Index >> Waste Water Microbiology >>Chemical Oxygen Demand

The COD test measures not only the oxygen equivalent of the waste organic matter but also that of the microbial cells. The oxygen demand associated with the microbial cells is only partially exerted during a BOD test, also some of the organic compounds measured by the COD determination may not be metabolised by the microorganisms in either the BOD bottle or the biological treatment process. There are many different analytical approaches to quantify the presence of oxidisable contaminants in waters. Over a century ago, a potassium permanganate solution (i.e. a strong, dark purple coloured oxidant) was added to waters. In turn, if there were readily oxidisable materials present, the colour of this solution would progressively turn less purple, and perhaps even turn completely clear if all of the oxidising permanganate were to be completely reduced.

This reaction is allowed to continue for 90 minutes, at which time the sample is cooled and analysed for the residual presence of the dichromate using a colorimetric method (Le. where the orange colour gives way to a kelly-green colour as the oxidised chrome (hexavalent chrome) reduces to the reduced form (trivalent).

The organic matter is of two types:

that which is oxidised biologically and called biologically active.

that which cannot be oxidised biologically and called biologically inactive.

By performing a COD one gets an idea about the total or entire organic matter and actually what one is interested to find out is the biologically active matter which becomes the basis for further treatment.