Key Takeaways

• I&I can account for 30 to 50 percent of total flow reaching a treatment plant, more during storms.
• Inflow enters through surface connections like downspouts and sump pumps; infiltration seeps through underground cracks and joints.
• Untreated I&I raises treatment costs, forces capacity upgrades, increases regulatory risk, and can cause basement backups.
• Detection ranges from flow monitoring and smoke testing to sensor networks that pinpoint sources within a single storm.
• A structured program built on baseline data, prioritization, and verification turns I&I into a manageable line item.

Inflow and infiltration (I&I) is the unintended entry of stormwater or groundwater into sanitary sewer systems, increasing treatment costs, reducing capacity, and accelerating infrastructure deterioration. 

Every municipality depends on a reliable wastewater collection system to protect public health, preserve environmental quality, and support community growth. While many utilities focus on maintaining aging sewer infrastructure, one of the most persistent and expensive challenges often goes unnoticed: inflow and infiltration (I&I). Unlike visible pipe failures or emergency sewer overflows, I&I gradually introduces large volumes of clean water into sanitary sewer systems, consuming valuable capacity and increasing operating expenses without providing any benefit.

As sewer networks age, joints loosen, pipes crack, manholes deteriorate, and unauthorized connections become more common. During rainfall or periods of elevated groundwater, these defects allow water to enter the collection system, forcing wastewater treatment facilities to process significantly higher flows than they were designed to handle. Over time, municipalities may experience recurring sanitary sewer overflows, basement backups, rising energy costs, regulatory concerns, and expensive infrastructure upgrades that could have been delayed through earlier intervention.

Understanding what is inflow & infiltration (I&I) is the first step toward managing these risks. By identifying where excess water enters the sewer system and implementing data-driven maintenance strategies, municipalities can make smarter investment decisions, improve system reliability, and maximize the lifespan of existing infrastructure.

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.

What Is the Difference Between Inflow and Infiltration?

Although municipalities frequently discuss inflow and infiltration together, understanding the distinction is essential when selecting the appropriate corrective actions.

The primary difference lies in both the source and the pathway. Inflow enters from the surface through direct openings or improper connections and typically responds immediately to rainfall. Infiltration, on the other hand, originates below ground as groundwater enters through defects in buried sewer infrastructure over a longer period.

Another important difference is how each issue is resolved. Reducing inflow often involves removing illegal connections, repairing damaged manhole covers, or improving stormwater drainage. Addressing infiltration generally requires structural rehabilitation such as joint sealing, pipe lining, grouting, manhole restoration, or pipe replacement.

Because both problems often exist simultaneously, municipalities benefit from comprehensive assessment programs that quantify each source independently before prioritizing rehabilitation investments.

Why Is Inflow & Infiltration a Problem for Municipal Wastewater Systems?

I&I is easy to dismiss as an unavoidable cost of an aging system. In practice, it compounds across nearly every part of utility operations.

Increased Wastewater Treatment Costs

Every gallon of clear water entering the system gets conveyed and treated alongside actual sewage, diluting influent and wasting chemical and energy capacity. At $3 to $5 per thousand gallons, one million gallons a day of unnecessary I&I can mean $1 million to $1.8 million a year in avoidable expenses.

Capacity Constraints and Expensive Infrastructure Upgrades

When I&I eats into pipe and pump capacity, the traditional response is building bigger: larger pipes, lift stations, treatment capacity, at a cost of tens or hundreds of millions. Grand Rapids, Michigan, cut a projected billion-dollar program to an estimated $30 to $50 million using a targeted sensor network instead.

Regulatory Compliance and Environmental Risks

Once capacity is exceeded, the result is often a sanitary sewer overflow, an unauthorized discharge of untreated sewage. SSOs violate the Clean Water Act and can trigger consent decrees and fines, on top of harm to local waterways.

Basement Backups and Loss of Public Trust

When the system surcharges, sewage can back up into basements. Beyond property damage and health hazards, these events erode public confidence quickly and put direct political pressure on utility staff and elected officials.

How Do Municipalities Detect Inflow & Infiltration?

