When the City of Brownsville, Tenn., required expansion to their wastewater treatment plant in 2001, they turned to unique technology to provide superior aeration and basin mixing efficiency while lowering power consumption.

Brownsville's influent flow (1.97 mgd) goes through a grinder before entering a complete mix, aerated lagoon (570 foot by 410 foot by 12 foot deep), then to a partially aerated lagoon and finally to a settling cell where it is chlorinated.

Originally, the design called for thirty 25 HP surface aerators with a total of 750 HP worth of surface aerators to both mix and aerate. Because of the potentially high power and maintenance costs, the city considered and ultimately purchased three 85 HP BIOMIXERTM Aeration and Mixing Systems. This resulted in lowering the total connected horsepower from 750 HP to 255 HP, a savings of 495 HP, by using the benefits of fine bubble efficiency, including the production of more pounds of oxygen per HP-Hr.

In order to ensure mixing would be sufficient with a minimal number of units, the BIOMIXER™ Directed Flow™ discharge option was added. This feature allows the discharge to be diverted in one direction, and the three units were positioned such that the combined effect of their individual discharge patterns would create circular mixing in the lagoon.

Testing confirmed the high oxygen transfer rates (the D.O. averaged 9.1 mg/l) and mixing throughout the lagoon – even in the corners and slopes. The extremely high D.O. level later resulted in a recommendation that the City take advantage of the BIOMIXER's unique ability to adjust its diffuser aeration submergence levels from the surface. By reducing submergence levels, the city saved more horsepower and produced less oxygen.

The BIOMIXER™ Aeration and Mixing System – winner of the Water Environment Federation's 2002 Innovative Technology Award for process equipment – is a fine bubble efficiency aerator that installs from the surface. It has the ability to adjust the submergence level of its slowly rotating diffuser blades. This is important, as it gives the treatment plant the flexibility to operate the Biomixer diffuser blades at various levels. When the treatment plant requires more oxygen, the blades are submerged deeper in the wastewater. When the requirement for dissolved oxygen is reduced, the diffuser blades can be raised in the wastewater and this, in turn, reduces the horsepower required by the positive displacement air blowers.

In addition, the diffuser blades can also be quickly raised above the water to allow for easy diffuser inspection and service.

The slowly rotating, fine bubble diffuser blades generate tremendous mixing by creating a 25-foot diameter airlift pump. This air lift pump generates a high volume, low velocity mixing pattern, in contrast to mechanical surface aerators which exhibit high velocity, low volume mixing.

To learn more about the benefits of the BIOMIXER™ Aeration and Mixing System, please contact Smith & Loveless' Biomixer Division at 562-407-0670, via e-mail at biomixer@smithandloveless.com, or online at www.biomixer.net.

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