Tennessee Wastewater Operator Practice Exam

If the influent BOD concentration increases and sludge age is held constant, what will happen to the MLSS concentration?

1. It will decrease

2. It will remain constant

3. It will increase proportionally to the load

4. It will fluctuate

The correct answer is: It will increase proportionally to the load

When the influent BOD (Biochemical Oxygen Demand) concentration increases while maintaining a constant sludge age, the MLSS (Mixed Liquor Suspended Solids) concentration is expected to increase. This is due to the fact that a higher BOD implies that there is more organic material available for the microorganisms present in the activated sludge to metabolize. As the microbial population processes the increased BOD, it will proliferate to handle the additional organic load. This increase in microbial mass contributes directly to a rise in the MLSS concentration. Essentially, the system is compensating for a greater inflow of organic material by increasing the biomass, which directly correlates with the concentration of solids suspended in the mixed liquor of the treatment process. In summary, when there is an increase in the influent BOD and the sludge age remains constant, the MLSS concentration will rise, reflecting the system's adjustment to the increased organic loading.