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What is COD?

Chemical Oxygen Demand (COD) is defined as the quantity of a specified oxidant that reacts with asample under controlled conditions. The quantity of oxidant consumed is expressed in terms of its oxygenequivalence. COD is expressed in mg/ !".

C99 A Complete COD System

#anna COD vials use internationally recognised chemistry which means you can use #anna vials in placeof other suppliers$ vials% and you can &e assured that the results you will o&tain will &e 'ust as accurateand relia&le. The vials are supplied with the precise volume of COD reagent premeasured maing it &othquic and easy to add the water sample.

*n accordance with international standards the vials are then heated to +,!-C and held at thattemperature for " hours. This procedure is straightforward using the #anna heating &loc specificallydeveloped for COD measurement. t the end of this time a colour forms in the vials which is measuredcolorimetrically using the new #anna C meter. COD readings are displayed on the meter directly inmg/l.The new #anna COD meter is &oth sophisticated and easy to use. 0ot only does it measure COD with 1ranges providing complete measurements in the range ! to +,%!!!mg/l COD% it is also programmed with12 other standard water measurements allowing the operator to test for parameters from luminium to3inc. ll readings are automatically displayed in mg/l% removing the need to select filters or use loo upta&les and charts.

Why Measure COD?

COD is often measured as a rapid indicator of organic pollutant in water. *t is normally measured in &othmunicipal and industrial wastewater treatment plants and gives an indication of the efficiency of thetreatment process. COD is measured on &oth influent and effluent water. The efficiency of the treatment

process is normally expressed as COD 4emoval% measured as a percentage of the organic matterpurified during the cycle.

Test *nfluent COD 5ffluent COD COD removal+ +"+6 6,+ 2"7" 68 1"8 2171 +16+ 1!9 997Due to its unique chemical properties% the dichromate ion (Cr"!9") is the specified oxidant in the ma'orityof cases. Cr"!9" is reduced to the chromic ion (Cr1:) in these tests.;oth organic and inorganic constituents of the sample are su&'ect to oxidation% however the organic

component predominates and is of greater interest. COD is a defined test< digestion time% reagentstrength and sample COD concentration all affect the extent of sample oxidation.

COD is often used as a measurement of pollutants in natural and waste waters and to assess thestrength of waste such as sewage and industrial effluent waters. COD has further applications in powerplant operations% chemical manufacturing% commercial laundries% pulp = paper mills% environmentalstudies and general education. *n pota&le drining water plants% COD values should &e less than +! mg/!"at the end of the treatment cycle.

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Methods

COD can &e measured &y the closed reflux titrimetric method and the closed reflux colorimetric method(>tandard ?ethods ,,"! C. and ,,"! D. respectively).Reactor:

The heater% or reactor% is used to o&tain fast organic reactions. >pecifications for this reactor are written in>tandard ?ethod ,,"! C. with specific temperature and vessel requirements. >ince it is vital that thereaction tae place at +,!-C (@"-C) for " hours it is important to ensure accurate preheating. light onthe #anna *nstruments COD reactor indicates that the temperature has reached the correct level. Thereactor is also equipped with a timer to notify the operator when the reaction is completed.Titration:

sample is refluxed in strongly acidic solution with a nown excess of potassium dichromate (A"Cr"!9).fter digestion the remaining unreduced A"Cr"!9 is titrated with ferrous ammonium sulphate todetermine the amount of A"Cr"!9 consumed and the oxidiBa&le matter is calculated in terms of oxygen

equivalent. This procedure is applica&le to COD values &etween 6! and 6!! mg/. #igher COD valuescan &e o&tained &y careful dilution or &y using higher concentrations of dichromate digestion solution(>tandard ?ethod ,,"! C.).Colorimetric:

hen a sample is digested% COD material in that sample is oxidiBed &y the dichromate ion. The result isthe change in chromium from the hexavalent (*) to the trivalent (***) state. ;oth chromium species exhi&ita color and a&sor& light in the visi&le region of the spectrum. *n the 6!! nm region the dichromate ion(Cr"!9") a&sor&s strongly while the chromic ion (Cr1:) a&sor&s much less. *n the 2!! nm region it is thechromic ion that a&sor&s strongly and the dichromate ion has nearly Bero a&sorption. (>tandard ?ethod,""! D.)

This method covers the ranges from ! to +,!!! mg/ !"E+) ! +,! mg/ near 6"! nm") !+!!! (+,!!) mg/ near 2!! nm1) !+,!!! mg/ near 2!! nmOfficial 4eporting ?ethod% (dichromate)The F> 5nvironmental Grotection gency specifies that the only accepta&le reporta&le measuring methodfor COD is the colorimetric dichromate method. dvantages in using this method include high accuracy%certifia&le results and a&ate chloride interference.COD measurement is usually com&ined with other important tests to gain a complete understanding ofthe effectiveness of the treatment plant.

There is a generally accepted classification of >ewage >trength rated &y Oxygen Demand% measured

either &y the ;iochemical method (;OD) or the COD method.

ater Classification ;OD CODea H "!! H 6!!?edium 1,! 9!!>trong ,!! +!!!ery >trong 9,! +,!!>ewage >trength 4ated &y Oxygen Demand

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BOD & COD

;iochemical Oxygen Demand (;OD) is an empirical test that determines the relative oxygen requirementsof wastewater% effluent and polluted waters. ;OD tests measure the molecular oxygen utiliBed during aspecified incu&ation duration for the &iochemical degradation of organic material (car&onaceous demand)and the oxygen used to oxidiBe inorganic material such as ferrous iron and sulfides. The most common

;OD test consists of a , day period in which a sample is placed in an airtight &ottle under controlledconditions temperature ("!-C @ +-C)% eeping any light from penetrating the sample to preventphotosynthesis. The Dissolved Oxygen (DO) in the sample is measured &efore and after the , dayincu&ation period% and ;OD is then calculated as the difference &etween initial and final DOmeasurements. ;OD can &e considered a more InaturalI test in determining the oxygen required tooxidiBe organic matter% however it does not account for rapid changes in conditions. COD is oftenpreferred for daily analysis since it is inherently more reproduci&le% accounts for changing conditions andtaes a short time to complete

TOC & COD

Organic car&on in water and wastewater is composed of a variety of organic compounds in variousoxidation states. >ome can &e oxidiBed further &y chemical or &iological processes% and the chemicaloxygen demand (COD) and &iological oxygen demand (;OD) methods can &e used to characteriBe thesefractions. hile Total Organic Car&on (TOC) is a more convenient and direct expression of total organiccontent than COD or ;OD% it does not provide the same ind of information. *f a repeata&le relationshipcan &e esta&lished &etween TOC and either COD or ;OD% then TOC can &e used as an estimate forCOD or ;OD for a specific source of water. The correlations must &e esta&lished independently for eachset of conditions including various points in the treatment process. >ince TOC is independent of theoxidation state of the organic matter and does not measure inorganic or organically &ound matter that cancontri&ute to COD or ;OD% TOC measurement cannot tae the place of COD or ;OD testing.