How to find it, measure it, and fix it—before it drains your budget.

If you manage a wastewater collection system, you already know the problem even if you have not yet put a dollar figure on it. During heavy rains, your lift stations run nonstop. Treatment plant flows spike far beyond what the population should produce. Basement backups trigger angry calls from residents, and regulators start asking uncomfortable questions about sanitary sewer overflows.

The cause, in most cases, is inflow and infiltration—commonly abbreviated as I&I. It is one of the most expensive, most widespread, and least visible challenges facing municipal infrastructure in North America. By some estimates, I&I accounts for 30 to 45 percent of the total annual flow arriving at wastewater treatment plants, and in peak wet-weather conditions, that number can climb dramatically higher. King County, Washington, has reported that I&I makes up roughly 75 percent of peak sewer flows during winter months.

This guide is written for public works directors, collection system managers, utility engineers, and municipal decision-makers who need a clear, practical understanding of what I&I is, why it matters, how to detect it, and what modern technology now makes possible. We will walk through the fundamentals, compare detection methods, examine real-world case studies, and provide a framework for building a proactive I&I management program.

What Is Inflow and Infiltration?

Though often grouped together under a single abbreviation, inflow and infiltration are two distinct phenomena with different sources, behaviors, and detection approaches.

Inflow

Inflow is stormwater or surface water that enters the sanitary sewer system through direct connections. It is driven by rainfall events and typically produces rapid, sharp spikes in flow that correspond closely with storm intensity. Common sources include:

• Roof downspouts and rain leaders connected directly to the sewer lateral
• Basement sump pumps illegally piped to the sanitary system
• Foundation drains tied into sewer lines rather than the storm system
• Cross-connections between storm drains and sanitary sewers
• Defective or missing manhole covers that allow surface water to pour in
• Catch basins improperly connected to the sanitary system

The volume from a single inflow source can be staggering. A single residential sump pump operating at full capacity during a storm can discharge more than 7,000 gallons of water into the sewer system—roughly equivalent to the average daily wastewater flow from 18 homes. When dozens or hundreds of these connections exist across a collection system, the cumulative impact overwhelms pipe capacity and downstream treatment.

Infiltration

Infiltration is groundwater that seeps into the sewer system through structural defects below the ground surface. Unlike inflow, infiltration tends to be slower, more persistent, and harder to pinpoint. It is driven primarily by groundwater levels, which rise in response to prolonged rain, snowmelt, and seasonal water table fluctuations. Common entry points include:

• Cracked or broken pipe segments
• Deteriorated or offset pipe joints
• Root intrusion openings
• Degraded manhole walls and chimney seals
• Failed service lateral connections

Infiltration can occur even on dry days when the water table is high enough to exert hydrostatic pressure on the pipe. However, the worst infiltration episodes typically follow significant or extended precipitation, when the surrounding soil becomes fully saturated and groundwater levels peak.

Why the Distinction Matters

The difference between inflow and infiltration has direct implications for how you detect and address the problem. Inflow sources are generally easier to locate—they tend to be at or near the surface—and less expensive to correct. Infiltration is more insidious because it enters through subsurface defects that are invisible from the surface and often distributed along long stretches of pipe. Effective I&I management requires understanding which type dominates in each part of your system so you can deploy the right investigation method and the right repair strategy.

The True Cost of I&I: Why It Cannot Be Ignored

Municipal leaders sometimes view I&I as an unavoidable cost of operating an aging system. The reality is that untreated I&I compounds expenses across every function of the wastewater utility:

Treatment Costs

Every gallon of groundwater or stormwater that enters the sanitary system must be conveyed to the treatment plant and processed alongside actual sewage. This clear water dilutes influent concentrations, reduces treatment efficiency, and consumes chemical, energy, and capacity resources that should be reserved for actual wastewater. For a municipality treating wastewater at $3 to $5 per thousand gallons, an I&I volume of just one million gallons per day represents $1 million to $1.8 million per year in avoidable treatment expense. Larger systems with higher I&I rates can face annual costs exceeding $5 million.

Capacity Constraints and Capital Costs

I&I flows consume pipe and pump station capacity that was designed for sanitary flow. As systems approach hydraulic limits, the conventional response is to build larger pipes, bigger pump stations, and expanded treatment facilities—capital projects costing tens or hundreds of millions of dollars. In many cases, reducing I&I at its source is a fraction of the cost. The city of Grand Rapids, Michigan, famously reduced a $1 billion capital improvement estimate to $30–50 million by deploying a sensor network to identify and address I&I at targeted locations rather than building system-wide capacity.

