18.2 Inflow & Infiltration (I&I) Assessment & Pipeline Rehabilitation

18.2 Inflow & Infiltration (I&I) Assessment & Pipeline Rehabilitation

[!NOTE] Environmental Compliance & Asset Management: Under the federal Clean Water Act and Arizona Pollutant Discharge Elimination System (AZPDES) rules enforced by ADEQ, public wastewater utilities are legally prohibited from discharging untreated or partially treated sewage into the environment. Uncontrolled extraneous water entering the collection network dilutes sewage, exhausts hydraulic pumping capacity, triggers catastrophic Sanitary Sewer Overflows (SSOs), and disrupts biological processes at municipal Water Reclamation Facilities (WRFs). Systematic I&I reduction and trenchless asset renewal are central to modern collection utility management.

Wastewater collection systems are designed to convey only sanitary wastewater generated by residential domestic fixtures, commercial establishments, and pretreated industrial facilities. However, no underground piping network is completely hermetic. Extraneous surface stormwater and subsurface groundwater continuously enter the piping matrix through two distinct hydraulic phenomena: Inflow and Infiltration (collectively termed I&I). Operators must diagnose these entry pathways using targeted field assessment techniques and deploy trenchless rehabilitation solutions to restore hydraulic and structural integrity.

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Inflow versus Infiltration: Physical Mechanisms & Hydraulic Signatures

Understanding the physical distinction between inflow and infiltration is essential because their entry pathways, hydraulic hydrographs, and mitigation techniques differ fundamentally.

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|                        Inflow vs. Infiltration Comparison Matrix                        |
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| Characteristic        | Inflow                                  | Infiltration          |
+-----------------------------------------------------------------------------------------+
| Water Source          | Direct surface stormwater runoff        | Subsurface groundwater|
|                       | and ponded rainwater                    | and saturated soil    |
| Entry Mechanisms      | Roof downspouts; yard area drains;      | Cracked/fractured pipe|
|                       | submerged manhole pickholes; unsealed   | barrels; open joints; |
|                       | street covers; cross-connected catch    | root intrusion; porous|
|                       | basins; illegal basement sump pumps     | manholes; broken taps |
| Hydrograph Response   | Instantaneous, sharp vertical spike;    | Delayed, slow rise;   |
|                       | correlates directly with rainfall onset | gradual flat plateau  |
| Duration              | Dissipates rapidly within hours after   | Persists for days,    |
|                       | precipitation event ends                | weeks, or months      |
| Seasonal Relationship | Correlates with localized storm events  | Correlates with high  |
|                       | (e.g., Arizona summer monsoons)         | seasonal water tables |
| Primary Detection     | Low-pressure smoke testing; fluorescent | Night flow isolation; |
|                       | dye water testing; visual storm surveys | CCTV crawler cameras  |
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1. Inflow: Surface Stormwater Entry

Inflow refers to extraneous water that enters the collection system directly from surface sources via gravity or pumped connections. Inflow is entirely dependent on precipitation events:

• Direct Sources: Unauthorized residential roof leaders (downspouts) plumbed into sewer laterals, surface area yard drains, driveway channel drains, illegal commercial parking lot catch basins cross-connected to sanitary lines, and sump pumps discharging foundation drainage.
• Manhole Inflow: During intense desert monsoon rainstorms in Arizona, street gutters frequently exceed curb capacities, flooding pavement. Standard cast iron manhole covers feature open pickholes and unsealed lid perimeters. A single submerged manhole cover with two open 1-inch pickholes can admit 20 to 50 gallons per minute (gpm) of storm runoff directly into the sanitary sewer.
• Hydrograph Signature: On a collection system flow hydrograph, inflow appears as an immediate, nearly vertical flow spike occurring within minutes of rainfall onset. As soon as the rain ceases and surface water drains away, the hydrograph exhibits a steep, rapid falling limb returning near baseline.

