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Understanding Biochemical Oxygen Demand (BOD)

Sep 18, 2024

Lecture Notes: Biochemical Oxygen Demand (BOD) and Water Quality

Recap from Last Class

  • Hydrology Concepts
    • Darcy Velocity vs. Actual (Linear) Velocity
      • Actual velocity is higher due to soil's porosity.
      • Equation: Actual velocity = Darcy velocity / porosity.
    • Importance of Porosity
      • Water flows through porous soil, affecting velocity.
  • Water Pollutants
    • Point Sources: Direct discharges like sewage.
    • Non-Point Sources: Indirect sources like fertilizers.

Oxygen Demand in Water

  • Types of Oxygen Demand
    • Oxygen-demanding materials deplete oxygen, affecting aquatic life.
    • Dissolved Oxygen (DO): Max ~8 mg/L under normal conditions.
    • Oxygen Demand: Needed to oxidize organics.

Types of Oxygen Demand

  • Theoretical Oxygen Demand
    • Based on known composition of waste, e.g., glucose.
    • Impractical due to complex waste compositions.
  • Chemical Oxygen Demand (COD)
    • Uses oxidizing agents like chromic acid.
    • Can overestimate demand due to non-biodegradable materials.
  • Biochemical Oxygen Demand (BOD)
    • Most realistic measure using microorganisms to mimic natural processes.
    • Measures oxygen consumed over time (DO initial - DO final).

Relationship Between Oxygen Demand Types

  • BOD is typically smaller than COD or theoretical oxygen demand.
    • Microorganisms convert some organics into body mass or waste, not just CO2.
    • BOD reflects biodegradability and actual environmental conditions.

Measuring BOD

  • Kinetics of BOD
    • Reaction modeled as first-order kinetics (exponential decay).
    • BODt = L0 - L0 * exp(-kT).
  • Calculating Ultimate BOD
    • Use short-term measurements (e.g., BOD5) and decay constant (k) to estimate ultimate BOD.*

Factors Affecting BOD and Decay Rate (k)

  • Nature of Waste: Simple sugars vs. complex organics affect decomposition rate.
  • Microorganism Characteristics: Type and concentration.
  • Temperature Effects
    • Higher temperatures increase k value; colder conditions slow decomposition.
    • Use equations to adjust k based on temperature differences.

Practical Application Example

  • Example Calculation
    • Given BOD3 and k, calculate ultimate BOD.
    • Use formulas: BODt = L0 - L0 * exp(-kT).
  • Dilution Factors
    • Importance of proper dilution to accurately measure DO changes.
    • Sample size ratio affects BOD calculations.*

Steps to Measure BOD

  1. Sample Preparation
    • Use 300 ml BOD bottle, fill with sample, ensure no air gaps.
    • Dilution to ensure detectable DO consumption.
  2. Introduce Microorganisms
    • Add seed to sample, seal bottle.
    • Use control blanks to account for microorganism respiration.
  3. Measure DO Over Time
    • Compare DO at time zero and after five days to calculate BOD5.

Key Takeaways

  • BOD provides a good measure of organic pollution and water quality.
  • Temperature, composition of waste, and dilution significantly impact BOD measurement accuracy.
  • Understanding kinetics and proper experimental procedures are vital for accurate assessments.

Study and review these notes to understand the principles of BOD and how it is measured and calculated, as well as its significance in assessing water quality.

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