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How It Works
Biochemical Oxygen Demand (BOD) measures how much dissolved oxygen microorganisms consume while breaking down organic matter in water. The BOD equation relates oxygen demand at any time to the ultimate BOD and a deoxygenation rate constant. The standard 5-day BOD test (BOD₅) is the most common water-quality benchmark.
For samples with few native bacteria (e.g., industrial effluent), a seeded BOD test adds a known microbial culture. The seed's own oxygen consumption is subtracted to isolate the sample's demand.
Example Problem
A wastewater sample has an ultimate BOD (L) of 300 mg/L and a deoxygenation rate constant (k) of 0.23 per day. What is BOD₅?
- BOD₅ = 300 × (1 − 10−0.23×5)
- 10−1.15 ≈ 0.0708
- BOD₅ = 300 × 0.929 = 278.8 mg/L
About 93% of the ultimate oxygen demand is exerted within 5 days at this rate.
Frequently Asked Questions
What is a normal BOD level for wastewater?
Untreated domestic wastewater typically has a BOD₅ of 200–300 mg/L. After secondary treatment, effluent BOD₅ usually drops below 20 mg/L to meet discharge permits.
What is the difference between BOD and COD?
BOD measures oxygen consumed by biological decomposition over days, while Chemical Oxygen Demand (COD) uses a strong oxidant to measure total oxidizable matter in a few hours. COD is always equal to or higher than BOD because it includes non-biodegradable organics.
Why is the BOD test done over 5 days?
Five days was chosen historically because it approximates the travel time of major English rivers from source to sea. It captures roughly 60–70% of the ultimate oxygen demand and provides a practical, reproducible measurement window.
When should you use a seeded BOD test?
Use a seeded test when the sample lacks sufficient microorganisms for decomposition, such as chlorinated effluent, industrial waste, or high-temperature discharges. The seed provides the bacteria needed for accurate results.
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