Chemical Oxygen Demand (COD)

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What is Chemical Oxygen Demand (COD)?

Chemical oxygen demand (COD) is the amount of dissolved oxygen that must be present in water to oxidize chemical organic materials, like petroleum. COD is used to gauge the short-term impact wastewater effluents will have on the oxygen levels of receiving waters.

COD versus BOD

Like COD, biochemical oxygen demand (BOD) measurement can be used to estimate the amount of pollution in a water sample. COD describes the amount of oxygen required to chemically break down pollutants, while BOD indicates the amount of oxygen required to breakdown organic pollutants biologically with microorganisms.

There is a correlation between COD and BOD, however, it must be experimentally established before using one parameter to express another. Usually COD analysis (which is a much faster and more accurate method) is used to estimate BOD using the established correlation.


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Why Measure Chemical Oxygen Demand (COD)?

When treated wastewater is discharged into the environment, it can introduce pollution in the form of organic content to receiving waters. High levels of wastewater COD indicate concentrations of organics that can deplete dissolved oxygen in the water, leading to negative environmental and regulatory consequences. To help determine the impact and ultimately limit the amount of organic pollution in water, oxygen demand is an essential measurement.

At Hach ®, find the testing equipment, resources, training and software for COD measurement and management during water treatment.

Featured Products to Measure Chemical Oxygen Demand

Spectrophotometers

Hach has been innovating with spectrophotometric technology for over 70 years and offers many of the leading spectro- photometers in the water analysis market.

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COD Reagents

Hach is dedicated to providing high-quality reagents for routine and challenging water analysis.

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EZ Series COD Analysers

The EZ Series Online Analyzers offer multiple options to monitor COD in water.

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DRB200

The DRB200 is designed to save time and ensure accuracy during testing.

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Which Processes Require Chemical Oxygen Demand Monitoring?

Municipal and Industrial Wastewater Treatment

Influent waters entering wastewater plants are high in organics and the wastewater plant must reduce the “organic loading” before discharging water to a receiving body.

Oxygen demand is useful for measuring waste loadings, evaluating the efficiency of the treatment process, and ensuring compliance with regulations for the oxygen demand of effluent.

  • Primary Treatment:

    Clarifiers, or sedimentation basins, slow the flow of the wastewater to allow suspended solids to settle. Surface skimmers collect any floating fats, oils and greases. With the use of this mechanical and physical means, approximately 30% of organic matter is removed from the wastewater and is routed to the solids management area of the plant.

  • Secondary Treatment:

    This process uses living organisms to aid in reducing organics. In the aeration basin, bacteria and microorganisms convert biodegradable organic matter to carbon dioxide and water. With this conversion, organics are reduced, thereby reducing oxygen demand.

  • Discharge Limits:

    Discharge limits vary from plant to plant depending on the characteristics of the receiving water, effects on aquatic life, recreational uses and other factors. Discharge permits may stipulate a specific maximum concentration for BOD or COD, or a percent removal. Some plants require achieving as much as 90% removal of oxygen demand.

For compliance, it’s necessary to measure BOD or COD in the influent water as it enters the plant, before mechanical screening processes and at the end of the treatment at the discharge point.

When selecting a method for analyzing oxygen demand, it’s important to consider the following:

  • Specific testing application
  • Oxidant that will be used
  • Completion time
  • Accuracy and precision of the measurement
This sedimentation basin slows the flow of wastewater to allow suspended solids to settle. 

How is Chemical Oxygen Demand Monitored?


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Laboratory Testing

ISO 6060-1989, DIN 38409-H41-H44 Standard LCK Vials


Hach EZ Series COD Analyzer

Online Process Testing

EZ Series COD Analyzers

COD Testing Advantages

  • COD is most suitable for rapid and frequent monitoring of treatment plant efficiency and water quality.
  • This method is more accurate than BOD (with a 5-10% relative standard deviation) and it offers a relatively short analysis time (2-hour digestion time), compared to the 5-day BOD test.
  • The COD oxidant is not affected by toxic materials in the sample.
  • Changes in the COD value between influent and effluent may parallel BOD content and supplement BOD results.

COD Testing Limitations

  • Some organic compounds are not oxidized completely with the COD method.
  • COD measurement can experience interference from chloride ions.

Frequently Asked Questions

Can COD be measured while the vial is still hot?

It is not recommended to measure the vials before letting them cool to room temperature. The measurement may be inaccurate, and a hot reagent vial could damage the digital instrument.

Is the inversion step in the COD method important after the digestion is performed?

The most missed step by customers in the COD analysis is the reagent vital inversion step. After the digestion and before the vial is fully cooled, the vial must be inverted several times. This returns any condensation to the reacted sample. Do not invert the vial again immediately before inserting the vial into the instrument. It is necessary to allow time for undissolved particles to settle to the bottom and out of the light path of the digital instrument.

What are the major interferences for COD?

Chloride is the primary interference when testing for COD. Each COD vial used with the dichromate method contains mercuric sulfate that will eliminate chloride interference up to the level specified in the procedure.

Why not simply measure TOC to assess the effects of an organic load on receiving water?

TOC measures organic carbon but different organic carbons will generate different oxygen demand. Measuring TOC alone will not necessarily indicate how much oxygen will be consumed by the organics in the environment. For example, oxalic acid and ethanol produce identical TOC results. However, due to different oxidation states, the oxygen demand of ethanol is 6 times greater than oxalic acid, meaning ethanol will have a greater effect on the dissolved oxygen content of a receiving water. Measuring oxygen demand rather than TOC produces a clearer picture of how the receiving waters will be affected by the organic-containing wastewaters.