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
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-analysers 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 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 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).