As is known to all, BOD, COD, TP, TN & SS are the five most common concepts in the wastewater treatment industry. It’s quite easy to understand from the literal meanings of TP(Total Phosphorus), TN(Total Nitrogen) & SS(Suspended Solids). However, while talking about another 2 concepts - BOD(Biochemical Oxygen Demand) & COD(Chemical Oxygen Demand), I believe a lot of us will get confused...
Actually, these two oxygen demands represent the content of organic matter in the wastewater.
1. Why use BOD & COD as pollutant indexes?
Most of the pollutants from the wastewater are organic matters. There may be hundreds or thousands of different kinds of them or even more. It is impossible for us to take out each organic matter as the water output index. At this very moment, there must be a uniform indicator to represent the organic matter in the wastewater.
We all know that organic matter is composed of hydro & carbons which can be oxidized by strong oxidants or oxidized by microorganisms. Then, carbon dioxide and water are produced. The more oxygen consumed in the oxidation process, the more organic matters exist in the wastewater, so the Chemical Oxygen Demand and Biochemical Oxygen Demand, as known as COD & BOD, represent the organic matter in the wastewater.
2. What are BOD and COD?
BOD refers to the amount of dissolved oxygen required for microbial decomposition of organic matter. In the actual measurement of dissolved oxygen consumption in water, generally, it will take 5 days as a cycle and was called BOD5. COD refers to the amount of oxidant consumed by a reducing substance in a unit volume of water sample under certain conditions. Generally referred to as CODcr by potassium dichromate method. The higher the BOD and COD values, the more serious the water was polluted.
3. What is the difference between BOD and COD?
The organic matter in the wastewater is divided into degradable organic matter and non-degradable organic matter. Only the degradable organic matter can be utilized by the microorganism. Strong oxidants can oxidize all organic matter, so BOD means degradable organic matter. COD means all organic matter. The difference between these two data represents a non-degradable organic matter. If there are no measurement errors, the COD’s readings must be bigger than BOD’s. The value of BOD/COD can usually be used to roughly indicate the biodegradability of wastewater. Generally, when the value is bigger than 0.3, the wastewater is considered to be biodegradable. The bigger the ratio, the higher the biodegradability of the wastewater.
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