Inflow and Infiltration Investigation
Sewer systems are designed to transport wastewater to treatment facilities where it can be safely processed and returned to the environment. However, these systems often carry more than just wastewater. When stormwater or groundwater enters the system unnecessarily, it increases the volume of water that must be treated, driving up costs for municipalities, utilities, and ultimately, the communities they serve.
This unwanted water, known as inflow and infiltration (I&I), is a common but often hidden issue within aging or compromised infrastructure. Identifying and addressing I&I begins with a proactive investigation process that allows infrastructure owners to understand exactly how their systems are performing and where problems exist.
What Is Inflow and Infiltration (I&I)?
Inflow and infiltration refer to two different ways that clean water can enter a sanitary sewer system:
- Infiltration occurs when groundwater enters the system through defects such as cracks, joint separations, or deteriorated pipe walls or manholes. This often happens below the surface and may increase gradually following rainfall events.
- Inflow occurs when stormwater enters the system more directly, often through improper connections such as sump pumps or stormwater conveyance systems. Inflow typically results in sharp increases in system flow during rain events.
While these sources differ, both contribute to the same problem: excess clean water that must be transported and treated, placing unnecessary strain on infrastructure systems.
Why I&I Is a Problem for Infrastructure Systems
The presence of inflow and infiltration can significantly impact the performance and cost of a sewer system. Even moderate increases in flow can lead to:
- Higher wastewater treatment costs
- Reduced system capacity during peak events
- Increased risk of sanitary sewer overflows
- Accelerated wear on pipes, pumps, and treatment facilities
Because these issues often develop gradually and occur below ground, they can go unnoticed until they begin to affect system performance or budgets. This makes early identification through investigation critical for long-term system management.
The First Step: Flow Metering
One of the most effective ways to begin an I&I investigation is through flow metering. This process provides measurable data on how a sewer system behaves under both normal and stressed conditions.
During flow metering, sensors are installed within strategically selected manholes or drainage structures throughout the system. These devices continuously measure flow rates and water levels over a defined monitoring period, capturing performance during both dry weather and wet weather events.
Once collected, the data is analyzed to identify trends and irregularities. Distinct flow patterns can help differentiate between inflow and infiltration:
- Sharp spikes in flow during rainfall events often indicate inflow sources
- Gradual increases following rainfall may suggest infiltration through structural defects
By understanding when and how excess water enters the system, infrastructure owners can begin narrowing down the areas most in need of further investigation.
Confirming the Source: Additional Investigation Methods
While flow metering provides valuable system-wide insight, additional investigation methods are often needed to confirm the exact location and cause of I&I. These techniques allow crews to move from data analysis to direct observation.
Wet Weather Inspections
Conducted during or immediately after rainfall events, wet weather inspections involve observing manholes and system access points under active conditions. This allows crews to identify visible inflow sources such as leaking covers, direct surface water entry, or defective structures.
CCTV Inspections
Closed-circuit television (CCTV) inspections provide a detailed view of the interior condition of sewer lines. Using NASSCO-certified methods and advanced robotic cameras, inspectors can identify cracks, joint failures, root intrusion, and other defects that allow groundwater infiltration. These inspections create a visual record that helps prioritize repairs.
Smoke Testing
Smoke testing is used to identify illegal connections and pipe defects. Non-toxic smoke is introduced into the system and observed at ground level. Smoke escaping from unintended locations, such as yards, buildings, or stormwater structures, indicate pathways where inflow may be entering the system.
System-Wide Assessment
A comprehensive I&I investigation often includes multiple assessment methods working together. Manhole inspections, cleaning, and additional monitoring may be incorporated to develop a complete understanding of system conditions.
From Data to Action
The true value of an inflow and infiltration investigation lies in what happens next. By combining flow data with targeted inspections, infrastructure owners can move beyond assumptions and implement solutions with precision.
Rather than applying broad or reactive repairs, this data-driven approach allows rehabilitation efforts to focus on the specific areas contributing to excess flow. This not only improves efficiency, but also supports better budgeting, long-term planning, and system performance.
With accurate information in hand, municipalities and private system owners can prioritize repairs, extend the lifespan of their infrastructure, and reduce unnecessary treatment costs.
USG: The Intersection of People, Safety, Service, and Technology
Inflow and infiltration may be hidden, but their impact is not. Addressing these issues requires a combination of advanced technology, experienced professionals, and a system-wide perspective.
USG provides comprehensive I&I investigation services that help clients identify problems, understand their systems, and take informed action. By integrating flow metering, field inspections, and data analysis, USG delivers solutions that are both targeted and effective.
Infrastructure systems are the backbone of our communities, and protecting them requires proactive, informed decision-making. USG stands at the intersection of people, safety, service, and technology; helping clients reduce costs, improve performance, and plan for the future with confidence.
Contact USG today to learn how our inflow and infiltration investigation services can support your infrastructure goals.