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. The water still has to be pumped, conveyed, treated, and sometimes reported when it contributes to an SSO. EPA notes that sanitary sewers are intended to carry wastewater, not widespread storm drainage; cracks, faulty seals, and improper connections can overload the system during wet weather.
The first decision is timing. If a manhole overflows only during rain, suspect I/I, inadequate capacity, or downstream wet-weather surcharge. If a basement backup happens on a dry day near a restaurant district, suspect blockage before I/I. If plant influent rises hours or days after rain and stays elevated, groundwater infiltration may be more important than direct inflow.
Investigation tools
| Tool | Best use | What a good answer includes |
|---|---|---|
| Flow monitoring | Quantify dry-weather flow, peak wet-weather response, and basin priority | Install in logical basins, compare rainfall, calibrate sensors |
| Rain gauges | Link flow peaks to rainfall amount and timing | Use local rainfall, not a distant airport only |
| Smoke testing | Locate inflow paths such as roof drains, yard drains, cross connections, defective cleanouts, and leaky laterals | Notify residents, check traps, document where smoke exits |
| Dye testing | Confirm a suspected connection or drainage path | Add dye at source and watch downstream manhole or sewer |
| CCTV | Identify cracks, offset joints, holes, roots, infiltration, sags, and protruding taps | Clean enough to see and code defects consistently |
| Manhole inspection | Find leaky covers, frame seals, chimney defects, bench/channel damage, and inflow at pick holes | Inspect during or soon after rainfall when possible |
Rehab selection logic
Choose rehabilitation based on the defect, not on the newest tool. Grouting can seal leaking joints or small defects but does not restore a badly broken pipe. Cured-in-place pipe (CIPP) can create a new liner inside the host pipe, but service laterals must be reopened and the host pipe must be suitable for lining.
Sliplining inserts a smaller pipe and may reduce diameter, so hydraulics matter. Pipe bursting can replace a deteriorated line with a new pipe while breaking the old one outward, but nearby utilities, laterals, ground movement, and access must be evaluated. Open-cut replacement is disruptive but may be necessary for collapse, severe grade defects, bad 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, and 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 | Lining follows the existing bad grade |
| Defective manhole chimney or cover inflow | Frame seal, chimney seal, insert, cover replacement, lining | Do not ignore private inflow sources in the same basin |
Emergency bypass logic
Bypass pumping is temporary conveyance around a failed pipe, pump station, or rehab work zone. The operator must maintain upstream service while protecting downstream capacity. A bypass plan should identify suction and discharge points, pump capacity, standby pump, fuel or power, hose route, traffic protection, air release or priming needs, alarms, spill containment, and monitoring.
Exam traps include undersizing the pump, discharging to a location that cannot handle the flow, forgetting backup power, blocking traffic without a plan, or assuming bypass pumping eliminates confined-space and gas hazards. During an emergency, the best answer usually protects life first, stops or contains the overflow, notifies according to the applicable rules, documents the event, and then corrects the cause.
A manhole overflows only during intense rainfall. Dry-weather inspections show normal flow. Which investigation sequence is most appropriate?
Smoke testing shows smoke coming 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