Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD) measures the total oxygen required to chemically oxidise the pollutants in a water sample, using a strong oxidising agent (potassium dichromate) under acidic, heated conditions. Unlike BOD, which captures only what micro-organisms can biodegrade, COD captures both biodegradable and non-biodegradable organic and oxidisable inorganic matter. COD is therefore always equal to or greater than BOD, and the COD:BOD ratio indicates how treatable the effluent is biologically. The inland-surface-water discharge limit is 250 mg/L.

COD is the faster, more complete pollution indicator — a COD test takes a few hours versus three days for BOD — so it is widely used for operational monitoring. A high COD with a low BOD signals the presence of persistent, non-biodegradable pollutants (solvents, certain chemicals, recalcitrant organics) that biological treatment alone cannot remove; this is the trigger for tertiary treatment (advanced oxidation, activated carbon, membranes).

For recyclers, COD is especially important in streams with non-biodegradable contamination. Plastic chemical recycling and pyrolysis wastewaters may carry dissolved hydrocarbons and additives; hydrometallurgical and chemical processing effluents carry oxidisable chemicals; and CBG digestate carries both biodegradable (high BOD) and recalcitrant (residual COD) fractions. A persistently high COD above 250 mg/L after biological treatment is the classic sign that the ETP needs a tertiary polishing stage.

The practical use is diagnostic and design-driving: compare COD with BOD to judge biodegradability, use COD for quick day-to-day ETP monitoring, and if COD stays high after secondary treatment, add tertiary treatment to meet the 250 mg/L limit. For chemical recyclers and pyrolysis operators, designing for residual non-biodegradable COD from the outset avoids the common failure of an ETP that removes BOD well but cannot meet the COD standard.