Infiltration and inflow investigation, smoke/dye testing, rehab selection, and emergency bypass logic
Key Takeaways
- Infiltration is groundwater entering through defects; inflow is stormwater entering through direct or rapid pathways such as roof drains, sump pumps, area drains, broken cleanouts, or leaky covers.
- Wet-weather SSOs point to I/I or capacity problems; dry-weather backups point first to blockages, sags, roots, grease, equipment failures, or collapsed pipe.
- Smoke testing finds inflow pathways, dye testing confirms suspected connections, flow monitoring quantifies rainfall response, and CCTV identifies pipe defects that allow infiltration.
- Rehabilitation selection depends on defect type, pipe shape, structural condition, hydraulics, lateral connections, access, service disruption, and whether the pipe needs cleaning, sealing, lining, bursting, or replacement.
- Emergency bypass pumping must be sized and controlled to maintain service, protect workers and the public, prevent SSOs, and avoid flooding downstream capacity.
I/I symptoms and investigation sequence
Infiltration and inflow (I/I) add unwanted water to sanitary sewers. Infiltration is groundwater seeping in through cracks, bad joints, and failed seals; inflow is stormwater entering rapidly through roof drains, sump pumps, area drains, broken cleanouts, and leaky manhole covers. Either way the water must be pumped, conveyed, and treated, and it may have to be reported when it causes an SSO. EPA notes that sanitary sewers carry wastewater, not widespread storm drainage; defects and improper connections overload the system during wet weather.
The first decision is timing. A manhole that overflows only during rain points to inflow, infiltration, or downstream wet-weather surcharge. A dry-day basement backup near a restaurant district points to a blockage first. If plant influent rises hours or days after rain and stays elevated, groundwater infiltration likely outweighs direct inflow. A quick screen compares dry-weather flow to peak wet-weather flow: a peaking factor far above the design value (for example, peak flow several times average dry-weather flow) flags an I/I-heavy basin.
Investigation tools
| Tool | Best use | What a good answer includes |
|---|---|---|
| Flow monitoring | Quantify dry-weather flow, peak wet-weather response, and basin priority | Place meters in logical basins, compare to rainfall, calibrate sensors |
| Rain gauges | Link flow peaks to rainfall amount and timing | Use local rainfall, not just a distant airport |
| Smoke testing | Locate inflow paths: roof drains, yard drains, cross connections, defective cleanouts | Notify residents, fill traps, document where smoke exits |
| Dye testing | Confirm a suspected connection or drainage path | Add dye at the source, watch the downstream manhole |
| CCTV | Find cracks, offset joints, holes, roots, infiltration, sags, protruding taps | Clean enough to see and code defects consistently |
| Manhole inspection | Find leaky covers, frame seals, chimney defects, inflow at pick holes | Inspect during or soon after rainfall when possible |
Rehab selection logic
Choose rehabilitation by the defect, not the newest tool. Grouting seals leaking joints or small defects but does not restore a broken pipe. Cured-in-place pipe (CIPP) forms a new liner inside the host pipe, but laterals must be reopened (usually by a robotic cutter) and the host must be suitable for lining. Sliplining inserts a smaller pipe and reduces diameter, so verify hydraulic capacity first. Pipe bursting replaces a deteriorated line with new pipe while fracturing the old one outward, but nearby utilities, laterals, and ground movement must be checked.
Open-cut replacement is disruptive but necessary for collapse, bad grade, poor bedding, or shallow accessible failures.
| Condition | Likely rehab family | Watch point |
|---|---|---|
| Leaking joints with sound pipe | Chemical grout or localized sealing | Not a fix for severe structural loss |
| Many cracks, roots, infiltration but stable shape | CIPP or other lining | Clean, inspect, verify diameter, reopen services |
| Undersized or badly deteriorated pipe | Pipe bursting or replacement | Check utilities, laterals, and capacity needs |
| Sag or belly causing solids settlement | Excavation and grade correction | A liner follows the existing bad grade |
| Defective manhole chimney or cover inflow | Frame/chimney seal, insert, cover replacement | Do not ignore private inflow in the same basin |
Emergency bypass logic
Bypass pumping is temporary conveyance around a failed pipe, pump station, or rehab zone. The operator maintains upstream service while protecting downstream capacity. Size the pump for peak flow, not average; a station passing 600 gallons per minute at peak needs pump and standby capacity above that, plus the right hose diameter to avoid excess head loss. A bypass plan identifies suction and discharge points, primary and standby pumps, fuel or power, hose route, traffic protection, priming or air-release needs, alarms, spill containment, and monitoring.
Exam traps include undersizing the pump, discharging where the receiving line cannot handle the flow, forgetting backup power, blocking traffic without a plan, or assuming bypass pumping removes confined-space and gas hazards. In an emergency, the best answer protects life first, stops or contains the overflow, notifies per the applicable rules, documents the event, then corrects the cause.
Prioritizing I/I work and the SSO response sequence
Not all I/I is worth chasing. The investigation produces flow and rainfall data that let the utility compare basins by the cost to remove a unit of peak flow against the cost to convey or treat it. Inflow sources such as a single roof drain or an open manhole pick hole are often cheap and fast to eliminate and remove large, sharp wet-weather peaks, so they are usually attacked first. Infiltration spread across miles of cracked, jointed pipe is expensive to chase and may favor targeted lining only where flow data justify it.
The exam favors answers that measure before spending: prioritize the basins and defects with the highest flow-removal value, rather than lining every leaking joint in the system.
When I/I or a failure does produce an SSO, the response order is the same one the exam tests repeatedly. First protect life and worker safety, including confined-space and gas hazards. Second stop or contain the overflow and keep it out of waterways and basements, using bypass pumping, blocking, or diversion. Third notify the public and regulators according to the permit and state rules, often within tight reporting windows. Fourth recover and clean up the spilled material and disinfect affected areas. Fifth document volume, duration, cause, and corrective actions, and feed the lesson back into the maintenance and capital plan.
An answer that jumps straight to repair while skipping safety, containment, or required notification is the trap, no matter how technically sound the repair sounds.
A manhole overflows only during intense rainfall. Dry-weather inspections show normal flow. Which investigation sequence is most appropriate?
Smoke testing shows smoke rising from a roof downspout connected to a sanitary lateral. What is the defect category and likely corrective action?
A CCTV report shows a long sag in a gravity sewer where solids settle, but the pipe wall is otherwise intact. Which rehabilitation choice best addresses the cause?
Which items are essential parts of an emergency bypass pumping plan? Select all that apply.
Select all that apply