Key Takeaways

• Four methods dominate I&I investigation: flow monitoring, smoke testing, CCTV sewer inspection, and high-density sensor networks.
• No single method does everything, each answers a different question about where and how groundwater and stormwater enter your system.
• Flow monitoring quantifies volume at the basin level but can’t localize sources; smoke testing finds surface inflow but misses infiltration.
• CCTV shows defect condition but can’t measure flow; sensor networks pinpoint both inflow and infiltration and quantify each source.
• The most cost-effective programs sequence these methods rather than choosing just one.

Inflow and infiltration detection methods include flow monitoring, smoke testing, CCTV inspection, and sensor networks; each finds different defects, so programs combine them. Every municipal collection system leaks. The question is never whether you have inflow and infiltration, it’s how much, where it’s entering, and which defects are worth fixing first. Inflow and infiltration detection methods exist to answer those questions, and choosing the right combination determines whether your investigation dollars produce an actionable repair list or just a stack of reports. This guide compares the four approaches engineers rely on most.

How to Detect Inflow & Infiltration: 4 Methods Compared

Detecting I&I means separating two problems that behave differently. Inflow is stormwater that rushes in through openings, roof downspouts, yard drains, cross-connected catch basins, during rain events. Infiltration is groundwater that seeps in slowly through cracked pipe, failed joints, and deteriorated manholes. A good detection method has to catch the abrupt spike of inflow, the slow rise of infiltration, or ideally both. The four methods below each capture a different piece of that picture.

I&I Detection Methods at a Glance

Method	What It Detects	Speed	Relative Cost	Best for
Flow Monitoring	Volume of Flows	3-6+ Months	High	Hydraulic modeling
Smoke Testing	Surface Inflow Connections	Days	Low	Locating Inflow Sources
CCTV Inspection	Structural Pipe Defects	Years	High	Rehab Design
Sensor Networks	Inflow + Infiltration, by pipe segment or basin	1-2 Rain Events	Moderate	Pinpointing Sources Fast

Flow Monitoring for I&I Detection

How Flow Monitoring Works

Flow monitoring places meters inside manholes at strategic downstream points. Each meter uses an ultrasonic or pressure sensor to record the depth and velocity of flow over weeks or months, capturing both dry-weather baseline and wet-weather response. Engineers then correlate flow spikes against rainfall records to estimate how much excess flow enters each monitored basin.

What Flow Monitoring Detects

This method excels at quantifying the overall volume of flow and ranking which basins contribute the most. It answers a single, important question: how much water is passing through this location?

Limitations of Flow Monitoring

The greatest limitation of flow monitoring is cost; the devices are tremendously expensive, limiting the number of sensors that can be used. Because of the cost limitations, few meters have to cover many miles of pipe; resolution is coarse.  Flow monitoring can tell you that somewhere upstream of the sensor (an area that can cover many miles of pipe, an I&I problem exists.You still need a second method to localize sources. Because of the cost, studies are usually short—often just two to three months, meaning you might not have enough rain during your study.

When to Use Flow Monitoring

Flow Monitoring’s best use case is for hydraulic modeling, not I&I studies. But it can be a first step for a municipality that has never studied its I&I and needs to understand the scope before committing to targeted work.

Smoke Testing for Sewer Inflow Detection

How Smoke Testing Works

A blower forces non-toxic, odorless smoke through the sewer main and its laterals. Technicians walk the street watching for smoke escaping from the ground, vent stacks, downspouts, yard drains, or defective manhole covers. Each emergence point is documented as a potential inflow source.

What Smoke Testing Detects

A smoke test for sewer inflow is the most direct way to find surface connections: illegal downspouts, cross-connected storm drains, broken cleanout caps, and leaking manhole lids. A crew can cover thousands of feet a day, and the results are immediate and intuitive.

Limitations of Smoke Testing

Smoke testing is a surface method. It cannot find infiltration (groundwater entering through subsurface cracks and joints) because there’s no path for smoke to reach those defects. It needs dry conditions, can’t quantify how much each source contributes, and tends to alarm residents, so advance notification is essential.

When to Use Smoke Testing

Use smoke testing where you know that your problem is inflow, not infiltration.You only use smoke testing as a follow on to other steps, it’s a second or third step in your investigation. The more you can target your testing, the better your results will be.

CCTV Sewer Inspection

How CCTV Inspection Works

A remotely operated camera travels the length of a pipe, transmitting live video while an operator codes defects to a standard like NASSCO PACP. Cracks, joint separation, root intrusion, corrosion, and active leaks are logged with precise location and severity.

What CCTV Inspection Detects

CCTV sewer inspection produces the most detailed structural condition data of any method. It’s the only approach that gives direct visual evidence of individual defects, which makes it essential for designing repairs.

Limitations of CCTV Inspection

It’s slow and expensive at scale. A crew covers 2,000 to 4,000 feet a day, and pipe usually has to be cleaned first. For a system with hundreds of miles of sewer, a blanket CCTV program runs for years and costs millions. Critically, the camera shows a defect but can’t confirm whether that defect actually contributes any I&I flow—not all defects produce flow. For CCTV to accurately find sources of I&I that source needs to be active..

