Florida Wastewater Practice Test

What is the typical RAS-to-Q ratio in an extended aeration activated sludge process?

1. 150 to 200%

2. 50 to 75%

3. 75 to 100%

4. 100 to 120%

The correct answer is: 75 to 100%

In an extended aeration activated sludge process, the RAS-to-Q ratio typically falls within the range of 75 to 100%. This ratio, which stands for Return Activated Sludge to Flow ratio, is crucial for maintaining an effective treatment process. A ratio within this range indicates that a significant portion of the activated sludge is being returned to the aeration basin to maintain an adequate concentration of microorganisms. This concentration is essential for the biological treatment of wastewater because it enhances the system's ability to break down organic matter effectively. Furthermore, operating within this RAS-to-Q ratio ensures that the treatment system can maintain its desired performance levels, ensuring the proper balance between the growth of biological mass and the removal of treated effluent. Maintaining an optimal RAS-to-Q ratio helps prevent issues such as washout of microorganisms or insufficient biological treatment due to low biomass concentration.