Biochemical oxygen demand (BOD) is defined as the amount of dissolved oxygen consumed by microorganisms during the biochemical process of decomposing certain oxidizable substances, especially organic matter, present in water under specified conditions. The process takes a long time, such as 100 days under 20 ℃ cultivation conditions. According to current international regulations, the result measured after five days of cultivation at a temperature of 20 ± 1 ℃ is called the five-day biochemical oxygen demand, denoted as BOD5, expressed in mg/L. For general domestic sewage and industrial wastewater, although they contain a lot of organic matter, if the sample contains sufficient microorganisms and oxygen, it can be directly measured. However, in order to ensure the growth of microorganisms, a certain amount of inorganic nutrients (phosphate, calcium, magnesium, and iron salts) need to be added. Some industrial wastewater that does not contain or contains few microorganisms, wastewater with high acidity or alkalinity, wastewater treated with high temperature or chlorination sterilization, etc., should be inoculated with microorganisms that can decompose organic matter in the water before measurement. This method is called inoculation. When there are organic compounds or toxic substances in some wastewater that are difficult to be degraded by microorganisms in general domestic sewage at a normal rate, the water sample can be appropriately diluted and inoculated with inoculated water containing adapted microorganisms after domestication. The BOD5 for general water quality testing only includes the oxygen consumption of carbonaceous organic matter oxidation and a small amount of inorganic reducing substances. Due to the presence of a large number of nitrifying microorganisms in the effluent of many secondary biochemical treatments and water bodies that have been polluted for a long time. These microorganisms can produce the biochemical process of nitrification once they reach a certain quantity. To suppress the oxygen consumption of nitrification, an appropriate amount of nitrification inhibitor should be added.