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22.01.2004 _basic COD_ Technical Sales Support EU page 1 Basics on COD measurement Christian Pust Technical Sales Support EU 18.02.2004.

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Presentation on theme: "22.01.2004 _basic COD_ Technical Sales Support EU page 1 Basics on COD measurement Christian Pust Technical Sales Support EU 18.02.2004."— Presentation transcript:

1 _basic COD_ Technical Sales Support EU page 1 Basics on COD measurement Christian Pust Technical Sales Support EU

2 _basic COD_ Technical Sales Support EU page 2 COD – Take Home Messages COD is chemical oxygen demand. COD is an indirect measure of organics. COD is measured by oxidizing organics with a strong oxidant (dichromate) and measuring the amount of oxidant consumed in the reaction. Correlation between COD and BOD is sample specific and may not always be possible.

3 _basic COD_ Technical Sales Support EU page 3 Outline Introduction to COD The Chemistry of COD COD Dichromate Digestion COD vs. BOD

4 _basic COD_ Technical Sales Support EU page 4 What is COD? COD is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. Standard Methods for the Examination of Water and Wastewater

5 _basic COD_ Technical Sales Support EU page 5 What is COD? In other words…. COD is an indirect measure of the amount of organics in a water sample.

6 _basic COD_ Technical Sales Support EU page 6 What is COD? The COD test uses a strong chemical oxidant (potassium dichromate), acid, and heat to oxidize organic carbon to carbon dioxide and water.

7 _basic COD_ Technical Sales Support EU page 7 2 KC 8 H 3 O K 2 Cr 2 O H 2 SO 4 16 CO H 2 O + 10 Cr 2 (SO 4 ) K 2 SO 4 What is COD? COD Reaction KHP Dichromate oxidant Acid Carbon Dioxide Water Chromic Ion

8 _basic COD_ Technical Sales Support EU page 8 What is COD? The COD test measures the amount of dichromate (oxidant) consumed in the breakdown of organic matter. More oxidant consumed = High levels of organics Less oxidant consumed = Low levels of organics

9 _basic COD_ Technical Sales Support EU page 9 What is COD? Why measure COD? Faster than BOD (2 hours versus 5 days) Fewer interferences (chloride) Stable methodology yielding repeatable results Easy to run!

10 _basic COD_ Technical Sales Support EU page 10 Where is COD Measured? MUNICIPAL WASTEWATER PLANT Influent Wastewater Return Sludge Bar Screen Grit Chamber Pump Primary Clarifier Secondary Clarifier Disinfection Contact Chamber Aeration Basin Pump Thickener

11 _basic COD_ Technical Sales Support EU page 11 The Chemistry of COD

12 _basic COD_ Technical Sales Support EU page 12 The Chemistry of COD COD is a two step process Digestion Determination

13 _basic COD_ Technical Sales Support EU page 13 The Chemistry of COD Required reagents for COD: Hach COD reagent vials

14 _basic COD_ Technical Sales Support EU page 14 The Chemistry of COD+ Reagent vials contain: Potassium dichromate 50% sulfuric acid Silver salt (catalyst) Mercuric salt (inhibits chloride interference)

15 _basic COD_ Technical Sales Support EU page 15 The Chemistry of COD Organic compounds are oxidized by potassium dichromate and sulfuric acid. COOH COOK H 2 SO 4 Cr 6+

16 _basic COD_ Technical Sales Support EU page 16 The Chemistry of COD The oxidation of organics results in the reduction of Cr 6+ to Cr 3+. Cr 6+ Cr 3+

17 _basic COD_ Technical Sales Support EU page 17 The Chemistry of COD Reduced chromium is the reaction product of interest. Other products of the reaction include: Carbon dioxide Water Potassium sulfate

18 _basic COD_ Technical Sales Support EU page 18 The Chemistry of COD Why is reduced chromium the reaction product of interest? Cr 6+ is yellow. Cr 3+ is green The reduction of Cr 6+ to Cr 3+ (and hence the consumption of oxidant) can be monitored colorimetrically.

19 _basic COD_ Technical Sales Support EU page 19 The Chemistry of COD As organics are oxidized and Cr 6+ is reduced to Cr 3+, the color of the solution changes from yellow to green.

