4.8 Sewer Cleaning, CCTV Inspection & Collection System Rehabilitation
Key Takeaways
- High-velocity hydraulic jetting is the dominant sewer cleaning method, using nozzle jets at 1,500 to 3,000 psi to scour the pipe and draw debris back toward the manhole for vacuum recovery.
- NASSCO Pipeline Assessment and Certification Program coding assigns each observed defect a structural or operations and maintenance grade from 1 to 5, allowing objective ranking of rehabilitation priority.
- Smoke testing rapidly screens a basin for inflow sources such as downspouts, area drains, and defective cleanouts, while dye testing confirms one specific suspected connection.
- Cured-in-place pipe lining rehabilitates a full segment without excavation and usually increases hydraulic capacity, because the liner is much smoother than the deteriorated host pipe.
- Grease and roots are the two leading causes of sanitary sewer overflows, so cleaning frequency should be set by observed segment condition and maintenance history rather than by a uniform system-wide interval.
Sewer Cleaning, CCTV Inspection & Collection System Rehabilitation
Sanitary sewer overflows are the most visible failure a wastewater utility experiences, they are reportable to DEQ, and they are overwhelmingly caused by conditions a maintenance program can find and fix: grease, roots, debris, and structural failure.
1. Cleaning Methods
| Method | Principle | Best for |
|---|---|---|
| High-velocity hydraulic jetting | A nozzle with rearward-angled jets at 1,500 to 3,000 psi propels itself upstream, then is winched back, scouring the wall and dragging debris to the manhole | The default method for 6 to 24 inch gravity sewers |
| Combination jet/vacuum truck | Jetting plus a vacuum to remove the debris pile from the manhole | Standard municipal unit |
| Mechanical rodding | A flexible rod with a cutting head driven by a power rodder | Hard roots and severe blockages |
| Bucket machine | Two winches pull a bucket back and forth | Heavy sand and grit in large pipe |
| Balling / flushing | An inflatable ball creates an annular high-velocity jet | Grease and light debris; being displaced by jetting |
| Chemical root control | Foaming herbicide (diquat or metam sodium) applied in the pipe | Root regrowth suppression; typically effective for 2 to 3 years |
| Chemical grease control | Bacterial or enzyme products | Supplementary at best; source control through a FOG program is the real fix |
Jetting cautions: excessive pressure can damage deteriorated clay or scour the invert of a corroded concrete pipe; jetting into an active surcharge can force sewage into building laterals; and jetting a segment with a known structural defect can cause it to collapse. Inspect first when the condition is unknown.
2. CCTV Inspection
A camera on a tractor or a float traverses the pipe while a technician records video and codes defects.
NASSCO PACP coding
The Pipeline Assessment and Certification Program is the industry-standard coding scheme, and PACP terminology appears in every consultant report an operator will read.
- Defects are recorded with a code, a clock position, a continuity (isolated or continuous), and a grade of 1 to 5, where 5 is the most severe.
- Codes are grouped into structural (cracks, fractures, breaks, holes, deformation, joint offsets, surface damage, lining failure) and operations and maintenance (roots, deposits, infiltration, obstructions, grease).
- Segment scores are computed as a quick rating, an overall rating, and a rating index, which allow objective ranking of thousands of segments for a rehabilitation program.
Grade guidance in practice:
| Grade | Meaning | Typical action |
|---|---|---|
| 5 | Failed or imminent failure | Immediate repair or replacement |
| 4 | Poor; failure likely in the near term | Schedule rehabilitation |
| 3 | Fair; moderate defect | Monitor, re-inspect on a defined interval |
| 2 | Good; minor defect | No action |
| 1 | Minimal defect | No action |
Related inspection tools: sonar for the submerged portion of a surcharged pipe, laser profiling for ovality and deformation in flexible pipe, multi-sensor platforms combining CCTV, sonar, and laser, and manhole inspection with a zoom camera or a pole-mounted camera.
