SVI 210 and Rising Blankets After a New Industrial Discharge
Your 4.5 MGD conventional activated sludge plant has two 0.75 MG aeration basins and two 70-ft secondary clarifiers. For three weeks the SVI has climbed from 115 to 210 mL/g; this morning's settleometer reads 610 mL/L with MLSS at 2,900 mg/L. Last-pass DO is 0.9 mg/L even though the blowers are at 92% of capacity, and the mixed liquor has a faint musty odor. Effluent TSS is 34 mg/L against a 30 mg/L permit limit, and clarifier blankets are at 5 ft and rising. A new food-processing discharger began sending high-strength, easily degradable waste five weeks ago. Return sludge is 60% of influent flow and wasting is 12,000 gpd at 6,000 mg/L. Nothing else at the plant has changed.
1. Check the settleometer reading against the reported MLSS and SVI, and explain what 610 mL/L means for the clarifiers. At this SVI, what MLSS would keep the 30-minute settled volume at or below 300 mL/L, and what does that tell you about the fix?
Read the worked answer
Consistency check SV30 = SVI × MLSS / 1000 = 210 × 2,900 / 1000 = 609 mL/L - the reported 610 mL/L is consistent, so the lab numbers can be trusted.
What 610 mL/L means More than half the cylinder volume is still sludge after 30 minutes of settling. In the clarifier that translates to a slow, deep blanket: the RAS pump has to move a large volume of thin sludge to keep the blanket off the weirs, and any upset (flow bump, short-circuit) pushes solids over the weir.
MLSS needed to hold SV30 ≤ 300 mL/L at SVI 210 MLSS = 300 / (210/1000) = about 1,430 mg/L
That is not a workable operating point: cutting the inventory by half roughly doubles the F/M, sharply reduces SRT, and puts BOD removal and any nitrification at risk, while the RAS rate required would still be high. So dilution by wasting is not the fix - the settling character of the sludge has to change.
Load check (is the clarifier actually overloaded?)
- Clarifier area per unit = π × 35² ≈ 3,848 ft²; both units = 7,696 ft²
- Solids loading = 4.5 × 8.34 × 2,900 / 7,696 ≈ 14 lb/day/ft²
- Overflow rate = 4,500,000 / 7,696 ≈ 585 gpd/ft²
Both are inside normal design ranges. The clarifiers are not mass- or hydraulically overloaded; the sludge simply will not settle or thicken well enough to be returned. That is the signature of filamentous bulking, not a clarifier capacity problem.
2. Lay out a two-week corrective action plan, in order, and state what would make you back out of each step.
Read the worked answer
Priority 1 - remove the oxygen limitation (days 1-3) Restore last-pass DO to 1.5-2.0 mg/L. In order of cost: verify diffuser condition and blower output; if the blowers are already maxed, reduce the solids inventory modestly (not down to 1,430 mg/L) and consider temporary step-feed to equalize oxygen demand. Back out if ammonia or BOD in the effluent starts climbing as you change the SRT - you may be trading bulking for an ammonia exceedance.
Priority 2 - close the nutrient gap if it exists (days 1-5) If the BOD:P ratio is above about 100:1, feed a soluble phosphorus source (and nitrogen if needed) at the aeration basin inlet. Back out if effluent phosphorus limits are tight, in which case dose to the minimum and rely on metal-salt polishing downstream.
Priority 3 - protect the clarifiers and the permit (ongoing) Increase RAS to hold blankets at 2-3 ft, add a low-dose polymer or coagulant to the secondary clarifier influent if effluent TSS stays above the limit, and increase wasting frequency while watching SRT. Back out if polymer carryover raises effluent BOD or if the sludge becomes deflocculated (pin floc, high filtrate turbidity).
Priority 4 - RAS chlorination only as a stopgap If SVI is still climbing after 7-10 days, chlorinate return sludge at roughly 2-5 lb/day per 1,000 lb MLSS, applied to the RAS line continuously, and stop the moment the SVI turns down or pin floc appears. Chlorination suppresses filaments but will not fix low DO or a nutrient deficiency, and it can deflocculate the sludge.
Priority 5 - structural fix Once conditions are stable, propose a selector (anoxic or anaerobic) or a re-aeration/plug-flow arrangement at the head of the basin. Selectors work on the substrate-gradient cause, which a new easily degradable industrial load has created.
Verification Track SVI and SV30 daily, blanket depth each shift, effluent TSS, microscopic filament index weekly, and ammonia if you are nitrifying. Improvement in the settling rate should show within one to two SRTs; if the SVI is flat after that, re-examine the industrial load - the discharger may be the problem, not the biology.