Water Pollution: BOD, COD and Eutrophication Explained

How do scientists measure "how polluted" a water sample is? Two standard tests do the job: BOD and COD. Both measure how much oxygen the pollution steals from the water, because fish and aquatic life die when dissolved oxygen runs out.

BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) are exam favourites in environmental chemistry. Together they reveal whether pollution comes mainly from biodegradable waste like sewage or from industrial chemicals, and their ratio helps engineers choose the right treatment. Untreated nutrient-rich waste also triggers eutrophication, the process that turns lakes green and then dead.

Key Concepts

• BOD: oxygen used by microbes to digest biodegradable organic waste, measured over 5 days at 20 °C, in mg/L of O2. High BOD means heavy organic loading, like sewage.
• COD: oxygen needed to chemically oxidise ALL organic matter (biodegradable or not) using hot potassium dichromate (K2Cr2O7) in acid, in mg/L of O2. Always ≥ BOD for the same sample.
• BOD/COD ratio: above ~0.5 means mostly biodegradable waste (biological treatment works); below ~0.3 means persistent chemicals dominate (chemical or physical treatment needed).
• Eutrophication stages: nutrient enrichment (nitrates, phosphates from fertilisers and sewage) → algal bloom → algae die → microbes decompose them, consuming oxygen → oxygen depletion (dead zone) where fish cannot survive.

Formulas

• BOD calculation: BOD5 = (DO initial − DO final) × dilution factor, in mg/L. DO is dissolved oxygen. Example: (8.0 − 3.0) × 10 = 50 mg/L.
• COD reagent: hot acidic K2Cr2O7 oxidises organics: Cr2O7²− + organics → Cr³+ + CO2 + H2O, with silver sulphate as catalyst. Takes ~2 hours, far faster than the 5-day BOD test.
• Algal growth: nutrients let algae photosynthesise: 6 CO2 + 6 H2O → C6H12O6 + 6 O2. When algae die, decay reverses this, consuming the same oxygen.
• Units: mg/L of O2. Clean river water: BOD below ~5 mg/L. Raw sewage: BOD above 300 mg/L.

Worked Example

A river sample diluted 1:10 has initial DO 9.0 mg/L and final DO 5.5 mg/L after 5 days at 20 °C. Its COD is 92 mg/L. Find BOD and the BOD/COD ratio, and interpret it.

• Step 1: Oxygen consumed = 9.0 − 5.5 = 3.5 mg/L.
• Step 2: BOD = 3.5 × 10 = 35 mg/L.
• Step 3: Ratio = 35 / 92 = 0.38.
• Step 4: Between 0.3 and 0.5 — a mix of biodegradable and persistent waste; biological treatment helps, but chemical treatment may also be needed.

This mirrors real lab work: the oxygen drop times the dilution gives the pollution strength, and the ratio tells the engineer which treatment to choose.

Common Misconceptions

• "BOD and COD measure the same thing." Wrong. BOD counts only what microbes digest in 5 days; COD counts all organics chemically, so COD is always higher and the ratio is meaningful.
• "Clear water is clean water." Wrong. Dissolved pollutants and fertiliser nutrients are invisible; a crystal-clear lake can still have high COD or be heading into eutrophication.
• "Algae in a lake are a good sign." Wrong in excess. A bloom blocks light, and the rotting algae consume all the oxygen, killing the very fish students assumed the algae would feed.

Real-World Applications

• Sewage treatment: primary removes solids, secondary (aeration tanks with bacteria) brings BOD down, tertiary removes nitrates and phosphates to prevent eutrophication.
• Factories must meet legal BOD and COD effluent limits before discharging into rivers.
• Precise fertiliser dosing cuts runoff, the main driver of lake eutrophication and coastal dead zones.
• Drinking-water plants use raw-water BOD/COD to decide filtration and chlorination levels.

BOD measures what microbes can digest, COD measures all organics chemically, and their ratio shows whether waste is biodegradable or persistent. Excess nutrients drive eutrophication — bloom, decay, oxygen loss, dead zones. Both tests together let engineers pick the right treatment and protect rivers and lakes.