift stations, and treatment plant.

If your capital plan is sized around peak wet-weather flow, and a meaningful share of that peak flow is water that shouldn't be in the system in the first place, you're not just planning for growth. You're planning around a problem you could be reducing instead.

How to Reduce I&I — and Why the Order Matters

Most I&I reduction programs start in the right place: flow monitoring to find where the water's getting in, then CCTV inspection and targeted repair — CIPP lining, manhole rehab, sump pump disconnection — to close the worst offenders.

That's necessary work, and there's no shortcut around it. See the full breakdown of what an I&I mitigation program covers if you're building this out from scratch.

What often gets missed is the second lever: how much of your system's design capacity is already tied up in FOG and biofilm buildup before I&I even shows up. A pipe running at reduced cross-section from years of grease accumulation has that much less room to absorb a wet-weather surge, no matter how well-sealed its joints are.

Addressing that buildup — treating the collection system biologically, upstream, so it doesn't accumulate in the first place — doesn't stop groundwater from finding a crack. It does free up headroom elsewhere in the system, so the I&I that does get in has more room to move without pushing the system into overflow.

Combine both, and you're not just closing entry points. You're changing how much of a surge your system can absorb while those entry points get closed over time.

What This Means for the Capital Plan

Here's the part worth bringing into a capital planning conversation directly: if a piece of that projected peak flow is reducible I&I rather than permanent demand growth, the timeline on the project tied to that flow isn't fixed. It's a function of how much of that I&I gets addressed, and how fast.

That's not a reason to shelve a capital project that genuinely needs to happen. It's a reason to ask, before locking in the number, how much of the projected peak is actually yours to reduce — because every gallon of I&I you take out of that peak is a gallon of expansion you didn't have to build for. For a deeper look at the capacity mechanics behind this, our earlier piece on I&I as a capacity problem walks through it in more detail.

What to Ask Before You Finalize the Number

• How much of our projected peak flow is I&I, versus genuine demand growth? Flow monitoring data usually already has this answer, even if it hasn't been pulled into the capital conversation yet.

• What's our current collection system capacity margin, separate from the I&I rehab program — is buildup already eating into it before any storm event hits?

• Does the capital timeline assume today's I&I load, or a reduced one? If it's the former, an active reduction program could be worth more to the budget than it's currently getting credit for.

See What's Actually Driving Your Number

If your next capital project is sized around peak flow, it's worth knowing how much of that peak is reducible before the budget gets locked in. Talk to In-Pipe about a collection system capacity assessment — see what's already recoverable, and what that could mean for the timeline on your next project.