Inflow and infiltration (I&I) means clean groundwater or stormwater sneaking into your sewer system through cracks, bad joints, or illegal connections, and it drives up treatment costs while raising the odds of overflows and backups. The fix starts with finding the entry points. If you have noticed backups, a soggy yard, or a municipal notice about excess flow, the next move is a sewer scope or flow check, not a guess.
TL;DR:
- Excessive I&I is typically indicated by wet-weather flows exceeding 275 gallons per capita per day after removing inflow, signaling a need for detailed evaluation.
- Identifying inflow and infiltration sources requires a combination of flow monitoring during storms, CCTV inspections, and smoke testing to pinpoint structural defects and illegal connections.
- Repairs range from simple fixes like disconnecting downspouts and sealing manholes to trenchless rehab methods such as pipe lining, which effectively reduce leaks without excavation.
- Full sewer system evaluation surveys focus on large, cost-effective leaks first, with detailed defect logs and video documentation guiding prioritized repairs.
- Homeowners and property managers should inspect connections and schedule sewer scopes when backups or green yard patches occur, while municipalities need to monitor storm flows to prevent overflows.
Table of Contents
- Definitions and the difference between infiltration and inflow
- Why I&I matters for treatment, capacity, and your property
- How professionals detect and measure I&I
- How to assess severity and calculate excessive I&I
- Repair and reduction options from quick fixes to trenchless rehab
- What a sewer inspection reveals and how contractors prioritize repairs
- What to do next based on your role
- How Trenchless Maine can help with inspection and repair
- FAQ
- Sources
Definitions and the difference between infiltration and inflow
Infiltration is groundwater that seeps into your sewer line through cracked pipes, deteriorated joints, or failing manhole structures. It is slow, constant, and tied to how saturated the soil is around your pipes. Inflow is different: it is surface water or stormwater that enters through a direct connection, like a downspout tied into a sanitary line, a sump pump discharging into a cleanout, or a yard drain plumbed where it should not be.
Both add clear water to a system built to carry sewage, and both strain capacity. There is also a related, less talked about problem called exfiltration, which is the reverse: sewage leaking out of a damaged pipe into the surrounding soil and, potentially, groundwater. Recognizing which one you are dealing with matters because the fixes are not the same. Sealing a cracked pipe solves infiltration. Disconnecting a downspout solves inflow.
Why I&I matters for treatment, capacity, and your property
Every gallon of groundwater or stormwater that enters a sewer line has to be conveyed and treated just like sewage, and that extra clear water adds real cost to pumping and treatment operations. During wet weather, the volume can spike fast enough to overwhelm pipes and pump stations that were sized for dry-weather sewage flow alone.
That overload is a leading contributor to sanitary sewer overflows, which release untreated wastewater into streets, basements, or waterways and carry real public-health and regulatory consequences. For homeowners, the same mechanism often shows up as a basement backup when the system downstream cannot keep up. For municipal operators, chronic I&I problems trigger compliance obligations under CMOM (Capacity, Management, Operation, and Maintenance) programs and NPDES permits, which require utilities to show they are actively identifying and correcting excess flow rather than letting it slide.
How professionals detect and measure I&I
Finding I&I is a process of narrowing down, starting broad and ending with pinpoint repairs. Here is the typical sequence professionals follow:
- Flow monitoring tracks volume over time at key points in the system, and the wet-weather peaks matter far more than the annual average, since excessive I&I is primarily a peak-flow problem that can hide behind an unremarkable yearly number.
- CCTV sewer camera inspection sends a video camera through the pipe to document cracks, offset joints, root intrusion, and lateral connections, producing the defect log that drives repair decisions.
- Smoke testing pushes non-toxic smoke through the system and watches where it escapes aboveground, which is excellent for finding inflow connections like illegal downspout ties but does little to reveal infiltration hidden under saturated soil.
- Air and hydrostatic testing pressurizes an isolated pipe section to detect leaks that are not visible on camera, useful for confirming a joint or seal failure.
- Dye and tracer tests, along with acoustic leak detection, help confirm specific connections or pinpoint a leak location when camera and smoke results are not conclusive.
This full approach, often called a Sewer System Evaluation Survey (SSES), moves from flow data screening to smoke and CCTV work, then to targeted tests on the specific sections that look suspect. Pairing smoke testing with CCTV is standard practice, since smoke finds inflow while the camera finds the structural defects behind infiltration.
Pro Tip: Ask for wet-weather flow data, not just an annual average. A quiet yearly number can hide a serious storm-driven spike.
How to assess severity and calculate excessive I&I
The EPA uses two numeric guidelines to judge whether I&I has crossed from normal to excessive. A dry-weather flow of 120 gallons per capita per day (gpcd) or less is considered a non-excessive infiltration indicator, while a wet-weather flow of 275 gpcd or less, after excessive inflow has been removed, is treated as the non-excessive benchmark for inflow. Gpcd simply means gallons per person per day, calculated from metered flow divided by the population served.
