What is DBO and COD?

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Some people before the analysis of either potable or residual water consider what are the DBO and COD parameters that are so intimately linked.

In previous posts we told you the main components of a water analysis. Now let’s focus on the analysis of these two parameters of water analysis!

1) What is COD in water?

COD stands for chemical oxygen demand and is expressed in mg/l O2. To understand it better we will have to notice that the water or a spill has in addition to pure water many other components. Those many other components may be susceptible to oxidation, therefore they will fall within our measurement, or they may not be susceptible to measurement.

It is a parameter that is used very frequently in municipal WWTPs (Wastewater Treatment Plants) or in industrial discharges,such as paper industries or agri-food companies, among others.

It is also very important in the purification processes,since for the decomposition of these components they will require the addition of a reagent in order for them to decompose.

Water with high COD values that is not properly treated can cause a severe eutrophication problem, leading to the death of all animal life in the water.

2) What is WBOD in water?

BoD is the Biological Oxygen Demand, that is, it expresses the amount of oxygen that living beings need to consume the organic matter of water. How long? That is the key question, which is why BOD5 is usually used, that is, the amount of oxygen necessary for bacteria to consume all the matter in 5 days.

It is a very useful parameter to be measured in wastewater and helps us evaluate if the wastewater is a load for the water receiving it.

When we exceed the reference values, the same problem determined in the previous point (the COD definition) will also be caused: eutrophication of water. And as a consequence of it the death of all the animals that live the water. Such as fish. They die precisely because of the consumption of water by bacteria.

3) Why are COD and BBO measured in wastewater?

The Chemical Oxygen Demand (whose acronym is COD) measures to know the level of pollution present in the water,that is why it is expressed in milligrams of biatomic oxygen per liter.

Precisely to know the concentration of matter present in the water, regardless of whether it is organic or inorganic matter that contaminates the water. While organic matter is the main influence on an increase in chemical oxygen demand, it is also influenced by some inorganic chemicals.

The Biological Oxygen Demand is also used to know the level of water pollution,but in this case we are going to look only at organic components. That is why its value will be less than that obtained by the chemical demand of oxygen.

Now that you know both concepts, it will be easy for you to reach the conclusion that they are measured to know the degree of water pollution. Therefore, it is measured mainly in discharges, that is, in water that will be returned to the receiving streams. For example, the water that is going to be returned by a factory to the river.

It is also used for water intended for irrigation, since this way we know in a relatively simple way if the water is contaminated and if such contamination is of a chemical type, so it would not be suitable for irrigation. Or if on the contrary it is a water that is not pure but that only contains natural or biological elements and a priori would be suitable for irrigation.

4) Difference between COD and DBO from a water analysis

COD tells us the amount of contaminants that are present in the water and that can be oxidized. However, the DBO tells us the contaminants, but not all those that can be oxidized but only those that can be biodegraded.

Therefore, the chemical demand for oxygen measures the presence of all kinds of contaminants, although it is always sensitive to organics. On the other hand, the BOD or biological oxygen demand measures only the organic components that are contaminating the water in the laboratory sample.

It is quite important to know the difference between the two since in the analysis of water or the analysis of the discharge both are usually examined because together they give us information that is more enriching.

5) Relationships between COD and DBO of a water analysis

The similarities between both parameters are many since both, both COD and DBO, measure the contaminants in the water.

In many analyses, if not in a very high percentage, we will see how both are analyzed. This way we will have a more global vision of water and we can know the level of pollution. COD values (biological oxygen demand) are always higher than BOD (biological oxygen demand) values; it is evident if we take into account that COD also collects inorganic pollutants.

Industrial discharges tend to have higher levels of COD as they have higher levels of non-biodegradable compounds.

6) Normal COD values in the water analyzed

The ratio of the Chemical Oxygen Demand (COD) is proportional to the level of pollution present in the water,regardless of whether it comes from biodegradable or non-biodegradable behaves.

The maximum value for Chemical Oxygen Demand is 500 mg/l.

7) Normal values of BBOD in the water analyzed

The values of the Biological Oxygen Demand (BOD) is proportional to the level of contamination found in the water of the analyzed sample. Therefore, at higher levels of BDR, we are facing water that is more contaminated by biodegradable components.

At the indicative level we could have one level like the following

• Pure Water: from 0 to 20 mg/liter
• Slightly Contaminated Water: from 20 to 100 mg/liter
• Moderately Contaminated Water: from 100 to 500 mg/liter
• Heavily Contaminated Water: from 500 to 3,000 mg/liter
• Extremely Contaminated Water: from 3,000 mg/liter

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