                      Hydrograph Signatures of I&I

       Flow Rate (MGD)
          ▲
          │             /\ ◄─── INFLOW: Rapid, Sharp Peak
          │            /  \     (Correlates Instantly with Rainstorm)
          │           /    \
          │          /      \──────────────┐
          │         /                      │ ◄─── INFILTRATION: Delayed, Prolonged
          │        /                       │      Plateau (Elevated Water Table)
          │       /                        └──────────┐
          │  ────┴────────────────────────────────────┴───────► Time (Days)
          │  [ Baseline Diurnal Sanitary Flow Curve ]

2. Infiltration: Subsurface Groundwater Seepage

Infiltration refers to extraneous water that enters the collection network from the surrounding ground through defective pipes, fractured fittings, and deteriorated structural joints:

• Direct Sources: Structurally cracked, sheared, or crushed pipe barrels; displaced, offset, or open bell-and-spigot joints where rubber gaskets have rotted or displaced; tree root intrusions that wedge open pipe joints; deteriorated customer service laterals; unsealed cleanout fittings run over by mowers; and porous, un-mortared brick or precast manhole walls below the water table.
• Hydrograph Signature: Unlike inflow, infiltration is driven by hydrostatic pressure from the surrounding groundwater table. Following significant precipitation or regional agricultural irrigation, surface water slowly percolates through the soil vadose zone. The groundwater table gradually rises above the buried sewer elevation, exerting hydrostatic head. Infiltration appears as a slow, delayed rise in baseflow that plateaus and persists for weeks or months until the regional aquifer recedes.

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System Impacts: Overflows, Clarifier Washouts & Regulatory Liability

When a collection system experiences severe I&I, the composite peak flow rate can surge to 5 to 20 times the average dry-weather flow (ADWF), overwhelming infrastructure designed only for sanitary waste.

1. Sanitary Sewer Overflows (SSOs)

When gravity pipes become hydraulically surcharged, the hydraulic grade line (HGL) rises above the pipe crown, transforming open-channel gravity flow into pressurized pipe flow. If the HGL exceeds ground surface elevation, raw wastewater blows off manhole covers and spills out onto public streets, residential properties, and into dry desert washes. SSOs represent a catastrophic public health hazard, exposing the public to waterborne pathogens (Giardia, Cryptosporidium, hepatitis, pathogenic E. coli) and triggering mandatory ADEQ 24-hour spill reporting, environmental remediation, and severe Clean Water Act financial penalties.

2. Lift Station Surcharging & Flooding

Extraneous water inundates lift station wet wells. Pumping equipment operates continuously at maximum capacity without cycling off. If inflow exceeds total pumping capacity, wet wells submerge electrical controls, flood surrounding terrain, and discharge untreated sewage directly to surface waters.

3. Treatment Facility Process Washouts

Water Reclamation Facilities are designed for specific hydraulic detention times (e.g., 2 to 3 hours in primary clarifiers; 6 to 12 hours in activated sludge aeration basins):

• Clarifier Solids Carryover: Surging hydraulic loading drives clarifier surface overflow rates (SOR) far above design limits. Settled biological solids and mixed liquor suspended solids (MLSS) are swept over effluent weirs, blinding tertiary sand filters and causing massive effluent turbidity spikes.
• Biological Process Disruption: Extreme hydraulic dilution flushes active nitrifying and heterotrophic biomass out of aeration basins, decimating the Mean Cell Residence Time (MCRT) and causing long-term ammonia and nitrogen permit violations under AZPDES standards.
• Economic Energy Waste: Utilities expend massive electrical energy pumping millions of gallons of clean groundwater and rainwater, and consume excessive chemical coagulants, polymers, and chlorine disinfectants treating unpolluted water.

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Diagnostic Field Assessment Methodologies

To eliminate I&I, collection system operators deploy four primary diagnostic detection technologies, each tailored to specific physical defects.

