One of the key factors to consider when evaluating performance characteristics of various aeration mixing system options is the predicted cost of energy to operate the system over its useful life. The challenge in calculating such costs is including efficiency data from manufacturers that reliably reflects performance one could expect to measure in the field.
The performance of any aeration device is dependent on specific field conditions inherent to the installation. Liquid depth, the diffuser array configuration, tank geometry, air flow, distribution piping arrangement, and other parameters directly impact oxygen transfer efficiency.
Helpfully, the American Society of Civil Engineers and other independent groups have developed rigorous performance criteria to evaluate the performance of mechanical aerators and diffusers. Most manufacturers use the standard methods approach and the ASCE Test Procedures for evaluating and presenting their products.
Fine pore aeration systems using either ceramic or flexible rubber membranes are the most energy efficient devices.
Advanced technology membranes can deliver oxygen transfer efficiencies approaching 4% oxygen transfer per foot of submergence, translating to as high as 12 pounds of oxygen per horsepower-hour.
There are some applications where the nature of the wastewater, temperature, and special conditions merit using coarse bubble aeration or some other aeration process. However, fine bubble aeration offers superior oxygen transfer and energy efficiency compared to these other systems. Combining these factors with the low-maintenance characteristics of membrane diffusers make them the leading choice by designers for most applications.
The table illustrates general surface aeration efficiencies.
This bulletin presents a useful tool for relating efficiencies of the most common aeration mixing devices currently on the market. The attached table presents a realistic comparison of the performance of various systems which allows one to conduct useful energy-efficiency assessments.
Surface aeration systems are typically limited to water depths up to 12 feet to optimize effective aeration and mixing. Further, the zone of influence of surface aerators may be dramatically reduced when submergence is less than 5-6 feet, due to diminishing aerator circulation capacity and corresponding oxygen transfer efficiency.
The table clearly shows that diffused aeration systems, specifically fine pore membrane aeration diffuser systems, offer significant energy savings compared to competing aeration devices. Significantly, fine pore aeration systems offer this benefit regardless of the depth and geometry of the basin, or the application.
General Oxygen Transfer Efficiencies
|MECHANICAL AERATION SYSTEMS|
|Rotors (brush aerators)
|2.5 to 3.5||1.52 to 2.13|
|Slow speed surface||3.0 to 3.5||1.82 to 2.13|
|High speed splash surface aeration||2.5 to 3.25||1.52 to 1.98|
|Induced surface aeration||1.0 to 1.5||0.61 to 0.91|
|Submerged Turbine (turbine
mixer and compressors)
|1.5 to 2.5||0.91 to 1.52|
|Jets (pumps with compressors)||2.0 to 3.5||1.22 to 2.13|
|Coarse Bubble System|
|Static tubes||2.0 to 3.0||1.22 to 2.13|
|Wide band grid||2.5 to 3.5||1.52 to 2.13|
|Miscellaneous coarse bubble||2.0 to 3.0||1.22 to 2.13|
|Traditional Fine Pore Aeration|
|Ceramic disc or ceramic
|5 to 7||3.04 to 4.26|
|Flexible Membrane Disc or Tubes Grid at Conventional
|4 to 7||2.43 to 4.26|
|Advanced Technology Membrane
Fine Pore Aeration
|up to 12||7.30|