Which investigation approach is right for your collection system?
When a municipality decides to investigate inflow and infiltration in its sewer system, one of the first questions is deceptively simple: how should we look for it? The answer has historically depended on budget, system size, and the preferences of the consulting engineer managing the study. But the emergence of sensor-based monitoring has expanded the toolkit considerably, and many collection system managers are now re-evaluating which methods—or combination of methods—deliver the best results for the investment.
This article provides a practical, side-by-side comparison of the four most widely used I&I detection approaches: traditional flow monitoring, smoke testing, CCTV inspection, and high-density sensor networks. For each, we will cover how it works, what it finds, what it misses, and where it fits in a comprehensive I&I program. For a broader overview of I&I fundamentals, see our Complete Guide on Inflow & Infiltration.
1. Traditional Flow Monitoring
How It Works
Flow monitoring places meters inside manholes at strategic downstream points in the collection system. Each meter uses a sensor (typically ultrasonic or pressure-based) to measure the depth and velocity of flow in the pipe. Data is logged continuously over a period of weeks to months, capturing both dry-weather baseline flows and wet-weather responses. Engineers then analyze the data, correlating flow spikes with rainfall records to estimate how much I&I is entering each monitored sub-basin.
What It Finds
Flow monitoring is excellent at quantifying the overall volume of I&I and identifying which major basins contribute the most excess flow. It answers the question: where in the system is the biggest I&I problem at a basin level?
What It Misses
Because traditional deployments use a relatively small number of meters (often 10 to 30 for a city-wide study), the data resolution is limited. Flow monitoring tells you that Basin A has a significant I&I problem, but it cannot tell you which specific pipes or manholes within that basin are the entry points. You still need secondary investigation to localize the sources. Studies also require multiple qualifying rain events, which means timelines of three to six months minimum and often longer in drier climates.
Best Used For
System-wide screening and baseline quantification. Flow monitoring is a strong first step for municipalities that have never studied their I&I and need to understand the overall scope before committing to targeted investigation.
2. Smoke Testing
How It Works
A blower is placed over an open manhole and forces non-toxic, odorless smoke through the sewer line and its connected laterals. Technicians walk the street and adjacent properties looking for smoke emerging from the ground surface, building plumbing vents, roof downspouts, yard drains, catch basins, or any other opening that connects to the sanitary sewer. Each location where smoke appears is documented and classified as a potential inflow source.
What It Finds
Smoke testing is the most direct way to identify surface-level inflow sources: illegal downspout connections, cross-connected storm drains, broken cleanout caps, and defective manhole covers. It is fast (a crew can test thousands of feet of sewer per day), relatively inexpensive, and produces immediately visible, intuitive results.
What It Misses
Smoke testing is fundamentally a surface detection method. It cannot detect infiltration—groundwater entering through cracks, joint failures, and subsurface defects—because there is no pathway for smoke to reach those entry points. It also requires dry conditions (smoke does not travel well through saturated soil or standing water), cannot quantify how much flow each source contributes, and can cause public concern when residents see smoke emerging near their homes. Advance notification and coordination are essential.
Best Used For
Targeted inflow source identification after a broader study (flow monitoring or sensor-based assessment) has identified the problem basins. Smoke testing is a confirmation and cataloging tool, not a comprehensive I&I investigation on its own.
3. CCTV Inspection
How It Works
A remotely operated camera is inserted into the sewer pipe and travels its length, transmitting live video to an operator on the surface. The operator records the footage and documents defects according to standardized coding systems (such as NASSCO PACP). Defects including cracks, joint separation, root intrusion, corrosion, and active leaks are logged with their precise location and severity.
What It Finds
CCTV provides the most detailed structural condition data of any method. It is the only approach that gives you direct visual evidence of individual defects inside the pipe. It is essential for rehabilitation planning because it shows you exactly what needs to be repaired and where.
