Bulking Sludge After a Dewatering Outage
A 4.5 MGD extended-aeration activated sludge plant receives 3.0 MGD with influent BOD5 of 210 mg/L and TSS of 220 mg/L. Mixed liquor in four 0.35 MG aeration tanks has risen to 3,200 mg/L, SVI is 300 mL/g in a 30-minute settleometer test, and the secondary clarifier sludge blanket is at 9.5 feet in a 12-foot sidewater depth. Dissolved oxygen in the last aeration pass is 1.0 mg/L. The belt filter press has been down six days awaiting a bearing, so wasting has been cut to about one-third of normal and the sludge holding tank is nearly full. Effluent ammonia has risen from 0.4 to 1.1 mg/L against a summer limit of 1.5 mg/L, and effluent TSS is 22 mg/L against a limit of 30 mg/L. Flows are normal and there is no unusual industrial load. Return sludge is at the maximum 60 percent of influent and cannot be increased.
1. Explain the mechanism linking the press outage to the clarifier blanket and the ammonia trend, and identify the biggest risk in the next 48 hours.
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The press outage cut wasting, so solids inventory and MLSS are climbing. Rising MLSS with constant organic load drives F/M down, which selects for low-F/M filamentous organisms; SVI 300 is the signature. The settleometer arithmetic shows the severity: settled volume after 30 minutes is SVI times MLSS in g/L, or 300 x 3.2 = 960 mL/L. The sludge occupies essentially the whole cylinder, so it cannot thicken. In the clarifier that means the blanket must rise to push the same solids through the return line, and RAS is already maxed at 60 percent.
On ammonia: at 1.0 mg/L DO in the last pass, nitrification is oxygen-limited. The rising inventory also stores sludge in the clarifier, where it can go anoxic, denitrify, and float.
Biggest 48-hour risk: the blanket reaching the weir and washing solids out, which would put both TSS and ammonia at risk at the same time, and could escalate to a permit exceedance within hours if a peak flow or a rain event arrives. Actions now: raise DO in the last pass to about 2 mg/L if blowers allow, check the blanket every few hours, and have a flow-reduction or diversion plan ready.
2. Wasting is constrained for at least a week. Rank your options and explain what each does and does not fix.
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- Restore wasting by any route. This is the only real fix. Options include hauling liquid sludge, renting a mobile press or centrifuge, sending sludge to a neighboring plant, or getting temporary approval to land-apply liquid. Even at low waste concentration, removing solids lowers MLSS and raises F/M, which attacks the cause.
- Cut MLSS deliberately to about 2,600 mg/L and haul the difference. This relieves clarifier loading and improves F/M. Confirm the resulting SRT still supports nitrification in warm weather before you go lower.
- Short-term polymer or coagulant feed to the clarifier to improve solids capture. It buys effluent quality, not SVI improvement, and it increases sludge production and dewatering demand downstream, so it is a bridge, not a solution.
- Low-dose chlorination of return sludge to suppress filaments. It can work, but it is slow (days), risky to nitrifiers, and needs residual targets and daily SVI and microscopic checks. Only with a written plan.
- Keep RAS at maximum and keep the clarifier blanket under control. What not to do: stop wasting entirely, or drop RAS to reduce blanket, both of which make the failure come faster. Track SVI daily and watch the ammonia trend as the leading indicator.
3. How many pounds and gallons of solids must be wasted to bring MLSS from 3,200 to 2,600 mg/L, and what operational check must accompany that target?
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Aeration volume is four tanks at 0.35 MG, or 1.4 MG. Pounds equals volume in MG times 8.34 times concentration in mg/L.
At 3,200 mg/L: 1.4 x 8.34 x 3,200 = about 37,400 lb of MLSS. At 2,600 mg/L: 1.4 x 8.34 x 2,600 = about 30,400 lb. Difference: roughly 7,000 lb of solids.
At a waste activated sludge concentration of 8,000 mg/L, the volume is 7,000 divided by (8.34 x 8,000), which is 7,000 divided by 66,720, or about 0.105 MG, roughly 105,000 gallons. Spread over three to four days that is about 26,000 to 35,000 gpd of wasting, which is feasible if hauling is available.
Check that must accompany it: SRT. Lower MLSS without changing the waste rate long term and you shorten sludge age. Compute SRT as solids inventory divided by solids leaving per day (waste plus effluent). In summer, nitrification needs enough SRT to hold the ammonia limit, so the 2,600 mg/L target must be reconciled with the SRT calculation, not adopted blindly. Also expect SVI to lag by several days; do not over-waste chasing the SVI number.