Chemical Oxygen Demand (COD)

Definitions and overview

Chemical oxygen demand (COD) is the amount of oxygen necessary to oxidize the organic and oxidizable inorganic compounds completely to CO2 & H2O.

Measurement of the polluting strength of wastewater and natural waters and is closely related to the Biochemical Oxygen Demand (COD), total organic carbon (TOC), and total oxygen demand (TOD). The COD will always be higher than the BOD since the COD includes substances that are both chemically as well as biologically oxidised. In untreated domestic wastewater, COD ranges between 250 and 1000 mg/L.

The ratio of COD: BOD provides a useful guide to the proportion of organic material present in wastewaters that is biodegradable, although some polysaccharides such as cellulose can only be degraded anaerobically and so will not be included in the BOD estimation. The COD:BOD relationship varies from 1.25 to 2.50 depending on the waste being analysed. The ratio increases with each stage of biological treatment as biodegradable matter is consumed but non-biodegradable organics remain and are oxidised in the COD test.

Analysis Protocol

The Standard Methods for the Determination of COD in Water and Wastewater

Reagents used:

1. Potassium dichromate (Standard solution): K2Cr2O7 – 0.004167 M (0.0250 N)

2. Mohr’s Salt: Ferrous ammonium sulphate (Standard solution): FeSO4.(NH4)2SO4 (0.025 M)

3. Mercuric Sulphate: Powdered HgSO4

4. Silver Sulphate: Powdered Ag2SO4

5. Phenanthroline ferrous sulphate indicator solution

6. Concentrated Sulphuric acid: H2SO4 18 M

Note: The Cr-COD method is used as the standard in USA and Taiwan. Compared to Japanese Mn-COD method, it has a higher detection limit and severe chloride ion interference because of the stronger oxidant and the longer digestion time. In order to reduce the chloride-ion interference. The mercury salt (e.g. HgSO4) is usually added to mask the chloride ion.

Overview of Protocol

The COD test employs a potassium dichromate reflux with concentrated sulphuric acid, using silver sulphate (Ag2SO4) as catalyst and mercuric sulphate (HgSO4) to complex any chlorides present which could interfere with the reaction. The sample is refluxed for 2 hours in an acidified potassium dichromate solution of known strength so that the amount of oxidisable organic matter in the sample is proportional to the potassium dichromate consumed in the oxidation reaction. The excess dichromate is titrated in the ferrous ammonium sulphide to calculate the amount of dichromate consumed.