Chemical Oxygen Demand

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The Chemical Oxygen Demand ( COD ; english demand oxygen chemical , COD ) than sum parameter is a measure of the sum of all the water , oxidizable substances under certain conditions existing. It indicates the amount of oxygen (in mg / l) that would be required for their oxidation if oxygen were the oxidizing agent . The terms "oxidisability Cr-VI" ( chromate consumption, if this were the oxidising agent) or "oxidisability Mn-VII" ( potassium permanganate consumption ) are also used. The chemical oxygen demand is used to assess pollutants that are released into the wastewater (g / kg of product) or that have been disposed of in a period of time (t / a, tons per year).

Determination procedure

To determine the COD, a water sample is strongly acidified with sulfuric acid and heated with a specified, precise amount of the strong oxidizing agent potassium dichromate (K 2 Cr 2 O 7 ), with the addition of silver sulfate as a catalyst . In the case of samples containing chloride, the chloride must first be removed or masked with mercury sulfate 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 the equivalent amount of oxygen O 2 is determined from this.

According to all variants in the German Standard Method (DEV), the remaining amount of dichromate is determined titrimetrically with ammonium iron (II) sulfate solution and ferroin indicator (DEV H41, H43 and H44 methods, DIN 38409).

However , the COD determination is mostly carried out by means of so-called cuvette quick tests, primarily for monitoring the COD in sewage treatment plants and other water technology systems in which no laboratory and trained laboratory personnel are available . These test kits can also be used with little previous knowledge, already contain all the necessary reagents and require only a few laboratory equipment. With this method, the dichromate consumption is determined photometrically - in contrast to the DEV.

COD of household wastewater

The chemical oxygen demand serves in particular as a sum parameter to quantify the pollution of waste water 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 domestic wastewater, the potassium chromate COD of 600 mg / l is generally about twice as high as the BOD 5 . The potassium permanganate COD of domestic wastewater is 300 to 400 mg / l. For industrial and commercial wastewater, the values ​​can be ten times higher. These values ​​are important parameters 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 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.

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

Areas of application

COD balance of a sewage treatment plant with primary clarifier. Activated sludge process with nitrification and denitrification as well as sludge digestion.

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

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

The COD balance is also used to measure the quality of drinking water . The COD of a complete river, the Rhine , was recorded for the first time in 2014 . In Mainz by 0.5 milligrams per liter of water, it rises continuously to 9.5 milligrams per liter of water at the mouth of the Rhine. A total of 128 organic substances were identified that together are responsible for the increase in COD: These are divided into pesticides , pharmaceuticals , biocides , industrial chemicals , sweeteners , narcotics and cosmetics .

See also

Web links

Individual evidence

  1. ^ Matthias Kramer, Jana Brauweiler, Klaus Helling: International Environmental Management Volume II: Environmental Management Instruments and Systems . Springer-Verlag, 2013, ISBN 978-3-322-87004-9 , pp. 109 ( limited preview in Google Book search).
  2. ↑ a b A. Denne, H. Rump, E. Staudte, W. Supperl, P. Doetsch, P. Dreschmann, K. Siekmann, S. Thomas: Abwassertechnologie emergence, discharge, treatment, analysis of waste water . Springer-Verlag, 1984, ISBN 3-662-05579-1 , p. 1007 ( limited preview in Google Book search).
  3. ^ A b c Karl Höll: Water use in the cycle: hygiene, analysis and evaluation . Walter de Gruyter, 2010, ISBN 978-3-11-022677-5 , p. 910 ( limited preview in Google Book search).
  4. ^ W. Hosang: Abwassertechnik . Springer-Verlag, 2013, ISBN 978-3-322-89544-8 , pp. 255 ( limited preview in Google Book search).
  5. ↑ Jana Handschag: Comparative studies on the rate of oxygen consumption in two aeration tanks with different pressure aerator elements . 2001, ISBN 3-8324-4137-9 , pp. 61 ( limited preview in Google Book search).
  6. ^ Willi Gujer: Urban water management . Springer-Verlag, 2013, ISBN 978-3-662-09885-1 , pp. 319 ( limited preview in Google Book search).
  7. ^ Andreas Fath: Rheines Wasser - 1231 kilometers with the current , Carl Hanser Verlag, Munich 2016, ISBN 978-3-446-44871-1 , pp. 132-134.