11.2 Inflow, Infiltration, Overflows & Trenchless Rehabilitation

11.2 Inflow, Infiltration, Overflows & Trenchless Rehabilitation

1. Inflow and Infiltration (I/I) Forensics

Extraneous clean water entering a sanitary sewer system reduces hydraulic conveyance capacity, triggers catastrophic Sanitary Sewer Overflows (SSOs), overloads wastewater pump stations, and dilutes influent organic strength ($BOD_5$ and $TSS$) at the treatment plant, causing severe biological process instability and spiking electrical aeration costs.

                        INFLOW vs. INFILTRATION HYDROGRAPH SIGNATURE

     Flow Rate
       (MGD)
         ▲
         │                     /\  ◄─── INFLOW: Sharp, instantaneous peak spike
         │                    /  \      directly tracking rainfall hydrograph
         │                   /    \
         │                  /      \___________
         │                 /                   \ 
         │   _ _ _ _ _ _ _/                     \_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
         │                                       \  INFILTRATION: Elevated sustained
         │                                        \ baseline flow tracking seasonal
         │═════════════════════════════════════════\══════════════════════════════
         │        Base Dry-Weather Sanitary Flow    \    groundwater table
         └───────────────────────────────────────────┴────────────────────────────►
           Rain Event Begins               Rain Ends (Days / Weeks Later)       Time

Inflow (Direct Stormwater Entry)

• Definition: Clean surface water and direct stormwater runoff that enters the collection network rapidly through intentional, illicit, or unsealed surface openings.
• Primary Physical Sources:
  1. Illicit residential and commercial sump pumps discharging groundwater/roof water into sanitary drains;
  2. Building roof downspouts (leaders) connected directly to sewer laterals;
  3. Foundation footing drains and basement interior perimeter dewatering channels;
  4. Cross-connections between municipal storm drain catch basins and sanitary mains;
  5. Flooded manhole covers containing perforated pickholes and unsealed frame chimneys submerged during street flooding; and
  6. Yard drains, driveway drains, and patio sumps.
• Hydrograph Signature: Instantaneous, flashy, sharp peak flow spikes. Flow rates skyrocket within minutes of precipitation onset, peak concurrently with the storm's peak intensity, and plunge back toward baseline within hours after the rain event terminates.

Infiltration (Groundwater Seepage Entry)

• Definition: Subsurface groundwater that slowly seeps into sewer mains, service laterals, and manholes through fractured structural walls, porous joints, and material voids.
• Primary Physical Sources:
  1. Cracked, fractured, or crushed vitrified clay or concrete pipe barrels;
  2. Open, misaligned, or deteriorated pipe bell-and-spigot joints where rubber gaskets have degraded or oakum/mortar has washed away;
  3. Fractured or unsealed building service lateral pipes on private property;
  4. Leaking lateral saddle connections (break-in tees) cored improperly into main collection lines;
  5. Porous brick manhole chimneys, deteriorated mortar joints, and unsealed precast riser barrel section joints; and
  6. Abandoned, unsealed building connections.
• Hydrograph Signature: Sustained, elevated baseline flow. Infiltration does not exhibit instantaneous spikes during storms. Instead, flows climb gradually over days or weeks as stormwater perches and recharges the surrounding groundwater table. Elevated flow remains sustained throughout entire wet seasons and recedes very slowly over months as the water table declines.

+-----------------------------------------------------------------------------------------+
|                        INFLOW vs. INFILTRATION DIAGNOSTIC MATRIX                        |
+---------------------+-------------------------------+-----------------------------------+
| Characteristic      | Inflow                        | Infiltration                      |
+---------------------+-------------------------------+-----------------------------------+
| Water Source        | Surface runoff / Stormwater   | Subsurface Groundwater Table      |
| Entry Pathways      | Sump pumps, downspouts, picks | Pipe cracks, open joints, saddles |
| Response Time       | Minutes to hours (Flashy)     | Days to months (Sustained lag)    |
| Volume Dynamics     | Massive instantaneous peaks   | High continuous background flow   |
| Seasonal Tracking   | Rainstorm events exclusively  | Seasonal groundwater elevations   |
| Primary Detection   | Smoke testing, dye testing    | Night flow isolation, CCTV PACP   |
+---------------------+-------------------------------+-----------------------------------+

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2. I/I Detection & Pipeline Inspection Methodologies

