Water Quality Monitoring Lecture Notes

Introduction

• Continuation of previous lectures on water quality monitoring.
• Previous topics included chemical parameters for wastewater assessment.
• Current focus: biological or biochemical parameters crucial for assessing organic matter and oxygen demands in water.

Biochemical Parameters

• Biochemical Oxygen Demand (BOD):

  • Key parameter for measuring the oxygen demand due to organic material in water.
  • Organic decay in water requires oxygen, primarily through microorganisms.
  • Important for understanding the impact of wastewater discharge on aquatic ecosystems.

• Importance of Oxygen in Water:

  • Essential for sustaining aquatic life (animals and plants).
  • High BOD indicates poor water quality due to high organic material content.

BOD Measurement

• BOD is expressed in mg of oxygen consumed per liter of water sample.
• Standard Methods:
  • Previously: 5 days at 20°C (BOD5).
  • Current standard: 3 days at 27°C (BOD3).
• Stages of Decomposition:
  • Carbonaceous Stage:
    • Initial oxygen demand due to conversion of organic carbon to CO2.
  • Nitrogenous Stage:
    • Later stage involving conversion of organic nitrogen to ammonia and nitrates.
    • Typically not included in standard BOD tests unless inhibitors are added.

BOD Test Procedure

1. Measure initial Dissolved Oxygen (DO) before incubation (DO initial).

2. Incubate the sample in darkness for 3 or 5 days at specified temperatures.

3. Measure final DO after incubation (DO final).

4. Calculate BOD using the formula:

   BOD = (DO initial - DO final) / P

   • Where P is the dilution factor (volume of sample / total volume).

Dilution Factors

• If final DO < 2 mg/L, dilution is required to ensure accurate BOD measurement.
• Ensure BOD test is valid, considering both initial and final DO values.

Calculating BOD

• When organic matter is low, you can directly measure initial and final DO to determine BOD.

• If high organic matter is present, dilute the sample with organic-free water to lower oxygen demand.

• Seeded Water:

  • Sometimes dilution water is seeded with microorganisms to ensure degradation occurs.

• Oxygen demand from seeded dilution water must be subtracted from total demand to get accurate BOD values.

Modeling BOD

• BOD can be modeled as a first-order reaction based on the remaining organic matter.

• BOD Formula:

  BOD_t = L_0 (1 - e^{-kt})

  • Where:
    • L_0 = ultimate BOD
    • k = BOD rate constant
    • t = time (in days)

• Temperature Dependence:

  • The k value is temperature-dependent, with a formula to relate k at different temperatures.

Example Problems

• Worked through examples to calculate BOD using the provided formulas and parameters.
  • Parameters included initial DO, final DO, k values, dilution factors, etc.

Conclusion

• BOD tests are critical for assessing the impact of organic matter in water bodies.
• Understanding each step of the process ensures accurate measurement and interpretation.
• Next lecture will build upon this foundation.

Thank you for your attention!