5-Day Biological Oxygen Demand Calculation: A Comprehensive Guide

Explore the essence of Biological Oxygen Demand (BOD) and its crucial role in environmental monitoring.

BOD5 Calculator

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Understanding Biological Oxygen Demand (BOD) is pivotal for environmental scientists and engineers, as it offers a crucial insight into the health of water bodies. This measurement is essential in assessing the quality of water and its ability to support life. Here, we delve into the concept of BOD, its significance, and how to accurately calculate it using a specific formula.

Definition and Formula

Biological Oxygen Demand, or BOD, refers to the amount of oxygen consumed by microorganisms in the decomposition of organic matter in water. It is a critical indicator of the organic pollution level in a water body. The higher the BOD, the greater the amount of organic matter or pollution. To calculate BOD, a standard formula is used:

\text{BOD}_5 = \frac{[(D_1 - D_2) - (B_1 - B_2) \times f]}{P}


  • D1 = oxygen level in the diluted sample initially (mg/l)
  • D2 = oxygen level in the diluted sample after 5 days (mg/l)
  • B1 = oxygen level in the dilution water initially (mg/l)
  • B2 = oxygen level in the dilution water after 5 days (mg/l)
  • V1 = volume of wastewater sampled for dilution (ml)
  • V2 = total volume of the diluted sample (ml)
  • f = dilution factor = \frac{V_2 - V_1}{V_2}
  • P = proportion of the sample in the diluted mixture = \frac{V1}{V2}

How to use Biological Oxygen Demand Calculator

  • Prepare the Samples: Collect the water sample and dilution water.
  • Measure Initial Oxygen Levels: Record the oxygen levels (D1 and B1) of both the sample and the dilution water.
  • Incubate for 5 Days: Keep the sample at a stable temperature for 5 days.
  • Measure Final Oxygen Levels: Record the final oxygen levels (D2 and B2).
  • Calculate the BOD: Use the formula to determine the BOD value.


  • What is the significance of the 5-day period in BOD testing?

    The 5-day period is is chosen as a standard because it strikes a balance between accuracy and practicality. Shorter timeframes might not capture all biodegradable organic matter, while longer periods could lead to inaccuracies due to nutrient depletion or algae growth. The 5-day period effectively reflects the level of pollution from biodegradable organic matter in water.

  • Can BOD be used to measure pollution?

    Yes, BOD is an effective measure of organic pollution in water bodies.

  • How does temperature affect BOD readings?

    Temperature influences the rate of microbial activity, hence affecting BOD readings. It’s essential to maintain a constant temperature during the incubation period.

Additional Tips

When conducting BOD tests, consistency in methodology is key. Ensure that samples are collected and stored properly to avoid contamination. Understanding the local environmental conditions can also provide insights into expected BOD levels. Sometimes, high BOD levels could indicate a nearby source of organic pollution.


BOD calculation is not just a scientific procedure; it’s a window into the health of our water bodies. By mastering this measurement, we can better understand and protect our aquatic environments. Whether you’re a student, a researcher, or just an environmental enthusiast, knowing how to calculate BOD is a valuable skill in the quest to preserve our planet’s precious water resources.

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  • Handbook of Chemical Engineering calculations, Chopey et al, McGraw Hill, 2004. Link