Chemical oxygen demand

The chemical oxygen demand (COD, Eng. Chemical oxygen demand, COD) is a measure of the sum of all the water available, oxidizable substances under certain conditions as a sum parameter. And represents the amount of oxygen (mg / l ), which would be needed for their oxidation when oxygen was the oxidant. The process is also known as " oxidizability Cr -VI " ( chromate consumption if this would be an oxidizing agent) the potassium permanganate ( " oxidizability Mn VII" ) are compared. In addition to this use as a measure of the concentration of chemically oxidizable substances in water, the chemical oxygen demand is also a measure of the chemically oxidizable substances emitted in the production of a quantity of product into the waste water (g / kg quantity of product) or be disposed of in a period (t / a, tons per year ) may be used.

Determination method

To determine the COD, a water sample is strongly acidified with sulfuric acid and heated at a predetermined precise amount of strong oxidizing agent potassium dichromate ( K2Cr2O7 ), with the addition of silver sulfate as catalyst. In chloride-containing samples, the chloride must first be removed or masked with mercury sulphate, so that its oxidation is not increased falsely chlorine the measured value. The amount of dichromate consumed is calculated by determining the residual dichromate and calculates the equivalent amount of oxygen O2.

After all variants in the German Standard Methods (DEV ) the remaining amount of the dichromate is titrimetric with ammonium iron (II ) sulfate solution and ferroin indicator determined ( method DEV H41, H43 and H44 ).

Especially for monitoring the COD in wastewater treatment plants and other water installations, where there is no laboratory and trained laboratory personnel is available, but the COD determination is usually done by means of so-called cuvette rapid tests. These test kits can also be used with little prior knowledge, contain all the necessary reagents and require only a few laboratory equipment. In this method, the determination of the dichromate consumption is - in contrast to the DEV - photometrically.

COD of household wastewater

The chemical oxygen demand is used in particular as a sum parameter for quantifying the contamination of wastewater with organic substances. He captures both biodegradable and non-biodegradable organic substances, but also some inorganic substances.

Together with the biochemical oxygen demand (BOD) is the COD clues to the quality of the strains contained.

In domestic wastewater, the COD is 600 mg / l usually about twice as high as the BOD5. It is an important parameter in the design, dimensioning and operational control of wastewater treatment plants.

The CSB can be divided into different sub- groups:

• " Particulate COD ". This means that particulate disperse meant existing oxidized by dichromate solids in the water, which are retained by a membrane filter with a pore diameter of 0.45 microns. They consist of biotic and biotic degradable non-degradable particulate matter.
• " Dissolved COD ". This refers dissolved and particulate membrane filter having a pore diameter of 0.45 microns passing oxidized by dichromate substances. They consist of biotic and biotic degradable non-degradable substances, dissolved or as very small particles.

The effluent of a biological wastewater treatment system are dissolved, biotic non-biodegradable organic substances when properly function mainly contain further minor extent biotic degradable organic matter and particulate organic matter, the amount of the latter depending on the function of the settler.

Applications

The COD can be used to describe the flows of organic carbon compounds in wastewater treatment plants ( COD balance ). This allows you to:

• The estimation of the oxygen demand in the aeration tank.
• The description of the conditions in the sludge stabilization.
• A plausibility check of measured values.
• Design of wastewater treatment plants for special waste water whose composition is not the default values ​​of urban waste water equivalent (eg increased proportion biotic not degradable organic substances).

Further, the COD balance is based on the description of the reaction kinetics of the activated sludge process. In addition, empirical data are available to allow the conversion of a COD in excess sludge in the dry matter ( DM) of the excess sludge. This COD / TS ratio varies mostly between 1.4 (excluding biomass in the activated sludge ) and 1.0 ( significant proportion of inorganic solids in the activated sludge ).