Measuring Oxygen: BOD and COD Explained Quiz

BOD measures the amount of dissolved oxygen consumed by aerobic bacteria as they break down organic waste over a specific period (usually 5 days). High BOD indicates that there is a large amount of organic material present, which can lead to oxygen depletion.

While BOD only accounts for biodegradable organic matter that bacteria can eat, COD uses a strong chemical oxidizing agent to break down almost all organic compounds, including those that are toxic or non-biodegradable by bacteria.

As bacteria consume organic waste, they use up the oxygen available in the water. If the BOD is too high, the dissolved oxygen (DO) drops below the level needed for fish and other aquatic organisms to survive.

Because COD measures the oxygen required to chemically oxidize all organic matter (both biodegradable and non-biodegradable), the value will naturally be higher than the BOD, which only tracks the biodegradable portion.

Raw sewage and animal waste from farms provide a massive "food source" for bacteria. As the bacteria rapidly multiply and eat this organic matter, they consume vast amounts of oxygen, causing the BOD to spike.

The standard test is called the $BOD_5$ test. It involves measuring the dissolved oxygen at the beginning and then again after 5 days of incubation at 20°C in the dark to see how much oxygen was consumed.

Eutrophication occurs when excess nutrients (like phosphorus) cause algae blooms. When the algae die, bacteria decompose them, causing a massive increase in BOD and creating "dead zones" where no fish can survive.

A COD test can be completed in about 2 to 3 hours using a chemical digester. This makes it a more useful tool for real-time monitoring of industrial wastewater compared to the 5-day wait required for a BOD test.

Potassium dichromate is a very strong oxidant used in COD tests. It is mixed with the water sample and sulfuric acid to ensure that virtually all organic substances are oxidized to carbon dioxide and water.

Clean water has very little organic waste, meaning bacteria have nothing to decompose (low BOD/COD). This allows the water to maintain high levels of dissolved oxygen for aquatic life.

A large gap between COD and BOD indicates that the water contains complex chemicals or toxins that bacteria cannot break down, which is common in industrial wastewater from chemical plants or textile mills.

Both BOD and COD are expressed as a concentration of oxygen (mg/L). For example, unpolluted water usually has a BOD below 2 mg/L, while raw sewage can have a BOD of several hundred mg/L.

While COD is fast, it can be misleading because it measures everything. It doesn't tell you how much of that waste will actually be "eaten" by bacteria in the natural environment, which is what BOD specifically determines.

Industries that deal with organic materials—like wood pulp for paper or fats and proteins from food processing—release high concentrations of organic compounds that require massive amounts of oxygen to break down.

If the bottles were in the light, any algae in the water would perform photosynthesis and release oxygen. This would cancel out the oxygen being used by the bacteria, leading to an inaccurate and falsely low BOD reading.