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.

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.

Due to tje 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 operatinal and subsequent costs are the consequence.

As a result, industries and operators are looking for online sum parameters and 'clean' methods, without 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.

Thermal Oxidation (High Temperature Method)

The thermal oxidation is well suited t determine the total oxygen demand in waste water (HT method). Whereby, the sample is lead into a reactor and resulting CO₂ will be detected (alike in the  online TOC-analysis). Usually, high temperature methods on the market reach a maximum temperature of about 1000°C. Since at these temperatures not all carbon compounds can be oxidized, it is necessary and common to use catalysts.

LAR's  QuickCOD-analysers use a special thermal combustion method at 1200°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  QuickCOD_o analyser 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 with 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 will be defined by comparing the measured online COD results to those of the laboratory method. This method suits applications well, where the concentrations of pollution are not subject to strong fluctuations.
The QuickCOD_i is based on the patented^* high temperature method with a complete non-catalytical combustion at 1200°C.

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 directly related to the COD again.

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

 

 

^{*pat. 1055927}