The biochemical oxygen demand (BOD) of water determines the impact of decaying matter on species in a specific ecosystem. Sampling for BOD tests how much oxygen is needed by bacteria to break down the organic matter.

Importance of Dissolved Oxygen

The oxygen in water which is available for species’ use is called “dissolved oxygen,” (DO). When organic matter from sewers, algal blooms, and other sources enters water, it is immediately broken down by bacteria, which requires some of the dissolved oxygen. When DO levels fall below a certain level, it adversely impacts aquatic life, sometimes causing mass fish kills. This, in turn, causes further problems because of the sheer number of decaying organisms requiring oxygen.

Conducting the Test

BOD is determined by analyzing the difference in dissolved oxygen from a sample for five days. Because of this standardized time, BOD is often referred to as BOD-5. The dissolved oxygen readings are usually in parts per million (ppm). 

Higher BOD indicates more oxygen is required, which is less for oxygen-demanding species to feed on, and signifies lower water quality. Inversely, low BOD means less oxygen is being removed from water, so water is generally purer. Cold water retains oxygen better than warmer water, so in summer months, dissolved oxygen is usually lower from the start.

Unpolluted rivers usually have BOD levels below 1 part per million (equivalent to 1 mg/L), while untreated sewage has between 200 and 600 ppm.