Biological oxygen demand (BOD) refers to the amount of dissolved oxygen that aerobic organisms need in order to break down organic material in water over a particular time period.
Nitrates and phosphates in a body of water can contribute to high BOD levels. Nitrates and phosphates are plant nutrients and can cause plant life and algae to grow quickly. When plants grow quickly, they also die quickly. This contributes to the organic waste in the water, which is then decomposed by bacteria. This results in a high BOD level.
A BOD level of 1-2 ppm is considered very good. There will not be much organic waste present in the water supply.
A water supply with a BOD level of 3-5 ppm is considered moderately clean.
In water with a BOD level of 6-9 ppm, the water is considered somewhat polluted because there is usually organic matter present and bacteria are decomposing this waste.
At BOD levels of 100 ppm or greater, the water supply is considered very polluted with organic waste.