Regulatory and Environmental Risk

When sewer capacity is exceeded, the result is sanitary sewer overflows (SSOs)—unauthorized discharges of untreated or partially treated sewage into the environment. SSOs violate the Clean Water Act, trigger enforcement action from state and federal regulators, and expose municipalities to consent decrees, fines, and legal liability. Beyond the regulatory risk, SSOs cause real environmental harm: contaminated waterways, impaired ecosystems, and public health hazards. A robust I&I management program is one of the most direct ways to reduce SSO frequency and demonstrate regulatory compliance.

Basement Backups and Public Trust

When the collection system surcharges during wet weather, sewage can back up into basements and lower-level fixtures in homes and businesses. These events are damaging, unsanitary, and deeply eroding of public trust. They generate property damage claims, legal exposure, and intense political pressure on elected officials and utility managers.

How to Detect I&I: Traditional Methods vs. Modern Technology

Historically, municipalities have relied on a toolkit of investigation methods that were developed decades ago. Each has value, but each also has significant limitations when used in isolation. The emergence of sensor-based monitoring technology has created a fundamentally new approach that addresses many of these limitations.

Flow Monitoring

Flow monitoring involves installing meters in manholes to measure the depth and velocity of wastewater flow over time. By comparing dry-weather flow profiles with wet-weather flows, engineers can quantify how much additional volume enters the system during rain events and identify which sub-basins contribute the most I&I. Flow monitoring is the standard first step in most I&I investigations. However, traditional flow monitoring deployments typically use a limited number of meters (often 10 to 30 across a study area), which provides basin-level data but cannot pinpoint specific pipes or manholes. Studies typically require three to six months and multiple qualifying rain events to produce reliable conclusions.

Smoke Testing

Smoke testing introduces non-toxic, visible smoke into the sewer system by placing a blower over a manhole. The smoke travels through the pipes and escapes at any point where there is an opening to the surface—revealing connected downspouts, broken cleanouts, cracked manholes, and cross-connections with storm drains. Smoke testing is relatively inexpensive and effective at finding inflow sources. However, it cannot detect infiltration (subsurface entry points), requires dry conditions, is disruptive to neighborhoods (it triggers alarm from residents who see smoke coming from their property), and must be supplemented by other methods for a complete picture.

CCTV Inspection

Closed-circuit television inspection sends a robotic camera through the interior of sewer pipes to visually identify structural defects: cracks, offset joints, root intrusion, corrosion, and active leaks. CCTV is the gold standard for condition assessment and is essential for planning rehabilitation work. However, CCTV is slow and expensive to deploy across an entire system (typically covering a few thousand feet per day), requires the pipe to be cleaned first, cannot quantify how much I&I a given defect is contributing, and identifies potential entry points rather than confirmed active sources.

Dye Testing

Dye testing involves introducing colored, non-toxic dye into suspected inflow sources (storm drains, yard drains, downspouts) and then observing whether the dye appears in the sanitary sewer. It is a confirmatory tool—effective at proving a specific connection exists—but it is labor-intensive, tests one location at a time, and requires suspected locations to already be identified by another method.

High-Density Sensor Networks: The Modern Approach

The most significant recent advancement in I&I detection is the deployment of high-density sensor networks that monitor entire basins simultaneously. Rather than placing a handful of flow meters at downstream points and inferring where I&I is coming from, this approach installs sensors at closely spaced intervals throughout the system—potentially every 300 to 800 feet—creating a continuous monitoring mesh that can detect flow changes at each manhole as they occur.

When a rain event hits, the sensor network captures the system’s response in real time. Abrupt flow spikes at a specific manhole indicate inflow entering at that location. Slow, gradual increases that persist after the rain stops indicate infiltration in the pipe segment upstream. By correlating the flow data with precise weather data and the known geometry of the collection system, engineers can isolate I&I down to specific pipe segments—often within a few hundred feet of the source.

RH Borden’s BASINiQ platform uses this approach, deploying radar-based level sensors that can monitor up to 200 manholes simultaneously. The system integrates sensor data with weather correlation, GIS mapping, and  to produce a complete, data-driven picture of where I&I is entering the system. In the largest deployment to date, BASINiQ used 423 sensors across 70 miles of collection system for the Village of Great Neck, New York, and identified 74 specific I&I locations in just four months—work that would have taken 5 to 10 years using traditional methods.

