8.3 Inflow and Infiltration (I&I) Sources, Flow Monitoring, Smoke Testing & CCTV Inspection

Inflow and Infiltration (I&I) Sources, Flow Monitoring, Smoke Testing & CCTV Inspection

Separate sanitary sewer systems are engineered exclusively to collect and transport domestic, commercial, and industrial wastewater. The entry of extraneous, unpolluted groundwater and surface stormwater—collectively termed Inflow and Infiltration (I&I)—is one of the most pervasive and costly challenges facing wastewater collection utilities. I&I overloads hydraulic capacity, triggers sanitary sewer overflows (SSOs), increases lift station electrical pumping costs, and severely disrupts biological treatment processes at downstream wastewater facilities.

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1. Defining Inflow vs. Infiltration & Hydraulic Impacts

Although often discussed together, Inflow and Infiltration enter through distinctly different pathways and exhibit completely different hydraulic signatures:

+-------------------------------------------------------------------------+
|                        INFLOW VS. INFILTRATION                          |
+-------------------------------------------------------------------------+
| Characteristic      | Inflow (Direct Surface Entry) | Infiltration (Groundwater)  |
+---------------------+-------------------------------+-----------------------------+
| Source Type         | Direct surface stormwater     | Subterranean groundwater    |
| Entry Pathways      | - Vented/pick-hole manholes   | - Cracked / crushed pipes   |
|                     | - Submerged frame rings       | - Deteriorated pipe joints  |
|                     | - Roof downspouts (gutters)   | - Failing lateral taps      |
|                     | - Foundation sump pumps       | - Leaking manhole barrels   |
|                     | - Yard & driveway drains      | - Root-penetrated fissures  |
|                     | - Storm sewer cross-links     |                             |
| Hydrograph Response | Immediate, rapid spike        | Delayed, gradual, sustained |
| Peak Timing         | Concurrent with rain event    | Peaks days/weeks post-storm |
| Duration            | Hours (subsides with storm)   | Days, weeks, or seasonal    |
+---------------------+-------------------------------+-----------------------------+

  Flow Rate (MGD)
       ^
       |          /\  <--- INFLOW: Sharp, instantaneous stormwater spike
       |         /  \
       |        /    \_________________
       |       /                       \  <--- INFILTRATION: Elevated baseflow
       |  _.-''                         ''-._   persisting after groundwater rise
       | /                                   \
       +---------------------------------------> Time (Days)
       [--- Rain Event ---]

The Operational Hazards of Excessive RDII

Rainfall-Derived Inflow and Infiltration (RDII) creates severe operational failures:

1. Hydraulic Overloading & SSOs: Wastewater volume exceeds pipeline carrying capacity, surcharging gravity mains and discharging raw sewage onto public streets or into local waterways.
2. Lift Station Drowning: Inflow floods lift station wet wells beyond pump capacity, tripping electrical alarms and burning out motors.
3. Treatment Process Disruption: Massive hydraulic surges wash active biomass (activated sludge) out of secondary clarifiers, drastically shorten hydraulic retention time in aeration basins, dilute wastewater temperature and BOD concentration, and dramatically increase chemical disinfection costs.

2. Diurnal Flow Monitoring & Hydrograph Analysis

Under dry-weather conditions, municipal wastewater collection networks exhibit a predictable, sinusoidal diurnal flow curve reflecting human behavioral patterns:

  Flow Rate (% of Average Daily Flow)
    200% |             .-.                       .-.
         |            /   \  <-- Morning Peak   /   \  <-- Evening Peak
    100% |           /     \                   /     \
         |          /       '._             _.'       \
     50% |         /           `'---------'`           \
         |  _.._  /                                     \  _.._
      0% +-'    `'---------------------------------------'`---->
         12 AM  3 AM   6 AM   9 AM   12 PM   3 PM   6 PM   9 PM  12 AM
                [ Nighttime Low ]

• Morning Peak (6:00 AM – 9:00 AM): Waking, showering, toilet flushing, breakfast preparation.
• Midday Plateau / Dip (11:00 AM – 2:00 PM): Reduced domestic occupancy as population commutes to workplaces.
• Evening Peak (6:00 PM – 9:00 PM): Cooking dinner, running dishwashers, laundry, bathing.
• Minimum Nighttime Flow (2:00 AM – 5:00 AM): Domestic water use ceases almost entirely.

