The pollution caused by organic and inorganic substances in water can be evaluated using a parameter called COD.
COD stands for Chemical Oxygen Demand and refers to the oxygen used during the oxidation of substances dissolved and suspended in water. By determining the COD it is possible to determine the quantity of chemically oxidizable substances with energy oxidants such as potassium dichromate present in a strongly acidic solution. The acid environment is created  by the addition of concentrated sulfuric acid.
Both inorganic compounds and organic substances can be oxidized. This method  can also be used to determine the quantity of  organic substances such as cellulose, that  may  not be detected by the equivalent biological method (BOD, Biochemical Oxygen Demand).

Thermoreaction (digestion) of a sample
Digestion is an extremely important step in many chemical reactions. The aim is to convert low-solubility compounds or substances present in the form of aggregates into soluble compounds in order to degrade organic substances into inorganic molecules, or to eliminate interfering substances and solubilize metallic ions. Digestion takes place by adding decomposition reagents to the sample which is then heated.