Metrolab Blog

COD and BOD – parameters for clean water

Chemical oxygen demand (COD) is a sum parameter that includes all dissolved or particulate substances in wastewater and determines the oxygen required for their oxidation, provided that oxygen is the oxidizing agent. DIN 38409-41 describes the procedure for its determination.

The Biochemical Oxygen Demand (BOD) is part of the COD, but in contrast to the COD it contains the biodegradable portion, caused by the respiration of the dissolved carbon load by microorganisms.

How did these tests come about? 

When sewerage systems were introduced in cities in the 19th century, it was found that unpleasant phenomena quickly occurred in the waters used for transportation, as the oxygen dissolved in the water was almost completely consumed. Rivers and ditches became evil-smelling broths, in which the fish and other creatures either died or the stocks were severely decimated, apart from the influence on the groundwater near the banks, which in turn was extracted in wells as drinking water for human consumption. Thus, one was forced to develop strategies and procedures for wastewater treatment and to determine the oxygen-consuming parameters.

The two tests mentioned above are still carried out in a modern and user-friendly version at wastewater treatment plants all over the world.


Measuring Approaches

The COD has established itself as a photometric cuvette test in which a standardized acidic dichromate digestion of a wastewater sample is carried out according to the measuring range in accordance with DIN 38409-41 and the photometrically determined dichromate consumption gives a measure of the COD. A typical value for municipal wastewater is approx. 600 mg/l COD in the inlet, in the outlet it is of course important to reduce this value as much as possible. The COD load contained in the waste water can be partly removed by mechanical processes at the screen, the grit chamber and by filtration or sedimentation in the secondary clarifier. Certain constituents can be eliminated chemically if necessary.

To perform the measurement Xylem Analytics supplies thermal reactors and photometers like the PhotoLab® 7600 UV-VIS with a selection of test sets for the corresponding measuring ranges and the modern reagent free method (OptRF), which allows a daily economic measurement of the COD without using the digesting method from above. It is also able to support the settings of the direct effluent measurement described below. The tests are designed in such a way that they can be carried out easily and safely without much effort and do not require any special laboratory knowledge.


                                  

COD cuvette test with barcode identification            Thermoreactor CR 3200 for digestion at a constant 148 ° C

PhotoLab® 7600 UV/VIS with OptRF