Operators utilize four standardized forensic techniques to pinpoint, quantify, and prioritize I/I defect repairs across collection networks:

1. Smoke Testing

• Mechanics: A sewer line segment is isolated using pneumatic plugs at adjacent upstream and downstream manholes. A portable blower (typically powered by a 5–8 HP gas engine or high-output electric motor) is placed over an open manhole, forcing non-toxic, odorless, white chemical smoke (derived from zinc chloride or pharmaceutical-grade glycol/glycerol canisters) through the pipe barrel under low positive pressure (0.1 to 0.5 psi).
• Diagnostic Capability: Outstanding for detecting direct inflow sources. White smoke rapidly travels through illicit pathways, venting visibly through:
  • Residential roof downspouts and gutters;
  • Storm sewer catch basins (confirming cross-connections);
  • Driveway, patio, and area drains;
  • Surface yard depressions indicating cracked laterals or missing cleanout caps; and
  • Foundation perimeter drains.
• Regulatory Protocol: Mandatory public notification is required 48 to 72 hours prior to smoke testing. Municipal police, fire departments, and 911 dispatch centers must be alerted to prevent false structural fire alarms. Door-to-door flyers must advise residents that smoke may enter living spaces if indoor plumbing P-traps are dry, cracked, or unvented.

2. Dye Water Flooding

• Mechanics: Non-toxic, biodegradable fluorescent dyes (such as uranine/fluorescein yellow-green or rhodamine WT red) are dissolved in water and introduced into suspected inflow sources, including storm sewer catch basins, ditch swales, or flat commercial roofs. Concurrently, the downstream sanitary sewer manhole is visually inspected or monitored using a fluorometer.
• Diagnostic Capability: Confirms direct, suspected hydraulic connections detected during smoke testing. It is particularly effective for proving that street catch basins or leaking storm culverts cross-connect into adjacent sanitary sewers.

3. CCTV Video Inspection & NASSCO PACP Standards

• Mechanics: Closed-Circuit Television (CCTV) robotic crawlers equipped with pan-and-tilt, high-definition zoom color cameras traverse pre-cleaned sewer mains. Distance-encoded footage provides continuous logging of structural and operational conditions.
• NASSCO PACP Standards: The National Association of Sewer Service Companies (NASSCO) Pipeline Assessment Certification Program (PACP) is the nationwide benchmark for defect grading. Defect conditions are categorized into two structural classes:
  • Structural Defects: Continuous or isolated fractures, circumferential/longitudinal cracks, broken pipes, missing wall fragments, holes, collapsed pipe crowns, and deformed thermoplastics.
  • Operational & Maintenance (O&M) Defects: Mineral encrustation, root intrusion (fine, medium, taproots), grease rings, settled debris, and pipeline sags (bellies).
  • Numerical Grading Scale: PACP assigns defect scores from Grade 1 (minor defect, unlikely to fail within 20 years) to Grade 5 (catastrophic structural defect, immediate failure or pipe collapse imminent; continuous heavy infiltration gushing at >50 gpm).

4. Night Flow Isolation (Diurnal Low-Flow Gauging)

• Mechanics: Flow monitoring performed strictly between 1:00 AM and 4:00 AM. During these early morning hours, municipal domestic water consumption reaches its absolute diurnal minimum (near zero baseline domestic sewage production).
• Diagnostic Capability: Sewer line sections are temporarily isolated by inserting pneumatic plugs into upstream manhole pipes while monitoring downstream discharge via portable V-notch weirs, acoustic Doppler sensors, or calibrated flumes. Any continuous, measurable flow observed between 1:00 AM and 4:00 AM is diagnosed as pure groundwater infiltration. Lines exhibiting baseflows exceeding 100 to 500 gallons per inch-diameter per mile per day (gpd/in-mi) are prioritized for immediate CCTV inspection and rehabilitation.

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3. Sanitary Sewer Overflows (SSOs) & NJDEP Reporting Mandates

A Sanitary Sewer Overflow (SSO) is an unpermitted discharge of untreated or partially treated raw sewage from a municipal sanitary collection system prior to reaching the headworks of a licensed wastewater treatment plant. SSOs contaminate surface waters, back up into residential basements, pool in public streets, and pose grave biological health hazards from waterborne pathogens (Salmonella, Shigella, Hepatitis A, enteroviruses, and protozoan cysts).

