Tennessee Wastewater Operator Practice Exam

What occurs when the rotational speed of a rotating biological contactor (RBC) is increased slightly?

1. Oxygen concentration will increase

2. Soluble BOD removal will decrease

3. Suspended solids concentration will remain unchanged

4. Wastewater flow rate will significantly increase

The correct answer is: Oxygen concentration will increase

When the rotational speed of a rotating biological contactor (RBC) is increased slightly, the primary effect is that the oxygen concentration will increase. This is because a higher rotational speed enhances the exposure of the microbial biomass to the atmosphere, facilitating greater oxygen transfer from the air into the wastewater. As the RBC rotates, its media become partially submerged, allowing for both biofilm growth and oxygen exchange. When the speed increases, it increases the mixing and turbulence at the surface of the media, which can lead to more efficient gas transfer dynamics. This results in a higher availability of dissolved oxygen in the water, which supports aerobic bacteria and improves the overall performance of the treatment process. Conversely, if the rotational speed were to increase significantly, it could lead to other complications, such as higher shear forces that might reduce the biomass attach to the media or disrupt the removal processes. However, in the context of a slight increase in speed, the immediate effect on oxygen concentration is beneficial and significant.