Chemical Oxygen Demand (COD) Calculator
Result
COD: -- mg O₂/L
About Chemical Oxygen Demand (COD)
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 widely used to assess the pollution strength of wastewater and surface water.
Calculation Formula
The COD is calculated using the following formula:
COD (mg O₂/L) = (A - B) × M × 8000 / Sample Volume
Where:
- A: Volume of FAS used for blank titration (ml)
- B: Volume of FAS used for sample titration (ml)
- M: Molarity of FAS solution
- Sample Volume: Volume of sample used (ml)
Precision and Accuracy
Based on interlaboratory studies with synthetic samples containing potassium hydrogen phthalate and NaCl:
- At COD of 200 mg O₂/L (no chloride): Standard deviation ±13 mg/L (CV 6.5%)
- At COD of 160 mg O₂/L with 100 mg Cl⁻/L: Standard deviation ±14 mg/L (CV 10.8%)
Importance of COD
COD is crucial for:
- Assessing wastewater treatment plant efficiency
- Monitoring industrial discharge compliance
- Evaluating surface water quality
- Determining organic pollution load
- Designing wastewater treatment systems
Typical COD Values
COD concentrations vary by water type:
- Raw Domestic Wastewater: 250-1000 mg/L
- Primary Effluent: 150-400 mg/L
- Secondary Effluent: 20-60 mg/L
- Surface Water: 5-20 mg/L
- Industrial Wastewater: 500-50,000 mg/L (varies by industry)
COD vs BOD
- COD: Measures all chemically oxidizable matter (faster, 2-3 hour test)
- BOD: Measures only biologically oxidizable matter (5-day test)
- The COD/BOD ratio indicates wastewater treatability (typically 1.5-3.0 for municipal wastewater)
Testing Methodology
Standard COD testing involves:
- Digesting the sample with potassium dichromate in strong acid at 150°C
- Titrating the remaining dichromate with FAS
- Using ferroin indicator for endpoint detection
- Calculating COD based on titrant used
Quality Control Considerations
- Use proper blank corrections
- Account for chloride interference when present
- Maintain consistent digestion temperature and time
- Use appropriate sample volumes based on expected COD range
- Verify FAS molarity regularly
Interferences and Limitations
- Chlorides >2000 mg/L require mercury sulfate addition
- Volatile organics may be lost during open reflux
- Some aromatic hydrocarbons resist oxidation
- Inorganic reducing agents contribute to COD
