I&I Analysis: Finding and Fixing Your Worst Sections

A practical guide to prioritizing infiltration and inflow repairs for maximum ROI.

If you’re managing a wastewater collection system, you already know the drain that infiltration and inflow (I&I) puts on your budget. During wet weather events, clean groundwater and stormwater flood into your sanitary sewers, overwhelming treatment plants and driving up operational costs. But here’s the challenge: with limited budgets and miles of pipe to maintain, how do you identify which sections to fix first?

At Modern Engineering Solutions, we’ve helped utilities across Colorado and neighboring states tackle this exact problem. Through systematic I&I analysis and strategic prioritization, our clients have reduced peak flows by 30-50% while maximizing their infrastructure investment returns. Here’s our proven approach to finding and fixing your worst I&I sections.

Understanding the True Cost of I&I

Before diving into detection methods, let’s quantify why I&I matters. Every gallon of clean water entering your sanitary system costs money to:

Transport through your collection system

Treat at your wastewater facility

Potentially bypass during peak events (risking regulatory violations)

Maintain infrastructure under increased hydraulic stress

For a typical mid-sized utility, excessive I&I can waste $500,000 to $2 million annually in unnecessary treatment costs alone. Add in the accelerated wear on pumps, increased energy consumption, and potential consent decree penalties, and the true cost becomes staggering.

Step 1: System-Wide Flow Monitoring

The foundation of effective I&I analysis is comprehensive flow monitoring. You can’t manage what you don’t measure, and random spot checks won’t reveal the full picture.

Strategic Monitor Placement

The most effective monitoring programs combine engineering analysis with local operational knowledge. Your system operators and public works staff are invaluable resources—they’ve walked the lines, responded to backups, and noticed patterns that don’t always show up in GIS databases. Before finalizing monitor locations, we always sit down with the local team to understand where they’ve observed chronic issues, seasonal problems, or unusual flow patterns.

We typically recommend installing flow monitors at:

Major trunk line intersections

Sub-basin discharge points

Upstream and downstream of suspected problem areas—particularly those flagged by operations staff based on their field experience

Key pump stations

Treatment plant influent (for baseline comparison)

Strategic points identified by local operators who’ve noticed recurring maintenance issues, odor complaints, or capacity concerns

This collaborative approach ensures your monitoring program captures both the obvious engineering checkpoints and the subtle problem areas that only come to light through years of hands-on system operation. Often, the data confirms what your operators have been telling you all along—but now you have the documentation to support capital improvement decisions and regulatory discussions.

Duration Matters

Short-term monitoring (2-4 weeks) might miss critical wet weather events. We advocate for minimum 60-90 day monitoring periods that capture multiple rain events of varying intensity. This data reveals not just peak flows, but the system’s response patterns to different precipitation levels.

Step 2: Identifying High-Priority Areas

Once you have flow data, the next step is isolation. Here’s our systematic approach:

Flow Isolation Method

Compare dry weather flows to wet weather peaks across each monitored basin. Calculate the I&I rate using:

I&I Rate = (Peak Wet Weather Flow – Average Dry Weather Flow) / Rainfall Volume

Basins with I&I rates exceeding 15% typically warrant immediate investigation. We’ve seen rates as high as 60% in deteriorated systems—essentially treating more stormwater than sewage.

Smoke Testing for Quick Wins

For rapid identification of inflow sources, smoke testing remains one of the most cost-effective tools. Common culprits we find include:

Illegally connected roof drains and area drains

Broken cleanout caps

Abandoned building laterals

Cracked manhole covers and frames

CCTV Inspection for Infiltration

While smoke testing catches inflow, CCTV inspection reveals infiltration sources:

Joint separations

Cracked pipes

Root intrusion points

Deteriorated service connections

Pro tip: Schedule CCTV work during high groundwater seasons. We’ve seen “dry” pipes that show massive infiltration when the water table rises.

Step 3: Prioritization Matrix

Not all I&I is created equal. Our prioritization matrix considers:

Volume Impact: Gallons per day of I&I removed

Cost per Gallon: Repair cost divided by daily I&I reduction

Risk Factors: Proximity to critical facilities, SSO history, regulatory status

Constructability: Access issues, traffic impacts, utility conflicts

The 80/20 Rule in Action

In our experience, 80% of I&I typically comes from 20% of the system. A recent study we completed for the Town of Oak Creek perfectly demonstrates this principle. Over a 48-day monitoring period, we analyzed five sub-basins and found that wastewater volumes significantly exceeded potable water usage across all areas, with total excess flow of 21,714,584 gallons—representing 89% of the total flow reaching the treatment plant.

The key finding: Areas 2 and 3 alone accounted for more than 60% of the total I&I volume, with Area 2 contributing 8,233,873 gallons of excess flow and Area 3 adding 5,500,263 gallons. This means that just two out of five monitored areas (40% of the system) were responsible for over 60% of the infiltration and inflow problem. By focusing rehabilitation efforts on these critical areas first, the Town can achieve maximum impact with targeted investment, addressing the majority of their I&I issues while working on less than half of their collection system.

Step 4: Cost-Effective Rehabilitation Strategies

Once you’ve identified priority sections, choose rehabilitation methods based on pipe condition, location, and budget:

Trenchless Solutions

For most I&I rehabilitation, trenchless methods offer the best value:

CIPP Lining: Ideal for structural issues and moderate infiltration

Pipe Bursting: When increased capacity is also needed

Chemical Grouting: Cost-effective for joint sealing

Manhole Rehabilitation: Often overlooked but critical for I&I reduction

Quick Fix Opportunities

Some of the highest ROI repairs are surprisingly simple:

Replace damaged manhole covers and frames

Install inflow dishes in manholes

Disconnect illegal connections

Repair service lateral connections

Step 5: Measuring Success

Post-rehabilitation flow monitoring is essential to verify results and justify continued investment. Key metrics include:

Peak flow reduction percentage

Annual treatment cost savings

Overflow event reduction

Capacity recovered for growth

Document everything. These success stories become powerful tools for securing future funding and SRF applications.

The Path Forward

Effective I&I reduction isn’t about fixing everything—it’s about fixing the right things. By following this systematic approach, utilities can maximize limited budgets while achieving measurable results.

Remember: every gallon of I&I you eliminate is capacity recovered for growth and economic development. In water-scarce regions, it’s also treated effluent available for reuse instead of being wasted on groundwater dilution.

Next Steps

Ready to tackle your system’s I&I challenges? Here’s where to start:

Assess your current flow monitoring coverage

Develop a monitoring plan targeting suspected high-I&I areas

Budget for comprehensive assessment (typically 1-2% of system replacement value)

Create a multi-year rehabilitation plan based on ROI

Track and document results for continued funding support

Modern Engineering Solutions specializes in I&I studies, flow monitoring programs, and rehabilitation design for water and wastewater utilities. Our team of licensed engineers has helped municipalities across the region reduce treatment costs, eliminate SSOs, and maximize infrastructure investments. Contact us to discuss how we can help optimize your collection system performance.

For more insights on water and wastewater engineering, listen to our podcast “Engineers for Communities” where we dive deep into infrastructure challenges and solutions with industry leaders.