Chemical Oxygen Demand (COD) Calculator

Calculate COD levels in water samples for water quality analysis and wastewater treatment

Chemical Oxygen Demand Calculator

COD Formula: COD = [(Vblank - Vsample) × M × 8000] / Vsample
Where: V = Volume (mL), M = Molarity of oxidant (mol/L)

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. 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.

COD Calculation Formula

Primary Formula:

COD (mg/L) = [(Vblank - Vsample) × M × 8000] / Vsample

Where:

  • Vblank = Volume of titrant for blank (mL)
  • Vsample = Volume of titrant for sample (mL)
  • M = Molarity of oxidizing agent (typically K₂Cr₂O₇)
  • 8000 = Conversion factor (1000 mg/g × 8 g O₂/mole)
  • Vsample = Volume of sample analyzed (mL)

⚗️ 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).

Practical Examples

Example 1: Municipal Wastewater

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

Example 2: Industrial Effluent

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

Applications of COD Analysis

🏭 Wastewater Treatment

Monitor treatment efficiency, ensure regulatory compliance, and optimize process control in municipal and industrial wastewater facilities.

🌊 Water Quality Monitoring

Assess surface water and groundwater quality, detect pollution sources, and evaluate environmental impacts of industrial activities.

📊 Regulatory Compliance

Meet EPA discharge limits, verify treatment effectiveness, and document environmental performance for permits and audits.

🔬 Process Optimization

Determine optimal treatment conditions, evaluate new processes, and troubleshoot operational issues in water treatment systems.

COD vs BOD: Understanding the Difference

ParameterCODBOD
Test Duration2-3 hours5 days (BOD₅)
MethodChemical oxidationBiological oxidation
MeasuresAll oxidizable matterBiodegradable organics only
Typical ValueHigher than BODLower than COD
AdvantagesFast, reproducibleEnvironmental 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.

Quick Reference

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

Related Calculators

Combustion Analysis

Elemental composition

Crude Protein

Kjeldahl method

Combustion Reactions

Balance equations

Where It's Used

  • •Environmental laboratories
  • •Wastewater treatment plants
  • •Industrial discharge monitoring
  • •Water quality research
  • •Regulatory compliance testing