Rising Sludge Blanket After Aeration Basin Flow Shift
A conventional activated sludge plant treats 1.8 MGD average flow. Aeration basin volume is 1.0 MG, so the design hydraulic retention time is about 13 hours. Over one weekend a nearby industrial user discharges a batch of high-strength wash water, and the plant diverts a portion of flow to the second aeration train to keep the first train from overflowing. By Monday the first train's mixed liquor suspended solids are 4,800 mg/L (normally 2,800 mg/L), the secondary clarifier sludge blanket has risen from 2.5 ft to 7 ft, and the return activated sludge rate is unchanged at 60 percent of influent flow. Effluent total suspended solids are climbing (from 8 to 35 mg/L), and dissolved oxygen in the first train has dropped to 0.8 mg/L. The sludge volume index from Friday's sample was 180 mL/g. The operator must decide what to adjust before the blanket reaches the weirs.
1. Identify the most likely mechanism causing the rising blanket and explain why simply increasing the RAS rate may not solve it.
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The combination of a high SVI (180 mL/g), rising MLSS, low DO, and an unchanged RAS ratio points to a solids-loading and settling problem rather than a simple inventory imbalance. At 180 mL/g the sludge is bulking, so the clarifier's ability to thicken and return solids is degraded; the blanket rises because solids enter faster than they can be settled and withdrawn.
Increasing RAS rate alone often makes things worse in this condition: it returns dilute, low-DO sludge to the aeration basin, shortens the actual detention time, and can increase the clarifier's hydraulic loading without improving compaction. The better first moves are to restore DO in the first train (check blowers, diffusers, and the diversion strategy), reduce the solids load reaching that clarifier (balance flow between trains, consider temporary wasting), and investigate the industrial discharge for biodegradable load and flow. Verify the SVI and MLSS with a fresh settleometer and gravimetric test before making large changes, since a single SVI value can be misleading.
2. The plant must decide whether to waste more sludge or reduce the RAS rate. Give a reasoned recommendation with the operational trade-offs.
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Wasting more is generally the safer lever here. The first train is carrying 4,800 mg/L MLSS, well above the normal 2,800 mg/L, so the system has excess inventory. Controlled wasting lowers MLSS over time, reduces the solids load on the clarifier, and is the standard response to a bulking sludge when the cause is high organic loading. The trade-off is that wasting removes biomass the plant may need once the industrial slug passes, and wasting too aggressively can drop the solids retention time and hurt nitrification if the plant nitrifies.
Reducing RAS is risky: it lowers the return of active biomass to the aeration basin, can worsen the blanket by allowing more solids to accumulate in the clarifier, and may cause septic conditions and rising effluent TSS. If RAS must be adjusted, small changes with close blanket monitoring are safer than large cuts. A reasonable plan is to increase wasting modestly, restore DO, rebalance flow between trains, and watch the blanket depth, effluent TSS, and SVI daily until the sludge settles again.