The practical workflow mirrors the thresholds: establish a dry-weather baseline flow, monitor flow during storm events to capture the wet-weather peak, and compare both numbers against the guidelines. When flows run well above either threshold, that is the trigger to commission a full SSES. From there, the goal is not eliminating every leak. It is targeting the largest, most cost-effective sources first, since that is where rehabilitation dollars go furthest.
Repair and reduction options from quick fixes to trenchless rehab
Not every I&I source needs major construction. Some of the most effective fixes are simple:
- Sealing manhole frames and covers to stop surface water from pouring straight into the system.
- Repairing or replacing damaged cleanout caps so storm runoff cannot enter through an open fitting.
- Disconnecting roof leaders and sump pumps that were illegally tied into the sanitary line and rerouting them to proper drainage.
- Correcting yard grading so water flows away from cleanouts instead of pooling around them.
For structural problems like cracked pipe or failed joints, trenchless rehabilitation is usually the more practical route than open excavation. Cured-in-place pipe (CIPP) lining seals the full length of a damaged section from the inside, closing cracks and joint gaps that would otherwise keep admitting groundwater, while a point repair addresses a single isolated defect without lining the whole run. Both reduce infiltration with far less digging than a traditional dig-and-replace job.
Full replacement still has a place when a pipe has collapsed, shifted out of alignment, or deteriorated beyond what lining can reinforce. For most leaking but structurally intact pipe, targeted rehab delivers the same result for less disruption and cost.
What a sewer inspection reveals and how contractors prioritize repairs
A proper sewer scope inspection delivers recorded video of the pipe’s interior, a written defect log, and a set of recommended repairs ranked by urgency. Contractors generally work the way EPA guidance frames rehabilitation economics: tackle the biggest, most cost-effective leaks first rather than treating every minor seep as equal priority.
Before authorizing any work, ask for the video footage itself, a written scope of repairs, clear warranty terms, and whether a trenchless, minimal-excavation option is available for your situation. A contractor who can document findings on camera and explain why a fix was chosen, rather than just quoting a number, is giving you what you need to make an informed decision.
What to do next based on your role
If you are a homeowner, check your downspouts and cleanouts for improper connections, and schedule a sewer scope if you have seen backups or a suspiciously green strip of yard. Property managers should audit laterals across a portfolio and require video evidence before signing off on any repair. Municipal professionals should run flow monitoring during storm events, launch an SSES where flows exceed EPA thresholds, and fund rehabilitation where it is most cost-effective.
— John
How Trenchless Maine can help with inspection and repair
If you need answers about what is happening in your line, not just a guess, Trenchless Maine offers sewer scope inspection that documents your pipe on video, identifies defects, and gives you a written plan instead of a vague estimate.
Depending on what the camera finds, our team can follow up with hydro jetting to clear roots and buildup, CIPP trenchless lining to seal cracks and joints without excavation, or underground pipe locating to map laterals before any work begins.
- Sewer scope inspection with recorded video and a defect log.
- Hydro jetting and drain clearing to prepare a line for lining or repair.
- CIPP trenchless lining, often completed without digging up your yard.
Jobs are typically scheduled promptly, methods are less invasive than full excavation, and completed lining work is backed by a warranty. Request an inspection through Trenchless Maine to find out what is actually going on underground.
FAQ
What is infiltration in simple terms?
Infiltration is groundwater that leaks into a sewer pipe through cracks, bad joints, or damaged manholes rather than water that was ever meant to be in the line. It tends to increase during wet seasons when the soil around the pipe stays saturated.
What does I&I stand for in engineering?
I&I stands for inflow and infiltration, the two main ways unwanted clear water enters a sanitary sewer system. Infiltration is groundwater seeping through defects, while inflow is stormwater or surface water entering through a direct connection like a downspout or sump pump.
How do you calculate infiltration?
Infiltration is typically assessed by measuring dry-weather flow in gallons per capita per day (gpcd) and comparing it against EPA’s guideline of 120 gpcd as the indicator of non-excessive infiltration. Flows consistently above that threshold usually trigger a closer look through an SSES.
What are examples of infiltration?
Common examples include groundwater seeping through a cracked sewer main, a separated pipe joint, or a deteriorated manhole structure where the surrounding soil stays wet. Root intrusion that cracks a pipe wall is another frequent cause of ongoing infiltration.
Sources
Public sewer mains fail from age, root intrusion, shifting soil, and joint separation, and those defects show up as cracks, offset joints, or collapsed sections during a camera inspection. Manholes are another common entry point when covers are not watertight or the structure has settled and opened a gap at the frame.
On the private side, homeowners and property managers should check foundation plumbing:
- Infiltration/Inflow I/I Analysis and Project Certification (EPA NEPIS)
- NPDES Compliance Inspection Manual – Chapter 13 (EPA)
Seasonal groundwater rise and heavy storm events make all of this worse. A pipe that seeps a trickle in August can take on a flood of water during spring thaw or a hard rain, which is why flow data collected only in dry months tells an incomplete story.