Infiltration Quantification from Nighttime Flow

During the minimum nighttime window (2:00 AM to 5:00 AM), flow measured by an area-velocity meter represents almost exclusively groundwater infiltration plus minor baseline industrial/commercial 24-hour discharges.

• Rule of Thumb: If dry-weather minimum nighttime flow exceeds 25% to 30% of average daily flow (ADF), the basin suffers from significant groundwater infiltration.

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3. Smoke Testing Protocols & Public Safety

Smoke testing is an efficient, cost-effective field method used to detect direct inflow sources, illegal connections, and shallow structural pipe breaks:

+-------------------------------------------------------------------------+
|                         SMOKE TESTING WORKFLOW                          |
+-------------------------------------------------------------------------+
| 1. Public / Agency Notification: Door hangers, emergency dispatch,      |
|    and local fire departments notified 24 to 48 hours prior.            |
| 2. Line Isolation: Isolate target sewer segment using sandbags or       |
|    pneumatic expandable pipe plugs at adjacent manholes.                |
| 3. Smoke Injection: Position high-capacity blower (1,500–4,000 CFM)     |
|    over manhole; ignite non-toxic zinc chloride/paraffin smoke candle   |
|    or liquid smoke generator to pressurize pipe with white smoke.       |
| 4. Surface Documentation: Field crews walk line inspecting and mapping  |
|    smoke emerging from downspouts, yard drains, pavement, or cleanouts. |
+-------------------------------------------------------------------------+

Mandatory Public Notification & Safety Protocols

• Pre-Notification: Residents must receive written notification 24 to 48 hours in advance. Individuals with chronic respiratory ailments (asthma, COPD) must be identified so special precautions can be taken.
• Emergency Dispatch Coordination: Local 911 dispatch centers and fire departments must be briefed on the exact testing schedule every morning to prevent false structure fire responses.
• Indoor Smoke Entry: Non-toxic white smoke should never enter a residence. If smoke does appear indoors, it indicates a dry plumbing trap (P-trap), a fractured internal drain-waste-vent (DWV) pipe, or an unvented fixture. Operators must inform the building owner immediately, as these defects allow lethal, flammable sewer gas ($\text{H}_2\text{S}$, $\text{CH}_4$) into occupied living spaces.

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4. Dye Testing & Cross-Connection Tracing

Dye testing confirms suspected direct hydraulic cross-connections between storm drainage structures and sanitary collection networks:

• Procedure: Non-toxic, biodegradable fluorescent tracing dyes (e.g., Fluorescein / Uranine or Rhodamine WT) are introduced into suspected roof downspouts, commercial area drains, parking lot catch basins, or stream beds and flushed with clean water from a water tanker.
• Detection: Downstream sanitary manholes are visually monitored or sampled with handheld fluorometers. The appearance of intense green/red color confirms a direct illicit physical cross-connection.

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5. Robotic CCTV Inspection & NASSCO PACP Standards

Internal pipe condition assessment relies on self-propelled robotic Closed-Circuit Television (CCTV) crawler tractors equipped with high-definition, pan-tilt-zoom color cameras, laser profiling rings, and auxiliary LED lighting.

