The Science of Chemical Oxygen Demand
24Pages

Hach Company, 1997. All rights are reserved. K71.37053Printed in U.S.A. >

Recent advances in oxygen demand research haveexpanded the number of options available for testing. The purpose of this technical bluebook is to 1) provide background information on oxygen demand testing, and 2) discuss the comparative advantages of methods cur- rently available, including the new Hach Manganese III COD Method designed to eliminate hazardous heavy metals waste. The Total Organic Carbon (TOC) test uses heat, ultra- violet light, and a strong chemical oxidant (or a combina- tionof these three) to oxidize organic compounds to CO > 2 and H > 2 O. Oxygen demand is measured...

Table 1: Comparison of COD, BOD and TOC Tests ParameterCODBODTOCOxidant Used K > 2 Cr > 2 O7Oxidation by microorganisms O > 2 Mn > 2 (SO > 4 ) > 3 Օ K > 2 S > 2 O > 8 HeatՕ Combination of the abovewith various catalysts Most Rapid and frequent monitoring Modeling treatment plant Measures amount of total Suitable Use of treatment plant efficiency process and the effects of organic carbon in samplesand water qualityorganic compounds on the dissolved oxygen content of receiving waters Test Completion 1-1/2 to 3 hours5 days (for standard BOD test)Several minutes to hours TimeAccuracy and 5 -...

Table 2:Major Oxidants Other than K > 2 Cr > 2 O > 7 and Mn > 2 (SO > 4 ) > 3 Used in COD Determinations OxidantAdvantagesDisadvantages KMnO > 4 Stable for several months, MnO > 2 must be excludedՕ Relatively slow-acting and is not quantitative Is used in acidic, neutral and basic mediaՕ Results may depend upon sample size Manganese is a non-hazardous metalՕ Does not oxidize volatile acids or amino acids Incomplete oxidation of many organic compounds Օ Unstable in solution: Forms MnO > 2 precipitate which catalyzes reagent decomposition.Ce(SO > 4 ) > 2 More complete oxidation of organic...

) will require 40 mg mercuric sulfate to effect-ively mask the chloride interference. In some instances, mercuric sulfate may not mask chloride well enough to prevent the combined interference of chloride and high concentrations of ammonia or other nitrogenous com- pounds. This occurs more often in strong dichromate solutions (high range reagents). One solution to this problem is to dilute the sample to a level where it can be tested using a weaker dichromate solution (low range reagent). The combined interference will usually not be observed with the lower-strength reagent.Hach Dichromate...

Wastewater samples were tested using both the dichromate and Mn III COD procedures. Sample data is presented in Table 5. Table 5:Comparison of Manganese III COD to Dichromate COD RatioManganese III CODDichromate CODMn III COD/SampleSamplemg/Lmg/LCr COD WWTP Influent #114284880.88WWTP Influent #214635100.91 Industrial Influent #111531690.90 Industrial Influent #212342480.94 Industrial Influent #312202500.88 WWTP Influent #3Day 13455230.66Day 23164960.64 Day 33125120.61 Day 43344430.75 Day 53445010.69 Day 63494680.75WWTP Effluent #3Day 156620.90Day 264631.02 Day 362511.22 Day 455570.96 Day...

An empirical relationship exists between COD and BOD, but the correlation must be established for a specific sample.Once this correlation has been done, COD test results can be used to estimate BOD test results, BOD test dilutions, and BOD performance in the plant. In Table 6 below, Mn III COD (with chloride removal) results are used to estimate BOD test results. The estimated BOD test results are compared to actual BOD test data. Table 6:Comparison Between Estimated BOD (from COD results) and Actual BOD Test Data SampleMn III COD Result Estimated BOD ResultActual BOD Resultmg/Lmg/Lmg/L...

4. Replace the vial cap tightly. Hold the vial by the cap and invert several times over a sink to mix. (CAUTION! Dichromate COD vials will become very hot during mixing.) 5. Place the vials into the pre-heated COD Reactor. Heatthe vials for 1 hour (Mn III COD Reagent) or 2 hours (Dichromate COD Reagents). 6. Remove the vials from the reactor and cool to room temperature. The Micro COD Digestion 7. Determine COD results either colorimetrically or by titration. Apparatus used in the micro digestion method consistsof a COD Reactor with a 25-vial capacity. A typical COD reactor is shown in...

An example of a completely illustrated, easy-to-follow procedure from Hach. > HACH PROGRAM: 2700COD, ULR λ ), 350 nm, is automaticallyselected. > Perform the digestionaccording to given instructions. > Press the soft keyunder HACH PROGRAM. Select the stored program number for ultra low range COD by pressing 2700 with the numeric keys. Press: ENTER > The display will show:HACH PROGRAM: 2700 COD, ULR. The wavelength ( > Insert the Test TubeAdapter into the sample cell module by sliding it under the thumb screw and into the alignment grooves. Fasten with the thumb screw. > Clean the outside...

9. Place the sample vial into the adapter. Close the light shield. Results in mg/L COD (or chosen units) will be displayed. Method Performance Precision Standard: 100.0 mg/L CODProgram95% Confidence Limits_______ ____________________2710 99.4 - 100.6 mg/L O > Standard Solution Method. Method Performance Precision Check the accuracy of the 0 to40 mg/L range with a 30 mg/L standard. Using Class A glassware, prepare a 1000-mg/L solution by diluting 850 mgof dried (120 C, overnight) potassium acid phthalate (KHP)in 1000 mL oforganic-free deionized water. Prepare a 30 mg/L dilution by diluting...

Method Performance Precision0-1500 mg/L range Standard: 1000 mg/L COD Program 95% Confidence Limits________ ____________________2720 998 - 1002 mg/L COD 0-15,000 mg/L range Standard: 10,000 mg/L COD Program95% Confidence Limits________ ____________________2720 9980 - 10,020 mg/L COD Estimated Detection Limit ProgramEDL_________________ ___________2720 (0 - 1500 mg/L)3 mg/L COD 2720 (0 - 15,000 mg/L)30 mg/L COD Sensitivity ∆ Abs ∆ Concentration______________ _____ ______________Entire Range 0.010 -23.5 mg/L Program Number: 2720 Portion of Curve > 1. Perform the digestion according to given...