COD METER: online UV 254nm continuous measurement of organic load

A photo of the COD meter

The measuring principle is based on the UV light absorption by unsaturated organic molecules at 245nm according to Beer-Lambert law:

[C] = k log (lin/lout)

[C]: concentration of the sample;
k: absorption coefficent (specific to each molecule);
lin: light intensity at the input of the sample;
lout: light intensity at the output of the sample.

The approximate ratio COD/UV (ppm.abs- 1.m) is given bellow
for a few chemicals:

Acrolein 3
Benzene 5
Chlorobenzene 5
Dimethyldisulfide 3.5
Hydrogen peroxide 1.5
Nitro benzene 0.15
Phenol 5
Styrene 0.25
Tetrachloroethylene 0.5
Toulene 5
Triethylamine 1
Xylene 7

Generally, the sample is a mixture of many different molecules
giving an average absorption coefficent.

That means that the instrument must be calibrate according
to a COD laboratory measurement before using.

Also, the ratio of the different molecules must remains approximately
constant to assume a good correlation with laboratory COD.

The UV absorption can be considered as an alterantive method
for COD (Chemical Oxygen Demand) when fast, reliable and inexpensive
measurement with very low maintenance are required.

The correlation for industrial wastewater may need an experimentation
to be validated while results are guaranteed on river water or urban wastewater.

Turbidity, suspended solid or dirty on the flow cell is automatically
compensated by a differential measurement with a second detector at a reference

This method is an accordance with DIN38404-C3 standard and can be
considerate as an alternative method refering to AFNOR XPT90-210 standard.


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