How To Reduce Inflow And Infiltration: A Municipal Guide To Sewer I&I Abatement

Reducing sewer inflow and infiltration (I&I) requires a systematic approach of continuous flow monitoring, diagnostic smoke and dye testing, and targeted trenchless remediation such as Cured-in-Place Pipe (CIPP) lining and manhole rehabilitation. By executing these targeted structural repairs under NASSCO guidelines, municipalities can lower peak wet-weather flows by up to 40% to 70%, reclaiming treatment plant capacity and avoiding regulatory fines.

Municipal Sewer System Diagnostic and Equipment Planning

To successfully reduce inflow and infiltration across a municipal collection system, utility managers must coordinate a structured diagnostic phase before deploying excavation or trenchless crews. This preparation requires identifying high-risk basins, securing specialized diagnostics machinery, and training field personnel in standardized pipeline assessment metrics.

Developing a baseline of dry-weather flow versus wet-weather flow allows engineers to isolate basins where clear water enters the sanitary sewer system. This initial phase prevents wasteful expenditures on uncompromised sewer mains and directs capital improvement budgets to the most severely degraded sub-basins.

Required Gear, Standards, and Program Benchmarks

• Diagnostic & Inspection Equipment: Multi-sensor CCTV inspection crawlers, high-volume smoke blowers with non-toxic zinc-free smoke agents, liquid dye packages (fluorescein yellow-green or rhodamine red), acoustic leak detectors, and portable area-velocity flow meters.
• Safety & Regulatory Standards: OSHA 29 CFR 1910.146 Confined Space Entry permits, gas detectors (LEL, H2S, O2, CO), traffic control setups complying with the Manual on Uniform Traffic Control Devices (MUTCD), and NASSCO PACP/MACP (Pipeline/Manhole Assessment Certification Program) standards.
• Rehabilitation Materials: ASTM F1216 compliant felt or fiberglass CIPP liners, hydrophilic seals, polyurethane chemical grouting resins, and epoxy-based manhole structural liners.
• Estimated Program Budget & Timeline: Diagnostic and flow monitoring programs typically span 3 to 6 months to capture seasonal groundwater fluctuations and storm events, with costs ranging from $2.00 to $5.00 per linear foot. Physical remediation timelines vary widely based on system scale, but trenchless lining programs average $65 to $150 per linear foot depending on pipe diameter and depth.

Systematic Phase-by-Phase I&I Assessment and Remediation Protocol

Step 1: Basin-Level Flow Monitoring and Data Isolation

To locate the primary sources of I&I, you must partition your sewer network into discrete sheds using area-velocity flow meters installed at critical gravity interceptor nodes.

1. Install temporary flow meters within key manholes for a minimum of 60 days, ensuring the monitoring window captures at least two significant rainfall events (minimum 0.5 inches of precipitation).
2. Calculate the Average Dry Weather Flow (ADWF) during low-groundwater periods, typically between 12:00 AM and 6:00 AM, to establish a diurnal baseline.
3. Measure the Peak Wet Weather Flow (PWWF) during and immediately after rain events.
4. Apply the "I&I Ratio" formula: Divide PWWF by ADWF. Any sub-basin exhibiting a ratio greater than 3.0 indicates severe inflow, rapid infiltration, or both, and must be prioritized for micro-level diagnostic isolation.

Step 2: Micro-Level Diagnostic Testing

Once problematic sub-basins are isolated, deploy targeted field diagnostics to identify the exact structural defects or illicit connections introducing clear water into the sanitary lines.

1. Smoke Testing: Force non-toxic, highly visible smoke through sewer segments using a specialized blower positioned over a manhole. Seal off adjacent upstream and downstream lines using inflatable pneumatic plugs. Document all surface discharge points. Smoke escaping from roof gutters, driveway drains, or yard areas indicates direct path "inflow" sources. Smoke emerging from the grass or pavement over the main indicates shallow pipe "infiltration" from broken joints or lateral connections.
2. Dyed Water Flooding: To confirm suspected cross-connections with storm sewers or to locate indirect leaks near creek crossings, plug the suspect storm sewer segment and flood it with water containing highly visible biodegradable fluorescent dye. Simultaneously monitor the downstream sanitary sewer manhole using portable CCTV cameras or visual observation. If dyed water appears in the sanitary line within minutes, a direct physical connection or structural void exists.
3. Closed-Circuit Television (CCTV) Inspections: Clean the sewer mains using high-pressure hydro-jetting equipment, then deploy a self-propelled, pan-and-tilt CCTV crawler. Document structural conditions, lateral connections, and soil-pipe interface points using NASSCO's PACP coding. Look specifically for dripping, running, or gushing water at joints, structural cracks, radial fractures, and root intrusions.

