Chemical Oxygen Demand (COD)
Calculate COD in mg/L using dichromate or permanganate methods. Analyze BOD/COD ratios for wastewater characterization and treatment design.
💧 Chemical Oxygen Demand (COD) Calculator
Dichromate | Permanganate | BOD/COD Ratio | Wastewater Characterization
📋 Sample Examples
🏛️ Municipal Wastewater
Typical domestic sewage sample
🍔 Food Processing Effluent
High organic load from food industry
👕 Textile Industry Wastewater
Dye-containing industrial effluent
🛢️ Petrochemical Effluent
Hydrocarbon-containing wastewater
📄 Pulp & Paper Mill
Lignin-rich industrial wastewater
💊 Pharmaceutical Wastewater
Complex organic compounds
🧪 Permanganate Method
Using KMnO4 for COD determination
Calculate COD
For educational and informational purposes only. Verify with a qualified professional.
📋 Key Takeaways
- • COD = (A − B) × N × 8000 × D / V (dichromate)
- • Measures all oxidizable matter (organic + inorganic)
- • BOD/COD > 0.5 → biodegradable; < 0.2 → refractory
- • Dichromate: 2–3 h; BOD₅: 5 days
What is 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. Unlike Biological Oxygen Demand (BOD), which measures only biodegradable organic matter, COD measures both biodegradable and non-biodegradable organic compounds, making it a more comprehensive indicator of water pollution.
Higher COD = More organic/inorganic pollutants = Poorer water quality
COD vs BOD: Key Differences
| Parameter | COD | BOD |
|---|---|---|
| What it measures | All oxidizable matter (organic + inorganic) | Biodegradable organic matter only |
| Test duration | 2-3 hours | 5 days (BOD₅) |
| Oxidizing agent | K₂Cr₂O₇ (dichromate) or KMnO₄ | Microorganisms |
| Typical values | Usually 1.5-2× higher than BOD | Lower, measures only biodegradable fraction |
| Use cases | Quick assessment, toxic samples, industrial wastewater | Biological treatment design, environmental impact |
How Does COD Measurement Work?
COD measurement involves oxidizing organic and inorganic matter in a water sample using a strong chemical oxidant under acidic conditions at elevated temperature. The amount of oxidant consumed is proportional to the oxygen demand of the sample.
🔬 Dichromate Method (Standard Method)
Principle
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Strong oxidizing agent in acidic medium
Oxidizes ~95-100% of organic matter
Procedure
1. Add known volume of K₂Cr₂O₇
2. Heat at 150°C for 2 hours
3. Titrate excess dichromate with Fe²⁺
4. Calculate COD from titrant difference
🧪 Permanganate Method
Principle
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Weaker oxidant than dichromate
Oxidizes ~60-80% of organic matter
Use Cases
• Less toxic than dichromate
• Quick screening test
• Lower COD values expected
• Not standard for regulatory compliance
COD Calculation Formulas
Dichromate Method Formula
COD (mg/L) = [(A - B) × N × 8000 × D] / V
Where:
- A = Volume of Fe²⁺ used for blank titration (mL)
- B = Volume of Fe²⁺ used for sample titration (mL)
- N = Normality of dichromate solution (N)
- D = Dilution factor (if sample was diluted)
- V = Sample volume (mL)
- 8000 = Equivalent weight of O₂ (8 g/eq × 1000 mg/g)
Note:
The difference (A - B) represents the volume of titrant consumed by the organic matter in the sample. This is multiplied by normality and the oxygen equivalent (8000) to determine COD in mg/L.
Permanganate Method Formula
COD (mg/L) = [(Vperm × Nperm - Voxalic × Noxalic) × 8000] / Vsample
Where:
- Vperm = Volume of KMnO₄ added (mL)
- Nperm = Normality of KMnO₄ (N)
- Voxalic = Volume of oxalic acid used (mL)
- Noxalic = Normality of oxalic acid (N)
- Vsample = Sample volume (mL)
When to Use COD Testing
COD testing is essential for wastewater treatment, environmental monitoring, and regulatory compliance. Understanding when and why to use COD helps in proper water quality assessment.
Industrial Wastewater
Quick assessment of organic load, especially for toxic or non-biodegradable compounds.
- Textile industry
- Petrochemical plants
- Pharmaceutical manufacturing
Treatment Plant Monitoring
Real-time process control and efficiency monitoring at wastewater treatment facilities.
- Influent characterization
- Treatment efficiency
- Effluent compliance
Environmental Monitoring
Assess water quality in rivers, lakes, and receiving waters for regulatory compliance.
- Surface water quality
- Discharge permits
- Impact assessment
BOD/COD Ratio: Wastewater Characterization
The BOD/COD ratio is a critical parameter for wastewater treatment design. It indicates the biodegradability of organic matter and helps select appropriate treatment processes.
Ratio > 0.6
Highly Biodegradable
Excellent for biological treatment
0.4 - 0.6
Moderately Biodegradable
Good for biological treatment
0.2 - 0.4
Moderately Refractory
May need pretreatment
< 0.2
Refractory
Advanced oxidation needed
| Wastewater Type | Typical COD (mg/L) | Typical BOD (mg/L) | BOD/COD Ratio |
|---|---|---|---|
| Municipal Wastewater | 250-800 | 100-400 | 0.40-0.60 |
| Food Processing | 1000-10000 | 500-6000 | 0.50-0.70 |
| Textile Industry | 500-2000 | 50-300 | 0.10-0.30 |
| Petrochemical | 500-5000 | 50-500 | 0.05-0.20 |
| Pulp & Paper | 1000-5000 | 200-1500 | 0.20-0.40 |
| Pharmaceutical | 2000-20000 | 200-2000 | 0.05-0.15 |
Practical Applications & Examples
Example: Municipal Wastewater Treatment
Given:
- Sample volume: 20 mL
- Blank titration: 24.5 mL
- Sample titration: 18.2 mL
- Dichromate normality: 0.25 N
- BOD₅: 180 mg/L
Solution:
COD = [(24.5 - 18.2) × 0.25 × 8000] / 20
COD = [6.3 × 0.25 × 8000] / 20
COD = 630 mg/L
BOD/COD = 180/630 = 0.29
Moderately refractory - suitable for biological treatment
Example: Food Processing Effluent
Given:
- Sample volume: 10 mL
- Blank titration: 25.0 mL
- Sample titration: 12.5 mL
- Dilution factor: 5
- BOD₅: 2500 mg/L
Solution:
COD = [(25.0 - 12.5) × 0.25 × 8000 × 5] / 10
COD = [12.5 × 0.25 × 8000 × 5] / 10
COD = 12,500 mg/L
BOD/COD = 2500/12500 = 0.20
High organic load - requires pretreatment
Limitations & Considerations
⚠️ Limitations
- • Chloride interference (forms Cl₂, consumes oxidant)
- • Nitrite interference (oxidized to nitrate)
- • Some compounds not fully oxidized
- • Toxic chromium waste (dichromate method)
- • May overestimate biodegradable fraction
✓ Best Practices
- • Use mercury sulfate to mask chloride
- • Run blank and standards with each batch
- • Dilute high COD samples appropriately
- • Follow standard methods (APHA, ASTM)
- • Combine with BOD for complete assessment
📚 Official Data Sources
⚠️ Disclaimer: This calculator uses EPA and APHA Standard Methods for COD determination. For regulatory compliance and precision work, consult the official sources above and follow ISO 6060 or approved laboratory procedures.
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