3. Mechanisms of BOD Removal
The removal of biochemical oxygen demand (BOD) by a biological floc (biomass) occurs in a defined sequence:
- Initial High-Rate Removal: Rapid adsorption and absorption of BOD onto the active biomass upon contact.
- Metabolism and Growth: Decomposable organic matter is utilized by cell enzymes for growth, synthesizing new cells and releasing end products.
- Endogenous Respiration: When the external food supply becomes limited, the biomass undergoes oxidation of its own cellular material.
- Conversion to Solids: The biomass is converted into settleable solids that can be removed from the wastewater.
The ratio of Food to Microorganisms (M/F) is a critical operational parameter.
- Low M/F Ratio: Corresponds to the log-growth phase with excess food. This results in a maximum rate of metabolism but produces biomass with poor settling characteristics.
- High M/F Ratio: Corresponds to the endogenous phase with limited food. This leads to starvation conditions but produces biomass with good settling characteristics and achieves high BOD removal efficiency.