COD (COD)

Chemical Oxygen Demand (COD) is the mass of oxygen required to chemically oxidise all organic and oxidisable inorganic matter in a water sample, expressed as mg/L. Where BOD measures only the biodegradable fraction over 5 days, COD captures the total oxidisable load — biodegradable plus refractory organics — in a 2-hour laboratory test. It is a faster, more comprehensive load indicator and a mandatory parameter in every SPCB effluent consent.

The mechanism is wet chemistry. A measured effluent sample is heated to 150°C for 2 hours with a strong oxidiser — potassium dichromate (K2Cr2O7) in concentrated sulphuric acid, with silver sulphate catalyst and mercuric sulphate to suppress chloride interference. The dichromate consumed during oxidation is measured by titration with ferrous ammonium sulphate or spectrophotometry; oxygen equivalent is reported as COD. The closed-reflux digestion method (IS 3025 Part 58) is the Indian standard.

India's discharge limits under the Environment (Protection) Rules 1986: COD ≤250 mg/L for discharge to inland surface water, no general limit for sewer or land disposal but industry-specific Schedule VI tightens this further (e.g. petroleum refinery COD ≤125 mg/L, distillery ≤200 mg/L). The COD:BOD ratio is the diagnostic that drives treatment design — a ratio below 1.5 indicates highly biodegradable effluent (food, dairy, sugar), 1.5-3 indicates moderately biodegradable (sewage, pulp), and above 4 indicates substantial refractory organics requiring advanced oxidation, activated carbon adsorption or Fenton's reagent before biological polishing.

For recycling plants, COD is the parameter that exposes problem effluents. Tyre pyrolysis wash water carries COD of 8,000-25,000 mg/L with COD:BOD ratios of 4-8 — refractory aromatic and phenolic compounds that biological treatment alone cannot meet. E-waste acid leaching wastewater from gold/copper recovery carries COD of 15,000-50,000 mg/L plus dissolved heavy metals, demanding chemical precipitation followed by Fenton's oxidation and clarification. PET wash effluent is friendlier at COD 1,500-4,000 mg/L, COD:BOD around 2 — UASB followed by aerobic clarification reliably hits 250 mg/L. The recurring trade-off is between capex (advanced oxidation Rs 6-12 crore for 100 m³/day) and opex (electricity 2-4 kWh/m³, hydrogen peroxide Rs 35-45/kg) on refractory streams.