Rising Sludge Blanket in a Low-Load Activated Sludge Plant
A 1.5 MGD extended-aeration activated-sludge plant (aeration volume 0.50 MG) treats municipal wastewater with a seasonal load from food-service dischargers. Design influent BOD is 200 mg/L; current MLSS is 4,000 mg/L and DO in the aeration basin holds at 1.2 mg/L. The secondary clarifier sludge blanket has risen from 3 ft to 9 ft, effluent TSS is 45 mg/L against a 30 mg/L permit limit, and the 30-minute SVI is 310 mL/g. Under the microscope, long filaments dominate the floc. Mixed liquor pH is 6.9, effluent ammonia is 0.5 mg/L, and effluent nitrate is 12 mg/L. RAS is being pulled at 60% of influent flow. A few small clumps rise and break apart at the clarifier surface, but the blanket is uniformly high across the tank.
1. Calculate F/M and use it, plus the other data, to decide whether this is a low-F/M bulking problem or a denitrification (rising sludge) problem.
Read the worked answer
F/M calculation
BOD load:
200 mg/L × 1.5 MGD × 8.34 = 2,502 lb BOD/day
MLSS inventory:
4,000 mg/L × 0.50 MG × 8.34 = 16,680 lb MLSS
F/M = 2,502 / 16,680 ≈ 0.15 day⁻¹
Interpretation: this is a low-F/M system, and low F/M combined with an SVI of 310 mL/g and long filaments that extend out of the floc is textbook filamentous bulking. Low substrate selects for filaments with high surface-area-to-volume ratio that can scavenge at low concentrations.
Supporting evidence
- SVI 310: poor settling. Normal is roughly 80–150 for this type of plant.
- Filaments dominant, not just present.
- DO at 1.2 mg/L is on the thin side and favors some filaments.
- Blanket uniformly high across the clarifier - a hydraulic/settling failure.
Why it is not primarily denitrification Rising sludge from denitrification would show the blanket lift in chunks, a clear supernatant above a rising blanket, and typically low RAS/ long sludge age with high nitrate in the clarifier. Here nitrate is 12 mg/L and there are only a few surface clumps - a secondary symptom. The uniform 9-ft blanket with SVI 310 is the primary event. Note: with high nitrate and a deep blanket, denitrification is contributing to the TSS; treat it as a second front, not the diagnosis.
2. What immediate actions reduce effluent TSS below 30 mg/L without wrecking nitrification, and in what order?
Read the worked answer
Immediate (hours to days) - stabilize the clarifier, then the biology.
- Increase RAS rate from 60% toward the range your clarifier can handle (often 100%+ of influent flow in a bulking event). Getting the blanket back down is the fastest way to cut effluent TSS. Watch RAS thickness so you are not just recycling water.
- Raise DO in the aeration basin to 2.0 mg/L. Filaments that thrive at low DO lose their edge, and you protect nitrification while you are at it. Do not overshoot much past 2.0–2.5; you are paying for it.
- Add a polymer/coagulant to the clarifier feed as a temporary settling aid, run a jar test first to pick the dose.
- Control the grease load. Talk to the food-service dischargers about FOG control; a slug of grease and readily degradable substrate is part of why this bloomed.
- Do not starve the plant further by wasting hard. You want to raise F/M, but a sudden huge waste can crash nitrification. Step the wasting up and watch SVI and effluent ammonia daily.
- Consider a short-term selector or anoxic zone if the plant has the flexibility - a plug-flow or anoxic front end favors floc-formers over filaments.
Record everything. Permit exceedances need documentation and often state notification.
3. What changes to wasting and process control will fix this over the next 4–8 weeks, and how will you know it is working?
Read the worked answer
Direction of travel: raise F/M by lowering the MLSS inventory, while keeping the sludge age high enough that nitrification survives.
Wasting plan
- Target reduction: to bring F/M from 0.15 toward roughly 0.25–0.35, you need MLSS in the neighborhood of 2,500–3,000 mg/L, not 4,000. Compute the inventory:
Target MLSS 2,800 mg/L × 0.50 MG × 8.34 ≈ 11,680 lb- That is about 5,000 lb of solids to waste - do it over weeks, not one day.
- Watch sludge age: extended aeration runs long SRTs and you still need nitrification. Check the effluent ammonia daily during the drawdown.
- Recheck F/M weekly with actual lab numbers; the design BOD is a guess, the influent sample is not.
Process control
- Settleability: SVI and diluted SVI every week, plus a weekly microscopic check on filament type and abundance.
- DO control: hold 2.0 mg/L in the aerated zones.
- Nutrient check: confirm you are not nutrient-limited (N and P in the influent relative to BOD), which also selects filaments.
- RAS/WAS flow meters calibrated - control based on numbers, not valve position.
How you know it is working
- SVI trending down toward 150 mL/g
- Blanket depth stable below about 4 ft
- Effluent TSS consistently under 30 mg/L
- Filament abundance dropping under the microscope
- Ammonia still low
If SVI has not improved in three weeks despite F/M and DO being right, revisit the industrial load - the problem may be arriving through the sewer, not growing in the basin.