What are COD and BOD

What is biochemical oxygen demand (BOD) and chemical oxygen demand (COD)

Biochemical oxygen demand (BOD) is also called biochemical oxygen consumption. It is a comprehensive indicator of the content of organic compounds and other aerobic substances in water. When organic substances in water come into contact with air, the amount of oxygen required to decompose due to the action of aerobic microorganisms to make them inorganic or gaseous is called biochemical oxygen demand, expressed in ppm or mg/L. The higher the value, the more organic pollutants in the water, the more serious the pollution. In fact, the time for complete decomposition of organic matter varies due to its type and quantity, the type and quantity of microorganisms, and the nature of water. It often takes tens or hundreds of days to complete oxidative decomposition, and sometimes, due to the influence of heavy metals and toxic substances in the water, it hinders the activities of microorganisms and even causes them to die.

Therefore, it is difficult to determine BOD accurately. In order to shorten the time, this kind of oxygen demand (BOD5) of five days is generally used as the basic estimation standard for organic pollutants in water. BOD5 is about 70% of the oxygen consumption of complete oxidation decomposition. Generally speaking, rivers with BOD5 below 4ppm can be said to be pollution-free.

Chemical oxygen demand (COD) is the amount of oxygen required by oxidizing agents (such as potassium dichromate or potassium permanganate) to oxidize organic pollutants and some reducing substances in water under certain conditions, expressed in milligrams of oxygen consumed per liter of water sample. Chemical oxygen demand (COD) is an important index used to evaluate water quality. Chemical oxygen demand has the characteristics of simple and rapid determination. The oxidant potassium chromate can completely oxidize the organic substances in water, and can oxidize other reducing substances. The oxidant potassium permanganate can only oxidize about 60% of organic matter. Neither method can reflect the actual degradation of organic pollutants in water, because neither method can express the amount of organics that can be oxidized by microorganisms. Therefore, biochemical oxygen demand (BOD) is often used in the study of water quality polluted by organic substances.