Warning: Never conduct smoke testing without notifying local fire departments, emergency dispatch centers, and adjacent residential properties at least 48 to 72 hours in advance. Lingering smoke can trigger false alarms and cause public panic if residents mistake diagnostic smoke for building fires.

Step 3: Targeted Manhole Rehabilitation and Chimney Sealing

Manholes represent up to 30% to 50% of a system's total I&I, primarily due to leaking frame chimneys, unsealed corbels, and cracked brickwork situated within the fluctuating groundwater table zone.

1. Chimney Seal Installation: Excavate the shallow pavement surrounding the manhole frame if necessary, or work internally to apply a flexible, cured-on-site polyurethane or rubber chimney seal. This seal must span the joint between the cast iron frame and the masonry chimney to absorb traffic-induced deflection while blocking water.
2. Cementitious and Epoxy Coating: High-pressure wash the manhole interior at a minimum of 4,000 PSI to remove all loose mortar, grease, and biological growth. Apply an active water-stop hydraulic cement to plug running leaks, then spray-apply a minimum 1/8-inch (125 mil) thick layer of structural epoxy or calcium aluminate cementitious mortar.
3. Inflow Dish Installation: Install high-density polyethylene (HDPE) or stainless-steel inflow dishes directly beneath the manhole cover grates. These dishes allow venting while catching surface water runoff that pours through the cover's pick-holes and vent openings.

Step 4: Trenchless Pipe Rehabilitation (CIPP and Slip-lining)

To resolve mainline pipe infiltration without disruptive open-cut excavation, deploy trenchless rehabilitation technologies to restore structural integrity and seal joint failures.

1. Cured-in-Place Pipe (CIPP) Lining: Select a needle-felt or fiberglass liner impregnated with a thermosetting epoxy, vinyl ester, or polyester resin. Pull or invert the liner through the prepared sewer main from an upstream manhole to a downstream manhole using hydrostatic pressure or compressed air.
2. Curing: Circulate steam or hot water through the inverted tube, or insert an ultraviolet (UV) light train to cure the resin according to the manufacturer's exact temperature and pressure specifications (typically adhering to ASTM F1216 or ASTM F2019 standards).
3. Lateral Reinstatement: Deploy a robotic cutter inside the cured pipe to precisely cut out and reinstate active service lateral connections.
4. Hydrophilic Connection Seals: Install hydrophilic gaskets at the reinstated lateral junctions. These gaskets expand up to 10 times their dry volume when exposed to water, creating an impenetrable seal between the new mainline CIPP liner and the existing lateral pipe.

Pro-Tip: Ensure the CIPP installer maintains the required curing pressure until the resin has fully cross-linked and cooled down below 100 degrees Fahrenheit (38 degrees Celsius). Premature depressurization can cause the liner to pull away from the host pipe, creating an annular space that allows groundwater to migrate and infiltrate downstream manholes.

Step 5: Post-Rehabilitation Flow Verification

To prove the return on investment and verify the physical reduction of I&I, you must conduct post-project flow analysis.

1. Re-install the area-velocity flow meters in the exact locations used during the baseline assessment phase.
2. Monitor flows over a seasonal cycle that experiences similar rainfall intensity and groundwater levels as the pre-rehabilitation monitoring phase.
3. Compare the pre- and post-rehabilitation hydrographs. Calculate the reduction in peak wet-weather volumes and verify that the base infiltration rate during dry hours has trended down toward zero.

