Floating Horizontal Aerators Enhance Eufaula, Alabama Activated Sludge Plant


Courtesy of S&N Airoflo Inc.

The Eufaula, Alabama, wastewater treatment plant (WWTP) is an extended aeration activated sludge facility designed for an average daily flow rate of 2.7 million gallons per day (mgd). Prior to February, 2005 this WWTP used vertical aerators to provide oxygen and mixing in its aerator basins. The plant has two aeration basins, each 1.87 million gallons in volume, which had 160 horsepower (hp) of vertical aerators in each basin. This facility was having difficulty maintaining adequate dissolved oxygen (DO) levels, with mixed liquor DO concentrations often in the range of 0.3 to 1.0 mg/L. The low DO levels sometimes caused filamentous bulking problems, and there were accumulations of sludge in the basin bottoms because of inadequate mixing. In addition, the plant was spending about $90,000 per annum on power costs. Moreover, the vertical aerators required significant maintenance to keep them in good operating condition.

Because of the problems noted above, the staff at Eufaula WWTP began to evaluate options to improve WWTP performance via changes in the areation system. One option was to replace the existing aerators with other vertical aerators. A second option was to replace the existing aerators witha  diffused aeration system. A third option was to install floating brush rotors maufactured by S&N AIROFLO of Greenwood Mississippi.

Because of the past problems with vertical aerators, WWTP staff was not inclined to install new vertical aerator units. Although diffused aeration is widely used the activated sludge processes, the staff decided not to use diffused areation because of high capital cost. Instead, the decision was made to install floating brush rotors.

Until the last few years, aeration of activated sludge was typically achieved by diffused aeration systems or vertical aerators. Also, fixed brush rotor aerators were usually used in oxdation ditches. These aeration systems have generally performed well and provided adequate oxygen transfer and mixing. Since 1998, floating brush rotors (FBRs) have become popular choices for oxidation ditches and other types of activated sludge facilities.

Because of the above problems with original fixed rotor designs, floating horizontal aerators have become popular when existing units need to be replaced. FBRs have a very high oxygen transfer rate. FBRs are set at their optimum operating depth to provide maximum oxygen tranfer regardless of the water level in the aeration basin. In addition, FBRs provide excellent mixing of liquor. The Floating Brush Rotors manufactured by S&N AIROFLO of Greenwood, Mississippi, have high pumping rates of about 2100 gpm per horsepower.

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