Root Causes of SSOs

1. Fats, Oils, and Grease (FOG) Blockages: Commercial food service facilities and residential kitchens discharge liquid grease that cools and saponifies with calcium inside sewer pipes, forming monolithic solid "fatbergs" that choke the pipe cross-section.
2. Tree Root Intrusions: Root tendrils penetrate hairline pipe joints seeking moisture and nutrients. Inside the pipe, they expand into dense fibrous mats that trap toilet paper and solids.
3. Pump Station / Power Failure: Mechanical impeller rag-binding, electrical panel burnouts, or loss of utility power without automatic standby generators cause wet wells to flood, backing up the upstream network.
4. Extreme Wet-Weather I/I Capacity Exceedance: Massive stormwater inflow during hurricanes or intense convective rainstorms overwhelms conduit capacity, forcing manhole covers open and surcharging into low-lying street basins.
5. Structural Pipe Collapse: Severe crown corrosion or bedding erosion causes pipe failure, physically blocking all downstream flow.

NJDEP Regulatory Spill Reporting Mandates (N.J.A.C. 7:14A)

Under the New Jersey Water Pollution Control Act and NJDEP regulations codified in N.J.A.C. 7:14A (NJPDES Rules), unpermitted sewage discharges carry strict, legally enforceable notification deadlines:

+-----------------------------------------------------------------------------------------+
|                        NJDEP MANDATORY SSO REPORTING TIMELINE                           |
+-----------------------+-----------------------------+-----------------------------------+
| Action Step           | Regulatory Deadline         | Communication Channel / Agency    |
+-----------------------+-----------------------------+-----------------------------------+
| 1. Immediate Verbal   | Within TWO (2) HOURS        | NJDEP Environmental Action Hotline|
|    Notification       | of discovery                | 1-877-WARN-DEP (1-877-927-6337)   |
+-----------------------+-----------------------------+-----------------------------------+
| 2. Local Health &     | Immediate (Concurrent)      | Municipal Health Department &     |
|    Police Alert       |                             | Local Emergency Management (OEM)  |
+-----------------------+-----------------------------+-----------------------------------+
| 3. Formal Written     | Within FIVE (5) DAYS        | NJDEP Regional Water Compliance   |
|    Incident Report    | of occurrence               | and Enforcement Bureau            |
+-----------------------+-----------------------------+-----------------------------------+

• The Two-Hour Verbal Rule: The licensed collection system operator or authorized municipal official must contact the NJDEP Action Line at 1-877-WARN-DEP within two (2) hours of becoming aware of an overflow event. Failure to notify within 2 hours constitutes a direct statutory violation subject to mandatory administrative civil penalties.
• The Five-Day Written Report: Within five (5) days, the utility must submit a detailed technical written report containing:
  1. Exact geographic location (street address and GPS coordinates);
  2. Estimate of total volume discharged (calculated via pipe hydraulic formulas, surcharging depth, or duration-rate estimates);
  3. Date, start time, and duration of the discharge;
  4. Receiving water body, storm sewer, or public land impacted;
  5. Precise root cause of the blockage, failure, or capacity exceedance;
  6. Immediate mitigation actions taken (vacuum extraction, hydro-jetting, disinfection with calcium hypochlorite, lime application, street sweep containment);
  7. Public notification measures enacted (signage, water testing, media notices); and
  8. Corrective engineering plan and preventative maintenance schedule implemented to permanently prevent recurrence.

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4. Combined Sewer Systems (CSS) & CSOs in New Jersey

In older, historic urban centers across New Jersey—most notably Newark, Jersey City, Paterson, Bayonne, Camden, Trenton, Hoboken, and Elizabeth—wastewater collection originated in the late 19th and early 20th centuries as a single, unitary conduit network known as a Combined Sewer System (CSS).