Wastewater Rehabilitation Materials and Performance Standards

The selection of trenchless rehabilitation technologies must align with the specific structural state of the host pipe, geological conditions, and expected hydrostatic pressure. Use the following comparative matrix to match system needs with industry standard specifications:

Diagnostic Exceptions and Field Construction Failures

Active Groundwater Hydro-static Pressure Blowing Out Wet Grout

• Root Cause: When field crews attempt to apply cementitious mortar or standard epoxy coatings inside a leaking manhole, high external groundwater pressure forces its way through the masonry joints, washing away the repair material before it has time to cure or set.
• Actionable Fix: Prior to applying any structural lining, drill injection ports directly through the brickwork or concrete at the points of active infiltration. Pump a rapid-reacting hydrophilic polyurethane chemical grout behind the manhole wall. This grout reacts instantly with the incoming groundwater, expanding into a dense, waterproof foam that seals the exterior soil void and stops the water flow. Once the active leaks are stopped, apply the final structural cementitious or epoxy liner over the dry substrate.

CIPP Liner Shrivelling or Developing Lifts and Wrinkles

• Root Cause: Inadequate internal pressure during the resin curing cycle, or installing a liner that was improperly measured for the host pipe's internal diameter, causes the liner to pull away or buckle under the weight of external groundwater.
• Actionable Fix: Use high-resolution CCTV to assess the severity of the lift. If the wrinkle restricts flow by more than 5% of the pipe diameter or threatens structural stability, deploy an internal robotic cutter equipped with a diamond-grit milling head to grind down the wrinkle. If the structural integrity of the liner is compromised, install a stainless-steel mechanical internal repair sleeve over the milled section, or perform a localized spot-repair lining using a high-performance vinyl ester resin patch.

False Negatives During System-Wide Smoke Testing

• Root Cause: Conducting smoke testing when the regional groundwater table is high, or when the surrounding soil is highly saturated from recent rain, blocks the smoke from escaping through underground pipe cracks and rising to the surface.
• Actionable Fix: Suspend smoke testing programs during wet-weather seasons or when local groundwater tables rise above the crown of the sewer main. Limit smoke testing to dry summer or early autumn months when soil moisture is low, allowing smoke to rise freely through dry soil pores to identify hidden structural breaks and illicit connections.

Frequently Asked Questions

What is the difference between inflow and infiltration?

Inflow refers to stormwater that enters the sanitary sewer system directly through deliberate or accidental surface connections, such as roof downspouts, foundation drains, street manhole covers, and storm sewer cross-connections. Infiltration occurs when groundwater seeps into the sewer system through structural failures, including cracked pipes, deteriorated pipe joints, damaged lateral connections, and compromised manhole walls.

How do you identify the exact source of groundwater infiltration?

Groundwater infiltration is identified using Closed-Circuit Television (CCTV) sewer inspection cameras, which allow operators to visually spot dripping joints, mineral deposits, and root intrusions. Other effective diagnostic methods include dye testing, where dyed water is introduced into nearby storm drains or wet soils to see if it migrates into the sanitary line, and acoustic leak detection, which senses the sound frequencies of water entering pressurized or gravity pipes.

Can chemical grouting permanently stop sewer infiltration?

Chemical grouting is a highly effective, semi-permanent solution designed to seal pipe joints and manhole cracks for 10 to 20 years. Because chemical grouts remain flexible, they accommodate soil movement and thermal expansion better than rigid cementitious patches, but they do not add structural strength to a collapsing pipe. For a permanent, structural fix exceeding 50 years, chemical grouting should be combined with structural CIPP lining.

Why does reducing I&I lower municipal wastewater treatment costs?

When clear rainwater and groundwater enter the sanitary sewer system, they consume valuable hydraulic capacity within the collection pipes and at the wastewater treatment plant. This forces municipalities to pay to pump and treat clean water, which accelerates wear on mechanical equipment, drives up chemical consumption, increases electricity costs, and can trigger costly, illegal wet-weather bypasses or sanitary sewer overflows (SSOs).

Optimize Your Municipal Infrastructure and Lower Treatment Costs

Addressing sewer system failures requires experienced technicians, advanced diagnostic equipment, and proven trenchless rehabilitation methods. Contact our municipal engineering team today to schedule an automated flow monitoring assessment and design a comprehensive, cost-effective I&I abatement program tailored to your city's budget.