Chemical oxygen demand (COD) is a critical waste treatment measurement in everything from municipal systems to food manufacturing waste streams.

Performing COD testing the right way is important in determining wastewater treatment effectiveness and can help diagnose any problems in treatment. In this blog, we’ll cover what chemical oxygen demand is, how to test it, and how to get the best equipment for your tests.

What is Chemical Oxygen Demand?

Chemical oxygen demand (COD) is an indirect measurement of the amount of organic matter in a sample. With this test, you can measure virtually all organic compounds that can be digested by a digestion reagent.

COD contrasts with biochemical oxygen demand (BOD), which relies on the use of microorganisms to break down the organic material in the sample by aerobic respiration over the course of a set incubation period (typically five days).

BOD and COD correlate with one another in virtually all samples, but BOD is always lower than COD as the biochemical breakdown of organics is often not as complete as the chemical method.

Importance of Chemical Oxygen Demand

As gauges of organic matter in a sample, BOD and COD are critical in wastewater for determining the amount of waste in the water. Waste that’s high in organic matter requires treatment to reduce the amount of organic waste before discharging into receiving waters.

If water treatment facilities do not reduce organic content of wastewater before it reaches natural waters, microbes in the receiving water will consume the organic matter.

As a result, these microbes will also consume the oxygen in the receiving water as part of the breakdown of organic waste. This oxygen depletion along with nutrient rich conditions is called eutrophication, a condition of natural water that can lead to the death of animal life.

Wastewater facilities reduce COD and BOD by using these same microbes under controlled conditions. These facilities aerate chambers injected with specialized bacteria that can break down the organic matter in an environment that does not harm natural waters. A reduction in BOD is used in these facilities as a benchmark for treatment effectiveness.

Since a BOD test takes five days to complete, COD is used to monitor the treatment process in day-to-day operations. The COD test takes only a few hours to complete.

If BOD were always used, treated wastewater would need to be held, and a problem with the treatment process wouldn’t be detected until five days later! This would mean that wastewater would need to be held until results could be verified.