Oxygen plant: BOD removal 90%, same as above 45.4 kg/hr O2

WASTEWATER: 157.7 m3/hr


47.3 kg/hr

MLVSS = 2043 KG; 6000 ppm V = 12000 CU FT = 340 m3 Retention Time = 2.2 hr

Agitators: 4.5 hp


4.73 kg/hr


15.3 kg/hr

FIG. 7.35.2 Comparison of air and oxygen in activated-sludge wastewater treatment.

lb of BOD removed. The low oxygen efficiency of 7.5% shows that mixing rather than dissolving oxygen is the determining factor of air sparging. Good mixing is essential for keeping bacteria in suspension, breaking up bacteria flocs, and promoting maximum contact of bacteria with the organics they use as a food source. A well-mixed aeration basin prevents raw wastewater from flowing directly from inlet to outlet. Sludge activation works better with pure commercial oxygen.

Pure oxygen accelerates the oxidative processes by at least a factor of four. Rapid oxygen transfer allows the plant to operate at high sludge concentrations. With oxygen, the sludge is less slimy and settles more readily, as shown in Figure 7.35.1. The SS can be increased threefold to 6000 ppm for the same settling velocity of 7 ft/hr. Figure 7.35.2 shows that at 6000 ppm MLVSS, a 1-mgd plant needs an aeration tank of only 12,000 cu ft. The clarifier can be operated at mass loadings of 45-65 lb/day/sq ft and hydraulic overflow rates of 600-1200 gpd/sq ft. In effect, the plant settler using air and the one using oxygen are the same size in spite of different MLVSS concentrations. For 1-mgd plants, these settlers have a surface of 1100 sq ft, a volume of 16,000 cut ft, and a mean hydraulic residence time of 2.9 hr. Better sludge settling also reduces sedimentation chemical use. Often these savings can pay for the cost of oxygen.

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