NASSCO PACP Standardized Defect Scoring

The National Association of Sewer Service Companies (NASSCO) Pipeline Assessment Certification Program (PACP) provides a uniform, objective coding standard for sewer defects:

+-------------------------------------------------------------------------+
|                   NASSCO PACP DEFECT SEVERITY SCALE                     |
+-------------------------------------------------------------------------+
| Grade 1: Minor / Blemish     | Minor defect; failure unlikely in 20+ yrs|
| Grade 2: Light Defect        | Defect not currently progressing; 10–20 yr|
| Grade 3: Moderate Defect     | Significant deterioration; 5–10 yr life  |
| Grade 4: Severe Defect       | High risk of collapse within 1 to 5 yrs  |
| Grade 5: Most Severe Defect  | Pipe collapsed or collapse imminent      |
+-------------------------------------------------------------------------+

Defect Classifications

1. Structural Defects (PACP-S): Cracks (longitudinal, circumferential, spiral), fractures, broken pipe walls, missing pipe fragments, hole in pipe with soil visible, deformed/deflected PVC pipe ($> 7.5%$), dropped joints, collapsed pipe.
2. Operation & Maintenance Defects (PACP-OM): Root intrusion (fine hair roots, medium root masses, taproots), grease coatings/plugs, mineral encrustation/calcium scaling, settled debris/grit, protruding lateral taps (hammer taps), standing water / pipe sags (bellies).

Multi-Sensor Inspection Enhancements

• Laser Profiling: Projects a circular laser ring onto the pipe wall to calculate exact cross-sectional ovality, proving whether flexible PVC pipe exceeds the maximum allowable 5% deflection threshold.
• Sonar Profiling: Utilizes high-frequency acoustic sound waves beneath the waterline to calculate the exact volume of settled grit, sand, and sludge in surcharged or deep interceptor pipes without requiring bypass pumping.

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6. Worked Inspection & I&I Calculations

Worked Example 7.3.1: Inflow Volume Calculation from Hydrograph Surcharge

A municipal collection basin serving 10,000 residents has a baseline dry-weather flow of 1.0 MGD ($694.4\text{ gpm}$). During a high-intensity summer thunderstorm, an area-velocity flow meter at the basin outlet records a peak flow spike reaching 4.5 MGD ($3,125\text{ gpm}$). The storm hydrograph surcharge lasts for a total duration of 4.0 hours, averaging 3.0 MGD ($2,083.3\text{ gpm}$) over that period. Calculate the total volume of direct stormwater Inflow (in gallons) that entered the collection network.

Step 1: Calculate the extraneous Inflow flow rate above baseline dry-weather flow: $\text{Average Total Storm Flow} = 3.0\text{ MGD}$ $\text{Baseline Sanitary Dry-Weather Flow} = 1.0\text{ MGD}$ $\text{Extraneous Inflow Rate} = 3.0\text{ MGD} - 1.0\text{ MGD} = 2.0\text{ MGD}$

Step 2: Convert Inflow rate from MGD to gallons per hour: $\text{Inflow Rate} = \frac{2,000,000\text{ gallons}}{24\text{ hours}} = 83,333.3\text{ gallons/hour}$

Step 3: Calculate total extraneous Inflow volume over the 4.0-hour event: $\text{Total Inflow Volume} = 83,333.3\text{ gal/hr} \times 4.0\text{ hours} = 333,333\text{ gallons}$

Worked Example 7.3.2: Groundwater Infiltration Baseline Evaluation

A sanitary sewer meter basin records an Average Daily Flow (ADF) of 800,000 gallons/day (0.80 MGD). Between 2:00 AM and 5:00 AM during a dry-weather period with zero industrial discharges, the meter records a steady flow rate of 240 gpm. Determine if this basin exceeds the 30% infiltration screening threshold.

Step 1: Convert the 2:00 AM to 5:00 AM nighttime flow rate to an equivalent 24-hour daily rate: $\text{Nighttime Flow Rate} = 240\text{ gpm} \times 1,440\text{ minutes/day} = 345,600\text{ gallons/day}$

Step 2: Calculate nighttime flow as a percentage of Average Daily Flow: $\text{Percentage} = \left( \frac{345,600\text{ gal/day}}{800,000\text{ gal/day}} \right) \times 100 = 43.2\%$

Conclusion: Because 43.2% exceeds the 25% to 30% screening threshold, this collection sub-basin is experiencing severe groundwater infiltration and should be prioritized for CCTV robotic inspection and joint sealing.