Infiltration and Inflow Forensics: How to Find I/I

What Is Infiltration and Inflow, and Why Should You Care?

Infiltration and inflow (I/I) is extraneous water that leaks into your collection system and forces your plant to treat rainwater and groundwater like it's sewage. It's usually lower in pollutant strength than sanitary wastewater, but it's not necessarily clean - it can carry sediment, nutrients, salts, or other contaminants. It's expensive, it blows out your hydraulics, and in bad cases it can wash your bugs right over the clarifier weirs.

Here's the deal. Your total collection system flow comes in three layers: base sanitary flow (the actual sewage from homes and businesses), groundwater infiltration (slow leaks through cracks and bad joints), and rainfall-driven inflow and infiltration, or RDII (the storm surge). When those last two layers get out of hand, you're paying to pump and treat water that never should've hit the plant.

And it's not a small problem. In severe cases the average wet weather flow can run several times the dry weather baseline, and in badly deteriorated systems I/I can make up 50 percent or more of a system's total flow. Some plants spend a large share of their pumping energy and chemical budget just moving and treating this clear water. That's real money going down the drain, literally.

How Does I/I Wreck Your Treatment Process?

The tricky part is that I/I dilutes your influent BOD and TSS concentrations, so on paper your loading looks light. But the massive hydraulic surge is what kills you.

High flow drops your hydraulic retention time (HRT) across the whole plant. In your secondary clarifiers, the real washout indicator is the surface overflow rate (SOR): when the upward velocity of the rising water beats the settling velocity of your solids, you lose containment. Clarifier capacity depends on more than one number - SOR, solids loading rate, sidewater depth, sludge blanket conditions, and the settling characteristics of your mixed liquor all factor in. When the surge pushes SOR past what your solids can settle against, active mixed liquor physically washes over the weirs and out to the receiving stream. Now you've got TSS and BOD violations, and you might be rebuilding your solids inventory for weeks.

Keep in mind that hydraulic dilution lowers the concentration of BOD in your influent, but it does not reduce the daily mass (lbs/day) of sanitary BOD entering the plant. Because F/M is a mass-to-mass calculation (lbs of BOD applied divided by lbs of MLVSS under aeration), clear-water dilution alone does not swing your F/M ratio or starve the biomass. The real danger from I/I is hydraulic washout, not F/M starvation. Any wasting changes should be based on your solids inventory, SRT, effluent quality, and settling data. And in winter, that cold I/I water lowers process temperatures enough to shock and inhibit your nitrifiers right when you need them. If you're watching your settleability numbers climb during wet weather, understanding how to read your sludge volume index helps you catch a washout before it becomes a permit problem.

How Do You Tell Infiltration From Inflow on a Hydrograph?

You read the shape of the flow curve and you watch the clock: inflow shows up as sharp peaks during rain, while infiltration shows up as a sustained rise in base flow that lingers long after the rain stops.

Inflow comes in through direct physical connections: roof downspouts, catch basins, sump pumps, and cross-connected storm sewers. It shows up as sharp, immediate peaks on your hydrograph right when it's raining. Rain starts, flow spikes. Rain stops, flow drops back down pretty quick.

Infiltration seeps in slowly through cracks, offset joints, and deteriorated manholes as the groundwater table rises. It shows up as a sustained elevation in your base flow that hangs around long after the rain quits.

One useful trick for spotting chronic infiltration is the nighttime minimum. In a strictly sanitary system with no industrial shift work, domestic flow reaches its low point overnight, typically between 2:00 AM and 4:00 AM. It won't drop to zero - toilets, appliances, commercial activity, pump cycles, and normal background flow all contribute - so you have to estimate the expected sanitary baseline first. If your flow stays well above that expected minimum during the window night after night, that's a strong indicator of groundwater leaking in around the clock. It's not conclusive proof on its own, though: leaking water services, industrial or commercial discharges, sump pumps, meter problems, and delayed conveyance all have to be ruled out before you pin it on infiltration.

To model the wet weather piece, operators lean on RDII tools like the EPA SSOAP toolbox and SWMM software. These compare your wet-weather hydrographs against your established dry-weather baseline to estimate how much RDII volume and peak flow you're dealing with. Keep in mind the results are estimates, not exact figures - they depend on rainfall data, flow-meter accuracy, baseline selection, antecedent moisture, groundwater conditions, and model calibration.

