The city of Brownsville, Tenn., in 2001 retained the firm of J. R. Wauford and Company, Consulting Engineers, Inc., of Jackson, Tenn., to design an expansion of the city?s wastewater treatment capacity.
Brownsville?s influent flow averages 1.97 mgd. The influent goes through a grinder before entering the new, aerated lagoon where the Biomixer units are installed. From this complete mix aerated lagoon, the flow goes to a partial aerated lagoon and then to a settling cell where it is chlorinated.
The original design called for thirty 25 hp surface aerators for a total of 750 hp worth of surface aerators to both mix and aerate a 570 ft by 410 ft by 12 ft deep lagoon. Due to concern over high power costs, a decision was subsequently made to purchase five 85 hp Biomixer Aeration and Mixing Systems. The saving of 325 hp was due to Biomixer being a fine bubble efficiency aerator, thereby producing more pounds of oxygen per hp-hr.
Due to project cost overruns at bid time, however, it was necessary to reduce the number of Biomixer units from five to three. This resulted in lowering the total connected horsepower from 750 hp to 255 hp, a savings of 495 hp. In order to ensure mixing would be sufficient with the smaller number of units, the 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.
The Biomixer Aeration and Mixing System 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 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 blades can be raised in the wastewater, and this in turn reduces the horsepower required by the positive displacement air.
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 air lift 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. In addition, since the Biomixer diffuser blades are not generating mixing by rotating a pitched blade like traditional mechanical surface aerators.
By use of a shallow baffle attached to one half the circumference of the Biomixer, a Directed Flow option allows all of the mixing energy to be discharged in one direction.
These units were started up in October, 2001 and testing was undertaken in February, 2002 to validate their performance.
The testing confirmed the high oxygen transfer rates, as D.O. averaged 9.1 mg/l throughout the lagoon. This 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. Specifically, it was recommended they reduce their submergence levels, thereby saving horsepower and producing less oxygen.
The mixing energy produced by the three units, using the Directed Flow discharge scheme, was able to create a circular flow in the lagoon. As evidenced by the suspended solids testing, this high volume, directed mixing energy was able to thoroughly mix the huge volume of this lagoon, including the corners and slopes, with the exception of the extreme middle of the basin.
While complete mix is currently not needed in this portion of the lagoon from a process standpoint, it could easily be achieved in the future by the addition of one Biomixer in the middle of the lagoon. Should the city wish to do so, this would still be one unit fewer than in the original design. Such an additional unit, however, would not need the Directed Flow option.
The Biomixer Aeration and Mixing system is providing optimum dissolved oxygen transfer and mixing at this lagoon, in spite of the number of units being reduced from the original design of five units down to three. This is being obtained with the dissolved oxygen levels throughout the basin averaging 9.1 mg/l. Even at the influent end of the basin where the loading rate is the highest, the dissolved oxygen readings were above 8.5 mg/l.
It has been recommended to the city that because of the high dissolved oxygen content, the blade submergence depth be reduced. This will reduce the dissolved oxygen concentration in the lagoon and reduce the horsepower draw on the Biomixer units. This will allow the city to save on energy costs while still maintaining mixing throughout the basin.
For additional information, phone Biomixer Corp. at 310-263-2560.