I/I Spike and SSO at a Low-Lying Manhole
A combined collection area of about 15,000 feet of 8- to 15-inch gravity sewer serves 900 connections and discharges to a lift station. During a two-inch rain in three hours, the lift station wet well rises from its normal 4-foot level to within six inches of the high-level alarm. A low-lying manhole in a residential area begins surcharging, and sewage is surfacing at the rim near a storm drain that discharges to a creek. The crew has a portable pump and a vacuum truck. The weather forecast calls for another inch of rain in four hours. The collection system has known root intrusion in two segments and a sagging section under a creek crossing. The state requires reporting of sanitary sewer overflows within 24 hours and public notification for overflows that reach surface water.
1. What are your immediate priorities, in order, and why?
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Priority order is public health and surface water first, then system protection, then documentation.
- Stop or contain the overflow at the manhole. Use sandbags, berms, or the vacuum truck to capture and haul the flow so it does not reach the storm drain and creek. Sewage entering surface water is the highest-consequence outcome.
- Notify. Because the overflow is reaching (or at risk of reaching) surface water, start public notification and call the state hotline. The 24-hour reporting clock has started; do not wait for the rain to end.
- Protect the lift station. The wet well is near the high-level alarm. Stage the portable pump to supplement the lift station and keep the wet well below alarm to avoid a pump-station overflow, which would be a larger release.
- Deploy the vacuum truck to the low manhole to draw down the surcharge and reduce surfacing.
- After the event, clean and disinfect the affected area and document everything: rainfall, levels, overflow volume estimate, photos, and actions.
The storm drain connection makes containment and notification non-negotiable even while the rain continues.
2. After the event, how do you use the I/I data to prioritize the repair program?
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Treat this as an inflow and infiltration (I/I) investigation with the rain event as the data source.
The two-inch rain in three hours is a defined event; compare it to the wet-well response and manhole surcharge to separate inflow (fast response, directly connected) from infiltration (delayed, groundwater-driven). The rapid rise to near-alarm during the storm and the surfacing at a low manhole suggest significant inflow, likely from the storm drain connection or direct connections, plus the known root intrusion and creek-crossing sag acting as infiltration pathways.
Prioritization:
- Smoke-test or dye-test the area around the low manhole and the storm drain connection to find direct inflow sources. Direct storm connections are the fastest, cheapest fix and give the biggest peak-flow reduction.
- CCTV the two root-intrusion segments and the creek-crossing sag. Root intrusion and sags are infiltration and blockage risks; the sag also traps solids and can cause dry-weather SSOs.
- Rank repairs by cost per gallon of peak flow removed. Inflow sources first, then structural infiltration.
- Re-evaluate the lift station capacity against the post-repair flows, and consider a flow monitor in the basin to measure the reduction.
Document the analysis so the repair program is defensible to the state and to ratepayers.