Storm-Driven SSO at a Lift Station and Low-Point Manhole
A collection system serves 18,000 people. A 12-inch vitrified clay gravity interceptor runs 1.2 miles to a two-pump lift station with 75-hp pumps. During a 2-hour storm of 1.5 inches, the wet well rises from 3.5 to 7.2 feet; the high-level alarm is 6.5 feet. The lead pump cycles every 90 seconds instead of every 6 minutes. The lag pump starts, but its discharge pressure is 8 psi lower than normal. A grease mat is visible in the wet well. Upstream, a manhole at the low point overflows about 300 gallons to a storm drain that discharges to a wash. The utility has a portable pump and a vactor truck.
1. What immediate response actions are required, including spill control and notification?
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Treat the overflow as an SSO and stop it first. Dispatch the crew to the upstream manhole; use the vactor to remove the grease mat and the portable pump to relieve the wet well. Place containment and absorbent around the storm drain, recover as much of the 300 gallons as possible, and disinfect the area. Post warning signs at the wash and notify the local health department and NDEP as required by the permit; record start and stop time, volume, receiving water, and corrective action. Do not wash the spill into the storm drain. Because the storm drain discharges to a wash, assume a waters-of-the-state notification is needed. Keep the public out of the area and document with photos. If the overflow continues, pump and haul to the headworks. Confined-space entry into the wet well requires a permit and is not the first response.
2. How would you determine whether the cause is inflow/infiltration, a pump problem, or a grease blockage?
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The 90-second cycle and 7.2-foot wet well show inflow and infiltration during the storm, but the lag pump's 8-psi lower discharge pressure suggests a pump or check-valve problem that reduced capacity. Check the lag pump suction for a grease or rag plug, verify the check valve and discharge valve position, and compare amp draw and flow. The grease mat indicates the interceptor is surcharged and grease is being pushed downstream. The low-point manhole overflowing to the storm drain confirms the gravity line is undersized or blocked. Use the vactor on the interceptor, not just the wet well; grease plus storm flow can seal a 12-inch line. Verify pump cycling after cleaning; if it returns to 6 minutes, rain inflow was the trigger.
3. What follow-up reporting and capital or maintenance actions would you recommend?
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Report and correct. Submit the SSO report with volume, duration, cause, and corrective actions, and keep the notification log. Clean and disinfect the affected storm drain and wash per the health department. For the root cause, televise the 1.2-mile interceptor and the low-point manhole, and smoke-test or dye-test the tributary area to find inflow sources such as roof leaders, yard drains, and broken laterals. Add a grease control program for restaurants and increase vactor cleaning frequency at the low point. For capacity, evaluate a larger interceptor or a storage/equalization basin at the lift station, and add a third pump or variable-speed control if the lag pump is marginal. Inspect the lag pump check valve and impeller and replace worn parts. Track rainfall versus wet-well level to prioritize I/I removal.