Biochemical Oxygen Demand

(BOD)

Biochemical Oxygen Demand, or BOD, is a measure of the quantity of oxygen consumed by

microorganisms during the decomposition of organic matter. BOD is the most commonly used

parameter for determining the oxygen demand on the receiving water of a municipal or industrial

discharge. BOD can also be used to evaluate the efficiency of treatment processes, and is an

indirect measure of biodegradable organic compounds in water.

Imagine a leaf falling into a stream. The leaf, which is composed of organic matter, is readily

degraded by a variety of microorganisms inhabiting the stream. Aerobic (oxygen requiring)

bacteria and fungi use oxygen as they break down the components of the leaf into simpler, more

stable end products such as carbon dioxide, water, phosphate and nitrate. As oxygen is

consumed by the organisms, the level of dissolved oxygen in the stream begins to decrease

Water can hold only a limited supply of dissolved oxygen and it comes from only two sourcesdiffusion

from the atmosphere at the air/water interface, and as a byproduct of photosynthesis.

Photosynthetic organisms, such as plants and algae, produce oxygen when there is a sufficient

light source. During times of insufficient light, these same organisms consume oxygen. These

organisms are responsible for the diurnal (daily) cycle of dissolved oxygen levels in lakes and

streams.

If elevated levels of BOD lower the concentration of dissolved oxygen in a water body, there is a

potential for profound effects on the water body itself, and the resident aquatic life. When the

dissolved oxygen concentration falls below 5 milligrams per liter (mg/l), species intolerant of low

oxygen levels become stressed. The lower the oxygen concentration, the greater the stress.

Eventually, species sensitive to low dissolved oxygen levels are replaced by species that are

more tolerant of adverse conditions, significantly reducing the diversity of aquatic life in a given

body of water. If dissolved oxygen levels fall below 2 mg/l for more than even a few hours, fish

kills can result. At levels below 1 mg/l, anaerobic bacteria (which live in habitats devoid of

oxygen) replace the aerobic bacteria. As the anaerobic bacteria break down organic matter, foulsmelling

hydrogen sulfide can be produced.

BOD is typically divided into two parts- carbonaceous oxygen demand and nitrogenous oxygen

demand