Infiltration is groundwater seeping into the sewer through cracked pipes. Inflow is surface water entering through illegal connections. Both end up at your treatment plant — and both are tested on the operator exam.
A wastewater treatment plant is designed to handle a specific flow — the average daily flow from the community it serves, with some capacity built in for peak conditions. I&I breaks that design assumption. When groundwater seeps into deteriorating pipes or surface water pours in through illegal connections, the collection system delivers far more flow to the plant than the sewage alone would generate. The treatment plant has to handle all of it.
During a heavy rainstorm, a system with significant I&I problems can see flows multiply by several times. Clarifiers get hydraulically overwhelmed. Detention times drop. Disinfection contact time shrinks. In severe cases, untreated wastewater bypasses secondary treatment entirely — a sanitary sewer overflow that triggers regulatory reporting and public notification requirements.
Understanding I&I — what it is, where it comes from, how to find it, and what can be done about it — is fundamental knowledge for any collections operator, and it's tested on Class I and Class II certification exams.
The exam will ask you to tell these apart. The distinction is in where the water comes from and how it enters the collection system.
Infiltration = groundwater IN through pipe defects (slow, steady, subsurface).
Inflow = surface water IN through direct connections (fast, storm-driven, above ground).
Together they're called I&I — and in regulatory and engineering documents you'll often see it written as I/I. Both terms refer to the same combined problem. The EPA uses "rainfall derived infiltration and inflow" (RDII) to describe the portion of I&I that is directly attributable to precipitation events, which helps distinguish it from the baseline groundwater infiltration that occurs regardless of weather.
Infiltration enters wherever the pipe or structure has lost its integrity. Sanitary sewers are not designed to handle groundwater — they're designed to be watertight. Over time, that watertightness degrades through a combination of age, ground movement, root intrusion, and corrosion. The most common entry points are:
Inflow sources are typically illegal or improper connections to the sanitary sewer. They bring surface water directly into the system during and immediately after rain events:
Every gallon of I&I that enters the collection system arrives at the treatment plant as wastewater that must be pumped, treated, and discharged. The volume is real even though the water is largely clean. That creates several compounding problems:
The most immediate impact. When storm-driven inflow multiplies the flow entering the plant, the hydraulic loading on the secondary clarifiers can exceed their settling capacity. Surface overflow rate increases, the sludge blanket is pushed upward, and solids wash over the effluent weirs. MLSS drops as solids leave the system. The result is elevated TSS and BOD in the effluent — potentially a permit violation.
Treatment processes depend on adequate contact time — time for biological organisms to consume BOD, time for solids to settle, time for disinfectant to inactivate pathogens. When I&I doubles or triples the flow through the plant, detention time in each unit process is cut in half or worse. Biological treatment efficiency drops. Disinfection contact time may fall below the minimum required for permit compliance.
I&I is largely clean water. When it enters the aeration basin, it dilutes the MLSS. A lower MLSS means less biological treatment capacity per unit volume. The F/M ratio spikes as the food load stays constant but the microorganism mass per gallon drops. This can push the process toward young, poorly settling sludge and elevated effluent BOD.
Before the flow even reaches the plant, it overloads lift stations in the collection system. Wet wells fill faster than pumps can handle, triggering high-water alarms. Force mains carry flow at or above their hydraulic capacity. In severe events, wet wells overflow — a sanitary sewer overflow that requires regulatory reporting.
More flow means more pumping energy, more chemical usage, more sludge production from diluted biological treatment, and more operator hours responding to alarms and process upsets. I&I is essentially a tax on the plant's operating budget — paid every time it rains significantly.
I&I doesn't just increase flow — it dilutes the biological process, reduces detention time, and can cause hydraulic washout of the clarifier. On the exam, if a question describes treatment plant performance degrading during or immediately after a rain event, I&I is the cause to consider first.
Before I&I can be controlled, it has to be found. The four standard detection methods each reveal different aspects of the problem. Operators use them in combination — starting with the least expensive reconnaissance methods to define the scope, then deploying more targeted tools to locate specific defects.
The simplest and least expensive method. In the early morning hours — typically 2:00 AM to 4:00 AM — wastewater flow from residential areas drops to its lowest point. Very little flush activity, very little shower and laundry water. In a healthy system, this early morning flow represents the true baseline domestic wastewater contribution.
Operators survey manholes during this window, looking for clear water flowing at near-ground temperature. Clear, cold water at 2 AM is not domestic sewage — it's groundwater infiltration. The volume and temperature of the flow give a rough indication of infiltration severity. This method works best in smaller systems where individual service areas can be isolated. In large systems, the signal gets diluted.
