Understanding Inflow & Infiltration – Keys to Successful Sanitary Sewer Evaluation Surveys (SSES)

Cities and municipalities rely on sanitary sewer systems to quietly do an important job: move wastewater where it needs to go without backups, overflows, or unexpected surprises. One of the biggest threats to that reliability is something most people never see—inflow and infiltration (I&I).

I&I happens when stormwater or groundwater enters the sewer system where it doesn’t belong. A little is inevitable. A lot becomes expensive, disruptive, and can strain a community’s wastewater infrastructure. That’s where Sanitary Sewer Evaluation Surveys (SSES) come in. These investigations help cities pinpoint the problems, prioritize repairs, and plan improvements.

Below is a straightforward look at I&I and why SSES is such a valuable tool for managing it.

What Are Inflow & Infiltration?

Inflow and infiltration are terms used to describe the ways in which groundwater and stormwater enter the sanitary sewer system.

Inflow

Is defined as stormwater that enters the sewer system quickly and directly—usually during rainfall or snowmelt. Common sources include:

• Sump pumps or downspouts incorrectly tied into the sewer
• Yard or area drains
• Leaky or vented manhole covers
• Open cleanouts or broken service caps

Inflow creates sharp, sudden spikes in flow during wet weather.

Infiltration

On the other hand is groundwater that seeps into the system slowly and continuously. It usually enters through:

• Cracked or aging pipe
• Bad or offset joints
• Defective service connections
• Deteriorated manhole walls or chimneys

Infiltration is more dependent on groundwater levels and can persist long after a storm has passed

Why Are I&I a Concern?

Even though this extra water isn’t wastewater, once it enters the system it needs to be converyed, pumped, and treated like it is. That leads to several issues:

Capacity and Treatment Overloads

Excess water can overload pipes, pump stations, and treatment facilities, increasing the risk of backups and overflows.

Increased Operational Costs

Handling the extra volume of water from I&I requires more energy and resources at treatment plants, leading to higher operational costs for municipalities.

Environmental and Public Health Rispects

Overflows or surcharging can lead to untreated wastewater reaching local waterways or causing basement backups.

Regulatory Compliance Issues

Communities with chronic I&I problems may face DNR or EPA compliance issues, including fines or required corrective action.

The Role of Sanitary Sewer Evaluation Surveys

Sanitary Sewer Evaluation Surveys (SSES) is a structured investigation used to pinpoint where I&I is getting in. It combines field observations, data, and engineering analysis to help communities make informed decisions.

Components of an SSES:

Sewer Inspection

Cameras are run through sewer pipes to document structural defects, service conditions, and potentila infilitration points.

Flow Monitoring

Temporary flow meters track how different sewer basins respond to rainfall. Comparing dry-weather and wet-weather flows highlights where I&I is occurring.

Smoke and Dye Testing

These tools help identify improper connections or leaks. Smoke testing shows where air escapes; dye testing traces water movement.

Manhole Inspection

Examining manhole, above and below the flowline, helps identify cras, leaks, corrosion, and other structural issues.

Best Practices for Conducting an SSES

A successful SSES requires careful planning and execution. Here are some best practices:

Prioritize Areas Key Area

Focus on basins with known issues, older infrastructure, or wet-weather flow spoikes. Data should drive the investigation.

Utilize the Right Tools

Not every area needs every test. Matching the technology to the problem produces better, more efficient results.

Engage with the Community

Keeping the public informed during smoke testing or field work reduces confusion and helps identify sources like improper sump pump connections.

Plan for Long-Term Results

The real value of an SSES is in what comes next: prioritizing repairs, planning rehabilitation, and tracking improvements over time.

Common Challenges in I&I Management

Funding Limitations

I&I reduction is a long-term effort. Phasing work and pursuing grants or low-interest loans can help communities manage costs.

Large Data Sets

SSES results generate a lot of information. Using good data management tools keeps things organized and actionable.

Aging Infrastructure

Many communities have pipes well past their design life. SSES helps determine what needs immediate attention and what can be planned for in future capital improvements.

Conclusion: Be Proactive Before Problems Grow.

Inflow and infiltration can’t be eliminated completely, but it can be managed. A proactive approach—guided by an effective SSES—helps communities:

• Protect their existing infrastructure
• Reduce treatment and operation costs
• Improve system performance
• Avoid regulatory complications
• Plan confidently for the future

Good information leads to good decisions. And SSES is one of the best ways for cities to understand what’s happening underground and invest wisely in their sewer systems.