1. Basics on COD measurement
Christian Pust
Technical Sales Support EU

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. Outline Introduction to COD The Chemistry of COD
COD Dichromate Digestion
COD vs. BOD
Here is where I introuduce all the topics we’ll be cover in the following presentation. Being true to Toastmaster’s form, I find it best to:
Tell them what you’re there to tell them
Tell them
Tell them what you’ve just told them

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
Here is the official Standard Methods definition of COD. I like to start with this one, so they understand what the verbage is like in the “real world”, then I simplify it by adding, “In other words…” See next slide.

5. What is COD? In other words….
COD is an indirect measure of the amount of organics in a water sample.

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.
Here I employ a favorite segway: “In other words…”. This is a simplified definiton of COD, which I read very slowly, so everyone can understand. I also add the entire chemical reaction for COD on the bottom, just in case anyone wants to know it.

7. COD Reaction 2 KC8H3O4 + 10 K2Cr2O7 + 41 H2SO4 
What is COD?
COD Reaction
2 KC8H3O K2Cr2O H2SO4 
16 CO H2O + 10 Cr2(SO4) K2SO4
Dichromate oxidant
KHP
Acid
Here I employ a favorite segway: “In other words…”. This is a simplified definiton of COD, which I read very slowly, so everyone can understand. I also add the entire chemical reaction for COD on the bottom, just in case anyone wants to know it.
Carbon Dioxide
Water
Chromic Ion

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
Here I employ a favorite segway: “In other words…”. This is a simplified definiton of COD, which I read very slowly, so everyone can understand. I also add the entire chemical reaction for COD on the bottom, just in case anyone wants to know it.

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!
Here I employ a favorite segway: “In other words…”. This is a simplified definiton of COD, which I read very slowly, so everyone can understand. I also add the entire chemical reaction for COD on the bottom, just in case anyone wants to know it.

10. MUNICIPAL WASTEWATER PLANT
Where is COD Measured?
MUNICIPAL WASTEWATER PLANT
Primary
Clarifier
Secondary
Clarifier
Bar
Screen
Grit
Chamber
Return
Sludge
Aeration
Basin
Influent Wastewater
Pump
Disinfection
Contact
Chamber
Pump
Thickener

11. The Chemistry of COD

12. The Chemistry of COD
COD is a two step process
Digestion
Determination

13. The Chemistry of COD
Required reagents for COD:
Hach COD reagent vials

14. The Chemistry of COD+ Reagent vials contain: Potassium dichromate
50% sulfuric acid
Silver salt (catalyst)
Mercuric salt (inhibits chloride interference)

15. The Chemistry of COD Cr6+ Cr6+ Cr6+ Cr6+ H2SO4 H2SO4 COOH COOK H2SO4
Organic compounds are oxidized by potassium dichromate and sulfuric acid.
Cr6+
H2SO4
H2SO4
COOH
COOK
Cr6+
Cr6+
H2SO4
H2SO4
Cr6+

16. The Chemistry of COD Cr6+ Cr3+ Cr6+ Cr6+ Cr6+
The oxidation of organics results in the reduction of Cr6+ to Cr3+.
Cr6+
Cr3+
Cr6+
Cr6+
Cr6+

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. The Chemistry of COD
Why is reduced chromium the reaction product of interest?
Cr6+ is yellow. Cr3+ is green
The reduction of Cr6+ to Cr3+ (and hence the consumption of oxidant) can be monitored colorimetrically.

19. The Chemistry of COD
As organics are oxidized and Cr6+ is reduced to Cr3+, the color of the solution
changes from yellow to green.

20. The Chemistry of COD
The amount of green color formed is proportional to the COD of the sample.

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 Cr6+ that has been reduced to Cr3+.

22. The Chemistry of COD Colorimetric measurement
Won’t 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. 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. 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. The Chemistry of COD Interferences Bromide also interferes
Bromide interference is not controlled by mercuric sulfate
Samples must be diluted to minimize interference

26. COD Dichromate Digestion Procedure
Water Analysis Handbook – page 737
Let’s take a look!
Introducing the next part: the actual COD dichromate digestion procedure.

27. Hach COD Methods EPA Approved Methods
Dichromate Reactor Digestion Method
High Range – mg/L COD
Low Range – 0-150mg/L COD
Introducing… the EPA approved Hach COD methods.

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
Introducing… the EPA approved Hach COD methods.

29. Hach COD Methods Methods for Process Control (not EPA Approved)
COD2 – Mercury-free formulation of Hach COD vials
Introducing… the EPA approved Hach COD methods.

30. COD vs BOD

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. COD vs BOD
BOD must still be run to comply with NPDES permit regulations!

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. 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. COD vs BOD
Potassium dichromate will oxidize regardless of water conditions.

36. COD vs BOD COD BOD Picky bugs vs Clean Plate Club chemicals
COD measurements will always be higher than BOD measurements
BOD
COD

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. 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. COD vs BOD
In this example, the correlation calculates out to COD = 1.75 BOD

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.