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Determination of Biochemical Oxygen Demand (BOD) – Environmental Engineering Lab Experiment

Determination of Biochemical Oxygen Demand (BOD) – Lab Experiment

Biochemical Oxygen Demand (BOD) is the amount of oxygen required by microorganisms to decompose organic matter present in water or wastewater under aerobic conditions. It is an important indicator used to measure the pollution level of water bodies.

BOD is usually expressed in milligrams per liter (mg/L) and is commonly measured over a 5-day incubation period at 20°C.

Aim

To determine the Biochemical Oxygen Demand (BOD) of the given water or wastewater sample.

Apparatus Required

  • BOD bottles (300 mL capacity)
  • Incubator maintained at 20°C
  • Pipettes
  • Measuring cylinder
  • Conical flask
  • Burette

Chemicals Required

  • Manganous sulfate solution
  • Alkaline iodide azide reagent
  • Concentrated sulfuric acid
  • Standard sodium thiosulphate solution
  • Starch indicator
  • Distilled water

Theory

Microorganisms present in water consume dissolved oxygen while decomposing organic matter. The amount of oxygen consumed during this biological process indicates the level of organic pollution in the sample.

In the BOD test, the dissolved oxygen (DO) of the sample is measured initially and again after incubating the sample for 5 days at 20°C. The difference between the initial and final DO values gives the BOD of the sample.

Procedure

  1. Fill the BOD bottle completely with the water sample without trapping air bubbles.
  2. Measure the initial dissolved oxygen (DO) using the Winkler method.
  3. Seal another BOD bottle containing the same sample.
  4. Place the bottle in an incubator at 20°C for 5 days.
  5. After 5 days, determine the final dissolved oxygen using the same method.
  6. Record the initial and final DO values.

Observation Table

Sample Initial DO (mg/L) Final DO after 5 days (mg/L) BOD (mg/L)
Water Sample ______ ______ ______

Calculation

Biochemical Oxygen Demand (BOD) =

Initial DO − Final DO

If dilution is used:

BOD (mg/L) = (D₁ − D₂) × Dilution Factor

Where:

  • D₁ = Initial dissolved oxygen (mg/L)
  • D₂ = Final dissolved oxygen after incubation (mg/L)

Result

The Biochemical Oxygen Demand (BOD) of the given water sample was determined after 5 days of incubation.

BOD of the sample = ______ mg/L

Precautions

  • Avoid trapping air bubbles while filling BOD bottles.
  • Use clean glassware to prevent contamination.
  • Maintain the incubator temperature at 20°C.
  • Perform dissolved oxygen determination carefully.

Conclusion

The BOD test helps determine the biodegradable organic pollution in water and wastewater. Higher BOD values indicate a greater amount of organic pollution and a higher oxygen demand for microbial decomposition.

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