                     COMBINED SEWER OVERFLOW (CSO) REGULATOR DYNAMICS

   Dry Weather: 100% Flow to WWTP           Wet Weather: Storm Overwhelms Interceptor
   ══════════════════════════════           ═════════════════════════════════════════
        Combined Sewage                          Combined Sewage + Stormwater Runoff
   ──────────────────────────────►          ────────────────────────────────────────►
           │                                         │                     │
           ▼                                         ▼ Overflow Weir       ▼
    ┌─────────────┐                           ┌─────────────┐       ┌─────────────┐
    │ Interceptor │                           │ Interceptor │       │ CSO Outfall │
    │ to Regional │                           │ to Regional │       │ to Receiving│
    │    WWTP     │                           │    WWTP     │       │ River/Tidal │
    └─────────────┘                           └─────────────┘       │    Water    │
                                                 (Capacity)         └─────────────┘

Operation of Combined Sewer Systems

• Dry-Weather Conditions: The combined sewer conveys 100% of sanitary domestic sewage and commercial waste directly into regional interceptors, transporting it safely to the regional WWTF for complete secondary/tertiary purification.
• Wet-Weather Overflows (CSOs): When rainfall or snowmelt enters street catch basins, the combined volume quickly dwarfs the hydraulic conveyance capacity of the downstream interceptor pipes and treatment plant. To prevent catastrophic sewage backups into city basements and streets, mechanical regulator structures (gravity overflow weirs, float-actuated gates, or vortex throttle valves) divert the excess mingled stormwater and raw wastewater directly into local water bodies (such as the Passaic River, Hackensack River, Hudson River, Arthur Kill, or Delaware River) as a Combined Sewer Overflow (CSO).

NJDEP CSO Regulatory Policy & Long-Term Control Plans (LTCPs)

Under federal Clean Water Act mandates and NJDEP NJPDES Combined Sewer System General Permits, all 21 New Jersey CSO permittees must develop and implement comprehensive Long-Term Control Plans (LTCPs):

1. Implementation of the Nine Minimum Controls (NMC): Baseline technology-based operations including proper operation and regular maintenance programs, maximizing collection system storage capacity, review and modification of pretreatment requirements, maximizing flow to the WWTP, prohibition of dry-weather CSOs, control of solid and floatable materials (netting facilities and bar screens), pollution prevention programs, public notification, and monitoring of CSO impacts.
2. Long-Term Capital Engineering Measures:
   • Sewer Separation: Physically excavating roadways to install a secondary, dedicated storm sewer pipe network, separating municipal stormwater entirely from sanitary sewage.
   • Deep Underground Storage Tunnels: Mining massive subterranean bedrock storage tunnels (such as 20- to 30-foot diameter bored rock tunnels) to store tens of millions of gallons of wet-weather combined sewage until the storm subsides, after which pumps dewater the tunnel back to the treatment plant.
   • Off-Line Storage Basins: Constructing regional underground reinforced concrete detention tanks.
   • Green Infrastructure (GI): Installing permeable pavements, bioretention swales, engineered rain gardens, and tree trenches to infiltrate stormwater into the upper soil profile, preventing runoff from ever entering the combined collection system.

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5. Trenchless Rehabilitation & Line Renewal

Once inflow and infiltration sources are located, they have to be fixed. WPI's Wastewater Collection blueprint devotes 23 of 100 questions to Collection System Operation, Maintenance, and Restoration, and rehabilitation methods are a large share of that content.

Selecting a Method

Reinstating laterals: after CIPP or fold-and-form, service connections are reopened robotically with a cutter guided by CCTV. A missed or partially cut reinstatement causes an immediate backup, so post-lining CCTV of every lateral is standard.

Excavation Support Methods

• Hydro excavation (vacuum excavation) uses pressurized water or air plus a vacuum to remove soil without mechanical contact. It is the preferred method for potholing to expose existing utilities and for daylighting a line before conventional excavation, because it will not cut a cable, a gas service, or a fiber duct.
• Conventional open-cut remains appropriate for complete replacement, deep collapses, and locations where alignment or grade must change. It requires full compliance with trenching and shoring rules covered in Chapter 14.

Restoration and Records

Every rehabilitation project ends with the same three administrative steps that examiners ask about: post-work CCTV acceptance, updating the GIS and asset management record with the method, materials, date, and location (including GPS coordinates and photographs), and restoring the surface to the requirements of the roadway opening permit and any easement or right-of-way agreement.

Exam Trap Alert: Pipe bursting is the only listed trenchless method that increases pipe capacity. CIPP, fold-and-form, and sliplining all reduce internal diameter, although CIPP's smooth liner often recovers much of the lost capacity through a better roughness coefficient. If a question asks how to fix a hydraulically undersized sewer without open-cut, the answer is pipe bursting.

