What are BOD and COD?
These terms refer to biological oxygen demand and chemical oxygen demand.

Biological oxygen demand

This measure identifies the amount of oxygen required to oxidize organic and inorganic matter in an aqueous sample. It is measured by a test known as BOD5.
To perform this test, a water sample must be incubated at 20°C for a period of 5 days in the dark. At the time of incubation and after 5 days, the amount of dissolved oxygen in the sample must be measured. The differential will be expressed in mg/L and represents the biological oxygen demand required to oxidize the contaminants in the water.

Chemical oxygen demand 

This measurement allows the identification of all oxidizable materials in the sample. In other words, unlike the biological oxygen demand, the chemical oxygen demand does not represent only the materials that can be oxidized in a biological way.

This measure is most often used for monitoring oxidizable matter in water since the test results for COD concentrations are automatic. Indeed, it is sufficient to add potassium dichromate, a powerful oxidizing agent, and to calculate the residual quantities to know the BOD and COD concentration.

The addition of potassium dichromate to identify the presence of COD also allows the identification of BOD, since the presence of the former undoubtedly means the presence of the latter.


Understanding the impact of BOD / COD

Although it may seem trivial, the concentration of dissolved oxygen required for the oxidation/decomposition of organic and inorganic matter can have a significant impact on the health of a water body.

This means that when water heavily contaminated with BOD/COD is discharged into a water body, the dissolved oxygen in the water body can be completely consumed. When this happens, aquatic life such as fish, algae or any other organism living underwater are forced to migrate elsewhere or they will drown due to lack of oxygen. 
In short, to learn more about BOD/COD and to better understand their impact as well as the techniques to reduce these contaminants, consult this article: 

What is Biofilm and what do we do with it?
Typically, biofilms are structured bacterial accumulations coated with a polymer matrix that attaches to some surface.