Lagoon TSS and Ammonia Excursion After Storm
A town of 2,100 operates a three-cell facultative lagoon permitted at 0.35 MGD under an Oklahoma OKPDES permit. The lagoon normally holds about 7.5 MG, giving roughly 21 days of detention. After a 2.4-inch storm, influent reaches 1.05 MGD for two days and the collection system has known infiltration. Effluent at the outfall is TSS 48 mg/L, BOD 24 mg/L, ammonia 9.0 mg/L as N, and DO 1.1 mg/L; permit limits are 30 mg/L TSS, 20 mg/L BOD, and 6.0 mg/L ammonia from May through October. There is no permitted bypass, but the plant has a diversion to an emergency storage basin that is only 40 percent empty. Algae mats are visible in the final cell, and the chlorinator/dechlorinator is operating but the contact basin is short-circuiting. The operator must decide whether to discharge, store, or recirculate, and what to tell DEQ.
1. Is the exceedance more likely caused by the storm or by chronic lagoon performance? What immediate steps would you take?
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Both storm and chronic conditions are involved, but the storm is the trigger. At 1.05 MGD, the 7.5-MG lagoon detention falls from about 21 days to about 7 days, which shortens settling and washes algae and solids toward the outfall. Low DO of 1.1 mg/L and high ammonia of 9.0 mg/L indicate incomplete nitrification and possibly organic overload, while the TSS of 48 mg/L is consistent with algae and solids carryover. Immediate steps: maximize aeration in the first cells, reduce or stop recirculation that short-circuits the final cell, inspect and repair the contact basin baffling, collect flow-weighted samples, and preserve the chain of custody. Use the emergency basin if it can hold the excess without creating a worse overflow; do not bypass. Notify DEQ of the exceedance and the operational response. The immediate goal is to protect the stream and restore detention, not to mask the numbers.
2. How should the operator weigh emergency storage against discharge, and what should the DEQ notification cover?
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Emergency storage is preferable to an unauthorized discharge if the basin has capacity and can be emptied without causing a later shock load. With the basin 40 percent empty, estimate usable volume and the time to fill at 1.05 MGD before deciding. If storage is insufficient, recirculate to the head of the lagoon only if it does not further reduce detention, and prioritize aeration. A discharge that exceeds the permit is not automatically a bypass, but it must be reported and evaluated. The long-term answer is infiltration/inflow removal and lagoon solids management; the storm merely exposed those weaknesses. Document flow, rainfall, DO, ammonia, and TSS before, during, and after, and propose a corrective action schedule to DEQ. If the basin is used, plan a controlled return rate so the treatment units are not shocked.