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Environmental Engineering DISCUSSION
At a small activated sludge plant the clarifier is carrying solids over the weirs and the sludge blanket is rising. Data: flow 5,000 m³/day, influent BOD 200 mg/L, aeration tank volume 1,500 m³, MLSS 3,000 mg/L with MLVSS 2,500 mg/L, waste sludge flow 50 m³/day at 8,000 mg/L, effluent suspended solids 15 mg/L. The 30-minute settled volume is 600 mL/L.
How are F/M ratio, sludge age and SVI calculated from these, what ranges are normal, and which parameter should the operator actually adjust?
F/M is the food supplied per unit of biomass per day: F/M = Q × S_0 / (V × X_v). Food is 5,000 m³/day × 0.2 kg/m³ = 1,000 kg BOD/day. Biomass is 1,500 m³ × 2.5 kg/m³ = 3,750 kg MLVSS. F/M = 1,000 / 3,750 = 0.27 per day.
Conventional activated sludge normally runs at about 0.2 to 0.4, and extended aeration at roughly 0.05 to 0.15, so your loading is in the normal conventional range. The hydraulic retention time is 1,500 / 5,000 = 0.3 day, or 7.2 hours, which is also typical.
Sludge age (SRT, or mean cell residence time) is the mass of solids in the system divided by the mass leaving each day: SRT = V × X / (Q_w × X_w + Q_e × X_e). Solids in the aeration tank: 1,500 × 3.0 = 4,500 kg. Wasted: 50 × 8.0 = 400 kg/day. Lost in the effluent: 4,950 × 0.015 = 74 kg/day. SRT = 4,500 / 474 = 9.5 days.
Use the same solids basis throughout, total or volatile, and notice that the effluent loss is already 16 percent of the total. If the clarifier loses more solids, the SRT falls without anyone touching the waste pump. For conventional plants 5 to 15 days is usual, and nitrification needs the upper part of that range or more in cold water.
SVI is the volume in millilitres occupied by one gram of sludge after 30 minutes of settling: SVI = settled volume (mL/L) × 1,000 / MLSS (mg/L) = 600 × 1,000 / 3,000 = 200 mL/g. A well-settling sludge is roughly 50 to 150 mL/g. At 200 you have bulking sludge, and that, rather than F/M or SRT, is what your numbers point to: the sludge compacts poorly, the blanket rises in the clarifier, and solids are carried over the weir.
Look at the mixed liquor under a microscope. Filamentous bacteria extending from the flocs and bridging between them are the usual cause of an SVI this high. Low dissolved oxygen in the aeration tank, nutrient deficiency (nitrogen or phosphorus) in industrial wastewater, septic influent and very low F/M in completely mixed tanks are the common triggers.
On what to adjust: the operator's main control is the wasting rate, which sets the SRT; F/M follows from it, since wasting less raises the MLSS and lowers F/M. Make changes in small steps, no more than about 10 to 15 percent of the waste rate per day, and judge the result over two or three sludge ages, because the biology responds slowly. Controlling to a steady SRT is more stable than chasing a fixed MLSS.
For the immediate problem, check the dissolved oxygen (around 2 mg/L in the aeration tank is a usual target) and the return sludge rate. Returning too little lets the blanket build up; returning too much adds to the clarifier solids loading. Track the blanket depth and the SVI daily until the settling recovers.
For reference, the appearance of the settling test tells you which way the plant has drifted. Young sludge (high F/M, short SRT) settles slowly and leaves a turbid liquid with dispersed, fluffy floc. Old sludge (low F/M, long SRT) settles fast and dense but leaves fine pin floc in an otherwise clear liquid. Filamentous bulking settles very slowly yet leaves an unusually clear supernatant above the blanket, because the filament network strains out fine particles.
Against that, solids rising in clumps after an hour or two in the test cylinder, with gas bubbles, indicate denitrification in the clarifier, which is a different problem solved by removing sludge faster or denitrifying deliberately.
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