Question

9. What is the importance of controlling the F/M ratio in suspended mixed activated sludge systems? What are the implications of a low or high F/M ratio to the settling properties of the biological floc? Please, elaborate your answer.

Answer 1

• Part A

1. The food to microorganism (F/M) ratio is one of the significant design and operational parameters of activated sludge systems.
2. A balance between substrate consumption and biomass generation helps in achieving system equilibrium.
3. The F/M ratio is responsible for the decomposition of organic matter.
4. The type of activated sludge system can be defined by its F/M ratio.

• Extended aeration, 0.05 < F/M < 0.15
• Conventional activated sludge system, 0.2 <F/M < 0.5
• Completely mixed, 0.2 < F/M < 1.0
• High rate, 0.4 < F/M < 1.5

PART B

• a low F/M ratio, means you have too many bacteria in your wastewater system relative to the incoming Biochemical Oxygen Demand (BOD).
  
• This causes starvation of healthy floc forming bacteria, and issues such as bulking, or high effluent Total Suspended Solids (TSS).
  

• Small, weak flocs can be formed in activated sludge that are easily sheared and subject to hydraulic surge flotation in the final clarifier leading to a turbid effluent. These small flocs, termed pin floc, consist only of floc-forming bacteria without a filament backbone and usually <50um in diameter. Pin floc occurs most commonly at starvation conditions -- a very low F/M and long sludge age. Chronic toxicity can also cause a pin floc condition

• Low F/M conditions also favor the growth of filamentous bacteria  some of whom often cause bulking issues in wastewater treatment systems.