Tennessee Wastewater Operator Practice Exam

What is the main method by which mixer aerators transfer dissolved oxygen (DO) into process tanks?

1. Mixing heavy solids with lighter ones

2. Creating large bubbles at the surface

3. Shearing bubbles from the sparger ring and mixing them into the process liquid

4. Using chemical dispersants

The correct answer is: Shearing bubbles from the sparger ring and mixing them into the process liquid

Mixer aerators primarily transfer dissolved oxygen (DO) into process tanks through the shearing of bubbles produced by the sparger ring, which then get mixed into the liquid. This method is effective because it maximizes the surface area of the bubbles in contact with the water, enhancing oxygen transfer efficiency. As the bubbles rise, they are sheared apart, breaking into smaller bubbles that have a greater total surface area. Smaller bubbles have a longer residence time in the water column and provide more opportunity for oxygen to be absorbed into the liquid. This vigorous mixing action also helps to evenly distribute the dissolved oxygen throughout the tank, ensuring that microorganisms have adequate oxygen to support their metabolic processes. This method is distinct from just creating large bubbles, which may not provide the same level of efficiency in oxygen transfer due to their size and the tendency to rise quickly to the surface without effectively dissolving. Additionally, mixing heavy solids with lighter ones or using chemical dispersants would not accurately describe the functioning of mixer aerators concerning transferring oxygen into the liquid phase.