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|                        I&I Diagnostic Methodologies Overview                            |
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| Methodology         | Target Defect                           | Primary Advantage       |
+-----------------------------------------------------------------------------------------+
| Night-Time Flow     | Subsurface groundwater infiltration;    | Rapidly screens entire  |
| Isolation           | leaking submerged mains                 | drainage sub-basins     |
| Smoke Testing       | Surface inflow; illegal downspouts,     | Fast, low-cost pinpoint |
|                     | yard drains; broken surface cleanouts   | of direct surface leaks |
| Fluorescent Dye     | Suspected cross-connections to storm    | Verifies direct physical|
| Testing             | drains; flooded ditch/curb seepage      | hydraulic connections   |
| Closed-Circuit TV   | Structural cracks, root masses, broken  | Direct visual/laser     |
| (CCTV) Inspection   | taps, joint offsets, sagging inverts    | rating under PACP codes |
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1. Night-Time Flow Isolation (Baseflow Monitoring)

Domestic water consumption exhibits a predictable diurnal cycle. Between 1:00 AM and 5:00 AM, municipal water usage drops to near zero as community residents sleep. In a clean, tight collection system, flow in residential collection mains should be virtually non-existent during these early morning hours.

• Field Procedure: Crews install temporary flow measurement devices (such as portable V-notch weirs, Palmer-Bowlus flumes, or acoustic area-velocity sensors) in key junction manholes. Operators systematically isolate upstream pipe segments by inserting inflatable pneumatic plugs during night hours. If significant flow (e.g., > 10–20% of average daily flow) is measured between 2:00 AM and 4:00 AM, the segment is flagged for severe groundwater infiltration.

2. Low-Pressure Smoke Testing

Smoke testing is the most cost-effective technique for identifying illegal surface inflow sources and shallow structural defects open to the atmosphere.

• Field Procedure: Crews isolate a 300- to 600-foot sewer reach by sandbagging or plugging adjacent upstream and downstream manhole flow channels. A high-volume, gasoline-powered blower (1,500 to 4,000 cubic feet per minute [CFM]) is positioned over an open manhole. Non-toxic, odorless white chemical smoke (typically generated from zinc chloride smoke candles or liquid petroleum-hydrocarbon / food-grade glycol vaporizers) is injected into the sewer under low pressure (0.1 to 0.5 psi / 2 to 4 inches of water column).
• Observations: The low pressure forces smoke through any open pathway to the surface. Smoke plumes emerge within minutes from illegal roof downspouts, outdoor patio drains, driveway grates, storm sewer catch basins, missing or cracked lateral cleanout caps, and cracks in street asphalt. Prior public notification (notifying fire departments, police, and residents) is legally required to prevent false structure fire alarms.
• Crucial Limitation: Smoke testing cannot detect infiltration below the groundwater table. If a crack, open joint, or broken lateral is submerged beneath groundwater or saturated soil, the liquid barrier prevents smoke from escaping to the surface.

3. Fluorescent Dye Testing

Dye testing is utilized to confirm suspected hydraulic cross-connections identified during smoke testing or visual surveys. Concentrated non-toxic, biodegradable fluorescent tracer dyes (such as Fluorescein Sodium, which glows brilliant neon green, or Rhodamine WT, which fluoresces bright red-orange) are introduced into suspected inflow sources: commercial area drains, flooded roadway swales, or storm drain inlets. Crews flush the fixture with clean water from a water truck while simultaneously monitoring the downstream sanitary sewer manhole. Visual observation or detection using a handheld fluorometer or ultraviolet (UV) blacklight provides undeniable physical proof of a direct connection.

4. Closed-Circuit Television (CCTV) Robotic Inspection

Robotic CCTV inspection is the gold standard for definitive underground condition assessment. Self-propelled crawler tractors equipped with high-definition, pan-tilt-zoom (PTZ) color cameras, laser profiling rings, and inclinometers are traversed through pre-cleaned sewer reaches.

• Data Captured: CCTV reveals circumferential and longitudinal cracks, broken pipe barrels, joint offsets, root intrusion, protruding service taps, invert sags (bellies), heavy mineral scale, and active infiltration leaks (visible as dripping, running, or gushing water through joints and fractures).

