Which compound increases the BOD in water

Chemical Oxygen Demand



The Chemical oxygen demand (CSB, engl. chemical oxygen demand, COD) is a measure of the sum of all substances in the water that can be oxidized under certain conditions. It indicates the amount of oxygen (in mg / l) that would be required for their oxidation if oxygen were the oxidizing agent. The process is also compared with the consumption of potassium permanganate ("Oxidisability Mn-VII") under the name "Oxidisability Cr-VI".

To determine the COD, a water sample is strongly acidified with sulfuric acid and treated with a specified, precise amount of the strong oxidizing agent potassium dichromate (K2Cr2O7) heated, with the addition of silver sulfate as a catalyst. In the case of samples with a high chloride content, the chloride must first be removed or masked so that its oxidation to chlorine does not falsely increase the measured value. The amount of dichromate consumed is calculated by determining the remaining dichromate and from this the equivalent amount of oxygen O2 calculated.

According to all variants in the German standard procedure (DEV), the remaining amount of dichromate is determined titrimetrically with ammonium iron (II) sulphate solution and ferroin indicator (procedures DEV H41, H43 and H44).

In order to monitor COD in sewage treatment plants and other water-technical systems in which no laboratory and trained laboratory personnel are available, the COD determination is mostly carried out using so-called cuvette quick tests. These test kits can be used without any prior knowledge, already contain all the necessary reagents and only require a small amount of laboratory equipment. With this method, the dichromate consumption is determined - unlike with the DEV - photometrically. The associated special photometer also shows the result converted as oxygen in mg / l.

The chemical oxygen demand serves in particular as a sum parameter to quantify the pollution of wastewater with organic substances. It covers both biodegradable and non-biodegradable organic substances, but also some inorganic substances.

Together with the Biochemical Oxygen Demand (BOD), the COD gives clues about the quality of the contained pollution.

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

The COD can be divided into different sub-fractions:

  • "Particulate COD". This means that there are particulate-disperse solids in the water that can be oxidized by dichromate and that are retained by a membrane filter with a pore diameter of 0.45 µm. They consist of biotically degradable and biotically non-degradable particulate matter.
  • "Dissolved COD". This means dissolved and particulate, membrane filters with a pore diameter of 0.45 µm that can be oxidized by dichromate. They consist of biotically degradable and biotically non-degradable substances, dissolved or as very small substance particles (such as non-aggregated bacteria of normal size).

In the course of a biological wastewater treatment plant, if it is functioning properly, it mainly contains dissolved, biotically non-degradable organic substances, furthermore, to a small extent, biotically degradable organic substances and particulate organic substances, the amount of the latter depending on the function of the secondary clarifier.

The COD can be used to describe the material flows of organic carbon compounds in sewage treatment plants (COD balance). This enables:

  • Estimation of the oxygen demand in the aeration tank.
  • The description of the conditions in the sludge stabilization.
  • A plausibility check of the measured values.
  • Dimensioning of sewage treatment plants for special wastewater, the composition of which does not correspond to the standard values ​​of municipal wastewater (e.g. increased proportion of biotically non-degradable organic substances).

The COD balance is also the basis for describing the reaction kinetics of the activated sludge process. In addition, empirical values ​​are available that enable the COD in the excess sludge to be converted into the dry matter (TS) of the excess sludge. This COD / TS ratio is mostly between 1.4 (exclusively biomass in the excess sludge) and 1.0 (considerable proportion of inorganic solids in the excess sludge).

Category: Analytical Chemistry