Activated Sludge Bulking and Ammonia Excursion
An extended-aeration activated-sludge plant treats 0.45 MGD with influent BOD5 of 210 mg/L and TSS of 240 mg/L. MLSS is 4,200 mg/L, SVI is 240 mL/g, and the secondary clarifier sludge blanket is 3.5 feet above the collector. Dissolved oxygen in the aeration basin is 1.2 mg/L, RAS is 6,000 mg/L at 60% of influent flow, effluent TSS is 38 mg/L, and effluent ammonia is 9.8 mg/L against a permit limit of 4.0 mg/L. Influent pH is 6.9 and alkalinity is 90 mg/L as CaCO3. The plant has been receiving a septic-hauler discharge of 8,000 gallons per day for two weeks, and the blowers are at maximum output. The permit also has a monthly average flow limit of 0.5 MGD.
1. What is the most likely explanation for the high SVI and the high effluent ammonia, and what additional data would you check?
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The high SVI of 240 mL/g indicates poor sludge settling, likely filamentous bulking. Low dissolved oxygen at 1.2 mg/L and a low food-to-microorganism ratio from MLSS at 4,200 mg/L both favor filamentous organisms. The high clarifier blanket and effluent TSS of 38 mg/L are consistent with solids washout from bulking sludge. The ammonia result of 9.8 mg/L against a 4.0 mg/L limit points to incomplete nitrification. Low DO limits nitrification directly, and alkalinity of 90 mg/L is marginal because nitrification consumes about 7.1 mg/L of alkalinity as CaCO3 per mg/L of ammonia nitrogen oxidized. The septic-hauler discharge adds ammonia and BOD, increasing both oxygen demand and alkalinity demand.
Check the actual hauler discharge volume and strength, influent and effluent ammonia, aeration basin DO profile, blower output and diffuser condition, clarifier blanket depth and return rates, and whether any industrial or septage load contains toxic substances. Check pH at several points because nitrification lowers pH and can drive it below 6.5. Verify the SVI calculation and look at a microscopic slide for filament type; that helps distinguish low-DO bulking from nutrient-deficiency bulking.
2. What three operational changes would you prioritize, and what could go wrong with each?
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- Raise DO in the aeration basin. Target at least 2.0 mg/L, and higher in the nitrification zone if the blowers can deliver it. Risk: the blowers are already at maximum, so the operator may need to reduce flow, reduce return rates, or rent a blower. Simply opening valves without checking diffusers may not help.
- Control the septic-hauler load. Reduce or stop the 8,000 gpd septage discharge until ammonia and alkalinity are under control, or require the hauler to disclose strength. Risk: if the plant has a contractual obligation, stopping it may create a legal problem; a staged reduction and monitoring plan is safer.
- Adjust solids handling. Increase wasting to lower MLSS and reduce the clarifier solids load, but do not crash the solids retention time so low that nitrifiers wash out. Increase RAS cautiously to return sludge from the clarifier, but if the sludge is bulking, too much RAS can recycle filaments and increase the blanket. Risk: wasting too aggressively will worsen ammonia; wasting too little leaves the blanket high. The operator should make one change at a time and track SVI, blanket depth, effluent TSS, and ammonia daily.
Alkalinity may also need supplementation if pH falls below about 6.5, but adding chemicals is not a substitute for controlling the load.
3. What permit and reporting actions are required, and what monitoring would you add?
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The effluent ammonia of 9.8 mg/L is more than twice the 4.0 mg/L limit, and effluent TSS is elevated. The operator should notify the supervisor and the state as required by the permit, document the dates and lab results, and determine whether the exceedance is a monthly average or a single sample violation. Do not wait for the monthly report if the permit requires immediate oral or written notice. The monthly flow limit of 0.5 MGD is not currently exceeded at 0.45 MGD, but the septage discharge counts toward flow and should be tracked.
Increase monitoring to daily for influent and effluent ammonia, effluent TSS, DO, pH, alkalinity, and clarifier blanket depth until the plant stabilizes. Collect a microscopic sample and track SVI. Review the permit for toxicity, bypass, and upset provisions, because a treatment upset may have specific reporting and defense requirements. Keep a log of operational changes and their effects; the state will want a credible explanation and a corrective-action plan, not just a resampled result.