What is the effect of operating at a very low F/M on sludge production?

• A. Sludge production increases
• B. Sludge production is reduced
• C. Sludge settles faster
• D. Sludge remains unchanged

Answer: (B)

In activated sludge, the amount of substrate available per unit of biomass (the F/M ratio) controls how much new microbial growth can occur. When F/M is very low, there isn’t enough food for rapid growth, so the microorganisms use most of the available energy just to maintain themselves rather than to produce new cells. That lowers the biomass yield, meaning less excess sludge is generated. So sludge production is reduced under very low F/M conditions. The question isn’t about how fast sludge settles or about sludge staying the same; those aspects aren’t directly driven by F/M in this context.