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NASSCO PACP Defect Scoring & Condition Assessment

To eliminate subjective reporting and provide rigorous data for asset management, the wastewater industry relies universally on the Pipeline Assessment Certification Program (PACP) developed by the National Association of Sewer Service Companies (NASSCO).

Defect Families: Structural vs. Operational & Maintenance (O&M)

PACP standardizes sewer defect coding by dividing anomalies into two distinct operational categories:

1. Structural Defects: Physical damage to the pipe barrel or joint that compromises the structural integrity of the conduit. Examples include longitudinal cracks, circumferential cracks, spiral cracks, multiple fractures, broken pipe, holes in the barrel wall, collapsed sections, deformed pipe walls (in flexible pipe), and defective/separated joint gaps.
2. Operational and Maintenance (O&M) Defects: Non-structural physical obstructions or deposits that restrict the hydraulic flow area or impede operational cleaning. Examples include root intrusion (fine roots, root balls, root masses), heavy grease deposits, settled gravel and sediment, mineral encrustation, biological slime, and hanging gasket materials.

PACP Severity Grades (1 to 5)

Every identified defect is assigned a standardized severity grade ranging from 1 to 5 based on its severity and risk of imminent pipe failure:

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|                            NASSCO PACP Defect Severity Grades                           |
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| Grade | Severity Level      | Structural Meaning                 | O&M Meaning          |
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| **1** | **Minor Defect**    | Minor flaw; hairline surface crack;| Light slime / minor  |
|       |                     | zero structural loss; low priority | root hair (<5% area) |
| **2** | **Minor/Moderate**  | Early deterioration; small open    | Medium grease ring;  |
|       |                     | joint; no immediate failure risk   | light roots (5-10%)  |
| **3** | **Moderate Defect** | Medium crack or spall; deformation | Medium roots (10-25%);|
|       |                     | < 5%; stable in foreseeable future | moderate sediment bed|
| **4** | **Significant**     | Severe fracture; open hole; pipe   | Heavy root mass      |
|       |                     | deformed 5-10%; failure in 5-10 yrs| (25-50%); major grit |
| **5** | **Most Severe**     | Pipe collapsed; missing wall chunk;| Total blockage; roots|
|       |                     | deformation > 10%; failure IMMINENT| > 50%; sewage back-up|
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PACP algorithms combine individual defect grades into an overall Pipe Structural Score and Pipe O&M Score, enabling utility engineers to prioritize capital improvement investments, schedule chemical root treatments, or dispatch emergency trenchless rehabilitation crews.

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Trenchless Pipeline Rehabilitation Technologies

When a sewer main suffers from widespread structural failure or severe infiltration, open-cut excavation is often prohibitively expensive or socially disruptive, particularly beneath paved urban arterials, light-rail corridors, or historic business districts. Utilities deploy trenchless rehabilitation technologies to reconstruct the damaged conduit from inside without disturbing the surface.

                        Trenchless Rehabilitation Methods

       Cured-In-Place Pipe (CIPP)                   Pipe Bursting
   ┌────────────────────────────────┐       ┌────────────────────────────────┐
   │ Existing Deteriorated Host Pipe│       │ Host Pipe Fractured Outward    │
   │ ┌────────────────────────────┐ │       │ ░░░ ┌────────────────────┐ ░░░ │
   │ │ Seamless Thermoset Resin   │ │       │ ░░░ │ New Full-Diameter  │ ░░░ │
   │ │ Impregnated Tube Liner     │ │       │ ░◄══┤ Continuous HDPE    │ ░░░ │
   │ │ (Cured: Hot Water/Steam/UV)│ │       │ ░░░ │ Pipe Towed Behind  │ ░░░ │
   │ └────────────────────────────┘ │       │ ░░░ └────────────────────┘ ░░░ │
   └────────────────────────────────┘       └────────────────────────────────┘
      No Excavation; Smooth Invert             Expands or Upsizes Diameter

1. Cured-In-Place Pipe (CIPP — ASTM F1216 / ASTM F1743)

Cured-In-Place Pipe is the most widely specified trenchless renewal technology for collection pipes ranging from 6 to 96 inches in diameter.

