Tennessee Wastewater Operator Practice Exam

If the SRT is currently 12 days and is decreased to 8 days, what will happen to the F/M?

1. It will remain the same

2. It will increase

3. It will decrease

4. It will fluctuate

The correct answer is: It will increase

When the sludge retention time (SRT) is decreased from 12 days to 8 days, it directly affects the food-to-microorganism ratio (F/M). The F/M ratio is a key parameter in wastewater treatment, representing the amount of food (typically in the form of biochemical oxygen demand, or BOD) available to the microorganisms in the system, relative to the mass of microorganisms present. A decrease in SRT means that the microorganisms are retained in the system for a shorter period, which typically leads to a reduction in the concentration of active biomass. Since the available food (BOD) remains relatively constant for a given flow and loading, the reduced biomass results in a higher F/M ratio. This is because there is less microbial mass available to consume the same amount of food, effectively increasing the ratio of food to microorganisms. Essentially, as the SRT decreases, the balance shifts towards more food per unit of biomass, leading to an increase in the F/M ratio. This relationship underscores the importance of SRT in managing the dynamics of microbial populations in wastewater treatment processes.