11.3 Collection System Operation, Cleaning & Inspection

11.3 Collection System Operation, Cleaning & Inspection

A proactive Collection System Operation and Maintenance (O&M) program is vital to maintain hydraulic capacity, extend infrastructure service life, and eliminate illegal Sanitary Sewer Overflows (SSOs). Wastewater collection systems face severe environmental stresses: fat, oil, and grease (FOG) encrustation, invasive tree root penetration, abrasive grit accumulation, structural earth loading, and lethal biogenic sulfuric acid corrosion. Certified operators must master both hydraulic and mechanical cleaning techniques, robotic pipeline assessment standards, trenchless rehabilitation methods, and chemical odor/corrosion mitigation strategies.

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Preventive Maintenance & Hydraulic Sewer Cleaning

Hydraulic sewer cleaning using high-pressure water jetting (combination jet-vac trucks) is the primary method for maintaining municipal gravity sewers.

                 HYDRAULIC SEWER JETTING OPERATIONAL DYNAMICS

  UPSTREAM                                                          DOWNSTREAM
  MANHOLE                                                           MANHOLE
  ┌──────┐                                                          ┌──────┐
  │      │                 Gravity Sewer Pipe                       │VACUUM│◄── Debris
  │      │  ◄══════════════════════════════════════════════════════ │TRUCK │    Extraction
  │      │                 Direction of Sewage Flow                 │HOSE  │
  │      │                                                          │      │
  │      │                 High-Pressure Water Hose                 │      │
  │      │ ═════════════════════════════════════════════════════════│══════│◄── High-Pressure
  │      │            ◄── [Jetting Nozzle Propels Upstream]         │      │    Hose Reel
  │      │                                                          │  ▼   │
  │      │                   /│ ┌─────────┐                         │ ┌──┐ │
  │      │   Forward Thrust / │ │ Jetting │  Rear Propulsion Jets   │ │  │ │ Dislodged Debris
  │      │   Jet (Bores)   <  │ │ Nozzle  │ ===► ===► ===►         │ │  │ │ Sump / Vacuum Tube
  │      │                  \ │ └─────────┘ (Scours & Drags Debris) │ └──┘ │
  └──────┘                   \│                                     └──────┘

Pressure, Flow & Public Protection

Jet pressure and flow are selected for the nozzle, diameter, blockage, pipe material/condition, and equipment rating. Values such as 1,500–2,500 psi and 30–80 gpm describe common equipment, not a mandatory operating band. Use the utility’s procedure to manage hose/nozzle hazards, upstream flooding, customer plumbing pressure effects, and weakened pipe.

Cleaning Direction & Debris Capture

A common jet-vac setup inserts the nozzle at the downstream manhole and sends it upstream, then retrieves it while capturing debris at the downstream vacuum. This is a sound default where access, pipe condition, and hydraulics allow it, but the job plan—not a universal rule—controls direction and setup.

• Why this setup is useful:
  1. The rear-facing high-pressure water jets propel the nozzle forward up the pipe while scouring the walls.
  2. When the hose is retrieved slowly backward toward the downstream manhole, the rear jets act as a hydraulic squeegee, dragging all dislodged grease, grit, and rocks downstream directly into the lower manhole.
  3. A heavy-duty vacuum suction tube positioned at the bottom of the downstream manhole continuously extracts the debris into the truck's debris tank, preventing dislodged solids from washing further downstream to create a massive blockage in lower collection mains.

Jetting Nozzle Classifications

1. Penetrator / Chisel Nozzles: Feature a powerful forward-facing water jet ($15^\circ - 30^\circ$ angle) combined with rear propulsion jets. Used to bore through complete blockages, ice dams, and dense root plugs.
2. Flusher / Scour Nozzles: Feature wide-angle rearward jets ($30^\circ - 45^\circ$ angle) designed for maximum wall-scouring thrust and sediment dragging in heavily silted lines.
3. Rotating Spinner / Chain Cutter Nozzles: High-speed spinning water jets or water-driven rotating chain flails that slice circumferential root rings, mill hard calcium/grease scale, and polish pipe walls.

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Mechanical Sewer Cleaning Equipment

When heavy obstructions cannot be cleared by hydraulic jetting alone, mechanical cleaning tools are utilized:

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Closed-Circuit Television (CCTV) & NASSCO PACP Standards

Robotic Closed-Circuit Television (CCTV) inspection systems are the primary non-destructive tool for evaluating structural condition and prioritizing capital sewer renewal.