• The Lining Tube: A flexible, non-woven polyester needle-felt or fiberglass composite tube manufactured to match the exact diameter and length of the host pipe. The interior of the felt tube is vacuum-impregnated with a liquid thermosetting chemical resin (typically unsaturated polyester, vinyl ester for chemical resistance, or epoxy) blended with a thermal or photo-initiator catalyst.
• Installation Mechanics: The resin-saturated tube is inserted into the host pipe via an existing manhole. It is inverted (turned inside-out) using hydrostatic water column head or compressed air pressure. The inversion process forces the resin-saturated felt tightly against the interior wall of the deteriorated host pipe, pressing resin into cracks, open joints, and structural voids.
• Curing (Polymerization): Once fully inverted and held under continuous internal pressure against the host pipe, the resin is cured into a rigid, structural pipe-within-a-pipe:
  • Hot Water Curing: Circulating heated water (160°F–180°F) through the liner.
  • Pressurized Steam Curing: Injecting high-pressure steam, which provides rapid heat transfer and shorter curing cycles.
  • Ultraviolet (UV) Light Curing: Common with high-strength fiberglass liners. An optical light train emitting specific UV wavelengths is pulled through the pressurized tube, instantly triggering photopolymerization.
• Hydraulic & Structural Properties: The cured CIPP liner conforms to ASTM F1216 as a fully structural, stand-alone conduit capable of supporting all soil and surface live loads for a 50+ year design life. Although the internal pipe diameter is reduced slightly (typically by 1/4 to 1/2 inch due to the liner wall thickness of 6 to 12 mm), the new lining's exceptionally smooth glass-like interior (Manning's $n = 0.009 - 0.010$) significantly decreases boundary friction, frequently increasing total hydraulic discharge capacity compared to the rough, deteriorated host pipe.
• Lateral Reinstatement: After curing and cooling, a robotic remote-controlled cutter tractor equipped with a high-speed diamond router bit is driven through the lined pipe under CCTV guidance to cleanly drill out and reopen customer lateral connections from inside the pipe.

2. Slip-Lining

Slip-lining is an established trenchless method where a continuous or segmented flexible pipe of smaller outside diameter is inserted directly into the existing deteriorated host pipe.

• Procedure: Standard materials include continuous butt-fused High-Density Polyethylene (HDPE), fusible PVC, or short gasketed sections of fiberglass reinforced polymer mortar pipe (FRPM). The liner is pulled or pushed into the host pipe from an excavated insertion pit.
• Annular Grouting: Once the new liner is anchored in place, the annular void between the exterior of the new liner and the interior of the host pipe is completely filled with low-density cellular grout (lightweight cementitious slurry) pumped under low pressure. Grouting locks the liner in place, transfers structural loads, and seals off groundwater migration along the annular space.
• Limitation: Slip-lining causes a substantial reduction in cross-sectional pipe diameter (often a 15% to 30% reduction), which can severely restrict hydraulic capacity unless the host pipe was originally substantially oversized.

3. Pipe Bursting

Pipe bursting is a trenchless replacement method that fractures the existing host pipe while simultaneously pulling a new, equal-diameter or larger-diameter pipeline into the resulting cavity.

• Mechanics: A cone-shaped bursting head is driven through the host pipe using pneumatic percussive hammers, hydraulic expansion jaws, or high-tonnage static pulling winches. The expanding bursting head breaks brittle host pipes (vitrified clay, unreinforced concrete, asbestos cement, or cast iron) outward into the surrounding native soil matrix. Directly attached behind the expanding bursting head is a continuous string of butt-fused HDPE or restrained-joint PVC pipe.
• Upsizing Capability: Pipe bursting is the only trenchless methodology capable of upsizing the collection main (e.g., expanding an undersized 8-inch main into a 10-inch or 12-inch main) without digging a continuous surface trench, making it the premier choice for relieving chronic hydraulic capacity bottlenecks.