Chemical Oxygen Demand Calculators

0 calculators tagged with “Chemical Oxygen Demand”

Chemical Oxygen Demand (COD) is the amount of oxygen required to chemically oxidize all organic and inorganic matter in a water sample using a strong oxidizing agent — typically potassium dichromate (K₂Cr₂O₇) under acidic conditions. Unlike BOD₅, which measures only biodegradable organic matter over 5 days, COD measures both biodegradable and non-biodegradable oxidizable substances in 2–3 hours. COD results are always ≥ BOD₅. The COD/BOD ratio indicates wastewater treatability: a ratio near 1.5–2 indicates easily biodegradable waste; ratios > 4 suggest significant non-biodegradable content requiring physical-chemical treatment.

All Calculators

No calculators found for this topic.

COD Test Method

Standard dichromate reflux method (APHA 5220): sample + K₂Cr₂O₇ + H₂SO₄ + Ag₂SO₄ (catalyst) → reflux at 150°C for 2 hours → oxidizable matter reduces Cr⁶⁺ to Cr³⁺ → measure residual Cr⁶⁺ by colorimetry (A₆₂₀ nm) or back-titration. COD (mg O₂/L) = (A−B) × M × 8,000 / V_sample. A = titrimetric mL (blank); B = sample; M = molarity; 8,000 = 8 g O/equivalent × 1,000 mg/g.

COD vs. BOD

  • COD: measures all oxidizable material chemically; takes 2–3 h; always ≥ BOD₅
  • BOD₅: measures biodegradable organic matter biologically; takes 5 days; closer to actual oxygen demand in receiving water
  • COD/BOD₅ ratio: 1.5–2.0 = easily biodegradable; 2–4 = moderately; > 4 = significant non-biodegradable fraction

Typical COD Values

Clean river: < 20 mg/L. Moderately polluted: 50–200 mg/L. Raw municipal sewage: 300–600 mg/L. Food processing wastewater: 1,000–10,000 mg/L. Pulp and paper: 1,000–5,000 mg/L.

Regulatory Context

US NPDES effluent limits commonly specify COD for industrial discharges. EU Urban Wastewater Directive: effluent COD < 125 mg/L for communities > 10,000 PE.

Glossary

COD (Chemical Oxygen Demand)
Oxygen equivalent of all oxidizable material (biodegradable + non-biodegradable) in water; measured by K₂Cr₂O₇ reflux (2–3 h); always ≥ BOD₅; mg O₂/L.
COD/BOD₅ Ratio
Biodegradability indicator; 1.5–2 = easily biodegradable; 2–4 = moderate; > 4 = significant non-biodegradable fraction → biological treatment alone insufficient.
Dichromate Reflux
Standard COD test: K₂Cr₂O₇ + H₂SO₄ + Ag₂SO₄ catalyst at 150°C, 2 h; Cr⁶⁺ reduced proportional to oxidizable matter; measured colorimetrically at 600 nm.

Frequently Asked Questions

COD (Chemical Oxygen Demand): the oxygen equivalent needed to chemically oxidize ALL oxidizable material in water — both biodegradable and non-biodegradable organics, plus some inorganic reducing agents (sulfides, nitrites). Uses K₂Cr₂O₇ in acidic solution; takes 2–3 hours. BOD₅: the oxygen consumed by microorganisms breaking down biodegradable organic matter over 5 days at 20°C. Only biodegradable substances counted. Takes 5 days. Relationship: COD ≥ BOD₅ always (COD counts more types of matter). COD/BOD₅ = 1.0–1.5: almost all COD is biodegradable. COD/BOD₅ = 2–4: significant non-biodegradable fraction. COD/BOD₅ > 4: industrial waste with refractory organics → biological treatment alone may be insufficient.

APHA Standard Method 5220B (closed reflux colorimetric): add 2.5 mL sample to a COD digestion tube. Add 1.5 mL K₂Cr₂O₇ solution (0.0417 M or 0.0833 M depending on range). Add 3.5 mL H₂SO₄ + Ag₂SO₄ reagent (catalyst). Cap tightly; digest at 150°C for 2 hours in a COD reactor. Cool; read absorbance at 600 nm (for high range; 420 nm for low range) using a spectrophotometer or pre-programmed photometer. Compare against COD standards to calculate concentration. Result is in mg O₂/L. Pre-made vials (Hach COD tubes) simplify field analysis.

High COD indicates a high concentration of oxidizable substances — usually organic matter. Sources: Food processing, brewery, dairy wastewater: very high COD (1,000–50,000 mg/L) from carbohydrates, fats, and proteins. Municipal sewage: 300–600 mg/L COD. Industrial: pharmaceutical, textile, petrochemical effluents often have high COD from synthetic organics. Receiving water impact: high COD discharge → when biodegradable fraction is respired by microorganisms → DO depletion → hypoxia → fish kills (same mechanism as BOD, but COD gives the total potential oxygen demand). Non-biodegradable COD: doesn't exert immediate oxygen demand but may accumulate as persistent organic pollutants (POPs).

The COD/BOD₅ ratio (or BOD₅/COD) indicates what fraction of total organic matter is biodegradable: COD/BOD₅ ≈ 1.5–2: easily biodegradable wastewater → conventional biological treatment (activated sludge) effective. COD/BOD₅ ≈ 2–4: moderately biodegradable → biological treatment with longer retention time; advanced treatment may be needed for non-biodegradable fraction. COD/BOD₅ > 4: significant refractory (non-biodegradable) organics → biological treatment alone inadequate; need Fenton oxidation, ozonation, UV/H₂O₂, or adsorption (activated carbon). Municipal sewage typically has COD/BOD₅ ≈ 2. Industrial effluents from synthetic chemical production may have COD/BOD₅ > 10 → require extensive pre-treatment before biological plant or alternative treatment.