WASTEWATER/Aerator’s sealed bearings reduce plant’s down time
The Concordia, Kan., wastewater treatment plant has been in operation in the north central Kansas community since 1979 and currently handles a flow of 1.2 mgd. The plant has three 10-foot-deep oxidation ditches used for activated sludge processing. One of them is 25 feet in diameter; the other two are 13 feet.
Officials found themselves dealing with bearings in an aerator that seized up repeatedly and required replacement at considerable expense. Propeller life was reduced to about one year.
Ultimately, a switch to a new type of aspirating aerator featuring a solid shaft and sealed roller bearings resolved the problem.
Raw sewage entering the plant averages 200 mg/1 BOD and 300 mg/1 total SS. The city uses a standard activated sludge process with four aerators in the outside ditch and two in each of the two inner ditches.
Dissolved oxygen levels in all three ditches range from 1.5 to 2.5 ppm. Discharge from the plant typically measures 5 mg/1 BOD and 5 mg/1 SS.
The malfunctioning aerator had a hollow driveshaft with a universal joint that coupled the bearing and sleeve to the motor. Wastewater entered holes on the bushing to lubricate the ceramic bearing. But sand, grit and other solids in the water greatly increased bearing wear, while hair or other stringy solids plugged the holes.
The aerator propellers also required annual replacement because of wear or corrosion. Replacement bearings cost about $156, and propellers averaged $700 depending on size. Twice a year, two people typically spent about four hours disassembling the unit, replacing the propeller, shaft and bearings and lubricating and inspecting the unit.
Plant managers decided to replace the aerator with one they believed would be more maintenance-free. They installed the Tornado aspirating aerator from Aeromix Systems, Minneapolis, Minn.
The aerator has a solid, stainless steel shaft, making it possible to use sealed mechanical bearings at each end that uptake or absorb the propeller thrust load.
The roller bearings eliminate thrust loading, allowing for use of a standard drive motor without thrust bearings, a type that is less expensive and more durable.
The aerator is mounted at an angle in the water with the motor and air intake above the surface and the propeller below. The motor rotates, turning the solid shaft and its attached propeller.
Water moves at a high velocity through and near the propeller blades, creating a low pressure zone at the hub. The low pressure draws air in through the large-diameter draft tube and out into the water at the propeller hub.
Turbulence and flow created by the propeller breaks up the air bubbles, mixes the basin contents and disperses oxygen. The horizontal movement maximizes oxygen transfer and mixing. In addition, velocity from the water surface to the basin floor does not vary as it would when brush rotors are used.
To maximize propeller life, the aerator is specially designed to operate at about half the speed of its predecessor. The propeller uses larger blades that provide similar mixing capability at slower speeds.
The aerator has been in service for three years. A recent inspection showed no discernable wear on the bearings or propeller. The bearings provide a minimum design life of 100,000 hours and a theoretical life of more than 1 million hours, assuming proper installation and lubrication.