20 _basic COD_ Technical Sales Support EU page 20 The Chemistry of COD The amount of green color formed is proportional to the COD of the sample.

21 _basic COD_ Technical Sales Support EU page 21 The Chemistry of COD How does chromium relate to COD? Each molecule of potassium dichromate has the same oxidizing power as 1.5 molecules of oxygen. Oxygen demand is directly related to the percentage of Cr 6+ that has been reduced to Cr 3+.

22 _basic COD_ Technical Sales Support EU page 22 The Chemistry of COD Colorimetric measurement Wont solids in sample interfere? No! Solids in sample are either digested or settle out of the light path. Do not invert vials once they have cooled.

23 _basic COD_ Technical Sales Support EU page 23 The Chemistry of COD Colorimetric measurement High Range – measures increase in green color at 620nm Low Range – measures decrease in yellow color at 420nm

24 _basic COD_ Technical Sales Support EU page 24 The Chemistry of COD Interferences Chloride is the primary interference Dichromate will oxidize chloride to chlorine, which results in high readings Interference controlled up to 2000mg/L Cl - with mercuric sulfate in Hach COD vials

25 _basic COD_ Technical Sales Support EU page 25 The Chemistry of COD Interferences Bromide also interferes Bromide interference is not controlled by mercuric sulfate Samples must be diluted to minimize interference

26 _basic COD_ Technical Sales Support EU page 26 COD Dichromate Digestion Procedure Water Analysis Handbook – page 737 Lets take a look!

27 _basic COD_ Technical Sales Support EU page 27 Hach COD Methods EPA Approved Methods Dichromate Reactor Digestion Method High Range – mg/L COD Low Range – 0-150mg/L COD

28 _basic COD_ Technical Sales Support EU page 28 Hach COD Methods Methods for Process Control (not EPA Approved) Dichromate Reactor Digestion Method Ultra Low Range – 0-40mg/L COD High Range Plus – 0-15,000mg/L COD

29 _basic COD_ Technical Sales Support EU page 29 Hach COD Methods Methods for Process Control (not EPA Approved) COD2 – Mercury-free formulation of Hach COD vials

30 _basic COD_ Technical Sales Support EU page 30 COD vs BOD

31 _basic COD_ Technical Sales Support EU page 31 COD vs BOD Why compare COD to BOD? Faster process control Know what you are sending downstream within two hours rather than five days. COD is a more stable measurement method

32 _basic COD_ Technical Sales Support EU page 32 COD vs BOD BOD must still be run to comply with NPDES permit regulations!

33 _basic COD_ Technical Sales Support EU page 33 COD vs BOD Why is COD more stable than BOD? The tests use different methods of oxidation BOD - Microorganisms COD – Chemicals (potassium dichromate)

34 _basic COD_ Technical Sales Support EU page 34 COD vs BOD Microorganisms are susceptible to pH, temperature, and other variables in the water. Oxidation efficiency depends on the condition of the microorganisms

35 _basic COD_ Technical Sales Support EU page 35 COD vs BOD Potassium dichromate will oxidize regardless of water conditions.

36 _basic COD_ Technical Sales Support EU page 36 COD vs BOD Picky bugs vs Clean Plate Club chemicals COD measurements will always be higher than BOD measurements BOD COD

37 _basic COD_ Technical Sales Support EU page 37 COD vs BOD Can COD be correlated to BOD? It depends! Correlation between COD and BOD may or may not exist depending on sample composition, seasonal variation, and other factors.

38 _basic COD_ Technical Sales Support EU page 38 COD vs BOD How can a correlation be determined? Collect empirical data COD and BOD data for the same water sample collected over the same period of time Graph data Graph COD and BOD data to determine whether or not a correlation exists

39 _basic COD_ Technical Sales Support EU page 39 COD vs BOD In this example, the correlation calculates out to COD = 1.75 BOD

40 _basic COD_ Technical Sales Support EU page 40 COD – Take Home Messages COD is an indirect measure of organics. COD is measured by oxidizing organics with a strong oxidant (dichromate) and measuring the amount of oxidant consumed in the reaction. Correlation between COD and BOD is sample specific and may not always be possible.


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