22.01.2004 _basic COD_ Technical Sales Support EU page 1
Basics on COD measurement
Christian Pust
Technical Sales Support EU
18.02.2004
22.01.2004 _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.
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Outline
Introduction to COD
The Chemistry of COD
COD Dichromate Digestion
COD vs. BOD
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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
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What is COD?
In other words….
 COD is an indirect measure of the amount of organics in a water
sample.
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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.
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What is COD?
COD Reaction
2 KC8H3O4 + 10 K2Cr2O7 + 41 H2SO4 
KHP
Dichromate
oxidant
Acid
16 CO2 + 46 H2O + 10 Cr2(SO4)3 + 11 K2SO4
Carbon Dioxide
Water
Chromic Ion
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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
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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!
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Where is COD Measured?
MUNICIPAL WASTEWATER PLANT
Bar
Screen
Influent
Wastewater
Primary
Clarifier
Grit
Chamber
Pump
Return
Sludge
Secondary
Clarifier
Aeration
Basin
Pump
Thickener
Disinfection
Contact
Chamber
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The Chemistry of COD
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The Chemistry of COD
COD is a two step process
 Digestion
 Determination
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The Chemistry of COD
Required reagents for COD:
 Hach COD reagent vials
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The Chemistry of COD+
Reagent vials contain:
 Potassium dichromate
 50% sulfuric acid
 Silver salt (catalyst)
 Mercuric salt (inhibits chloride interference)
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The Chemistry of COD
Organic compounds are oxidized by potassium dichromate and sulfuric acid.
Cr6+
H2SO4
H2SO4
COOH
Cr6+
Cr6+
COOK
H2SO4
Cr6+
H2SO4
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The Chemistry of COD
The oxidation of organics results in the reduction of Cr6+ to Cr3+.
Cr6+
Cr6+
Cr3+
Cr6+
Cr6+
Cr3+
Cr3+
Cr3+
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The Chemistry of COD
Reduced chromium is the reaction product of interest.
Other products of the reaction include:
 Carbon dioxide
 Water
 Potassium sulfate
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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.
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The Chemistry of COD
As organics are oxidized and Cr6+ is reduced to Cr3+, the color of the solution
changes from yellow to green.
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The Chemistry of COD
The amount of green color formed is proportional to the COD of the sample.
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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+.
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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.
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The Chemistry of COD
Colorimetric measurement
 High Range – measures increase in green color at 620nm
 Low Range – measures decrease in yellow color at 420nm
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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
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The Chemistry of COD
Interferences
 Bromide also interferes
 Bromide interference is not controlled by mercuric sulfate
 Samples must be diluted to minimize interference
22.01.2004 _basic COD_ Technical Sales Support EU page 26
COD Dichromate Digestion Procedure
Water Analysis Handbook – page 737
Let’s take a look!
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Hach COD Methods
EPA Approved Methods
 Dichromate Reactor Digestion Method
 High Range – 0-1500mg/L COD
 Low Range – 0-150mg/L COD
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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
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Hach COD Methods
Methods for Process Control (not EPA Approved)
 COD2 – Mercury-free formulation of Hach COD vials
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COD vs BOD
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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
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COD vs BOD
BOD must still be run to comply with NPDES permit regulations!
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COD vs BOD
Why is COD more stable than BOD?
 The tests use different methods of oxidation
 BOD - Microorganisms
 COD – Chemicals (potassium dichromate)
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COD vs BOD
Microorganisms are susceptible to pH, temperature, and other variables
in the water.
 Oxidation efficiency depends on the condition of the microorganisms
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COD vs BOD
Potassium dichromate will oxidize regardless of water conditions.
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COD vs BOD
Picky bugs vs Clean Plate Club chemicals
 COD measurements will always be higher than BOD measurements
BOD
COD
22.01.2004 _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.
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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
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COD vs BOD
500
400
300
200
100
0
Influent BOD
Influent COD
3/
1/
01
3/
3/
01
3/
5/
01
3/
7/
01
3/
9/
01
3/
11
/0
3/ 1
13
/0
3/ 1
15
/0
1
mg/L Value
Influent COD and BOD
Date
In this example,
the correlation
calculates out to
COD = 1.75 BOD
22.01.2004 _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.