Activated-Sludge Bulking and Effluent TSS Exceedance
An activated-sludge plant treats 5.0 MGD with an aeration basin volume of 1.5 MG and two secondary clarifiers. Current MLSS is 3,200 mg/L, SVI is 230 mL/g, RAS flow is 2.5 MGD, and WAS is 0.07 MGD. Influent BOD5 load is 8,500 lb/day. The NPDES permit limits monthly average TSS and BOD5 to 30 mg/L each; last month’s effluent was 36 mg/L TSS and 27 mg/L BOD5. Operators report a rising clarifier sludge blanket, small pin flocs in the effluent, and DO of 1.2 mg/L in the aeration basin. The permit requires reporting anticipated exceedances. The plant manager wants to increase RAS to 4.0 MGD and defer wasting until the blanket drops. A nearby industrial discharger recently increased its flow, and the collection system has no equalization.
1. Calculate the F/M ratio and solids retention time (SRT) using the given data. What do these values, together with the SVI and DO, suggest about the cause of the settling problem?
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F/M ratio:
- MLSS mass = 1.5 MG × 3,200 mg/L × 8.34 = 40,032 lb
- F/M = 8,500 lb BOD5/day ÷ 40,032 lb MLSS = 0.21 day⁻¹
SRT:
- WAS solids = 0.07 MGD × 3,200 mg/L × 8.34 = 1,868 lb/day
- SRT = 40,032 lb ÷ 1,868 lb/day = 21.4 days
Interpretation: The SVI of 230 mL/g is well above the usual 80–150 mL/g range for good settling, so the sludge is bulking. A low F/M of about 0.21 day⁻¹ with a long SRT of about 21 days, combined with DO of only 1.2 mg/L, is consistent with low-DO or low-F/M filamentous bulking. The pin flocs may reflect deflocculation under low DO rather than a young sludge. The clarifier blanket rise and effluent TSS of 36 mg/L are symptoms of poor settling and solids washout, not a problem that can be solved by returning more sludge alone.
2. Evaluate the manager’s plan to raise RAS to 4.0 MGD and stop wasting. What would you do instead, and what immediate risks does the plan create?
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Raising RAS can temporarily lower the blanket by returning more solids to the aeration basin, but it does not reduce the total solids inventory. Stopping wasting is counterproductive: it pushes SRT above 21 days, increases MLSS, worsens the already low F/M condition, and raises oxygen demand in a basin that is already at DO 1.2 mg/L. At SVI 230, the clarifier is flux-limited; a higher RAS rate may simply recycle more water and create a thicker blanket or more washout.
A better response is to increase wasting gradually to bring SRT toward roughly 10–12 days, raise DO above 2 mg/L, and investigate the filamentous organism type and possible nutrient deficiency. Maintain RAS at a rate that keeps the blanket stable, not simply at maximum. Check the industrial contributor for toxic or high-organic loads and consider flow equalization. Immediate risks of the manager’s plan include continued permit exceedance for TSS, solids carryover to the receiving water, possible ammonia permit problems if nitrification is affected, and loss of sludge inventory if the blanket rises over the weirs.
3. The effluent TSS is already above the monthly average limit. What reporting and operational actions should be taken before the next monthly discharge monitoring report?
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First, do not wait for the DMR to disclose the problem. Notify the state NPDES agency as required by the permit’s noncompliance reporting clause, and document the date, cause, and actions taken. The DMR must report the actual measured TSS and BOD5 values, not adjusted or estimated values intended to hide the exceedance.
Operationally, increase wasting in steps while monitoring MLSS and SVI, improve aeration to raise DO, inspect the clarifiers for hydraulic short-circuiting or sludge collector problems, and consider temporary polymer or coagulant addition only if it is compatible with the permit and sludge handling. Reduce or divert the industrial load if it is contributing to the upset. Increase effluent monitoring and clarifier blanket checks to daily until the blanket is stable. If the exceedance is anticipated to continue, the permit may require a written report and a corrective action plan. The key is to control solids inventory and settling, not merely to increase RAS.