3. Finding Inflow and Infiltration
Infiltration is groundwater entering through defects; it is steady, seasonal, and follows the groundwater table. Inflow is stormwater entering through direct connections; it is fast, peaks with the storm hydrograph, and is usually the larger and cheaper problem to fix.
| Technique | Finds | Notes |
|---|---|---|
| Flow monitoring / flow isolation | Which basin contributes the excess | Nighttime flow isolation identifies infiltration; storm-event monitoring identifies inflow |
| Smoke testing | Inflow sources - roof downspouts, area drains, uncapped cleanouts, defective laterals, manhole covers, cross-connected storm drains | Fast and cheap; requires public notification because smoke can enter buildings with dry traps |
| Dye testing | Confirmation of a specific suspected connection | Slower, definitive |
| Manhole inspection | Cover inflow, chimney and frame leakage, bench and channel defects | Manholes are a disproportionate share of inflow; a perforated cover in a ponding area can pass surprising volumes |
| CCTV | Infiltration at joints and defects, visible during high groundwater | Schedule for wet season |
| Rainfall-derived I&I analysis | Quantifies the response of each basin | The basis for prioritizing capital spending |
Manhole rehabilitation is often the highest return per dollar in an I&I program: replacing frames and covers with gasketed or bolted units, installing inflow dishes, sealing the chimney with a flexible seal, and lining the interior with cementitious or epoxy coatings.
4. Trenchless Rehabilitation
| Method | Description | Where used |
|---|---|---|
| Cured-in-place pipe (CIPP) | A resin-saturated felt or fiberglass tube is inverted or pulled into the host pipe and cured with hot water, steam, or ultraviolet light | The dominant method; rehabilitates a full segment manhole to manhole without excavation. Laterals are reinstated by a robotic cutter |
| Sliplining | A smaller-diameter pipe is pushed or pulled inside the host and the annulus grouted | Simple and cheap, but loses significant cross-section |
| Pipe bursting | A bursting head fractures the host pipe outward while pulling in a new HDPE pipe | Allows upsizing; requires launch and receiving pits and clearance from adjacent utilities |
| Fold-and-form / deformed liner | A folded thermoplastic liner is inserted and reformed with heat and pressure | Close-fit alternative to CIPP |
| Spiral wound lining | A profiled strip is wound in place | Large diameter, can be done in live flow |
| Spot repair / sectional liner | A short liner or a robotic repair addresses one defect | Where the segment is otherwise sound |
| Chemical grouting | Grout injected through a packer seals leaking joints | Stops infiltration at joints; does not restore structural capacity |
A hydraulic point candidates miss: CIPP reduces the internal diameter slightly, but because the liner's Manning roughness coefficient is much lower than the deteriorated host pipe (about 0.010 versus 0.013 or worse), the rehabilitated pipe usually carries more flow than before, not less.
Bypass pumping is required for most rehabilitation work, and it is where the safety and spill risk concentrates: sizing for peak wet-weather flow, redundant pumps, alarms, and a spill contingency plan are mandatory, not optional.
5. Building the Program
Set cleaning frequency by condition, not by calendar
A uniform "clean everything every five years" schedule wastes money on clean pipe and under-serves problem segments. A condition-based program classifies every segment:
| Classification | Cleaning frequency |
|---|---|
| Hot spot (known grease, roots, flat grade, repeated overflow) | Monthly to quarterly |
| Elevated | Semiannual to annual |
| Routine | Every 3 to 7 years |
| Low risk (steep, large diameter, clean at last inspection) | Inspection-driven only |
The data that drives the classification comes from the maintenance history (every blockage and overflow location), the CCTV assessment, and the debris recovered during the last cleaning.
Records that matter
- Segment identifier, date, method, footage cleaned, debris quantity and character.
- Every blockage and near-miss, with cause coded (grease, roots, debris, structural, capacity).
- Every sanitary sewer overflow: location, start and stop time, estimated volume, cause, receiving water, corrective action, and the DEQ notification made.
- CCTV inspection date, PACP scores, and re-inspection due date.
The connection to the plant
Collection system condition is a treatment plant issue. Excess I&I dilutes influent, raises hydraulic loading, drives clarifier washout, shortens detention times, and can push a plant over its permitted flow. Grease that is not intercepted at the restaurant becomes scum in the primary clarifier and foam in the digester. The operator who reads only the plant's data is reading half the system.
A collection system crew must decide between smoke testing and dye testing to locate the source of a large, rapid flow increase that coincides exactly with rainfall in one basin. Which technique should be used first and why?
A CCTV inspection report assigns a segment a NASSCO PACP structural grade of 5. What does this indicate?
A deteriorated 12-inch clay sewer with an effective Manning roughness of about 0.015 is rehabilitated with cured-in-place pipe having a roughness of about 0.010. What is the net hydraulic effect?