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| Water chemical oxygen demand
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| Definition |
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The chemical oxygen demand (COD) is the amount of oxygen consumed to completely chemically oxidise the organic water constituents to inorganic end products.
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| Rationale |
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COD is an important, rapidly measured variable for the approximate determination of the organic matter content of water samples. Some water samples may contain substances that are difficult to oxidise. In these cases, because of incomplete oxidation under the given test methods, COD values may be a poor measure of the theoretical oxygen demand. It should also be noted that the significance of the COD value depends on the composition of the water studied.
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| Users |
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Limnologists, ecologists, analysts, and modellers concerned with the determination of water quality parameters.
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| Assessment method |
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Done by the oxidation of oxidisable material with a known amount of potassium chromate, the titration of excess chromate, and the calculation of oxygen used. For the International Standard of the methodology please refer to ISO-6060. It is available at http://www.iso.ch/cate/cat.html
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| Units of Measure |
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mg/l
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| Frequency of measurement |
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Recommended sampling frequencies by the UN Global Environment Monitoring System/Freshwater Quality Programme (GEMS/WATER):
· rivers- every two weeks;
· lakes- every two months;
· ground water- every three months.
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| Spatial resolution |
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Tier 2-4
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| Accuracy/precision required |
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TBD
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| Associated measurements |
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Biological oxygen demand, and other parameters of water quality.
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| Present status |
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The GEMS/WATER programme has been identified as the provider of the most comprehensive database. It collects data from a world-wide network of about 700 stations measuring water quality parameters. COD is on the list of variables recommended for measurement by GEMS, but its measurement is not implemented at all stations.
Some data on North America may also be found in the Storage and Retrieval database (STORET) which is provided by the US Environmental Protection Agency. However, COD is not routinely measured, so the data coverage may be very incomplete.
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| R and D needed |
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Implementation of COD as a routine measurement of wastewater and freshwater quality descriptions.
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| Global/Regional data holders |
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- Three statistical reviews covering the data collected by GEMS/WATER from 1988- 1996 are available at http://www.cciw.ca/gems/.
- A selective summary of GEMS/WATER data (Annotated Digital Atlas of Global Water Quality) can be found at http://www.cciw.ca/gems/atlas-gwq/intro.html.
- The STORET database can be accessed online at http://www.epa.gov/storet/.
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| Data policy |
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Open for all the above mentioned databases.
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References |
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ISO 1989.
ISO 6060: Water quality - Determination of the chemical oxygen demand. Geneva,
Switzerland, International Organization of Standardization
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UNEP/
WHO/UNESCO/WMO 1992. GEMS/WATER Operational Guide. -3rd
ed. by M. Allard (ed.). Burlington, Ontario, Canada, WHO Collaborating
Centre for Surface and Ground Water Quality.
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