Chemical Oxygen Demand Presentation By Douglas Rittmann

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Chemical Oxygen Demand Presentation By Douglas
Rittmann, Ph.D., P.E. Water/Wastewater
Consultant To CE 4153 On April 18, 2006
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General Considerations
COD test allows measurement of a waste in terms
of total quantity required for oxidation to
carbon dioxide and water
The COD tests oxidizes organic compounds by the
action of strong oxidizing agents under acid
conditions
The amino nitrogen will be converted to ammonia
nitrogen and organic nitrogen will be
converted to nitrates.
Organic is converted to carbon dioxide and
water regardless of the biological
assimilability of the substances. Ex glucose
lignin are both oxidized completely, hence,
COD is higher than BOD.
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Advantages and Disadvantages
One of the limitations of COD is its inability
to differentiate between biologically
oxidizable and biologically inert organic matter.
It does not provide any evidence of the rate at
which the biologically active material would
be stabilized under conditions in nature.
Major advantage is the short time required for
evaluation in 3 hours versus 5 days for the
BOD test.
COD data can be interpreted in terms of BOD
values after sufficient experience has been
accumulated to establish reliable correlation
factors.
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History of COD Test
Chemical oxidizing agents have long been used
to measure the oxygen demand of sewage and
polluted waters.
Potassium permanganate were used for many
years, and the results were referred to as
oxygen consumed from Potassium Permanganate.
Oxidation by potassium permanganate was highly
variable due to strength of oxidant and with
respect too various types of compounds.
Oxygen consumed values were always less than
the 5-day BOD because of the inability of
permanganate to carry the oxidation to particular
endpoint
Potassium dichromate is the most practical
oxidizing agent with a silver ion catalyst in
an acid state and refluxed at high temperatures
to completely oxidize organics to carbon
dioxide and water.
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Chemical Oxygen Demand by Dichromate
COD results are reported in mg/L of oxygen.
A 0.25 N dichromate solution is equivalent to 2
mg of oxygen.
An excess of oxidizing agent must be present to
ensure that all organic matter is completely
oxidized.
Ferrous ammonium sulfate titrant is an excellent
reducing agent to measure the excess dichromate
after oxidation. 6Fe Cr2O7 14H
6Fe3 2Cr3 7H2O
Blanks are required because extraneous organic
matter is impossible to exclude.
Indicator Ferroin is an excellent indicator for
dichromate reduced by ferrous
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Calculations
COD (mg/L) 8000 (blank titr.- sample titration)
(norm. Fe(NH4)2(SO4)2)/
ml sample
Alternate Procedures COD test is precise and
accurate for samples with A COD of 50 mg/L or
greater. It is important in any modification that
the Volume of sample plus dichromate solution be
maintained at 11 ratio. If it is smaller, the
oxidizing power of the solution will decrease,
while If it is larger, the blank consumption of
dichromate becomes excessive
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Inorganic Interferences
Chlorides can cause high results but eliminated
by the addition of mercuric sulfate prior to the
addition of other reagents.
Nitrites and nitrates interference can be
eliminated by sulfamic acid.
Ferrous and sulfides can interfere but are low in
concentration.
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New Manganese III COD HACH Method
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Manganese III Method for COD
Virtually eliminates hazardous metals waste
disposal Concerned about mercury, chromium or
silver in waste streams? With the new Manganese
III COD (chemical oxygen demand) Method, disposal
of these heavy metals wastes is virtually
eliminated. The patented Manganese III COD
Reagent1 eliminates using characteristic heavy
metal oxidizers, catalysts and chloride removal
agents used in traditional COD testing. Hach
researchers have applied many innovations used in
the original USEPAapproved2 micro COD system to
develop a new method for monitoring wastewater
treatment processes. The Manganese III COD Method
can be used for Municipal and industrial
wastewater Municipal and industrial process
waters Environmental monitoring Food and
beverage industry Pulp and paper industry
Manufacturing plants
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Manganese III Method for COD
Simplifies COD analysis In less than 90 minutes,
up to 25 COD tests can be completed. Like Hachs
reliable USEPA-approved Dichromate COD Method,
the new Manganese III COD Method relies on a
closed reflux micro method, which is both easier
and less labor intensive than the open reflux
macro method. Plus, analysts at all skill levels
can get accurate results with this new method.
The Manganese III COD System uses prepared
reagents, a simplified procedure, and reliable
Hach equipment, including the COD Reactor and a
colorimeter or spectrophotometer. The new method
also includes a Vacuum Pretreatment Device (VPD)
and patent-pending Chloride Removal Cartridges,
which eliminate the use of mercury-containing
reagents for sample chloride removal. After
preparing the samples, simply digest them in the
COD Reactor for one hour.
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Manganese III Method for COD
The Manganese III COD Reagent contains Manganese
3, an oxidizer, and concentrated sulfuric acid.
The reagent does not contain metal catalysts or
mercuric sulfate. The Manganese III COD Reagent
offers several advantages when compared to open
reflux COD testing Provides a distinct color
change for easy analysis Simplifies process
control testing Minimizes handling and exposure
to corrosive chemicals Virtually eliminates a
hazardous metals waste stream and disposal costs
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Application of COD Data
Used extensively in analysis of industrial
samples
Used to determine losses in sewer systems
Due to short time for analysis, errors can be
corrected the same day
BOD test with COD Test can determine toxic
conditions more effectively
COD test used widely for better operation of
wastewater facility because of its speed in
analysis.