                    ROBOTIC CCTV CRAWLER DEPLOYMENT

                 CCTV Inspection Van (Video Monitor / PACP Data)
                       ┌────────────────────────────┐
                       │ [Video Feed] [PACP Defect] │
                       └─────────────┬──────────────┘
                                     │ Cable Reel
  Manhole Grade ═════════════════════╪═════════════════════════════════
                       ┌─────────────┴──────────────┐
                       │ Downstream Manhole         │
                       │                            │
  Incoming Main        │ ┌────────────────────────┐ │
 ══════════════════════╪═╡ Robotic Crawler Camera │ │
                       │ │ [Pan-Tilt-Zoom Lens]   │ │
  ◄── (Travels Upstream│ │ [Laser Profiler]       │ │
       at <= 30 ft/min)│ └───────┬────────┬───────┘ │
                       │         ▼        ▼         │
                       └────────────────────────────┘

Camera Technology & Operational Requirements

• Crawler Platform: Steerable, multi-wheel or track-driven robotic transporters equipped with Pan-and-Tilt-Zoom (PTZ) camera heads, internal laser ring profilers (to measure pipe ovality and deflection), and digital inclinometers (to record pipe grade and locate sags/bellies).
• Maximum Travel Speed: To ensure every pipe joint and surface defect is thoroughly evaluated, crawler speed must not exceed $30\text{ feet per minute}$ ($0.15\text{ m/s}$).

NASSCO PACP Defect Classification

The National Association of Sewer Service Companies (NASSCO) established the Pipeline Assessment Certification Program (PACP) as the North American industry standard for sewer defect coding:

1. Structural Defects: Physical damage to the pipe barrel that impairs structural load-bearing capacity:
   • Cracks (Longitudinal, Circumferential, Spiral)
   • Fractures (Multiple cracks where pipe pieces remain in place)
   • Broken Pipe (Missing fragments / displaced walls)
   • Hole (Pipe wall completely missing, exposing surrounding soil)
   • Deformed Pipe (Flexible plastic/HDPE pipe ovalization exceeding $5\%$
   • Collapsed Pipe (Total structural failure with loss of sewer cross-section)
2. Operation & Maintenance (O&M) Defects: Blockages and biological/chemical accumulations that restrict hydraulic carrying capacity:
   • Root Intrusion (Fine hair roots, tap roots, medium root masses, root balls)
   • Deposits (Encrusted grease, settled grit, rocks, construction debris)
   • Obstacles (Protruding service lateral taps, solid grout penetration)
3. Infiltration Defects: Visual evidence of groundwater entry:
   • Weeper (Slow moisture sheen on pipe wall)
   • Sweater (Water droplets forming across wall)
   • Dripper (Continuous dripping into flow)
   • Runner (Continuous trickling stream)
   • Gusher (High-pressure continuous pressurized jet of groundwater)

PACP Condition Grades (1 to 5)

PACP assigns condition grades from 1 (least severe) to 5 (most severe) to coded observations. A grade describes observed condition; it does not by itself impose a universal repair deadline. The owner combines the coding with consequence of failure, service history, hydraulic capacity, location, inspection quality, and engineering review to set rehabilitation priorities.

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Smoke Testing Protocols & Public Safety Notifications

Smoke testing uses purpose-designed, labeled test smoke and forced air to locate inflow pathways. Equipment capacity and isolation layout are selected for the pipe reach and manufacturer instructions; a single cfm range is not a regulatory requirement.

Public-Safety and Notification Plan

Test smoke can enter a building through a dry trap or defective plumbing, so plan outreach before testing. Follow the utility’s approved procedure and coordinate with local fire/dispatch authorities and other agencies they identify. Notify affected occupants with the locally specified lead time and explain that test smoke is not a fire but should still be reported if its source is uncertain. Advise occupants to restore water seals in seldom-used traps as appropriate. The exact 24–48-hour window, agency list, contact frequency, and water quantity are local program choices—not universal federal or Missouri mandates.

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Trenchless Sewer Rehabilitation Technologies

Replacing failing collection sewers via traditional open-cut trench excavation is extraordinarily disruptive and expensive in urban streets. Trenchless rehabilitation methods renew pipelines with minimal surface disruption:

                  CURED-IN-PLACE PIPE (CIPP) INVERSION

      Water Inversion Column / Pressure Vessel (Grade Level)
             ┌────────────────────────┐
             │  Water / Air Pressure  │
             └───────────┬────────────┘
                         │ Inversion Head
  Manhole ═══════════════╪═════════════════════════════════════════════
             ┌───────────┴────────────┐
             │                        │ Existing Deteriorated Host Pipe
             │ ┌──────────────────────┼──────────────────────────────┐
             │ │ Inverting Resin Tube │◄── Saturated Felt Liner      │
 ════════════╪═╡  (Hot Water/Steam    │    Pressed Against Host Wall │
             │ │   or UV-Cured)       │                              │
             │ └──────────────────────┼──────────────────────────────┘
             │                        │ Smooth, Seamless New Pipe
             └────────────────────────┘

1. Cured-in-Place Pipe (CIPP)

• Process: A flexible non-woven polyester felt tube impregnated with liquid thermosetting resin (polyester, vinyl ester, or epoxy) is inserted into the host pipe via water hydrostatic inversion or compressed air. The liner is pressed tightly against the host pipe walls and cured into a rigid, structural pipe using circulating hot water, pressurized steam, or ultraviolet (UV) light trains.
• Reinstatement: Robotic internal cutting tools enter the cured line under CCTV guidance to drill open and reinstate lateral service connections from inside the pipe.
• Advantages: Jointless, smooth interior ($n = 0.009 - 0.011$), design life established by the engineered liner; a continuous liner can seal many host-pipe joints, but laterals, terminations, defects, and installation quality still control infiltration.

