Infiltration and Inflow Through Manholes: Diagnosis, Quantification, and Cost-Effective Solutions

Infiltration and inflow (I/I) through manholes costs wastewater utilities hundreds of millions of dollars annually in unnecessary treatment costs, system capacity constraints, and SSO events. Yet manholes as a source of I/I are consistently underestimated compared to pipe defects — even though a single severely deteriorated manhole can contribute more I/I than hundreds of feet of cracked pipe.

This guide covers how to identify and quantify manhole I/I, prioritize it in your overall I/I reduction program, and select the most cost-effective rehabilitation approaches.

What Is Infiltration? What Is Inflow?

While often grouped together as “I/I,” infiltration and inflow are distinct phenomena with different sources and different management implications:

Infiltration is groundwater that enters the collection system through defects in the structure — cracked or deteriorated pipe, offset or separated joints, or deteriorated manhole walls and joints. Infiltration is chronic and roughly proportional to groundwater elevation above the invert. It’s highest after wet weather as groundwater tables rise, but it occurs year-round wherever groundwater is above sewer elevation.

Inflow is stormwater or surface water that enters the collection system through direct connections or through open defects at or near the surface. Common manhole-related inflow sources include: deteriorated or improperly fitted frame-to-cover seals, unsealed lifting holes, unmortared grade rings, and deteriorated chimney sections (the cone/corbel section between the barrel and the frame).

The distinction matters for prioritization: infiltration correlates with groundwater elevation and requires structural rehabilitation of below-grade defects. Inflow can often be addressed far more cost-effectively by sealing surface connections — chimney seals, cover replacements, and grade ring repairs that cost a fraction of full structural rehabilitation.

The Scale of the Problem

Industry research consistently shows that manholes account for a disproportionate share of collection system I/I. Studies in various systems have found manholes contributing 25–50% of total system I/I despite representing far less than 25% of the collection system’s surface area. This concentration effect means manhole I/I reduction can deliver outsized return on investment compared to pipe rehabilitation spending.

The EPA estimates that excessive I/I costs wastewater utilities approximately $2 per gallon in additional treatment costs — meaning a single manhole contributing 50,000 gallons per day in I/I costs a utility approximately $36,500 per year in treatment costs alone. A $5,000 rehabilitation that eliminates that I/I pays back in under two months.

Identifying Manhole I/I Sources

Visual Inspection

During MACP inspection, inspectors should specifically document:

• Active infiltration (water entering through the barrel, joints, or at pipe connections — note location, estimated flow rate, and whether it’s a seep, drip, or flow)
• Staining from historical infiltration or inflow (calcium carbonate deposits, rust staining from rebar)
• Frame-to-cover fit and seal condition
• Chimney/cone section condition (mortar joints, grade ring condition, any visible gaps or cracks)
• Lifting hole condition (mortared or open?)

Smoke Testing

System-wide smoke testing, while primarily associated with identifying inflow connections to pipe systems, also reveals open or deteriorated manhole covers and frames where smoke escapes to the surface. Smoke visible rising from manholes during a test indicates surface inflow pathways.

Dye Testing

Dye testing during wet weather events can confirm the connection between surface water and collection system flow at specific manholes — useful when you need to document the source of a known I/I problem for a regulatory compliance demonstration.

Flow Metering

Area-based flow monitoring during and after wet weather events identifies collection system segments with high I/I rates. Once problem segments are identified, manhole-by-manhole visual inspection of that segment typically identifies the dominant sources.

Quantifying Manhole I/I

For regulatory compliance, funding applications, and capital planning, you’ll often need to quantify I/I from specific manholes, not just characterize it qualitatively. Methods include:

• Weir measurement — install a temporary weir plate across the invert and measure flow over the weir from above; effective for quantifying infiltration at specific points during dry weather
• Bucket and stopwatch — for lower flow rates, a simple timed fill of a known-volume container provides adequate precision for planning purposes
• Flow meter installation at the downstream manhole — comparing flows at consecutive manholes isolates I/I contribution from the segment between them

Cost-Effective Rehabilitation Solutions

For Surface Inflow (Frame, Cover, Chimney)

The most cost-effective I/I reduction in most systems comes from addressing surface inflow sources at the frame-cover-chimney interface. Options include:

• Chimney seals — flexible rubber boot seals or spray-applied polymer membranes applied to the cone/corbel section and frame base create an effective seal against surface water intrusion. Typically $500–$1,500 per manhole installed, with 20+ year service life when properly applied
• Frame and cover replacement — watertight frame and cover assemblies (sealed with gaskets or continuous weld) eliminate the frame-cover interface as an inflow source; particularly effective in areas with flat or negative grade near the manhole rim
• Grade ring sealing — mortar injection or polymer grout injection into deteriorated grade ring joints seals a common infiltration pathway in the upper section of the manhole

For Below-Grade Infiltration

Barrel infiltration requires direct structural rehabilitation of the defect locations:

• Chemical grout injection — polyurethane or acrylamide grout injected through the manhole wall at active infiltration points seals the defect from the outside, cutting off the water pathway. Effective for discrete infiltration points; not a substitute for comprehensive rehabilitation in severely deteriorated manholes
• Full spray-applied polyurea lining — a seamless polyurea liner applied over the entire manhole interior seals the structure comprehensively. This is the most thorough approach for manholes with multiple infiltration points or diffuse infiltration through deteriorated concrete
• Structural relining — for manholes with significant structural deterioration combined with I/I issues, structural relining (CIPM, FRP insert, or shotcrete rebuild) may be required before or alongside a sealing liner system

Building an I/I Reduction Program

The most effective I/I reduction programs address manholes and pipes together in a systematic, data-driven way. The framework:

1. Flow monitor all segments to identify high-I/I areas
2. Within high-I/I areas, conduct MACP inspections to characterize manhole conditions and quantify I/I sources
3. Implement surface inflow controls first (highest ROI, fastest payback)
4. Address structural I/I in a condition-priority order
5. Track system flow response after each phase to verify I/I reduction and document program performance

For more detail on I/I program design and funding, see our Federal Funding Guide. Join the Manhole Recovery community to connect with engineers and contractors who run I/I programs in systems like yours.