Utilities have relied on a handful of core methods for decades. Each has a role, and each has real limits.

Flow Monitoring

Meters in manholes track depth and velocity, letting engineers compare dry and wet-weather flow to estimate storm-driven volume. It is the standard starting point, but with typically 10 to 30 meters across a study area, it flags sub-basins rather than specific pipes, and studies often take three to six months.

Smoke Testing

Non-toxic smoke is blown through a manhole, and technicians watch for where it escapes: downspouts, broken cleanouts, cracked lids. It is inexpensive and can be effective for inflow but cannot detect infiltration and needs dry weather.

CCTV Sewer Inspections

A robotic camera travels through the pipe to document cracks, offset joints, and root intrusion. CCTV is the gold standard for condition assessment, but it is slow, requires cleaning first, and shows potential entry points rather than quantified sources.

Dye Testing

Colored dye is introduced at a suspected inflow source, and crews watch for it in the sanitary sewer. It confirms a connection with certainty but tests one location at a time.

Traditional vs. Modern I&I Detection Methods

Where Traditional Methods Fall Short

Flow monitoring, smoke testing, CCTV, and dye testing all work, but stitched together they can take years, and none alone quantifies volume and pinpoints location.

How High-Density Sensor Networks Changed the Approach

Sensor networks installed every 300 to 800 feet monitor an entire basin at once. A sharp spike during a storm points to inflow at that manhole; a slow, sustained rise afterward points to infiltration upstream. Correlated with weather data and system geometry, this isolates sources within a few hundred feet. RH Borden’s BASINiQ platform deployed 423 sensors across 70 miles of collection system in the Village of Great Neck, New York, and identified 74 I&I locations in four months, work that would otherwise take five to ten years.

How Can Municipalities Reduce Inflow & Infiltration

Reducing I&I means matching the repair to the confirmed defect: manhole sealing and lining for chimney and wall infiltration, cured-in-place lining for cracked segments, point repairs for localized breaks, lateral rehabilitation for defective private connections, and disconnecting illegal sump pumps for inflow. Trenchless methods are generally preferred since they limit surface disruption and cost less.

How to Build an Effective Inflow & Infiltration Management Program

Step 1: Quantify the Baseline

Start with system-wide flow monitoring or a sensor-based assessment to see how much I&I exists and where, including peak wet-weather flow versus dry-weather flow and top-contributing basins.

Step 2: Prioritize High-Impact Areas

Direct resources toward basins closest to hydraulic capacity, with the highest treatment cost impact, or a history of SSOs.

Step 3: Investigate and Confirm Sources

Deploy CCTV, smoke testing, and dye testing in areas flagged by monitoring data, rather than searching the whole system on rotation.

Step 4: Repair and Rehabilitate

Match repair methods to confirm defects, favoring trenchless approaches where they fit.

Step 5: Verify Results and Continue Monitoring

Conduct follow-up monitoring after rehabilitation to confirm flow reduction and demonstrate ROI. Many utilities now favor continuous monitoring over periodic studies for this reason.

Take a Proactive Approach to Managing Inflow & Infiltration

Inflow and infiltration will not resolve itself, and every year it goes unaddressed adds to treatment costs, capacity strain, and regulatory exposure. RH Borden & Company helps municipalities move toward smarter, more efficient wastewater management by combining sensor technology, AI analytics, and digital modeling, helping cities cut costs, improve reliability, and make data-backed maintenance decisions.

Depending on your needs, RH Borden offers several solutions. For rapid sewer evaluation and lower cleaning costs, use Acoustic Assessments powered by SL-RAT technology. To pinpoint groundwater intrusion, BASINiQ I&I Location Services provide high-density, sensor-based detection. To understand structural conditions, Manhole Virtual Models use AI-powered digital twins to guide rehabilitation. Cities can centralize data with the GIS Dashboard, and shift from reactive repairs to Condition-Based Maintenance when ready.

RH Borden is committed to wastewater solutions that support long-term infrastructure resilience across the United States. To explore the right fit for your municipality, reach out through our contact channels. 

Frequently Asked Questions

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.