I&I Analysis: Finding and Fixing Your Worst Sections

An I&I analysis finds your worst sections by measuring, not guessing. Meter each sewershed through a wet season, normalize the clear water so basins compare fairly, send field crews only to the basins that rank worst, and fix what removes the most flow for the money. Then meter again to prove it worked.

What does I&I really cost your utility?

Capacity, pumping, treatment and maintenance, plus compliance risk when it causes overflows. Past a point, removing clear water is cheaper than carrying it.

EPA's construction grants definitions in 40 CFR 35.2005 put that point in writing: excessive I&I is the amount that can be economically eliminated, judged by comparing the cost to correct it with the cost to transport and treat it. EPA New England's Guide for Estimating Infiltration and Inflow (June 2014, archived by EPA) adds that I&I causing overflows is considered excessive whatever the cost math says.

Where should flow monitors go?

Where each sewershed discharges to a trunk, upstream of known overflows, and on every plant influent line. Not in series, and not right next to a lift station.

Start with what your operators know: EPA's guide for evaluating CMOM programs (2005) has a capacity evaluation locate the overflows, surcharged lines and basement backups they already answer. EPA's report Computer Tools for Sanitary Sewer System Capacity Analysis and Planning (EPA/600/R-07/111, 2007) adds the siting rules:

• Sewershed discharge points into trunk sewers are the primary meter locations.
• Meter upstream of known overflows.
• Avoid sites close to pump stations. The analysis needs flow data with minimal fluctuation.
• Avoid meters in series. Meter error can reach plus or minus 20 percent, enough to swamp the flow from the area between them.

Rainfall data is just as critical. The report cites a study finding that relying only on the nearest rain gauge can give inappropriate results.

How long should you monitor flow?

Long enough to catch several storms of different sizes in your wet season. Usually months, not weeks.

The CMOM guide says temporary meters typically stay in 30 to 90 days and move on a roving basis to find high wet weather flow. For a full analysis, the computer tools report calls four to six months under normal precipitation generally adequate. Monitor when groundwater and soil moisture are high, and start just before the wet season to capture dry weather base flow. If the storms do not come, extend.

How do you rank sewersheds by I&I?

Normalize first. Raw gallons always point at the biggest basin.

• Percent of rainfall captured (R-value): I&I volume / (rainfall depth x sewered area). The computer tools report calls it easy to calculate and a guide to the relative number and size of defects. It does not predict peak flow.
• Unit rates: gallons per acre per inch of rain, or per inch-diameter-mile of pipe.

Get the area right. R moves inversely with the area you use, so parks, golf courses and septic areas left inside the boundary make a basin look better than it is.

Then split inflow from infiltration. EPA's Review of Sewer Design Criteria and RDII Prediction Methods (EPA/600/R-08/010, 2008) explains that a high fast-response R1 means mostly inflow, while more of the total in the slower R2 and R3 means mostly infiltration. That picks your field tests, and whether point repairs or wider rehabilitation fit.

Which field tests find the defects?

Smoke and dye for inflow, CCTV and manhole inspection for infiltration, only in the basins that ranked worst.

• Smoke testing quickly and cheaply finds downspouts, yard drains and faulty service connections. CMOM says to notify the public and test in no rain or snow, little wind and daylight.
• Dyed water testing confirms connections and, flooded around a suspect manhole, shows leaks at the frame, chimney, cone and walls.
• CCTV shows the inside of the pipe and leaves a permanent record.
• Manhole inspection catches covers in runoff paths and cracked barrels.

Time the CCTV for high groundwater, the period the New England guide uses to analyze infiltration. A joint that looks dry at low groundwater may run when the water table rises.

How do you decide which repairs come first?

Overflows and backups first. Then rank by clear water removed for the money, against what it costs to keep pumping and treating it.

• Risk: overflow and backup history, critical facilities nearby, permit pressure.
• Volume: gallons per day the repair should remove.
• Cost-effectiveness: repair cost per gallon per day removed, set against the cost to transport and treat it, as the federal definition does.
• Constructability: access, traffic, utility conflicts.

Inflow fixes such as disconnecting a roof leader are often the quick wins. The New England guide suggests putting I&I work in the normal public works budget, an incremental approach that can help defer capacity expansion.

