Biochemical Oxygen Demand (BOD) refers to the amount of dissolved oxygen consumed in water during the microbial decomposition of organic matter in surface water, commonly abbreviated as BOD with the typical unit of milligrams per litre.

The degradation of most organic substances under microbial action proceeds in two phases. The first phase involves the conversion of organic matter into carbon dioxide, ammonia and water. The second phase, known as the nitrification process, describes the further transformation of ammonia into nitrites and nitrates mediated by nitrosifying bacteria and nitrifying bacteria.

BOD generally denotes the oxygen consumption from the biochemical reactions in the first phase. The rate and extent of microbial decomposition of organic matter depend on temperature and time, with the optimal temperature ranging from 15 to 30 °C. Theoretically, complete bio‑oxidation of organic matter takes an infinitely long period. For practical purposes, however, the reaction is considered finished within 20 days, defined as BOD₂₀. Practical experience shows that the BOD measured after a five‑day incubation accounts for roughly 70–80 % of the total BOD and can represent the oxygen‑demanding capacity of organic pollutants in water. To ensure comparability of BOD results, a standard analytical method is adopted: measuring dissolved‑oxygen consumption after five‑day incubation at 20 °C, termed the 5‑day Biochemical Oxygen Demand, denoted as BOD₅. BOD quantifies the total amount of microbially degradable organic matter in a water body, expressed as milligrams of dissolved oxygen consumed per litre of water. A BOD value below 1 mg/L indicates clean water, while values exceeding 3–4 mg/L signal organic pollution. Nevertheless, BOD measurement requires a long incubation time. For highly toxic wastewater, inhibited microbial activity prevents accurate BOD determination.

The BOD₅ of ordinary turbulent rivers usually does not exceed 2 mg/L; an index above 10 mg/L will produce a foul odour. Water used for industry, agriculture and aquaculture requires a BOD below 5 mg/L, whereas drinking water must have a BOD less than 1 mg/L.

According to China’s Integrated Wastewater Discharge Standard, the permissible concentration for the Class‑II BOD limit at factory discharge outlets is 60 mg/L, and the BOD of surface water shall not exceed 4 mg/L.

Chemical Oxygen Demand (COD) is defined as the quantity of oxidising agent consumed during the chemical oxidation of oxidisable substances in a water sample under specified conditions. It is expressed as milligrams of oxygen consumed per litre of water sample and is generally abbreviated as COD.