Chemical oxygen demand

Chemical oxygen demand (COD) is the oxygen-equivalent amount consumed when a water sample’s chemically oxidizable substances are oxidized under specified test conditions. It is commonly reported as milligrams of oxygen per liter.

Dichromate COD method: A COD procedure that digests a water sample with excess potassium dichromate in strongly acidic conditions. This standard method converts oxidant consumption into an oxygen-equivalent result.
Biochemical oxygen demand: The oxygen consumed by microorganisms as they biologically degrade organic matter under specified conditions. BOD measures biological degradation, whereas COD measures chemical oxidation.
Wastewater treatment: Processes that remove contaminants from used water before discharge or reuse. Treatment operators track COD to assess organic-load removal and process performance.
Permanganate index: A measure of oxidizable substances based on their consumption of potassium permanganate under specified conditions. It uses a milder oxidant and generally captures a different fraction of water contaminants.
Potassium dichromate: An orange chromium(VI) salt used as a strong oxidizing agent in acidic solutions. It is the principal oxidant in the conventional COD digestion.
Total organic carbon: The amount of carbon bound in organic compounds in a sample, usually measured after converting it to carbon dioxide. TOC measures organic carbon directly rather than its oxygen-equivalent oxidation demand.
Activated sludge: A wastewater treatment process in which aerated microbial flocs consume dissolved and suspended pollutants. Changes in influent and effluent COD help evaluate this biological process.
Chloride interference in chemical oxygen demand: Positive bias in COD measurements caused when chloride is oxidized during the test. This interference can make saline samples appear more polluted than they are.
Closed reflux method: A COD test in which sealed tubes are heated to digest samples while limiting loss of volatile compounds. Sealed digestion makes the dichromate test practical for routine samples.
Dissolved oxygen: Molecular oxygen dissolved in water and available to aquatic organisms and chemical reactions. COD estimates demand that can deplete the dissolved oxygen present in receiving waters.

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