EVALUATION OF FACTORS AFFECTING THE OXYGEN TRANSFER
FROM HIGH-SPEED FLOATING SURFACE AERATORS
Richard A. Poduska, Senior Environmental Engineer
Eastman Kodak Company
Rochester, New York 14650
Rob F. Hager, Jr., Chemical Engineer
Tennessee Eastman Company
Kingsport, Tennessee 37662
INTRODUCTION
In response to a sudden and prolonged deficit imbalance in dissolved oxygen (DO) in an activated
sludge system using only high-speed floating surface aeration, a study was conducted to evaluate the
primary factor causing this condition. Based on preliminary evaluations that the influent biochemical
oxygen demand (BOD) load had not increased during this imbalance, the primary efforts were concentrated on identifying the cause of a decrease in the supply of oxygen from the aeration equipment.
The results of this evaluation demonstrate that changes in the mechanical integrity of the aerators
had a significant impact on the overall oxygen transfer capacity of the system.
These studies were conducted at an industrial wastewater treatment system that treats only the
industrial wastewater generated from the manufacture of chemicals, plastics, and fibers. The average
5-day biochemical oxygen demand (BOD,) load is nearly 114,000 kg/d (250,000 lb/day) at a flow of
68,130 mVd (18 mgd). Detailed descriptions of the system and its operation and control are reported
elsewhere [1, 2]. The system consists of three aeration basins, three secondary clarifiers, aerobic
sludge digestion, and belt-filter sludge dewatering. No primary clarifiers are needed. The three aeration basins have a combined capacity of 204,390 m1 (54 million gallons) that provide a 3-day hydraulic retention time at the average flow. Aeration and mixing in the three aeration basins are supplied
by one hundred and nine 56-kW (75-hp) high-speed floating surface aerators manufactured by Aqua-
Aerobic and Ashbrook. The sludge age averages approximately 15 days with a range from 8 to 25
days. The averaged mixed liquor suspended solids are approximately 2000 mg/1 with a range from
1500 to 3500 mg/1.
As shown in the schematic in Figure 1, the system is operated in the step-feed mode. The flow
of wastewater into Basin 1 is adjusted so that the DO in that basin is maintained at approximately
0.5 mg/1 with all the aerators in operation. The DO at the outfall of Basins 2 and 3 is targeted between
1-2 mg/1 by varying the number of aerators operating in these basins. Reductions in DO are made
by turning off aerators nearest the outfall first in a pattern that still provides adequate mixing in
each basin.
OPERATING HISTORY
Since the start-up of the activated sludge system in 1976, there were no significant problems in
maintaining the minimum desired residual DO concentrations of 1 mg/1 in Basins 2 and 3. However,
beginning in May, 1981 the DO concentrations abruptly decreased to less than 0.5 mg/1 in Basins 2
and 3. This condition persisted for several months even though all the aerators in these basins were
operating. During this time there were no changes in the BOD load or the composition of the
wastewater and therefore, no direct cause of the loss of DO could be assigned. The studies described
herein forcus on determining the cause of a reduced oxygen transfer efficiency of the aeration equipment in the system.
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