Project Watershed Central New York

In ecology, biochemical (biological) oxygen demand (BOD) is an indicator for the concentration of organic waste, bacteria, and other microorganisms in a sample of water. Because these microorganisms require oxygen for cellular respiration, their numbers, and thus the concentration of dead organic matter, can be gauged by measuring the amount of oxygen consumed by the water sample during a fixed period of time.

BOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organic compounds in water. However, COD is less specific since it measures total organic levels rather than just levels of biologically active organic matter.

The rate of decomposition in lakes and rivers is seasonally variable, as the amount of dissolved oxygen varies with the water temperature. It is also influenced by the availability of organic matter. If the decomposition rate is high, decay microorganisms can consume all of the available oxygen, denying it to aquatic animals such as fish.

BOD is used as a gauge of the effectiveness of wastewater treatment plants. Various commercial devices are available for its determination.

The BOD in England is carried out by taking two freshwater samples, testing for dissolved oxygen in one sample, then sealing the other sample (to prevent further oxygen dissolving in), and placing it in the dark (to prevent photosynthesis and thereby the addition of oxygen). This second sample is kept at room temperature and tested for dissolved oxygen after five days. The ratio of the dissolved oxygen in both samples is called the BOD5. Five days are chosen because this is supposedly the longest time that river water takes to travel from source to estuary in the UK. In the US, an environmental chemist claims that after 5 days the nitrogen cycle "kicks in" to influence the available oxygen in the sample.