Question
What do BOD, COD, and DO mean in the context of water quality, and how are they related to each other?
Solution — Step by Step
DO (Dissolved Oxygen): Amount of oxygen dissolved in water. Higher DO = healthier water. Fish need at least 5-6 mg/L to survive.
BOD (Biochemical Oxygen Demand): Amount of oxygen consumed by microorganisms to decompose organic matter in water over 5 days at 20 degrees C. Higher BOD = more organic pollution = dirtier water.
COD (Chemical Oxygen Demand): Amount of oxygen needed to chemically oxidise ALL organic matter (biodegradable + non-biodegradable). Always higher than BOD because it measures total organics, not just what microbes can break down.
graph LR
A[Sewage enters water body] --> B[Organic matter increases]
B --> C[Microbes multiply to decompose it]
C --> D[Microbes consume oxygen - BOD rises]
D --> E[DO drops - dissolved oxygen depleted]
E --> F{DO too low?}
F -->|Yes| G[Fish die, anaerobic conditions, foul smell]
F -->|No| H[Ecosystem recovers downstream]Key relationship: As BOD increases, DO decreases. They are inversely related.
| Parameter | Measures | Clean Water | Polluted Water | Relationship |
|---|---|---|---|---|
| DO | Oxygen available in water | High (8-10 mg/L) | Low (below 4 mg/L) | Decreases with pollution |
| BOD | Oxygen needed by microbes for bio-decomposition | Low (below 5 mg/L) | High (above 17 mg/L) | Increases with organic pollution |
| COD | Oxygen needed for total chemical oxidation | Low | Very high | Always COD greater than or equal to BOD |
BOD measures only the biodegradable fraction (what microbes can eat). COD measures everything — biodegradable + non-biodegradable organics (like plastics, certain industrial chemicals). Since COD covers more substances, it is always the larger number.
The ratio BOD/COD indicates how much of the pollution is biodegradable:
- BOD/COD close to 1: mostly biodegradable (treatable by biological methods)
- BOD/COD close to 0: mostly non-biodegradable (needs chemical treatment)
Why This Works
These three indicators together give a complete picture of water quality. DO tells you the current state (how much oxygen is available for aquatic life). BOD tells you the biological load (how much oxygen microbes will consume). COD tells you the total organic contamination. Together, they help water treatment plants decide what level of treatment is needed.
NEET frequently asks: “What does high BOD indicate?” Answer: high concentration of organic matter (high pollution). Also common: “Why does DO decrease downstream of a sewage outfall?” Because microbes decomposing the sewage consume the dissolved oxygen.
Alternative Method
Think of it with a simple analogy: imagine a pond as a bank account of oxygen.
- DO = current balance
- BOD = the “bills” that microbes will pay (withdrawals)
- COD = the total bills including ones that need chemical payment
When the bills (BOD) exceed the income (oxygen replenishment from atmosphere), the account (DO) crashes — and aquatic life suffers.
Common Mistake
Students write “high BOD means high oxygen in water.” This is backwards. High BOD means high oxygen DEMAND — the water NEEDS a lot of oxygen because it is heavily polluted. The actual oxygen present (DO) is LOW when BOD is high. BOD is a measure of pollution, not of oxygen availability.