Tennessee Wastewater Operator Practice Exam

What is the impact of increasing the return activated sludge (RAS) flowrate?

1. Increases the RAS concentration

2. Decreases the RAS concentration

3. Enhances nutrient removal

4. Improves effluent quality

The correct answer is: Decreases the RAS concentration

Increasing the return activated sludge (RAS) flowrate has a direct impact on the concentration of RAS in the system. When the flowrate of RAS is increased, it leads to a dilution effect on the remaining activated sludge in the treatment process. As more RAS is reintroduced back into the aeration tank, there is a proportionate increase in the volume of sludge being mixed with the influent wastewater. This dilution results in a lower concentration of solids in the mixed liquor, which means the actual concentration of RAS available to provide biological treatment is reduced. In essence, while the total amount of biomass is increased due to the higher flowrate, the concentration on a volume basis decreases. This relationship is crucial for operators to understand, as a lower RAS concentration can affect the efficiency of biological processes and the overall treatment performance in wastewater treatment facilities. Adjusting RAS flow rates is a critical aspect of maintaining the desired operational parameters for effective treatment.