A laboratory activated-sludge system was used to evaluate chemical-biological phosphorus removal by applying alum to the aeration tank. The laboratory apparatus was a dif- fused-air complete-mixing aeration tank, separate gravity clarifier, and airlift return sludge pump. Twelve liters of settled municipal wastewater were applied daily at a constant rate to the aeration tank, which had a volume of 3.6 l. The aeration tank MLSS concentration was held near 2000 mg/l by wasting 200-250 ml of mixed liquor each day. The temperature was 22°-24°C, pH 7.3-7.7, and sludge volume index varied from 90-130 ml/g. The alum solution feed to the aeration tank had a strength of 10.0 mg of commercial alum per milliliter. The following data were collected at various alum feed rates after arriving at steady-state conditions.
| Influent | ||||
Waste- water Feed (l/d) | Alum Applied (ml/d) | BOD (mg/l) | SS (mg/l) | P (mg/l) | Alk (mg/l) |
12.0 | 0 | 150 | 94 | 10.3 | 350 |
12.0 | 70 | 158 | 104 | 10.9 | 350 |
12.0 | 140 | 169 | 114 | 10.6 | 340 |
12.0 | 210 | 173 | 135 | 10.4 | 360 |
12.0 | 390 | 173 | 123 | 9.3 | 320 |
| Effluent | ||||
| Influent | ||||
Waste- water Feed (l/d) | Alum Applied (ml/d) | BOD (mg/l) | SS (mg/l) | P (mg/l) | Alk (mg/l) |
12.0 | 0 | 6 | 7 | 7.9 | 230 |
12.0 | 70 | 6 | 8 | 5.4 | 200 |
12.0 | 140 | 5 | 11 | 2.4 | 160 |
12.0 | 210 | 10 | 10 | 1.0 | 140 |
12.0 | 390 | 8 | 7 | 0.5 | 80 |
| Effluent | ||||
(a) Calculate the phosphorus removal and the weight ratio of alum applied to total phosphorus in the influent wastewater for each run. Plot a graph of percentage of phosphorus removal versus the weight ratio of alum to phosphorus.
(b) Calculate the average BOD and SS (suspended solids) removal efficiencies. Suggest reasons why the effluent BOD and SS values are lower and resulting efficiencies higher than are normally achieved by a full-scale treatment plant.
(c) Why did the concentration of alkalinity in the effluent decrease with increasing alum dosage?
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