Editorial

Installation of low speed aerators at gold mine proves successful.

Up to a few years ago, high-speed floating aerators were widely used in South Africa to solve a wide range of aeration problems without the need for ancillary equipment or structures. Today the general trend is to use low speed units that, although of a greater capital cost, offer higher oxygen transfer efficiencies and mixing capacities, greater stability and lower maintenance costs.

A typical case scenario is where four 22 kW high-speed floating aerators were originally installed to treat domestic sewage effluent at a gold mine in the western area. These units operated with limited success due to insufficient pumping action required to keep the solids in suspension and a low oxygen transfer rate resulting in the basin contents turning septic. Apart from the process aspect the aerators were plagued with frequent mechanical problems and rarely was the full compliment running together. Such was the problem that an independent consultant was called in and low speed floating aerators were recommended to replace the existing units.

Three 22 kW Lektratek low speed, tri-float, units were supplied to replace the existing four units and after sixteen days the sludge was again a healthy activated brown colour and the obnoxious smell completely eliminated. It has also been found that one of the new replacement units can be switched off as the aeration and pumping capability of two aerators is sufficient for the application. The aforementioned indicates that two low speed aerators can do the job of four high-speed units that could not meet the oxygenation demand nor properly mix the basin contents in this instance. Alternatively, four 11 kW low speed units could be installed resulting in better coverage of the basin surface area.

The following power consumption figures show that low speed aerators, although initially more expensive, can recover the total capital outlay in less than two years. Four 22 kW units would consume R115 632 worth of electricity per year at a cost of 15c/kWh, whereas two 22 kW units would consume half the amount (assuming full load conditions and disregarding motor efficiency and power factor).

Lektratek aerator impellers are rag free in operation, have a high pumping capacity suitable for low and high-energy applications and produce a stable wave formation with a minimum of erratic splashing and negligible airborne mist.

The motor is situated high above the liquid level thus minimising risk of clogging fins and fan with sludge, which often causes overheating, and ingress of moisture into the motor.

The motor is a standard four pole unit normally obtainable off the shelf with bearings used as intended by the manufacturer, not to support a long shaft, impeller and erratic unbalanced loads as on the high speed units.

A parallel shaft speed reducer, with a service factor greater than 2 on absorbed power, acts as the main drive component to cater for the torque and loads typically associated with aerators. No underwater bearings or bushes to support the impeller shaft.

Impeller operates at a tip speed of typically 5 m/sec whilst providing a high oxygen transfer rate without the possibility of cavitation that can cause erosion of the impeller and loss of efficiency. It has been proved that the oxygen transfer rate decreases at tip speeds higher than 5 m/sec.

Impeller life span is expected to equal or exceed that of the whole assembly and the rag free design of impeller eliminates shock loads on the drive assembly.

Support structures provides a safe platform that can accommodate two maintenance personnel without the risk of capsizing.

Lektratek aerators can be used in shallow lagoons and where the depth varies or falls below the minimum operating level with no negative effect on the equipment, e.g. tilting on its side or impeller clogged by or fouling lagoon floor.

Low speed aerator on day of installation - note the dark septic effluent colour caused by the inadequate aeration from high-speed units.

Same 22 kW low-speed aerator as above after sixteen days operation - note that the effluent colour has changed to a healthy activated sludge brown.

Before	After

Originally published in Water Sewage and Effluent, volume 17 No.4, dated 4 December 1997.