6. Bypass Pumping for Emergencies and Planned Line Work

Every trenchless method in the previous part requires the sewer to be taken out of service, and every lift station failure, force main break, or collapsed segment demands the same response. Bypass pumping — temporarily routing wastewater around an out-of-service reach — is an explicitly scored collection job task, and it is the operation where a mistake becomes a sanitary sewer overflow and therefore an N.J.A.C. 7:14A reportable event.

The Bypass Train

A bypass setup is a suction side, a pump, and a discharge side, all sized to the peak flow the reach will see, never the average:

Sizing and Redundancy

Size to the peak hourly wet-weather flow, not the daily average. An 8-inch sewer flowing full at the 2.0 ft/s minimum scouring velocity carries roughly 700 gpm, and that is the floor for a bypass on that line, before any allowance for rain. Standard practice is N+1 redundancy: a standby pump that auto-starts on a high-level float when the duty pump fails or loses prime, plus an auto-dialer or SCADA alarm on the upstream manhole level. A bypass is never left unattended during wet weather.

Sequence — Order Matters

1. Set and fuel the pumps, run the discharge line, and confirm the discharge manhole.
2. Start and prime the bypass pump first, and verify it is passing flow.
3. Only then set and inflate the upstream plug, watching the upstream level.
4. Monitor continuously; secure the plug against blowout and log levels.
5. Deflate the plug before shutting down the pump when the work is complete.

Exam Trap Alert: The single most common bypass question reverses steps 2 and 3. Plugging the line before the pump is primed and discharging backs the system up immediately and causes an overflow at the first upstream manhole or basement connection. Pump first, plug second; plug out, pump last.

Site Safety and Public Impact

Hose runs crossing a roadway need ramps or trench-and-cover with MUTCD-compliant work zone traffic control; hose across a driveway without a ramp is both a trip hazard and a claim. Diesel units near residences run into municipal noise ordinances for night work, which is why sound-attenuated pumps are specified for overnight bypasses. Fuel tanks need secondary containment, and every connection is a potential spill point that must be inspected each shift.

7. Practical Operational Scenarios & Exam Traps

Practical Operational Scenario

A collection system superintendent oversees an older municipality containing 85 miles of vitrified clay pipe (VCP). The local wastewater plant receives an average daily dry-weather flow of 2.0 MGD. Following a 2.5-inch autumn rainstorm, the plant influent meter jumps to 7.8 MGD within 90 minutes. Over the next five days of completely dry weather, plant influent remains abnormally elevated at 4.2 MGD, declining at only 0.1 MGD per day.

• Forensic Diagnosis:
  1. The rapid, instantaneous jump from 2.0 to 7.8 MGD within 90 minutes is a classic Inflow signature, resulting from thousands of unpermitted residential sump pumps, roof downspouts, and flooded manhole pickholes discharging stormwater directly into sanitary mains.
  2. The prolonged, five-day plateau at 4.2 MGD during dry weather proves severe Infiltration. The 2.5-inch rainfall recharged the local shallow groundwater aquifer, submerging deteriorated VCP pipe joints and broken building laterals, which will continue to leak until the water table drops.
• Action Protocol: The superintendent deploys smoke testing crews to identify illegal sump pumps and roof leaders, while scheduling night flow isolation (1:00 AM–4:00 AM) and robotic CCTV inspection to map cracked pipes for trenchless Cured-In-Place Pipe (CIPP) lining.

Critical Exam Traps

• Trap 1: The Drop Manhole Height Standard. Exam questions frequently test the exact threshold requiring a drop manhole: it is strictly 2.0 feet (24 inches) between incoming and outgoing inverts. Do not select 1.0 foot, 18 inches, or 3.0 feet.
• Trap 2: Self-Cleansing Velocity Magnitude. Candidates often confuse minimum self-cleansing velocity (2.0 ft/s) with maximum erosion velocity (10.0 ft/s). Remember that 2.0 ft/s applies when the pipe is flowing full or half-full.
• Trap 3: Minimum Public Sewer Diameter. Even if calculations show a 4-inch or 6-inch pipe can handle design flow, public gravity sanitary mains must be at least 8 inches (200 mm). Building laterals are 6 or 4 inches, but never public mains.
• Trap 4: NJDEP Spill Notification Deadlines. The verbal telephone notification to 1-877-WARN-DEP is 2 hours, not 24 hours. The formal written follow-up report is 5 days, not 30 days.

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