When to Use CCTV Inspection

Reserve it for segments already identified as I&I contributors, so the inspection characterizes known problems instead of searching the whole system blind.  While you probably don’t want to have a camera in the pipe during a heavy rain storm, CCTV can be very effective if you know that infiltration flows will be high for days or weeks after a storm. This window allows you to see active infiltration without the danger of blocking a pipe.

High-Density Sensor Networks for I&I

How Sensor Networks Work

This approach deploys sensors at close intervals, about every  800 feet, creating a monitoring mesh across entire basins rather than sampling a few downstream points. RH Borden’s radar-based level sensors are installed in manholes without confined space entry and without disrupting operations. When rain falls, the network captures the system’s response everywhere at once, and analytics tied to local weather data reveal exactly where flow enters and whether the signature reads as inflow or infiltration.

What Sensor Networks Detect

Sensor networks deliver both macro-level quantification and micro-level localization: which basins contribute most, and which specific segments and manholes are the entry points. They catch both inflow and infiltration and quantify each. In RH Borden’s Great Neck Village deployment, 423 sensors identified 74 specific I&I locations across 70 miles of sewer in four months.

Limitations of Sensor Networks

Sensors tell you where I&I enters, not the physical nature of the defect. A follow-up CCTV or visual inspection at the flagged location is still the best way to characterize the defect for repair, but because the data directs that camera to confirmed problem spots, the inspection scope shrinks dramatically.

When to Use Sensor Networks

Use them when you need actionable, location-specific results faster and cheaper than the old tools (CCTV, smoke testing, and flow monitoring) especially under regulatory pressure or tight timelines.

The Best I&I Detection Strategy: Combining Methods

No single method does everything, so the strongest programs sequence them. Start with sensor-based monitoring to quantify I&I system-wide and pinpoint contributing segments. Perform a visual inspection (i.e. walk around and look for drains) or deploy smoke testing in basins where inflow dominates to catalog surface connections. Send CCTV to the specific segments the data flagged to characterize defects and design repairs. Don’t forget to inspect manholes: they account for 30-40% of your system’s surface area and are often sources of I&I.  Finally, monitor again after rehabilitation to verify the flow reduction you paid for. This replaces “inspect everything” with a targeted, data-driven process that directs every dollar to the highest-impact locations.

Choosing the Right I&I Detection Method for Your Municipality

The right method depends on where you are. Never studied I&I? Start with a sensor or flow study. Know you have a problem but can’t pinpoint it? Sensor networks were built for exactly that gap. Already identified specific streets or sub-basins? Move to smoke testing and CCTV. Finished repairs? Verify with post-rehab monitoring. RH Borden & Company LLC plays a critical role in helping municipalities transition toward smarter, more efficient wastewater management systems. By combining sensor technology, AI analytics, and digital modeling, the company enables cities to reduce costs, improve infrastructure reliability, and make data-backed maintenance decisions. For rapid sewer evaluation and routine cleaning savings, municipalities can use Acoustic Assessments (SL-RAT technology). For pinpointing groundwater intrusion, BASINiQ I&I Location Services provide high-density, sensor-based detection with high accuracy. Manhole Virtual Models (MVM) use AI-powered digital twins to guide rehabilitation, the GIS Dashboard centralizes system data and visualization, and Condition-Based Maintenance programs help cities shift from reactive to proactive operations. We at RH Borden are committed to delivering innovative wastewater solutions that support long-term infrastructure resilience and smarter city development across the United States. To explore the right solution for your municipality or to work with our team, please reach out through our official contact channels. Contact us now!

Frequently Asked Questions:

What’s the difference between inflow and infiltration?

Inflow is stormwater that enters through direct connections like downspouts and cross-connected drains, producing abrupt spikes during rain. Infiltration is groundwater seeping in through cracks and failed joints, producing slow, sustained increases.

Which I&I detection method is most accurate?

For pinpointing exact entry points, high-density sensor networks are the most precise because they localize and quantify both inflow and infiltration. If inflow is the problem, a visual inspection or smoke testing is accurate. If infiltration is the problem, CCTV is most accurate for characterizing the physical defect once a location is known.

How much does an I&I study cost?

It varies with system size and method. Sensor networks carry moderate cost, smoke testing is relatively inexpensive, and blanket CCTV flow metering is the most expensive at scale; using data to focus efforts lowers total cost.

How long does it take to find I&I sources in our system?

Traditional flow monitoring needs multiple rain events, so three to six months or more. A sensor network can produce actionable results after one to two rain events, Great Neck Village reached 74 located sources in about four months.

Do we have to dig up streets or disrupt service to install sensors?

No. RH Borden’s radar-based level sensors install directly in manholes without confined space entry and without disrupting sewer operations, so there’s no excavation and no service interruption.