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What is Chemical Oxygen Demand and Biological Oxygen Demand?

What is Chemical Oxygen Demand and Biological Oxygen Demand?

  • Just like composting or how tree leaves rot; when organic chemicals (carbon and hydrogen compounds) or biological matter enter into water, it eventually breaks down.
  • Oxygen is necessary for breaking down material. For breaking down more materials, more oxygen is needed.
  • Additionally, aquatic organisms need oxygen to breathe.
  • When too much organic matter, either from a chemical or biological source is added to an aquatic system, the natural balance gets out of synchronisation and aquatic animals start dying.

Measures to Monitor Oxygen Demand

Chemical oxygen demand (COD):

  • COD is the demand for oxygen in a water body from chemical sources.
  • It is a measure of the amount of oxygen required by chemical organic matter (organic compounds) added to a water body.
  • Organic pollutants at a site could be things like chemicals, petroleum, solvents, cleaning agents, etc.

Biological oxygen demand (BOD):

  • It is similar to COD. The difference is that the material which puts demand for oxygen is of biological and not of chemical origin.
  • The biological origin matter includes plant and animal matter.
  • Both COD and BOD are expressed in milligrams per liter (mg/l), which indicates the mass of oxygen consumed or required per litre of solution.

Linkages between Biological Matter and Organic Matter

  • It is to be noted that chemically, biological matter occurring in nature is also organic.
  • Thus, one indicator namely BOD is also used for both chemical and biological oxygen demands.

Why Does Oxygen Dissolve More in Cold Water?

  • The amount of dissolved oxygen that the water can hold depends upon the temperature and salinity of the water.
  • Cold water can hold more dissolved oxygen than warm water and freshwater can hold more dissolved oxygen than saline water.
  • Thus, warmer and saltier the water, the lesser is the dissolved oxygen in it.
  • Cold water holds more oxygen than warm water because water molecules are closer in cold water which makes it harder for oxygen molecules to escape.
  • The tight structure of water molecules is also conducive for a more consistent attraction between the oxygen and water molecules.
  • The saline water is already saturated due to the presence of salts. As a result, its ability to hold oxygen molecules is low.
  • The solubility of oxygen in water depends upon other factors as well. For instance, oxygen is more soluble in water with a lot of surface area.
  • Water that splashes over a rock, like in rapids, has a larger surface area and is likely to contain more oxygen than the still water.


Environment & Biodiversity

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