Calculating Biochemical Oxygen Demand (BOD) involves determining the amount of oxygen consumed by microorganisms while decomposing organic matter in a water sample. The BOD test is crucial for assessing water quality and pollution levels.
Here's the process and formula:
1. Collect and Prepare Water Samples:
- Collect water samples carefully, ensuring minimal disturbance.
- Dilute the samples if necessary, especially if the pollution level is expected to be high. Dilution is important to ensure there's enough dissolved oxygen available for the microorganisms to consume during the incubation period.
2. Measure Initial Dissolved Oxygen (DO1):
- Determine the dissolved oxygen (DO) concentration in the diluted sample immediately using a dissolved oxygen meter or a Winkler titration. This is your initial DO (DO1).
3. Incubate the Sample:
- Place the diluted sample in a sealed BOD bottle.
- Incubate the bottle in the dark at a controlled temperature of 20°C (68°F) for a specified period, typically 5 days. This allows microorganisms to consume the organic matter.
4. Measure Final Dissolved Oxygen (DO2):
- After the incubation period, measure the dissolved oxygen concentration in the incubated sample using the same method as before. This is your final DO (DO2).
5. Calculate BOD:
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Use the following formula to calculate the BOD value:
BOD = (DO1 - DO2) * Dilution Factor
Where:
- BOD is the Biochemical Oxygen Demand (mg/L or ppm)
- DO1 is the initial dissolved oxygen of the diluted sample (mg/L or ppm)
- DO2 is the final dissolved oxygen of the diluted sample after incubation (mg/L or ppm)
- Dilution Factor is the ratio of the volume of diluted sample to the volume of original sample. If you used a 1:10 dilution, the dilution factor is 10.
Example:
Let's say:
- DO1 = 8 mg/L
- DO2 = 2 mg/L
- Dilution Factor = 10
Then:
BOD = (8 mg/L - 2 mg/L) * 10 = 60 mg/L
This means the Biochemical Oxygen Demand of the original water sample is 60 mg/L.
Important Considerations:
- Seeding: Sometimes, a "seed" of microorganisms needs to be added to the diluted sample, particularly if the original sample is disinfected or lacks sufficient microbial activity.
- Nutrient Addition: Nutrients may be added if they are limited in the sample to ensure proper microbial growth.
- Temperature Control: Maintaining a consistent temperature during incubation is critical for accurate results.
- Dilution Selection: Choosing the appropriate dilution factor is essential. Too little dilution, and all the oxygen will be consumed (DO2 = 0). Too much dilution, and the difference between DO1 and DO2 will be too small to accurately measure.
- Units: BOD is typically expressed in milligrams of oxygen per liter of water (mg/L) or parts per million (ppm).
