When a brush aerator needed to be replaced at the wastewater treatment plant in Liberty, New York, the plant decided to upgrade its operations. A new Sanitaire Silver Series aeration system and Flygt mixer from Xylem were installed, which has led to a 43 percent reduction in energy consumption.
The wastewater plant for the town of Liberty, in southeast New York, has a secondary treatment process that consists of two separate oxidation ditches. Each ditch is one million gallons in volume and capable of treating the permitted capacity of two million gallons per day, followed by clarification and UV disinfection.
Previously each oxidation ditch was equipped with two brush aerators, each driven by a 50-horsepower (37-kW) motor, for a total of 100 horsepower (74.6 kW) per basin. The brushes provided both aeration and mixing, maintaining complete solids suspension while providing flow circulation in the extended aeration basin.
Replacing an energy-intensive system
These brush aerators keep solids suspended, but they are highly energy intensive for sustaining dissolved oxygen levels. Their added disadvantage of having one speed and one fixed depth makes it almost impossible to adjust them for changing conditions.
When a shaft broke and caused a malfunction in one of the brush aerators, the Liberty treatment plant worked with its engineers, Delaware Engineering, to develop a plan for repair and upgrade.
After evaluating several options for retrofitting the basins, Delaware Engineering recommended a Xylem Sanitaire aeration system and a separate, highly efficient Flygt 4410 mixer. This was identified as the most cost-efficient treatment, and one that would also improve control and flexibility.
Separating aeration from mixing
The new solution, which involves separating oxygen transfer from mixing, allows each device to do what it does most efficiently, thus significantly reducing energy. Combining highly efficient submersible mixers with efficient fine bubble diffused aeration provides superior oxygen transfer and treatment at a fraction of the energy costs of brush aerators.
Oxygen transfer is a function of water depth, with deeper submergence of the fine bubble diffusers allowing more contact between the bubbles and the water, yielding higher transfer efficiency. With a depth of approximately 14 feet (4.3 meters), the basins at the Liberty plant proved ideal for a fine bubble diffused aeration retrofit.
Keeping aeration separate from mixing also allows operators to control the dissolved oxygen concentration more efficiently. This allows reduced aeration during periods of reduced oxygen demand without sacrificing circulation around the oxidation ditch.
The upgrade has produced substantial energy savings at the Liberty plant, reducing aeration and mixing energy consumption by 43 percent, from 100 horsepower (74.6 kW) to less than 57 horsepower (42.5 kW). This has led to savings of more than $36,000 per year while providing more stable treatment.
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The solution in detail
Due to their superior oxygen transfer efficiency, Silver Series II membrane disc diffusers are among the most efficient fine bubble diffusers in the industry. With a Standard Oxygen Transfer Efficiency (SOTE) of 6.56 percent per meter (2 percent per foot) of submergence, the diffusers deliver an approximate SOTE of 24 percent for the basins at the Liberty plant. Higher transfer efficiency means less total air is required to meet the demand, and with an extremely low pressure drop across the membrane, blower power consumption can also be minimized.
The Silver Series aeration system is known for its robust service, with its special high-grade EPDM elastomeric membranes delivering a long service life of 10 years or more. The system at the Liberty plant distributes air through 720 diffusers, 360 on each side of the basin. Two 50-horsepower (37-kW) blowers, equipped with variable frequency drives (VFDs), provide air to the diffusers at peak flow and loading, with a third identical blower installed as a standby.