THE biochemical oxygen demand (BOD) corresponds to the amount of oxygen consumed by microorganisms present in a certain effluent sample (such as domestic and industrial sewage). As these microorganisms carry out the decomposition of organic matter in the aquatic environment, knowing the amount of this gas is an effective way to analyze the level of pollution existing in this environment.
Microorganisms (aerobic bacteria, for example) act as catalysts for oxidation reactions, in which organic compounds, together with oxygen gas, are transformed into new compounds.
The oxidation of glucose, for example, leads to the production of carbon dioxide and water, as we can see in the following equation:
Ç6H12O6 + 6 O2 → 6 CO2 + 6 H2O
You organic compounds that are oxidized in nature are mainly made up of carbon, hydrogen and oxygen, in addition to nitrogen, phosphorus, sulfur. The main source of organic substances found in rivers and seas is sewage, in which we find carbohydrates, proteins and oils.
Biological decomposition generates the
biochemical oxygen demand, and this has a fundamental role in the environment, as the degradation of organic matter returns its elements and substances to nature.However, as most cities discharge their sewage into rivers, it is important to have a balance in the BOD of these effluents, which can be achieved as follows:
The relationship between the water flow and the amount of sewage released should be concerned with;
Intensify the aeration, that is, the amount of oxygen dissolved in the water.
→ BOD and water quality
THE biochemical oxygen demand is used to determine the level of water pollution. Waters with a low dissolved oxygen concentration are considered polluted, therefore, with a high BOD, as this substance is used in the decomposition of organic compounds. Unpolluted or clean waters, in turn, have high dissolved oxygen concentrations, low BOD, bordering on the saturation point.
→ BOD and the sewage treatment
In sewage treatment plants, the BOD is a parameter used to verify the efficiency in the decomposition of organic matter, because if the BOD is high, it means that the organic matter is being consumed. According to the legislation, the BOD maximum in sewage should be 60 mg/L.
So, in general, the biochemical oxygen demand acts as an indicator of water pollution. The greater the amount of effluent released into a watercourse, the greater the amount of organic matter, which will favor a large consumption of oxygen gas (O2) by microorganisms, increasing BOD and harming aerobic living beings.
That's because, when raising the BOD, anaerobic living beings start to carry out the oxidation reaction of organic compounds, which leads to the production of unpleasant-smelling substances, such as hydrogen sulphide (H2S).
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/quimica/demanda-bioquimica-oxigenio.htm
