Chemical oxygen demand

Chemical oxygen demand (COD) is a measure of the ability of chemical reactions to oxidize matter in an aqueous system. The results are expressed in terms of oxygen so that they can be compared directly to the results of biochemical oxygen demand (BOD) testing. The test is performed by adding the oxidizing solution to a sample, boiling the mixture on a refluxing apparatus for two hours and then titrating the amount of dichromate remaining after the refluxing period. The titration procedure involves adding ferrous ammonium sulfate (FAS), at a known normality, to reduce the remaining dichromate. The amount of dichromate reduced during the test—the initial amount minus the amount remaining at the end—is then expressed in terms of oxygen. The test has nothing to do with oxygen initially present or used. It is a measure of the demand of a solution or suspension for a strong oxidant. The oxidant will react with most organic materials and certain inorganic materials under the conditions of the test. For example, Fe²⁺ and Mn²⁺ will be oxidized for Fe³⁺ and Mn⁴⁺, respectively, during the test.

Generally, the COD is larger than the BOD exerted over a five-day period (BOD₅), but there are exceptions in which microbes of the BOD test can oxidize materials that the COD reagents cannot. For a raw, domestic wastewater , the COD/BOD₅ ratio is in the area of 1.5–3.0/1.0. Higher ratios would indicate the presence of toxic, nonbiodegradable or less readily biodegradable materials.

The COD test is commonly used because it is a relatively short-term, precise test with few interferences. However, the spent solutions generated by the test are hazardous. The liquids are acidic, and contain chromium, silver, mercury , and perhaps other toxic materials in the sample tested. For this reason laboratories are doing fewer or smaller COD tests in which smaller amounts of the same reagents are used.

[Gregory D. Boardman ]

RESOURCES

BOOKS

Corbitt, R. A. Standard Handbook of Environmental Engineering. New York: McGraw-Hill, 1990.

Davis, M. L., and D. A. Cornwell. Introduction to Environmental Engineering. New York: McGraw-Hill, 1991.

Peavy, H. S., D. R. Rowe, and G. Tchobanoglous. Environmental Engineering. New York: McGraw-Hill, 1985.

Tchobanoglous, G., and E. D. Schroeder. Water Quality. Reading, MA: Addison-Wesley, 1985.

Viessman, W., Jr., and M. J. Hammer. Water Supply and Pollution Control. 5th ed. New York: Harper Collins, 1993.

Chemical oxygen demand

Chemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals such as ammonia and nitrite. COD measurements are commonly made on samples of waste waters or of natural waters contaminated by domestic or industrial wastes. Chemical oxygen demand is measured as a standardized laboratory assay in which a closed water sample is incubated with a strong chemical oxidant under specific conditions of temperature and for a particular period of time . A commonly used oxidant in COD assays is potassium dichromate (K2Cr2O7) which is used in combination with boiling sulfuric acid (H2SO4). Because this chemical oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.

Chemical oxygen demand is related to biochemical oxygen demand (BOD), another standard test for assaying the oxygen-demanding strength of waste waters. However, biochemicaloxygen demand only measures the amount of oxygen consumed by microbial oxidation and is most relevant to waters rich in organic matter. It is important to understand that COD and BOD do not necessarily measure the same types of oxygen consumption. For example, COD does not measure the oxygen-consuming potential associated with certain dissolved organic compounds such as acetate. However, acetate can be metabolized by microorganisms and would therefore be detected in an assay of BOD. In contrast, the oxygen-consuming potential of cellulose is not measured during a short-term BOD assay, but it is measured during a COD test.

Chemical Oxygen Demand

Chemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation (the loss of an electron from a compound) of inorganic chemicals such as ammonia and nitrite. COD measurements are commonly made on samples of waste waters or of natural waters contaminated by domestic or industrial wastes.

Chemical oxygen demand is measured as a standardized laboratory assay (analysis) in which a closed water sample is incubated with a strong chemical oxidant (an oxidant is a chemical that causes an electron to be removed from a compound) under specific conditions of temperature and for a particular period of time.

Chemical oxygen demand is related to biochemical oxygen demand (BOD), another standard test for assaying the oxygen-demanding strength of waste waters. However, biochemical oxygen demand only measures the amount of oxygen consumed by the activity of microoganisms and is most relevant to waters rich in organic matter. Chemical oxygen demand and biological oxygen demand do not necessarily measure the same types of oxygen consumption. For example, chemical oxygen demand does not measure the oxygen-consuming potential associated with certain dissolved organic compounds such as acetate. However, acetate can be processed by microorganisms and so could be measured by means of the biological oxygen demand.

chemical oxygen demand (COD) An indicator of water or effluent quality, which measures oxygen demand by chemical (as distinct from biological) means, using potassium dichromate as the oxidizing agent. Oxidation takes 2 hours and the method is thus much quicker than a 5-day BOD (see biological oxygen demand) assessment. Since the BOD:COD ratio is fairly constant for a given effluent, COD is used more frequently for routine monitoring of an effluent once this ratio has been determined.

chemical oxygen demand (COD) Indicator of water or effluent quality, which measures oxygen demand by chemical (as distinct from biological) means, using potassium dichromate as the oxidizing agent. Oxidation takes two hours and the method is thus much quicker than an assessment of biological oxygen demand (BOD), which takes five days. Since the BOD: COD ratio is fairly constant for a given effluent, COD is used more frequently than BOD for the routine monitoring of an effluent once this ratio has been determined.