5 typical questions about COD

What does the abbreviation COD stand for?

The abbreviation COD stands for ‘Chemical Oxygen Demand’. This value gives the amount of oxygen that would be required to fully oxidise (almost) all organic compounds. This is a measure to quantify the amount of organics in water and is most often used to measure the pollution level. This is also used in environmental legislation which obliges most industrial companies to reduce COD levels below specific values.

What is the difference between COD and BOD?

BOD or ‘Biological Oxygen Demand’ is the amount of dissolved oxygen which is required by microorganisms to degrade organic material that is present in the water over specific time frame. BOD is often used by waste water treatment plants to get an idea how effective their treatment will be. COD includes everything that can be oxidised, while BOD only looks towards the part that can be biologically degraded.

What are typical sources of COD?

As with VOCs in air emissions, COD in water can be the result from either natural sources or human activities. Degradation of leaves, algae or dead fish are examples of natural sources. COD from human activities is often related to industry and households via the usage of cleaning products, solvents and pesticides but also from waste handling or recycling activities.  

What is the effect of COD on the environment?

Typically, high COD levels suggest higher levels of pollution but this is not always the case as COD can also come from natural sources. However, high COD levels can cause oxygen depletion in the water, independent from the COD source. This can result in mass die-off of aquatic live. Each year large numbers of fish die due to oxygen depletion caused by pollution. COD caused by pollution can also be toxic for the environment or can accumulate in the food chain when they do not degrade naturally. Then we’re talking about persistent molecules (POPs). Typical examples are pesticides.

How can COD be removed from water?

The most optimal COD removal technique very much depends on the composition and biodegradability. Biodegradable COD can often be easily removed via a biological treatment steps. Physico-chemical treatment can also help to reduce the concentration, while activated carbon is often used to remove recalcitrant COD. In general, pre-treatment steps or multiple technologies are often required to reach the environmental discharge limits.