Which rehabilitation methods fit I&I?

It depends on the defect. Sound pipe with leaking joints gets sealed. Cracked or broken pipe gets lined or replaced.

The CMOM guide calls joint sealing in sound pipe non-structural. Structural repairs line or replace pipe, by digging a segment or two (point repairs) or by trenchless methods from existing manholes. It lists pipe bursting and cured-in-place pipe, and says the choice depends on pipe size, type, location, flow and the severity of the I&I.

Do not skip manholes or laterals. CMOM calls service laterals a serious source of I&I, and the 2008 EPA review, citing a 1972 study, says building laterals can contribute as much as 70 to 80 percent of it, so fixing only the street sewers leaves a significant amount in. We wrote more on why Colorado manholes fail and what fixes them.

How do you prove the repairs worked?

Meter the same basins after the work, through a comparable wet season, and compare like storms.

The CMOM checklist asks whether post-rehabilitation flow monitoring is used. The computer tools report notes that antecedent moisture significantly affects how a basin responds to rain, so compare R-values from similar storms on similar ground. Document the results. The New England guide notes that State Revolving Fund applicants are generally required to evaluate I&I. For Texas utilities, our guide to the Texas Water Development Board's Clean Water State Revolving Fund covers that program.

Is your utility ready to start an I&I analysis?

You are ready to plan monitoring if you can answer yes to the first four. Each no is your first task.

1. Do you have at least 12 months of daily plant influent flow?
2. Do you have rainfall records for the same period?
3. Do you have a sewer map with sewershed boundaries?
4. Do you log overflows, backups and surcharging by location?
5. Do your lift stations record flow or pump run times?

Then ask your engineer which basins they will meter, for how long, and how they will prove the result. We cover more planning topics for utilities and municipalities.

A note on limits: this is general education from public sources, current as of October 2026. The 40 CFR Part 35 numbers come from EPA's construction grants program and are screening benchmarks, not permit limits. Your state rules and permit govern.

Frequently asked questions

What is the difference between inflow and infiltration?

Infiltration seeps in from the ground through defective pipes, joints, connections and manholes. Inflow enters from roof leaders, yard and area drains, manhole covers, storm sewer cross connections and similar sources. Both definitions are in 40 CFR 35.2005.

How much I&I is too much?

There is no single number. EPA's construction grants rule, 40 CFR 35.2120, generally calls for further study when plant flow tops 120 gallons per capita per day during high groundwater, or 275 total during storms. The New England guide puts state screening standards at 100 to 150 gallons per person per day. I&I that causes overflows counts as excessive regardless.

Will smoke testing get into houses?

It can, and the CMOM guide notes that smoke inside a building means sewer gas may be getting in too. Notify residents first.

I'm the public works director for a Texas city, and our plant flow jumps after every big storm. We can only afford a handful of temporary meters. Which basins do we meter first?

Start with the overflow, backup and surcharge spots your crews know, and the sewersheds upstream of them. Meter where those sewersheds discharge to a trunk, not in series. Then rove the meters, as the CMOM guide describes, until every basin has a wet weather record you can rank.

I manage a small Colorado sanitation district. We lined our worst mains two years ago, but the plant still spikes when it rains. What did we miss?

Often laterals and manholes. The 2008 EPA review puts laterals at as much as 70 to 80 percent of I&I. Smoke test for direct connections, dye test suspect manholes, and compare post-lining R-values against similar storms before lining. A wetter year can hide real progress.

Related resources

• Optimizing Collection Systems: Advanced Solutions for I&I Challenges in Rural Communities
• Key Take Aways From EP11: How Inflow and Infiltration Impacts Your Collection System and Wastewater Treatment Plant
• Understanding Your Community's Wastewater Collection System
• Capital Improvement Planning for Small Municipalities

Not sure which sewersheds to fix first?

Send us your plant flow, rainfall records and sewer map. We will help you plan the monitoring, rank the basins and turn the field results into a repair list you can fund and build.

Modern Engineering Solutions, McKinney, Texas, with offices in Golden, Colorado and San Jose, California. 525+ projects completed.

Talk to an engineer about your collection system or call (214) 833-6748.