COD Formula: COD = [(Vblank - Vsample) × M × 8000] / Vsample
Where: V = Volume (mL), M = Molarity of oxidant (mol/L)
Chemical Oxygen Demand (COD) is a critical water quality parameter that measures the amount of oxygen required to chemically oxidize organic and inorganic matter in water. It's a key indicator of organic pollution and is widely used in wastewater treatment, environmental monitoring, and industrial discharge analysis.
Unlike BOD (Biological Oxygen Demand), which measures biodegradable organics only, COD measures both biodegradable and non-biodegradable organic matter, providing a more comprehensive assessment of water contamination.
Primary Formula:
COD (mg/L) = [(Vblank - Vsample) × M × 8000] / Vsample
Where:
⚗️ Standard Method:
The dichromate reflux method is the most common COD test. It uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent in acidic conditions at high temperature. Excess dichromate is titrated with ferrous ammonium sulfate (FAS).
Given:
• Sample volume = 25 mL
• Blank titration = 10.5 mL
• Sample titration = 8.2 mL
• FAS molarity = 0.025 M
Calculation:
COD = [(10.5 - 8.2) × 0.025 × 8000] / 25
COD = [2.3 × 0.025 × 8000] / 25
COD = 460 / 25 = 18.4 mg/L
✓ This indicates clean water with minimal organic pollution
Given:
• Sample volume = 10 mL
• Blank titration = 12.0 mL
• Sample titration = 3.5 mL
• FAS molarity = 0.025 M
Calculation:
COD = [(12.0 - 3.5) × 0.025 × 8000] / 10
COD = [8.5 × 0.025 × 8000] / 10
COD = 1700 / 10 = 170 mg/L
⚠️ High pollution level - treatment required before discharge
Monitor treatment efficiency, ensure regulatory compliance, and optimize process control in municipal and industrial wastewater facilities.
Assess surface water and groundwater quality, detect pollution sources, and evaluate environmental impacts of industrial activities.
Meet EPA discharge limits, verify treatment effectiveness, and document environmental performance for permits and audits.
Determine optimal treatment conditions, evaluate new processes, and troubleshoot operational issues in water treatment systems.
| Parameter | COD | BOD |
|---|---|---|
| Test Duration | 2-3 hours | 5 days (BOD₅) |
| Method | Chemical oxidation | Biological oxidation |
| Measures | All oxidizable matter | Biodegradable organics only |
| Typical Value | Higher than BOD | Lower than COD |
| Advantages | Fast, reproducible | Environmental relevance |
Typical COD/BOD Ratio: For municipal wastewater, the COD/BOD ratio is usually 1.5-2.5. A higher ratio indicates more non-biodegradable organic matter.
Clean Water
< 20 mg/L COD
Good Quality
20-40 mg/L COD
Moderate
40-80 mg/L COD
High Pollution
80-200 mg/L COD
Severe
> 200 mg/L COD