The organic matter is oxidized by potassium dichromate. Is more used than the BOD because most of the organics materials are resistant to the microbial oxidization.

The enrichment of the water with algal nutrients are contributed by the sewage and is a serious economical problem due the increasing of the BOD causing a deterioration of the water quality.

The bacteria's oxidize the organic material and in the process they consume oxygen dissolved in water; the consequences of this are that the water flow became anaerobic killing animals and plants, the process of water purification is much more slower because of the scarcity of oxygen and this causes bad odors and the creation of toxic materials in the water. The consume of the oxygen by the bacteria is what we called BOD.

After water contamination, the water needs to start a process of purification and when this occurs without man involving is called self-purification and is caused by physical, chemical and microbiological changes.

This indicators provides a way to monitored natural water, searching for sewage pollution. There are two types of indicators:
Coliform group: Any gram-negative, rod-shaped, facultative anaerobic bacterium.
Fecal coliforms: Are coliforms that can grow in a temperature of 44.5°C.

Sewage is a mix of organic and inorganic natural materials that have a small quantity of man-made substances. Human excreta is the main source of sewage pollution and the domestic wastewater come generally from residences, buildings, hospitals, schools and in the other hand, the industrial waste water comes from manufacturing plants.