COD (Chemical Oxygen Demand) Calculators

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Chemical oxygen demand (COD) is one of the most widely used measurements in water quality analysis. It quantifies the total amount of oxygen required to chemically oxidize organic and inorganic matter in a water sample — giving a fast, reliable picture of pollution load. COD testing is essential in wastewater treatment, industrial discharge monitoring, and environmental compliance. Unlike BOD, which takes five days to measure, COD results are available in a few hours, making it the go-to metric for rapid pollution assessment.

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What Is Chemical Oxygen Demand (COD)?

Chemical oxygen demand (COD) is a measure of the total quantity of oxygen required to completely oxidize all organic and oxidizable inorganic compounds in a water sample, expressed in milligrams of O₂ per liter (mg/L). It is an indirect measure of the amount of organic matter in water — the higher the COD, the more polluted the water.

COD is measured chemically rather than biologically. A strong oxidizing agent — typically potassium dichromate (K₂Cr₂O₇) in sulfuric acid — is used to oxidize the sample under controlled conditions, and the oxygen equivalent consumed is calculated from the amount of dichromate reduced.

COD Formula and Calculation

For the dichromate reflux method, COD is calculated as:

COD (mg/L) = (A − B) × M × 8000 / V

Where:

  • A — volume of ferrous ammonium sulfate (FAS) titrant used for the blank (mL)
  • B — volume of FAS used for the sample (mL)
  • M — molarity of the FAS solution
  • 8000 — milliequivalent weight of oxygen × 1000 mL/L
  • V — volume of sample (mL)

For colorimetric (spectrophotometric) COD tests using pre-made reagent vials — common in field and routine lab testing — absorbance is measured at 600 nm and compared to a standard curve.

COD vs. BOD: Key Differences

  • COD measures all chemically oxidizable material — both biodegradable and non-biodegradable organic matter, plus oxidizable inorganics (e.g., sulfides, ferrous iron)
  • BOD (Biochemical Oxygen Demand) measures only the oxygen consumed by microorganisms to biologically degrade organic matter over 5 days (BOD₅)
  • COD is always ≥ BOD because it includes everything BOD measures plus non-biodegradable organics
  • COD takes 2–3 hours; BOD takes 5 days
  • The COD/BOD ratio indicates treatability: ratios close to 1 suggest highly biodegradable waste; high ratios (>3) suggest significant non-biodegradable content resistant to biological treatment

Typical COD Values

  • Clean river water: < 10 mg/L
  • Lightly polluted water: 10–100 mg/L
  • Domestic wastewater: 250–800 mg/L
  • Industrial effluents: 1,000–100,000 mg/L depending on the industry
  • Treated effluent discharge limits: Typically 75–125 mg/L under US EPA guidelines

COD in Wastewater Treatment

COD is a primary parameter for sizing and monitoring biological treatment systems. The organic loading rate — COD per unit volume per day — determines the capacity of activated sludge systems, anaerobic digesters, and biofilm reactors. Effluent COD is routinely measured to verify compliance with discharge permits.

Glossary

Chemical Oxygen Demand (COD)
The amount of oxygen required to chemically oxidize all organic and oxidizable inorganic substances in a water sample, expressed in mg O₂/L. A key indicator of water pollution and organic load.
Dichromate Reflux Method
The standard laboratory procedure for measuring COD, in which a water sample is oxidized with potassium dichromate in sulfuric acid at 150°C. The remaining oxidant is titrated to calculate oxygen consumed.
COD/BOD Ratio
The ratio of chemical to biochemical oxygen demand. Values near 1.0 indicate highly biodegradable waste suitable for biological treatment. Ratios above 3 suggest significant non-biodegradable or toxic content that may inhibit biological treatment.

Frequently Asked Questions

COD (Chemical Oxygen Demand) measures all chemically oxidizable material in water using a chemical oxidant — results in 2–3 hours. BOD (Biochemical Oxygen Demand) measures only biologically degradable organic matter via microbial activity — takes 5 days. COD is always ≥ BOD. The COD/BOD ratio indicates how biodegradable the waste is: ratios near 1 mean highly treatable; high ratios indicate significant non-biodegradable content.

Context determines whether a COD value is high. Clean river water has COD below 10 mg/L. Domestic sewage typically ranges 250–800 mg/L. Industrial effluents can reach tens of thousands of mg/L. Most regulatory discharge limits require treated effluent COD below 75–125 mg/L. Values exceeding permit limits indicate inadequate treatment and can lead to regulatory penalties.

The standard method is the dichromate reflux test: a measured water sample is digested with potassium dichromate in sulfuric acid at 150°C for 2 hours, oxidizing organic matter. The remaining dichromate is back-titrated with ferrous ammonium sulfate (FAS), and COD is calculated from the volume consumed. Colorimetric vial methods using spectrophotometry at 600 nm are faster and widely used for routine monitoring.

Not directly — the relationship between COD and BOD depends on the specific composition of the wastewater. However, once the COD/BOD ratio is established experimentally for a particular waste stream, BOD can be estimated from COD measurements. This is useful for process monitoring when you need frequent estimates but can only afford to run full BOD₅ tests periodically.