2. Pipe Bursting

• Process: A heavy-duty pneumatic, hydraulic, or static bursting head with expanding cutting blades is pulled through an existing brittle pipe (vitrified clay, concrete, or cast iron). The bursting head shatters the host pipe outward into the surrounding soil while simultaneously pulling a new, seamless string of High-Density Polyethylene (HDPE) pipe directly behind it.
• Advantage: Allows upsizing the pipe diameter (e.g., replacing an 8-inch line with a 10-inch or 12-inch HDPE main) without digging a continuous trench.

3. Sliplining

• Process: A new continuous or segmental pipe of slightly smaller diameter (HDPE, PVC, or fiberglass) is pulled or pushed into the host pipe. The annular void between the host pipe and the new liner is then pressure-injected with cellular cementitious grout.

4. Point Repairs & Chemical Grouting

• For isolated defects, short sectional CIPP patches ($3 - 10\text{ ft}$) or inflatable grouting packers that inject chemical acrylamide/polyurethane resin into leaking joints seal discrete infiltration sources without lining the entire reach.

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Hydrogen Sulfide ($\text{H}_2\text{S}$) Biogenic Corrosion & Odor Control

One of the most destructive degradation mechanisms in wastewater infrastructure is Biogenic Sulfuric Acid Corrosion (commonly called Crown Rot).

              THE BIOGENIC SULFURIC ACID CORROSION CYCLE (CROWN ROT)

                        SEWER HEADSPACE GAS
           ┌──────────────────────────────────────────────┐
           │  Moist Concrete Pipe Crown Above Waterline   │
           │  Acidithiobacillus bacteria oxidize H2S:     │
           │     H2S + 2 O2 ──► H2SO4 (Sulfuric Acid)     │
           │  Sulfuric acid converts CaCO3 concrete into  │
           │  crumbly gypsum (CaSO4), destroying crown!   │
           └──────────────────────┬───────────────────────┘
                                  ▲
                       H2S Gas Strips Out of Solution
                                  │
     Wastewater Surface ~ ~ ~ ~ ~ ╪ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
                                  │
               Dissolved Sulfide (H2S / HS-) in Solution
                                  ▲
           ┌──────────────────────┴───────────────────────┐
           │  Submerged Anaerobic Slime Layer (Pipe Invert)│
           │  Desulfovibrio bacteria reduce sulfates:     │
           │     SO4(2-) + Organic Carbon ──► S(2-) / H2S │
           └──────────────────────────────────────────────┘

The 4-Stage Crown Rot Biochemical Mechanism

1. Submerged Anaerobic Reduction: In slow-moving wastewater, an anaerobic biofilm layer develops on submerged pipe walls. Obligate anaerobic sulfate-reducing bacteria (Desulfovibrio) metabolize organic matter using sulfate ($\text{SO}_4^{2-}$) as an electron acceptor, reducing sulfate to dissolved hydrogen sulfide ($\text{H}_2\text{S}$ and $\text{HS}^-$).
2. Headspace Gas Stripping: At typical wastewater pH ($6.5 - 7.5$) and wherever turbulence occurs (manhole drops, bends, lift station force main discharges), dissolved $\text{H}_2\text{S}$ gas volatilizes out of the liquid stream into the humid sewer headspace.
3. Bacterial Colonization & Acid Production: Aerobic autotrophic bacteria (Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans) colonize the moist, unlined concrete pipe crown above the waterline. These bacteria consume gaseous $\text{H}_2\text{S}$ and atmospheric oxygen, producing concentrated Sulfuric Acid ($\text{H}_2\text{SO}_4$): $\text{H}_2\text{S} + 2\,\text{O}_2 \xrightarrow{\text{Acidithiobacillus}} \text{H}_2\text{SO}_4$
4. Chemical Concrete Dissolution: The sulfuric acid reacts violently with the alkaline calcium hydroxide and calcium carbonate ($\text{CaCO}_3$) in the concrete paste, converting structural concrete into calcium sulfate dihydrate (gypsum, $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$). The altered cementitious material loses strength and can wash away, progressively exposing reinforcement and reducing structural capacity.

Chemical Addition Strategies for Sulfide & Odor Control