Sludge Blanket Rising and Ammonia Breaking Through
A 4.0 MGD extended-aeration plant has two aeration basins totaling 2.0 MG, four secondary clarifiers with 10 ft side-water depth, and effluent limits of 25 mg/L CBOD5, 30 mg/L TSS, and 8 mg/L ammonia from April through October.
Over the last ten days: flow 3.6 MGD, influent CBOD5 240 mg/L, MLSS has fallen from 3,200 to 2,100 mg/L, SVI has risen from 140 to 340 mL/g, and clarifier sludge blankets stand at 7.5 ft. Effluent TSS is 42 mg/L, CBOD5 is 31 mg/L, and ammonia has climbed to 6.5 mg/L and is still rising. RAS pumps are at maximum capacity. Basin DO is 1.0 to 1.4 mg/L with all three blowers running and 0.6 mg/L in the last pass.
Three weeks ago a rendering plant began discharging 0.35 MGD averaging 1,800 mg/L CBOD5 and 400 mg/L oil and grease under a newly issued industrial permit. A grease-trap hauler also began dumping 8,000 gallons of septage per day at the headworks. Water temperature is 22 C. The collection system has had no reported overflows.
1. Diagnose the solids loss: what is the mechanism, which numbers prove it, and why is more RAS not the fix?
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Mechanism. The sludge can no longer be settled and returned as fast as it is being lost over the weirs. An SVI of 340 mL/g means 2,100 mg/L of MLSS occupies about 714 mL per liter after 30 minutes, leaving under 30 percent of the clarifier depth for clarification. That is why the blanket sits at 7.5 ft of 10 ft and effluent TSS is 42 mg/L.
Evidence that solids are escaping rather than being wasted. MLSS fell from 3,200 to 2,100 mg/L in ten days while effluent TSS rose. Unless wasting was deliberately increased, that inventory left over the weirs. Rising blanket plus rising effluent TSS confirms it.
Why more RAS will not fix it. RAS returns settled sludge; it does not make sludge settle. RAS is already at maximum, which is the correct first move, but pushing beyond design return rates adds hydraulic and solids loading to the clarifiers and can pull the blanket higher.
Most likely cause. Filamentous bulking. DO at 1.0 to 1.4 mg/L, and 0.6 mg/L in the last pass, is well below the 2.0 mg/L target and selects for filaments. The new rendering discharge, at 400 mg/L oil and grease and 0.35 MGD, feeds organisms such as Microthrix that thrive on fats and long-chain fatty acids. Confirm with a microscopic exam of the mixed liquor.
Immediate levers: hold RAS at maximum, cut clarifier hydraulic load by reducing throughput or equalizing flow, raise DO by cleaning diffusers and redistributing air, feed polymer to improve capture, and control grease at the source.
2. Effluent ammonia is 6.5 mg/L against an 8 mg/L permit limit and rising. What links the solids loss to nitrification, and what are your moves?
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Nitrification depends on holding a slow-growing population in the system, which means a long enough solids retention time. As solids wash out over the clarifiers, SRT falls and the nitrifiers, the slowest growers in the mixed liquor, are lost first. A 34 percent MLSS drop in ten days with 42 mg/L effluent TSS is a collapsing SRT, and ammonia climbing to 6.5 mg/L is the expected symptom.
Dissolved oxygen compounds it. At 0.6 mg/L in the last pass, ammonia oxidation is rate-limited; you want roughly 2.0 mg/L where nitrification is happening. Low DO also favors the filaments causing the washout, so this is one problem with two faces.
Moves, in order: stop wasting entirely until the blanket is under control; restore solids capture with polymer and reduced hydraulic load; fix air delivery by checking diffusers, blower filters, and whether air is short-circuiting to the first pass; check alkalinity and pH, because nitrification consumes about 7.1 mg of alkalinity per mg of ammonia-nitrogen and can drive pH down far enough to stall itself; reduce the nitrogen and grease load by holding septage deliveries and requiring pretreatment from the rendering plant.
On the permit: at 6.5 mg/L against a limit of 8, the monthly average is at risk. Call the state now, describe the upset and the corrective actions, and document everything. An exceedance you did not report is a much worse problem than the exceedance itself.
3. What do you do about the rendering plant and the septage hauler over the next two weeks?
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Treat this as a pretreatment and permitting problem, not only an operations problem.
Sample the rendering discharge with a composite sampler across a production day and analyze for CBOD5, TSS, oil and grease, pH, sulfide, and flow. Compare results to its permit limits and to the local limits that should exist for this plant. Oil and grease at 400 mg/L and 0.35 MGD is on the order of 11,600 pounds per day of grease, which is large relative to this plant's capacity and is also a collection-system problem waiting to happen.
Options: enforce local limits and issue a notice of violation if the discharge is out of compliance; require grease-interceptor installation, maintenance, and manifest records; require flow equalization or a scheduled discharge so the load arrives evenly; apply a surcharge for excess strength; and consider a temporary suspension of septage receipts until the plant recovers, since septage adds ammonia and solids at the worst possible moment.
Also examine how the industrial permit was written. If it was issued without adequate local limits or without a pretreatment evaluation, the fix includes reopening it. Document sampling, correspondence, and plant operating data, because any enforcement action or surcharge rests on that record.