Lagoon Ammonia Excursion in Cold Weather
A food-processing facility operates a facultative lagoon system with two cells in series. Cell 1 is 12 acres at 6 feet deep and receives 0.9 MGD of flow with influent ammonia of 45 mg/L as N and BOD of 600 mg/L. Cell 2 is 8 acres at 5 feet deep and serves as the polishing cell before discharge to a small stream. The permit limits ammonia to 8 mg/L as N and BOD to 30 mg/L. It is late November, water temperature has dropped to 8 degrees C, and ice is forming at the edges. Effluent ammonia has climbed from 4 to 14 mg/L over three weeks, effluent BOD is 38 mg/L, and dissolved oxygen in Cell 2 is 1.2 mg/L near the outfall. The facility cannot reduce production and has no chemical feed system installed.
1. Explain why the ammonia limit is being exceeded and why BOD is rising at the same time.
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Ammonia: Nitrification is a temperature-sensitive biological process. At 8 degrees C the growth rate of nitrifying bacteria is roughly half or less of its rate at 20 degrees C, so the population cannot keep pace with the 45 mg/L influent ammonia load. The nitrifier population washes out or becomes dormant faster than it can regrow, and ammonia passes through.
BOD: The rising BOD is linked to the same cold-weather slowdown. Heterotrophic BOD removal also slows at low temperature, and the lagoon's mixing and oxygen transfer are less effective with ice cover. The 38 mg/L effluent BOD may also include the oxygen demand exerted by ammonia (nitrogenous BOD) as well as unsettled solids and algae from the polishing cell. Low DO of 1.2 mg/L in Cell 2 limits both carbon oxidation and nitrification.
Why the two move together:
- Cold water slows all biological kinetics, so both carbon and nitrogen removal degrade.
- Ice and reduced mixing cut oxygen transfer and create stratification.
- The polishing cell has a limited biomass inventory and cannot buffer a load it was not sized for at this temperature.
Conclusion: The excursion is a cold-temperature capacity shortfall, not a toxic upset; the fix must address oxygen, mixing, and time, not just one parameter.
2. Without chemical feed, what operational changes can you make to protect the permit, and what are the risks of each?
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1. Maximize oxygen transfer. Keep aerators or surface mixers running and clear ice where safe. Risk: breaking ice can create hazards and may reduce insulation; verify safe access and never send personnel onto ice.
2. Extend detention and reduce short-circuiting. Adjust baffles, weirs, or flow splitting so water spends more time in Cell 1 where the biomass is denser. Risk: raising the water level increases the chance of overtopping or bank erosion; check freeboard.
3. Reduce the load per unit biomass. If any equalization or storage exists, hold high-strength flow and release it slowly. Risk: stored high-strength wastewater can go septic and create odors and a shock load when released.
4. Protect the polishing cell. Reduce solids carryover from Cell 1 by maintaining the inlet and outlet structures and avoiding scouring. Risk: if Cell 1 is overloaded, solids will still pass; this is a partial measure.
5. Use the permit's compliance tools. Notify the regulator promptly, document the cold-weather cause, and request a temporary variance or compliance schedule if allowed. Risk: notification does not remove the limit, but it reduces enforcement exposure and buys time.
6. Plan for next winter. Once the emergency is past, budget for aeration or mixing upgrades and a chemical feed system for alkalinity or supplemental nitrification support.