Biologycal Oxygen Demand (BOD)

Biologycal Oxygen Demand (BOD)

What is BOD?

Biological oxygen demand (BOD), also called biochemical oxygen demand, The BOD5 value indicates the amount of oxygen that bacteria and other tiny living beings consume for 5 days at a temperature of 20 ° C in a water sample for the aerobic degradation of the substances contained in the water. The value BOD is thus an indirect measure of the sum of all the biodegradable organic substances in the water. The value BOD indicates the amount of dissolved oxygen (mg / l) that is required during a certain time for the biological degradation of the organic substances contained in the wastewater. This value is an important parameter to assess the degree of load that residual water represents for the environment (receiving channel). As the substances contained in the wastewater are degraded in the receiving channel by the bacteria present there, the oxygen is partially or totally eliminated from the water. When these limit values ​​are exceeded, it can cause the death of living beings that breathe oxygen (crabs, fish, etc.).

The BOD is a pollution parameter to evaluate the quality of effluent or wastewater.
Drinking water is also evaluated for organic matter, this is measured through Total Organic Carbon (TOC or TOC) instead of BOD.

The biochemical decomposition of organic substrates is carried out by microorganisms. In this case we are talking about aerobic bacteria, which need energy that they produce from oxygen to complete decomposition. The oxygen is consumed and as a result the level of oxygen dissolved in the water is reduced. If there is a large amount of organic matter in the water, the oxygen demand is also higher for decomposition to take place.

The quality of the water is controlled by the authorities to protect the health of the users and other effects of poor water quality. A high BOD level may indicate fecal contamination or dissolved organic carbon particles from different sources other than humans or animals. This kind of pollution can seriously affect human health and cause problems in industry.

It is of great importance that governments ensure a low level of BOD in the effluent water coming out of sewage plants, because it is in the public interest to have rivers, lakes and seas with a high level of dissolved oxygen.

How to measure the level of BOD?

There are two methods to measure the BOD level, both are empirical tests.

  • Method I: It is the most common method. A special bottle for BOD is filled to the brim with the water test. The test is left for 5 days at a constant temperature of 20 ° C in the dark. After 5 days the oxygen content is measured compared to the original value, the oxygen consumption during this period indicates the oxygen demand of the water.
  • Method II: If a very high BOD is expected or if other toxic or inhibitory substances are present in the water, the sample can be diluted at first. In this way you can avoid having too little oxygen present to break down organic substances. This would falsify the measurement result. As with Method I, a comparison of before and after values ​​now serves as a measure of oxygen consumption during the measurement period.

After 5 days the dissolved oxygen is measured, with which the BOD level can be calculated. Drinking water should have a concentration of less than 1mg / l after 5 days. The wastewater concentration is accepted around 20mg / l.

As the methods are empirical, the BOD indicator does not give absolute results. What the indicator provides is a good test comparison but does not give an exact measure of contamination. An alternative to BOD is COD – Chemical Oxygen Demand.

Anaerobic bacteria like SRB do not need oxygen in the water to survive. These microorganisms live on sulfur, so they cannot be detected by measuring the biochemical oxygen demand.


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