Bulking Sludge Overwhelms Two Secondary Clarifiers
A 2.8 MGD conventional activated-sludge plant has two 70-ft-diameter secondary clarifiers with 12 ft side water depth, and 0.96 MG of aeration volume at an MLSS of 3,300 mg/L. Return sludge at 8,000 mg/L is pumped at 0.7Q and split evenly; influent BOD is 220 mg/L and flow is 2.8 MGD. Effluent TSS has risen from 12 to 38 mg/L and the sludge blanket sits at 10.5 ft in both clarifiers. A 30-minute settle test on the mixed liquor reads 650 mL/L, but the sample sat for 20 minutes before the test started and the cylinder was a 1,000-mL graduated cylinder. Staff suspect bulking sludge, yet the clarifiers have always had hydraulic capacity to spare.
1. Calculate the SVI, interpret what it means for the clarifier underflow and blanket, and critique how the sample was handled.
Read the worked answer
SVI. SVI = (settled volume in mL/L x 1000) / MLSS in mg/L = (650 x 1000)/3,300 = 197 mL/g. Above about 150 mL/g indicates poor settling; 197 mL/g is bulking.
What it means. At that SVI the sludge can only be thickened to about 1,000,000/197 = 5,100 mg/L in the underflow. The plant is reporting RAS at 8,000 mg/L, which is only possible because the blanket is being drawn deep - 10.5 ft in a 12 ft clarifier. That is not a thickening success; it is the clarifier failing to pass solids, and it explains effluent TSS climbing from 12 to 38 mg/L.
Sampling critique. The 20-minute delay is a real problem: mixed liquor held without aeration or gentle mixing starts to denitrify and to flocculate on its own, and both change the settle result. Pull the sample, aerate or gently stir it, and start the test within a couple of minutes. Use a purpose-made 1,000-mL settleometer and note whether you stirred slowly - stirred SVI is more reproducible than quiescent. Finally, one SVI is a snapshot; trend it against MLSS and flow.
2. Compute the hydraulic overflow rate, the clarifier solids loading rate and the F/M, then state where the real constraint is.
Read the worked answer
Geometry. Area per clarifier = pi x 35 squared = 3,848 ft2; both clarifiers = 7,696 ft2.
Hydraulic overflow rate (per clarifier, Q = 1.4 MGD): 1,400,000 / 3,848 = 364 gpd/ft2. Comfortably inside typical secondary-clarifier design values.
Solids loading rate. Return rate is 0.7Q, so each clarifier receives 1.4 + 0.98 = 2.38 MGD of mixed liquor. Solids = 2.38 x 8.34 x 3,300 = 65,500 lb/day. Divided by 3,848 ft2 = 17.0 lb/day/ft2, against a usual guideline of roughly 25-30 lb/day/ft2.
F/M. BOD load = 2.8 x 8.34 x 220 = 5,137 lb/day. Aeration-basin MLSS inventory = 0.96 x 8.34 x 3,300 = 26,400 lb. F/M = 0.19 lb BOD per lb MLSS per day - low-normal, which is exactly where low-F/M filamentous bulking lives.
The judgement. Every hydraulic and loading number says the clarifiers have capacity, so this is not a concrete problem and capital should not go to clarifiers. The constraint is the settling flux of this sludge: at SVI 197 the clarifier cannot simultaneously clarify and thicken at the RAS rate being asked of it. The fix is in the biology and the RAS/WAS strategy.
3. Lay out a corrective sequence - immediate, diagnostic, and design - that addresses the actual cause rather than the symptom.
Read the worked answer
Immediate (days).
- Do not raise the RAS rate. At SVI 197 more RAS returns thinner sludge and adds hydraulic and solids load; a deep blanket plus aggressive RAS is how you get 38 mg/L effluent TSS.
- Lower MLSS by wasting to waste, not by throttling RAS. Lower MLSS raises F/M, drops the blanket and buys the clarifier time.
- Feed polymer to the clarifier influent as a temporary crutch, with a jar test to pick the dose.
- Check DO in the last aeration pass and in the RAS line - low DO plus a long RAS line is a classic filament selector.
Diagnose the filament (weeks).
- Microscopy: Types 1701 and Haliscomenobacter point to low F/M or low DO; Microthrix points to grease, septicity and long MCRT; Thiothrix points to sulfide or nutrient imbalance.
- Check influent for septicity (sulfide, industrial sulfite) and for nutrient deficiency near 100:5:1 BOD:N:P.
- Confirm no sidestream - digester supernatant or centrate - is dumping load or sulfide at a bad time of day.
Design (months).
- Add an anoxic or aerobic selector ahead of the main basin: a small high-F/M, high-DO zone that favors floc-formers over filaments.
- Hold DO at 2.0 mg/L in aerated zones as a controlled setpoint and review MCRT - a low F/M is a design choice, and if the SRT exceeds permit needs, shorten it.
- Rewrite RAS/WAS control so the blanket is controlled by solids mass and blanket depth, not by a fixed pump setting.