Septage, Rising Blanket, and a Wasting Rate Nobody Wants to Change
A 2.5-MGD extended-aeration plant has a 1.0-MG aeration basin and one 60-ft-diameter clarifier. Three weeks ago a septage hauler began dumping about 8,000 gallons per day at the headworks. Since then MLSS has climbed to 3,200 mg/L, SVI has gone from 130 to 240, the clarifier sludge blanket sits at 9 ft in a 12-ft sidewater depth, and effluent TSS has risen from 12 to 32 mg/L. Ammonia is steady at 0.9 mg/L, DO holds 1.5 to 2.0 mg/L, influent BOD is 210 mg/L, and RAS is about 4,200 mg/L. Wasting runs at 15 gpm, and the operator is reluctant to waste harder because the blanket is already high.
1. Compute the solids inventory and the wasting rate needed for an 8-day SRT. How does the current 15 gpm compare, and what does that tell you about the blanket?
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Inventory: 1.0 MG x 3,200 mg/L x 8.34 = 26,700 lb of solids (aeration basin only; the clarifier adds roughly another 10 to 20%).
Solids that must leave at an 8-day SRT: 26,700 / 8 = 3,340 lb/day.
Solids already leaving in the effluent: 2.5 MGD x 32 mg/L x 8.34 = 667 lb/day.
Required wasting: 3,340 - 667 = 2,670 lb/day. At a WAS concentration of 4,200 mg/L (which is what the poor SVI gives you), flow = 2,670 / (4,200 x 8.34) = 0.076 MGD = 76,000 gpd = about 53 gpm.
Current condition: 15 gpm = 21,600 gpd x 4,200 x 8.34 = 755 lb/day, plus 667 in the effluent, so about 1,420 lb/day leaves and the actual SRT is roughly 26,700 / 1,420 = 19 days. The F/M is 2.5 x 210 x 8.34 = 4,380 lb BOD/day divided by 26,700 = 0.16/day, which is a very old, very low-F/M sludge.
The insight: the high blanket is not caused by wasting too little mass - it is caused by sludge that will not settle or compact, at an SVI of 240. Wasting more does not directly pull the blanket down; settleability, RAS rate, and clarifier loading do. But running at a 19-day SRT is feeding the problem, and the operator's caution is aimed at the wrong variable.
2. Is this bulking, solids overload, or hydraulics? What evidence would separate them, and how much blame does the septage deserve?
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Test the septage first, quantitatively. 8,000 gpd at, say, 5,000 mg/L BOD is about 334 lb/day, roughly 8% of the plant's BOD load - not enough by itself to double an SVI. But septage carries grease, sulfide, grit, and high TSS, and those affect filaments and settling out of proportion to BOD. Get a sample: BOD, TSS, grease, pH, sulfide, and total flow over a week.
Separate the mechanisms:
- Filamentous bulking: SVI 240 with a slow-settling, cloudy supernatant and a high filament count on the microscope. Look at filament type and abundance - that identifies the selector (low DO, low F/M, nutrient deficiency, septic/sulfide waste).
- Low-F/M (old sludge) bulking: consistent with an SRT near 19 days and F/M of 0.16, and septage grease can push this further.
- Hydraulic limit: 2.5 MGD over a 60-ft clarifier (2,827 ft²) is 884 gpd/ft² - at the top of the range for extended aeration. There is very little margin, so any settling deterioration immediately shows up as blanket rise and solids carryover.
- Denitrification: check for rising gas bubbles, floc that floats in the clarifier, and blanket that "fluffs" - a different fix entirely (reduce RAS hold time, adjust internal recycle).
Conclusion to defend: the septage is a contributor and possibly the trigger, but the plant was already running an old sludge at the edge of its clarifier capacity. Fixing only the septage will not restore the SVI.
3. What do you do this week, and what is the risk in simply cranking up RAS to pull the blanket down?
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Do this week:
- Step wasting up toward 40 to 50 gpm, in increments over several days, not in one jump. Verify with a daily solids balance and watch the blanket response. Sludge that old will not shed overnight.
- Sample and meter the septage and set an acceptance limit or a pretreatment requirement; if grease and sulfide are high, cut the volume while you diagnose.
- Microscope the sludge to name the filament. That determines whether the answer is a selector, DO control in the first cell, nutrient addition, or chlorination for a specific filament - chlorinating blindly at an SVI of 240 kills the good floc with the bad.
- Optimize RAS, do not maximize it. Set RAS to hold a stable blanket, and check that RAS is actually thickening (4,200 mg/L is thin, a sign of short-circuiting or a plugged RAS line).
- Protect the effluent now - polymer or coagulant aid ahead of the clarifier buys time while the biology is corrected, and it is cheap compared with a permit excursion.
The RAS risk: more RAS means more flow through an already hydraulically loaded clarifier, a higher surface overflow rate, and a thinner RAS. At 884 gpd/ft² you are already marginal. Crank RAS and you can pull the blanket down by pushing solids over the weir - the effluent TSS rises, the blanket looks better, and the permit violation gets worse. The control lever is wasting rate and settleability, with RAS adjusted to serve them.