Flow monitoring at key manholes over time — particularly comparing flows during dry periods to flows immediately following rain events — also reveals the magnitude of rainfall-derived I&I. A flow that doubles or triples during a storm and slowly returns to baseline over the following days is the characteristic RDII signature.
Smoke testing identifies both infiltration entry points and illegal inflow connections. A smoke-generating machine driven by an engine-powered fan forces non-toxic smoke into the collection system through a manhole opening. The smoke travels through the pipes under slight pressure and exits wherever there is a break, an open joint, or a direct connection to the atmosphere.
What smoke testing reveals:
Before smoke testing, operators must notify residents — smoke appearing in basements or at floor drains alarms homeowners who don't know the test is occurring. Dry plumbing traps that haven't been used can also allow smoke into buildings even when there's no defect. Proper notification reduces unnecessary emergency calls.
CCTV inspection is the most precise detection method. A purpose-built camera mounted on a wheeled crawler is inserted into the sewer line and travels the length of the pipe, recording video of the interior. The operator above ground watches the feed in real time and can pause, zoom, and log specific defect locations with GPS coordinates.
CCTV reveals what no other method can — the exact location and severity of individual defects: cracks, broken joints, root intrusion, offset joints, collapsed sections, and active infiltration seeping through the pipe wall. This precision is what makes CCTV the standard tool for pre-rehabilitation assessment and for confirming specific defect locations identified by smoke testing or flow monitoring.
One critical operational requirement: the pipe must be thoroughly cleaned before CCTV inspection. Grease, debris, and settled solids obscure the pipe wall and block the camera's view of defects. A thorough high-velocity jet cleaning immediately before the camera run is standard practice. Skipping this step produces poor-quality video that misses defects and wastes the cost of the inspection.
Treatment plant flow records — particularly chart recording flow measurement data — are one of the most accessible I&I detection tools. Operators don't need to enter the collection system at all. They need data.
Two patterns in the flow record indicate I&I:
Correlating flow records with local rainfall data — comparing flow response to specific storm events over time — allows quantification of the I&I load and identification of the worst-performing service areas within the system.
Once I&I is identified and quantified, the corrective options fall into two categories: rehabilitation of the collection system infrastructure to eliminate infiltration entry points, and elimination of direct inflow connections.
Most major rehabilitation work requires contractors and engineering involvement. Collections crews typically don't have the equipment or expertise to execute large-scale pipe rehabilitation. However, understanding the methods is exam-relevant and important for working with engineers and contractors:
While major rehabilitation typically requires contractors, collections operators can address specific I&I contributors within the capabilities of the crew:
Eliminating direct inflow connections requires a combination of inspection, enforcement, and customer education. Property owners with roof drains or sump pumps connected to the sanitary sewer must be identified and required to disconnect and redirect those connections to the storm system or to daylight. This is often a significant program requiring customer notification, inspection access, and sometimes regulatory enforcement authority.
Raising manhole covers in flood-prone areas to prevent surface water entry is a direct and immediate fix that collections crews can implement without contractor involvement.
You can't treat your way out of an I&I problem. The water is entering the collection system and arriving at the plant regardless of what operators do with it once it gets there. The only real fix is finding the sources and eliminating them — either through pipe rehabilitation or elimination of direct connections. Every gallon of I&I that enters the system is a gallon that costs money to pump and treat.
I&I is tested on both the collections operator exam and the wastewater treatment operator exam. The exam focuses on three areas:
Definitions and distinction. Know the difference between infiltration (groundwater through pipe defects) and inflow (surface water through direct connections). Know that infiltration is slow and steady while inflow is storm-driven and rapid. This is the most basic I&I exam question and appears frequently.
Detection methods. Know all four: late night survey, smoke testing, CCTV inspection, and flow records analysis. Know what each one detects and when it's used. The exam may describe a scenario — "an operator notices that treatment plant flow spikes sharply during rain events and returns to normal within 24 hours" — and ask which type of I&I is indicated (inflow — because it responds immediately to rain and drops off quickly) and which detection method would best identify the sources (smoke testing — because inflow comes from direct surface connections).
Treatment plant impact. Know that I&I causes hydraulic overloading, reduced detention time, diluted MLSS, and potential clarifier washout. Know that the problem cannot be solved by plant operation alone — it requires collection system rehabilitation.
A wastewater treatment plant operator notices that plant influent flow increases dramatically during heavy rainstorms and that effluent TSS and BOD climb above permit limits during these events. The most likely cause is:
Answer: Excessive inflow in the collection system — storm water entering through direct connections causes rapid flow surges that hydraulically overload the treatment plant. The corrective action is a collection system I&I study and elimination of the inflow sources — not a change in plant operation.
Our Fundamentals study guide covers the collection system topics tested on Class I and Class II exams — I&I, lift stations, force mains, pipe materials, and more. 150 exam-level questions with full answer explanations.
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