What Field Tools Find the Actual Leaks?

Reading hydrographs tells you how much I/I you've got and roughly what kind, but field tools find where it's coming from: smoke testing, dye testing, CCTV inspection, sonar and laser profiling, and manhole inspections. Here's the toolkit.

• Smoke testing: Crews force non-toxic smoke into an isolated sewer segment with high-volume blowers. Smoke pouring out of roof gutters, driveway drains, or yard cleanouts is a dead giveaway for illegal stormwater inflow. Smoke rising straight out of the ground points to a defect that communicates with the surface, which is a potential infiltration pathway worth confirming with follow-up inspection.

• Dye testing: When smoke flags a suspect connection, you confirm it with fluorescent dye. Flush bright dye down the suspect drain and watch for it to show up in the sanitary sewer. If it does, you've confirmed a cross-connection.

• CCTV inspection: Robotic camera tractors crawl the pipe and document cracks, intruding roots, sags, and offset joints. CCTV inspection is a standard method for gravity mains across a wide range of diameters, and you run it during late-night low-flow periods so the camera can actually see the pipe wall. Practical diameter limits depend on your camera equipment, access, flow depth, and pipe condition. Standardize what you find using NASSCO's Pipeline Assessment and Certification Program (PACP) coding so everybody's speaking the same language.

• Sonar and laser profiling: When the pipe is surcharged or running deep, the camera's useless below the waterline, so you switch to sonar. Laser profiling measures ovality, deformation, and settlement, mostly in flexible pipe.

• Manhole inspections: Old-school but valuable. High debris lines on the walls tell you about past surcharge events, and you check the bench and channel for structural rot.

For a solid rundown of collection system inspection and I/I analysis, the Massachusetts DEP I/I guidelines are a good reference.

Which Pipe Materials Fail, and How?

Every material leaves its own forensic signature, so knowing what's in the ground tells you what to look for.

• PVC: Resists corrosion just fine but deflects if the trench wasn't compacted or bedded right. Look for ovality on your laser profile.
• Ductile iron: Strong, but corrodes inside and out. Watch coatings and stray current.
• Concrete pipe and manholes: The classic biogenic sulfide corrosion victim. Bacteria turn hydrogen sulfide gas into sulfuric acid that eats away the pipe crown. If you've got septic, sluggish force mains upstream, this is where the acid attack shows up.
• HDPE: Common for force mains, joined by continuous fusion welds. Fusion quality and surge protection are everything here.

That sulfide problem ties right into disinfection and odor chemistry too. If you're managing sulfide-driven demand at the plant, the same logic behind chlorine dose, demand, and residual applies to how much oxidant those side reactions eat up.

How Do You Fix I/I Once You Find It?

The golden rule: fixing I/I at the source almost always beats paying to expand the plant to treat it.

For infiltration, you need structural repairs. Cured-in-place pipe (CIPP) lining installs a seamless resin pipe inside the old one. Chemical grouting seals weeping manholes and joints. Pipe bursting, sliplining, and point repairs handle the rest.

For inflow, enforcement against prohibited private connections is part of the answer - you disconnect the illegal storm sources, the downspouts and sump pumps and yard drains dumping into the sanitary sewer. But it usually takes public construction and maintenance too: sealing manhole covers, repairing structures, separating cross-connections, and providing alternatives for sump-pump or roof-drain discharge.

All of this is typically organized under a Capacity, Management, Operation, and Maintenance (CMOM) framework. CMOM is a collection-system management approach - it structures your condition assessment, maintenance, capacity planning, and long-term progress tracking. It is not a funding program, though; the money comes from utility revenues, loans, grants, or bonds.

I/I forensics isn't glamorous work, but it's some of the highest-value troubleshooting an operator can do. Every gallon of clear water you keep out of the system is a gallon you don't pay to pump, treat, and discharge. Learn to read your hydrographs, know your nighttime minimum, and understand what each inspection tool actually tells you, and you'll build the confidence to handle these questions on exam day and the real thing back at the plant.