Liberty, N.Y., WWTP replaces brush aerators with fine-bubble diffused aeration
When a broken shaft caused a malfunction in one of the brush aerators at the Village of Liberty, N.Y., wastewater treatment plant (WWTP), the village worked with its engineers, Delaware Engineering, to develop a plan for repair and upgrade.
The plant’s secondary treatment consists of two separate oxidation ditches, each 1 million gal in volume and capable of treating the permitted capacity of 2 mgd, followed by clarification and UV disinfection. Before upgrade, each racetrack-designed basin was equipped with two brush aerators, each driven by a 50-hp (37-kW) motor, for a total of 100 hp (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.
Brush aerators, such as those previously installed at the Liberty WWTP, 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. After evaluating several options for retrofitting the basins, Delaware Engineering recommended submersible mixing combined with fine-bubble diffused aeration as the most cost-efficient treatment, as well as one that would also improve control and flexibility.
Separating oxygen transfer from mixing allows each device to do what it does most efficiently, resulting in significant energy savings—the cornerstone of the bioloop process. 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.
Keeping aeration separate from mixing also allows operators to control the dissolved oxygen concentration more efficiently, allowing reduced aeration during periods of reduced oxygen demand without sacrificing circulation around the oxidation ditch and risking solids settling.
Oxygen transfer is a function of water depth—deeper submergence of the fine bubble diffusers allows more contact between the bubbles and the water, yielding higher transfer efficiency. With a depth of approximately 14 ft (4.3 m), the basins at the Liberty WWTP proved ideal for a fine-bubble diffused aeration retrofit.
Delaware Engineers recommended a Xylem Sanitaire aeration system, adding fine-bubble diffused aeration with Silver Series II membrane disc diffusers and a separate, highly efficient Flygt 4410 mixer. 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 ten years or more.
The Sanitaire Silver Series fine-bubble diffused aeration system distributes air through 720 diffusers—360 on each side of the basin. Two 50-hp (37-kW) blowers, equipped with variable frequency drives, provide air to the diffusers at peak flow and loading; a third identical blower is installed as a standby.
The upgrade has produced substantial energy savings at the Liberty WWTP, reducing aeration and mixing energy consumption by 43%—from 100 hp (74.6 kW) to less than 57 hp (42.5 kW)—saving more than $36,000 per year while providing more stable treatment.
Due to their superior oxygen-transfer efficiency, Sanitaire Silver Series II diffusers are among the most efficient fine-bubble diffusers in the industry. With a standard oxygen transfer efficiency (SOTE) of 6.56%/m (2%/ft) of submergence, the diffusers deliver an approximate SOTE of 24% for the basins at the village of Liberty. 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.