Building an I&I Management Program: A Practical Framework

Addressing I&I is not a one-time project—it is an ongoing program that should be integrated into your collection system management plan. Here is a practical framework that municipalities of any size can adapt:

Step 1: Quantify the Baseline

Before you can reduce I&I, you need to understand how much exists and where. Start with system-wide flow monitoring or a sensor-based assessment to establish your baseline. Key metrics to capture include: peak wet-weather flow vs. average dry-weather flow, estimated I&I volume as a percentage of total system flow, identification of the top-contributing basins, and seasonal variation patterns.

Step 2: Prioritize by Impact

Not all I&I is created equal. Focus remediation resources on the basins and pipe segments that contribute the most excess flow, are closest to hydraulic capacity, have the highest treatment cost impact, or are located in areas with SSO history or regulatory pressure. A sensor-based assessment like BASINiQ produces exactly this kind of prioritized, location-specific data.

Step 3: Investigate and Confirm

Once priority areas are identified through monitoring, deploy targeted investigation methods—CCTV inspection of the specific pipe segments flagged by sensor data, smoke testing in areas with suspected surface connections, and dye testing to confirm specific cross-connections. The key difference from the traditional approach is that you are now investigating confirmed problem areas rather than searching the entire system on a rotating schedule.

Step 4: Remediate

Match the repair method to the defect. Common rehabilitation approaches include manhole sealing and lining for chimney and wall infiltration, cured-in-place pipe lining (CIPP) for cracked or deteriorated pipe segments, point repairs for localized breaks, service lateral rehabilitation for defective connections, and source disconnection for illegal sump pump and downspout connections. Trenchless methods are preferred wherever possible because they minimize surface disruption, reduce cost, and can be completed faster than traditional open-cut replacement.

Step 5: Verify and Monitor

After rehabilitation work is completed, conduct a follow-up monitoring round to verify that the repairs achieved the expected flow reduction. This step is critical for demonstrating ROI to elected officials and ratepayers, validating compliance with regulatory requirements, and identifying any remaining or new I&I sources. Many municipalities are now moving toward continuous or semi-permanent monitoring systems that provide ongoing visibility into system performance rather than periodic snapshots.

Frequently Asked Questions About I&I

What percentage of I&I is considered acceptable?

There is no universal standard, but the EPA and most state regulatory agencies expect municipalities to maintain I&I at levels that do not cause SSOs or exceed treatment capacity. As a practical benchmark, many engineers consider a system with less than 10 to 15 percent of total flow attributable to I&I to be well-managed. Systems exceeding 30 percent should consider I&I reduction a high priority.

How long does an I&I study take?

Traditional studies using flow monitoring, smoke testing, and CCTV can take one to several years to complete for a moderately sized system. Sensor-based approaches like BASINiQ can produce actionable location data in as few as one to two qualifying rain events, with a typical project timeline of three to six months from deployment to final report.

What is the ROI of addressing I&I?

ROI varies by system, but the math is straightforward: reduce the volume of clear water being treated and you reduce treatment costs proportionally. A municipality treating one million gallons per day of I&I at $4 per thousand gallons is spending approximately $1.46 million per year on unnecessary treatment. Capital cost avoidance (not building larger pipes and plants) often represents an even larger savings. The Village of Great Neck’s BASINiQ study, for example, addressed a system where traditional I&I costs were estimated at $5.4 million annually.

Can I&I sensors detect both inflow and infiltration?

Yes. By analyzing hydrograph signatures correlated with weather data, sensor-based systems distinguish between the rapid flow spikes characteristic of inflow and the slower, sustained increases associated with infiltration. This distinction is critical for targeting the right repair methods at each location.

Does my municipality need to address I&I to comply with the Clean Water Act?

If your system experiences SSOs, your municipality is likely already under regulatory pressure to reduce them. I&I is the most common underlying cause of wet-weather SSOs. Proactively managing I&I is one of the most effective ways to demonstrate compliance, avoid consent decrees, and reduce the risk of enforcement action.

Next Steps: How RH Borden Can Help

RH Borden’s BASINiQ platform is designed specifically for municipalities that need fast, accurate, cost-effective I&I identification. Whether you are managing a small-town system of a few dozen miles or a large suburban collection network, BASINiQ scales to meet your needs. Our team has completed the largest high-density sensor I&I study in North America and serves municipalities across the southern and western United States. To set up a call Contact Us and we will have a local representative be in touch.