Tennessee Wastewater Operator Practice Exam

When should the SRT in an activated sludge system be increased to accommodate changing conditions?

1. When influent BOD decreases

2. When water temperature increases

3. When influent ammonia increases

4. When water temperature decreases

The correct answer is: When water temperature decreases

In an activated sludge system, the Sludge Retention Time (SRT) is a critical factor influencing the treatment efficiency and stability of the biological processes. Increasing the SRT helps to maintain a population of microorganisms that are capable of effectively breaking down organic material and nutrients. When water temperature decreases, the metabolic activity of the microorganisms also tends to decrease. This lower activity means that the microorganisms will require more time to achieve the same level of treatment. By increasing the SRT in response to lower temperatures, operators can ensure that the biomass in the system has adequate time to effectively treat the incoming wastewater. In contrast, when influent BOD decreases, the existing microbial population may be adequately maintained without needing to increase the SRT. Similarly, increases in influent ammonia and water temperature generally lead to situations that might not require adjustments to the SRT. For example, higher ammonia levels might necessitate a more rapid turnover of specific microbial populations, rather than extending the time they remain in the system. Thus, increasing the SRT in response to a decrease in water temperature is vital for maintaining operational efficiency and effectiveness in an activated sludge process.