Biochemical Oxygen Demand
What is Biochemical Oxygen Demand (BOD)?
Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required by microorganisms to biologically decompose biodegradable organic matter present in water or wastewater.
It is an important indicator of organic pollution and water quality.
How BOD works?
When organic matter enters a water body:
Organic waste → Microbial decomposition → Oxygen consumption → Lower dissolved oxygen
If the organic load is high, microorganisms consume more dissolved oxygen. This can reduce oxygen available to fish and other aquatic organisms.
BOD₅
The commonly used parameter is BOD₅, which measures the oxygen consumed over 5 days, generally under controlled conditions at 20°C.
It is expressed as:
mg/L of O₂
BOD vs COD
| Feature | BOD | COD |
|---|---|---|
| Full form | Biochemical Oxygen Demand | Chemical Oxygen Demand |
| Process | Biological oxidation | Chemical oxidation |
| Measures | Mainly biodegradable organic matter | Chemically oxidisable substances |
| Standard test | BOD₅ | Relatively rapid chemical test |
| Unit | mg/L O₂ | mg/L O₂ |
| Typical relationship | BOD < COD | COD > BOD |
Environmental Significance of BOD
High BOD generally indicates high organic pollution. Excessive BOD can cause dissolved oxygen depletion, leading to:
- Stress or death of aquatic organisms
- Anaerobic conditions
- Bad odour
- Deterioration of water quality
BOD Levels and Pollution
BOD is generally interpreted as follows for freshwater:
| BOD₅ (mg/L) | Approx. Pollution Level | Interpretation |
|---|---|---|
| < 1 | Very low / Clean | Very little biodegradable organic matter |
| 1–2 | Low | Relatively clean water |
| 2–5 | Moderate | Noticeable organic pollution |
| 5–10 | High | Significant organic pollution |
| > 10 | Very high | Heavily polluted; severe oxygen stress may occur |
Key principle:
Higher BOD → More biodegradable organic matter → Greater microbial oxygen consumption → Lower dissolved oxygen → Greater risk to aquatic life.
Point to Remember:
BOD ↑ = Organic Pollution ↑ = DO ↓ = Aquatic Stress ↑
Clean Water → BOD Low → DO High
Polluted Water → BOD High → DO Low
