COD - Chemical Oxygen Demand

The COD value indicates the amount of oxygen which is needed for the oxidation of all organic substances in water in mg/l or g/m3.

The COD (Chemical Oxygen Demand) is closely related to the laboratory standard method named Dichromate-Method. With this method the chemical oxygen demand is determined during chromic acid digestion of organic loads in waste water. Based on this method the COD became a commonly used sum parameter in waste water analysis. It is used for planning of waste water treatment plants, for controlling the cleaning efficiency and for the calculation of waste water taxes.

Common Detection Methods

Dichromate Method (Wet Chemical Oxidation)

As the dichromate method needs about 2 hours for oxidation and it uses hazardous chemicals such as chromic acid, mercury suphate, sulphuric acid and titration reagents, it is not suitable for online analysis. Also due to the heavy usage of toxic chemicals this method is not acceptable for laboratory personal. High requirements of occupational safety need to be observed, as well as the disposal regulations in respect of the environment. High operational and subsequent costs are the result and consequently industries and operators are looking for online sum parameters and 'clean' methods, without a second pollution due to the chemicals involved.

Clean methods to determine the oxygen demand

In the United States the TOD (Total Oxygen Demand) has been standardized and is used as a reference to the oxygen demand of organic substances in waste water. Another clean method to determine the chemical oxygen demand is the electrochemical oxidation using OH-radicals.

Definition of TOD - Total Oxygen Demand

Digramm with TOD/COD correlations

With this parameter the total oxygen demand of water can be measured. Even in the 1970s online analysers for the determination of TOD were available. After the rise in popularity of the COD-dichromate method the TOD had been suppressed in many countries as the COD prevailed in analysis of waste water. However, the total oxygen demand is a reliable and reproducable parameter to indicate the oxygen demand of water. In the United States it is standardized with the ASTM D6238 and very commonly used. The TOD correlates easily to the COD. Hence, this parameter is a preferable alternative to the COD measurement. Moreover, it is very suitable for online measurements, especially with the thermal combustion method at 1,200°C.

Online Measurement of COD and TOD

Thermal Oxidation (High Temperature Method)

LAR's QuickCOD-analyzers use a special thermal combustion method at 1,200°C, which allows a catalyst-free oxidization of the complete sample including any particles. Following which an oxygen detector determines the amount of oxygen consumed by the combustion. This very fast analysis has a cycle time of only 3 minutes and does not require any chemicals. The QuickCODultra analyzer measures the oxygen demand of all oxidisable substances in the waste water including organic nitrogen.

Correlation between TOC and COD

The COD concentration is calculated by use of this method that is correlated by a factor and the TOC concentration in the waste water is determined by use of a standard TOC analysis method. The COD/TOC correlation factor is defined by comparing the measured online COD results to those of the laboratory method. This method is well suited to applications where the concentrations of pollution are not subject to strong fluctuations.

Electrochemical measurement principle

With the patented Lead-Dioxide-Electrode by LAR OH-radicals are produced. Compared to other oxidants OH-radicals have a significantly higher oxidation potential. Thus, hard to oxidize substances can be fast and easily oxidized without using dangerous chemicals. This electrochemical principle is used for our Elox-series. During the reaction the electricity produced will be measured, which is proportional to the consumed OH-radicals. The OH-radicals are here again directly related to the COD.

This method offers a fast, simple, accurate and pollution-free alternative to the common dichromate method (wet chemical method).

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The total organic carbon (TOC) is one of the most important sum parameters in the assessment of the organic pollution of water. Since it includes all carbon compounds as one mass, it is exactly defined and an absolute quantity. Therefore, it may be determined directly.

Read more about TOC measurement methods.

Total organic carbon analyzers of LAR Process Analysers AG

The chemical oxygen demand (COD) indicates the amount of oxygen which is needed for the oxidation of all organic substances in water and is thus an important indicator for water analysis. It is considered in the planning and controlling of treatment, as well as assessing its efficiency, thus forming the basis for the calculation of waste water charges.

The COD can be determined in laboratory or online, whereby these methods significantly differ in duration and consumables used.

Read more about COD measurement methods.

COD analyzers of LAR Process Analysers AG

The biochemical oxygen demand (BOD) indicates the amount of oxygen which is needed for the biological degradation of organic substances in water.

Since the generally used BOD5 excludes the nitrification part of the process, this parameter is poorly suited for the control of waste water treatment plants. Alternatively, the total BOD is well-suited, due to the determination of the nitrogen as well as the carbonaceous part of biological degradation.

Read more about the online measurement of BOD.

Toxicity is described as the direct harmful effect of a substance on organisms. These effects can already occur at low concentraions of toxic substances and are dependent on the incubation period and the dosage.

Some test methods that are available on the market can detect toxicity by using fish, daphnia, molluscs, algae or luminous bacteria and testing whether a water sample has a toxic effect on the organisms. They do not, however, identify exactly which toxins are present.

Read more about online toxicity measurements.

BOD / toxicity analyzers of LAR Process Analysers AG

The total nitrogen bound (TNb) shows the pollution of water caused by nitrogen compounds. Nitrogen may be present as ammonia, ammonium salts, nitrites, nitrates and organic nitrogen compounds.

In contrast to single measurements of the above-mentioned components, the TNb determination contains all these substances in a single analysis process.

Read more about online TNb measurements.

The total phosphorus (TP) is a sum parameter that shows the organic and inorganic phosphorus compounds in water. Phosphorus is an essential nutrient for humans as well as flora and fauna. However, the substance – depending on its concentration– may cause serious damage.

Read more about online TP measurements.

TN TP analyzer of LAR Process Analysers AG