How to measure BOD in raw sewage

Measuring BOD (Biochemical Oxygen Demand) in raw sewage or a collection system involves determining the oxygen consumed by microorganisms while decomposing organic matter in the water sample over a specified period (typically 5 days). Here's the step-by-step process:

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1. Collecting the Sample

• Sample Collection:
  • Use clean, sterilized bottles to collect raw sewage.
  • Avoid excessive aeration during collection to prevent altering dissolved oxygen (DO) levels.
• Preservation:
  • Analyze the sample as soon as possible. If delays occur, store it at 4°C to minimize microbial activity.
• Dilution:
  • Raw sewage usually has a very high BOD, so dilution with distilled water is required to bring the oxygen demand within the measurable range.

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2. Preparing the Sample

• Dilution Water:
  • Prepare dilution water by aerating distilled water and adding nutrients like phosphate buffer, magnesium sulfate, calcium chloride, and ferric chloride. This ensures a conducive environment for microbial growth.
• Seed Microorganisms:
  • For some samples (especially treated wastewater), microbial seeding may be necessary to ensure sufficient bacterial activity. Raw sewage typically contains enough microorganisms and may not require seeding.

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3. Initial Dissolved Oxygen (DO) Measurement

• Measure the initial DO of the diluted sample using a dissolved oxygen meter or titration method (Winkler method).
• Record the DO value (in mg/L).

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4. Incubation

• Place the sample in a sealed BOD bottle to prevent air exchange.
• Incubate at 20°C ± 1°C in the dark to avoid photosynthesis, which could artificially increase oxygen levels.
• Incubation period is typically 5 days (referred to as BOD₅).

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5. Final Dissolved Oxygen (DO) Measurement

• After 5 days, measure the final DO of the sample using the same method.

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6. Calculating BOD

• Use the following formula:

$$\text{BOD (mg/L)} = (\text{Initial DO} - \text{Final DO}) \times \text{Dilution Factor}$$

• If seed microorganisms are used, subtract the oxygen demand of the seed (measured in a control sample).

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Example Calculation

• Initial DO: 9 mg/L
• Final DO: 3 mg/L
• Dilution Factor: 10

$$\text{BOD} = (9 - 3) \times 10 = 60 \, \text{mg/L}$$

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Key Considerations

1. Accuracy:
   • Ensure proper sealing of BOD bottles to prevent air intrusion.
   • Calibrate the DO meter regularly.
2. Dilution:
   • Choose an appropriate dilution to prevent complete DO depletion during incubation.
3. Replicates:
   • Test multiple samples to ensure reliability and account for variability in raw sewage.

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Alternative Method

• BOD Sensors:
  • Advanced sensors provide quicker results by simulating microbial oxygen demand under controlled conditions.

Accurately measuring BOD in raw sewage provides critical data for designing and operating sewage treatment processes effectively.