Biological Oxygen Demand (BOD)

Definition - What does Biological Oxygen Demand (BOD) mean?

Biological oxygen demand (BOD) refers to the amount of dissolved oxygen (DO) that aerobic organisms need in order to break down organic material in water over time. Likewise, BOD can also be used to describe the chemical procedure used for determining the amount of dissolved oxygen that said aerobic biological organisms need in their water supply.

In hydroponic gardening, knowing the BOD of all living things in your system is especially important so that you can oxygenate the water supply accordingly. Plants require a healthy number of microorganisms to be present in the water, as well as oxygen at their root zones, and BOD is one way to see that they have both.

MaximumYield explains Biological Oxygen Demand (BOD)

Most bodies of water contain some form of organic compounds that are used by microorganisms for food. In order for the microorganisms to feed, they need to utilize the dissolved oxygen in oxygenated water to convert this organic matter into a usable form.

Aquatic microorganism populations rise with a more readily available food source and available dissolved oxygen in the water. The amount of dissolved oxygen they use and the time it takes to do this is known as the biological oxygen demand. This measurement is useful for waste-water treatment facilities in gauging just how clean the treated water is.

Sometimes, microorganisms consume the oxygenated water faster than it can be replenished, causing an underlying shortage of oxygen. This kills off the fish and aquatic insects, and if you're growing aquatically, your plants may suffer as well.

Conversely, the decomposition from an abundance of organic matter present from leaf fall, biological runoff, or other environment pollution, will use up the oxygenated water more rapidly than the microorganisms can feed. This also can cause fish, plants, and insects to die off.

Measuring a water supply's BOD with instruments and meters can be done, but it's not a perfect science. BOD measurements are often expressed in milligrams of oxygen consumed per liter of sample over a five-day period (BOD5) of incubation at 20°C.

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