COD - chemical oxygend demand

Chemical oxygen demand, or in short COD, tells how much oxygen is needed to fully oxidize any quantity of some compounds containing either carbon, hydrogen or both.

Methane, CH4, can be used as an example.

To fully oxidize one mole of CH4, two moles of O2 is needed. Oxidation products are CO2 and H2O. The stoichometry

(1) CH4 + 2O2 => CO2 + 2H2O

It is of course also possible to calculate the chemical oxygen demand of other compounds. Take for instance the VFAs:

Acetate:
Propionate:
Butyrate:
Valerate:
CH3COOH + 2 O2
CH3(CH2)COOH + 3½ O2
CH3(CH2)2COOH + 5 O2
CH3(CH2)3COOH + 6½ O2
=> 2CO2 + 2H2O
=> 3CO2 + 3H2O
=> 4CO2 + 4H2O
=> 5CO2 + 5H2O
, 1 mole acetate is 64 gCOD
, 1 mole propionate is 112 gCOD
, 1 mole butyrate is 160 gCOD
, 1 mole valerate is 208 gCOD

Chemical oxygen demand can be measured experimentally by oxidizing a sample. Small kits are available, where the sample is just dropped into a small vial containing strong oxidizing agent and heated for a long period of time. Afterwards the vial is evaluated photometrically. The change in color reflects how much could be oxidized in the sample.

Usually, chemical oxygen demand is measured in gCOD or in kgCOD. In the example above, two moles of oxygen is needed to oxidize one mole of CH4. Therefore, the COD of 1 mole of CH4 is the weight of two moles of oxygen. That is 64 g.

So one mole of CH4 is 64 gCOD.

Some simulation models use COD as their basic unit. One example is the Anaerobic Digestion Model No. 1 (ADM1).

Below is a picture of the structure of the ADM1.

The ADM1 structure

COD in the different boxes: composite material, carbohydrates, proteins, lipids, monosaccarides, aminoacids, LCFA (long chained fatty acids), the volatile fatty acids (VFAs): valerate, butyrate, propionate and acetate, and finally hydrogen and methane.

In the ADM1, all the COD in e.g. LCFA is converted into digestion products from the anaerobic digestion of LCFA, and bacterial biomass. No COD is lost in such systems (mass balance) except loss due to bacteria, substrates and products being washed out of the system.


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