Biochemical Oxygen Demand (BOD) measures how much oxygen microorganisms need to break down organic matter in water. Think of it like a health check-up for your water body—if BOD is high, your water’s struggling to breathe. When organic waste enters water, bacteria rush to feast on it. But their digestion requires oxygen, reducing the dissolved oxygen (DO) level. BOD tells us how much was used.

BOD vs. COD: What’s the Difference?

BOD is biological. COD (Chemical Oxygen Demand) is chemical. COD is faster but can overestimate pollution because it includes non-biodegradable matter.

Equipment Needed for BOD Testing

Incubators and BOD Bottles

You'll need special Biochemical Oxygen Demand bottles (usually 300 ml) and a 20°C incubator to simulate natural water conditions during the test.

Oxygen Probes or Winkler Titration Setup

Measure dissolved oxygen (DO) before and after incubation using DO probes or the Winkler titration method for accuracy.

BOD Sensors & Meters

Modern labs often use automated BOD meters for faster, more consistent results. Great for high-volume testing.

Sample Collection for BOD Testing

Ideal Sampling Locations

Sample upstream and downstream of discharge points, or from the inflow/outflow of treatment plants.

Handling & Preservation Tips

Keep samples cool (below 4°C) and test within 24 hours. Use airtight bottles to prevent oxygen exchange.

Step-by-Step BOD Testing Procedure

Step 1: Sample Preparation

Remove chlorine if present. Sometimes nutrients or microbial seeding agents are added.

Step 2: Initial DO Measurement

Measure the starting DO using a DO probe or titration. Record it carefully.

Step 3: Incubation Period

Incubate sealed bottles at 20°C for 5 days (BOD5). No light—bacteria shouldn’t photosynthesize!

Step 4: Final DO Measurement

After 5 days, measure the remaining DO in the same sample.

Step 5: Calculating Biochemical Oxygen Demand

Use the formula:

BOD (mg/L) = Initial DO – Final DO

That’s it. If DO drops significantly, your sample was rich in biodegradable waste.

BOD Test Interpretation

What Do Your Results Mean?

• <1 mg/L: Very clean water
• 1–3 mg/L: Clean
• 3–5 mg/L: Moderately polluted
• >5 mg/L: Heavily polluted

Acceptable BOD Levels by Industry

Each country/industry has its own limits. For instance, treated sewage in the U.S. usually must be below 30 mg/L BOD.

Step-by-Step Guide to Measuring BOD

1. Understand What BOD Measures

• BOD (Biochemical Oxygen Demand) indicates the amount of dissolved oxygen (DO) consumed by microorganisms during the decomposition of organic matter over a specified time (usually 5 days at 20°C, known as BOD₅).
• High BOD = more organic pollution = less oxygen available for aquatic life.

2. Prepare Equipment and Reagents

You’ll need:

• BOD bottles (300 mL)
• DO meter or Winkler titration apparatus
• Incubator (set to 20°C)
• Nutrient buffer solution (phosphate buffer, magnesium sulfate, calcium chloride, ferric chloride)
• Seed microorganisms (if sample is treated or chlorinated)
• Dilution water (deionized and saturated with oxygen)

3. Sample Collection and Handling

• Collect the sample in clean, airtight bottles.
• Analyze it as soon as possible; if not, keep it at 4°C and test within 24 hours.
• Avoid bubbles and headspace to prevent oxygen contamination.

4. Prepare Dilution Water

• Add the nutrient buffer and seed microorganisms (if needed) to the dilution water.
• Aerate it to achieve oxygen saturation before use.

5. Dilute the Sample

• Mix sample with dilution water in several BOD bottles at different dilution levels (to ensure at least one falls within the measurable DO range).

6. Measure Initial DO (DOi)

• Immediately measure the initial DO in one of the prepared bottles using a DO meter or Winkler method.

7. Incubate the Sample

• Place the bottles in an incubator at 20°C in the dark for 5 days to prevent algae growth and additional oxygen production.

8. Measure Final DO (DOf)

• After 5 days, measure the final DO of the same bottles.

9. Calculate BOD₅

Use the formula:

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BOD₅ (mg/L) = DOi - DOf

If dilution was used, account for dilution factor in your final result.