Chromium Slug to an Activated Sludge Plant
A metal-finishing shop discharges to a municipal activated sludge plant under an industrial pretreatment permit. The shop has a batch treatment system using hydroxide precipitation and settling, but a valve failure on a rinse tank sent an untreated batch to the sewer. Two hours later the plant's mixed liquor dissolved oxygen rises from 1.8 to 5.5 mg/L, the food-to-microorganism ratio appears normal, effluent BOD climbs from 12 to 60 mg/L, and the sludge blanket in the clarifier begins to thin. The plant samples the influent and finds total chromium at 9 mg/L, well above the shop's 2.5 mg/L pretreatment limit. The plant serves 18,000 people and discharges to a river with a 30 mg/L BOD limit. The shop has a 4,000-gallon holding tank that is currently empty.
1. What does the sudden DO rise combined with falling BOD removal tell you, and how does the chromium finding fit?
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The DO rise is a warning sign of biological inhibition, not improved treatment. When the biomass stops consuming oxygen, the blowers keep supplying air and DO climbs. A normal food-to-microorganism ratio with rising effluent BOD means the food is present but the organisms are not using it.
Why chromium fits:
- Total chromium at 9 mg/L is far above the shop's 2.5 mg/L limit and is in the range where hexavalent chromium and other heavy metals inhibit activated sludge. Metals bind to cell surfaces and disrupt enzyme function.
- The thinning sludge blanket suggests the floc is losing integrity and settling poorly, consistent with metal stress and possible loss of biomass.
- The timing matches the valve failure and the two-hour travel time to the plant.
What it is not:
- A DO rise with normal loading is not a sign of over-aeration from a light load; the effluent BOD is rising, so the load is still there.
- It is not a simple hydraulic surge, which would dilute rather than inhibit.
Conclusion: The plant is experiencing heavy-metal inhibition of the biomass, and the chromium slug is the likely cause. Confirm with a mixed liquor respiration or oxygen uptake rate test and a metals scan of the influent.
2. What immediate actions protect the plant, and what should the pretreatment program require of the shop?
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Immediate plant actions:
- Isolate and contain. Work with the shop to divert any remaining flow to its empty 4,000-gallon holding tank and stop further discharge until the batch is characterized.
- Protect the biomass. If the slug is still arriving, consider diverting influent to equalization or storage if available; avoid wasting sludge during the event so the biomass inventory is preserved for recovery.
- Monitor intensively. Sample influent and mixed liquor for metals, run oxygen uptake rate and settleability tests, and track effluent BOD and TSS hourly until the slug passes.
- Notify. Inform the regulator, document the event, and warn downstream users if the discharge reaches the river.
Pretreatment program requirements for the shop:
- Issue a notice of violation for exceeding the 2.5 mg/L chromium limit and require immediate correction of the valve failure.
- Require the shop to characterize and properly treat the held batch before release, with verification sampling.
- Require a root-cause report, valve inspection and maintenance records, and a spill prevention plan for rinse tanks.
- Consider increased monitoring frequency and, if the violation is significant, an enforcement action with a compliance schedule.
Recovery: Once metals clear, the biomass should recover if it was not killed outright; seed with healthy sludge if the mixed liquor volatile suspended solids drop sharply.