What It Misses
CCTV is slow and expensive at scale. A typical crew covers 2,000 to 4,000 linear feet per day, and the pipe must usually be cleaned before the camera can be deployed. For a system with hundreds of miles of sewer, a comprehensive CCTV program takes years and costs millions of dollars. Critically, CCTV identifies potential I&I entry points (structural defects) but cannot confirm which defects are actually contributing significant I&I or quantify the volume. A small crack might leak a trivial amount, while a nearly invisible joint failure might be the primary source of infiltration in a segment. Without flow data to pair with the visual evidence, prioritization is based on judgment rather than measurement.
Best Used For
Condition assessment and rehabilitation design in specific pipe segments that have already been identified as I&I contributors through monitoring data. CCTV should follow a monitoring-based investigation, not precede it.
4. High-Density Sensor Networks
How It Works
This approach deploys sensors at closely spaced intervals throughout the collection system—typically every 300 to 800 feet—creating a monitoring mesh that covers entire basins rather than sampling a few downstream points. RH Borden’s uses radar-based level sensors that can be installed in manholes without confined space entry and without disrupting sewer operations. When rain events occur, the network captures the system’s response at every monitored point simultaneously. Flow analytics correlated with localized weather data then identify exactly where I&I is entering: which manhole, which pipe segment, and whether the signature indicates inflow (abrupt spikes) or infiltration (slow, sustained increases).
What It Finds
Sensor networks provide both macro-level quantification (which basins contribute the most I&I) and micro-level localization (which specific pipe segments and manholes are the entry points). They detect both inflow and infiltration, quantify the volume contributed by each source, and can produce actionable data after as few as one to two rain events. In the Great Neck Village deployment, 423 sensors identified 74 specific I&I locations across 70 miles of sewer in four months.
What It Misses
Sensor networks identify where I&I is entering the system but do not show the physical nature of the defect. Once a sensor network flags a manhole or pipe segment, a follow-up CCTV inspection of that specific location is still the best way to characterize the defect and design the repair. However, because the sensor data directs CCTV to confirmed problem locations rather than the entire system, the inspection scope is reduced by an order of magnitude.
Best Used For
System-wide I&I identification that produces actionable, location-specific results faster and at lower cost than the traditional sequence of flow monitoring followed by blanket CCTV. Particularly valuable for municipalities that need to demonstrate progress quickly, face regulatory pressure, or want to avoid multi-year investigation timelines.
The Optimal Approach: Combining Methods Strategically
No single method does everything. The most effective I&I programs combine methods in a logical sequence that maximizes information while minimizing cost and time:
- Start with sensor-based monitoring (or traditional flow monitoring if sensor technology is not available) to identify and quantify I&I system-wide and pinpoint the contributing basins and pipe segments.
- Deploy smoke testing in the basins where inflow is the dominant signal to catalog surface connections for disconnection or repair.
- Send CCTV to the specific pipe segments flagged by the monitoring data to characterize defects and design rehabilitation.
- Conduct post-rehabilitation monitoring to verify that repairs achieved the expected flow reduction and to adjust the priority list for the next round of work.
This approach replaces the old model of systematically inspecting everything with a targeted, data-driven process that directs every dollar of investigation and rehabilitation spending to the locations with the highest proven impact.
Making the Decision for Your Municipality
The right method depends on where you are in your I&I journey:
- Never studied I&I before? Start with a sensor-based assessment or flow monitoring study to establish your baseline and identify priority areas.
- Know you have a problem but cannot pinpoint it? Sensor networks are designed exactly for this situation—they bridge the gap between knowing you have I&I and knowing where it is.
- Already identified problem basins? Move to targeted smoke testing and CCTV in those areas to characterize sources and plan repairs.
- Completed repairs and need to verify? Post-rehabilitation monitoring (sensors or flow meters) validates your investment and identifies remaining work.
RH Borden’s BASINiQ platform can support municipalities at every stage of this process. Contact Us to discuss your Sewer System’s I&I challenges and we can discuss which approach makes the most sense for your situation.