Energy savings in mixing in activated sludge tanks

TOP_Process_tank_Drøsbro.JPGPHOTO: The biological process tank at Drøsbro is constructed as an annular shaped tank. Inflow to the tank takes place in the innermost ring to the anaerobic zone. From here the wastewater flows to the anoxic zone via an overflow weir before it via another overflow weir enters the aerobic zone in the outermost ring. Recirculation between the aerobic and anoxic zones is handled with a horizontal low head propeller pump.
Technical article

In a study of a Danish wastewater treatment plant, Computational Fluid Dynamics (CFD) simulations showed a potential for energy savings of more than 50% for mixing of horizontal flow systems. For submersible mixing in an activated sludge tank this can be realised by changing from medium speed, small diameter propeller mixers to slow speed, large diameter propeller flowmakers without altering the flow field in the tank.

Process tanks with unrestricted flow could potentially find easy and economically justified energy savings of up to 50% by using low speed, large diameter propeller flowmakers.


The study was based on an analysis of velocity flow fields using Computational Fluid Dynamics. The payback time for implementing the change in flowmakers will vary from plant to plant, and for the Drøsbo wastewater treatment plant in Denmark, the payback time was shown to be around four years. Furthermore, optimising the flowmakers reduces CO2 emissions by a substantial amount, reducing the burden on the environment.

Based on findings in this study, process tanks with unrestricted flow could potentially find easy and economically justified energy savings by using low speed large diameter propeller flowmakers compared to medium speed small diameter propeller mixers, despite the higher initial cost of a flowmaker.

Read the full article here (INSERT LINK TO PDF)



PHOTO: Velocity contour flow fields 0.3 m above the bottom of the tank.

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