Chemical Oxygen Demand

Presentation By

Douglas Rittmann, Ph.D., P.E.

Water/Wastewater Consultant

To

CE 4153

On

April 18, 2006

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.

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.

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.

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 agentto 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

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 withA COD of 50 mg/L or greater. It is important in any modification that theVolume of sample plus dichromate solution be maintained at 1:1 ratio.If it is smaller, the oxidizing power of the solution will decrease, whileIf it is larger, the blank consumption of dichromate becomes excessive

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.

New Manganese III COD HACH Method

Manganese III Method for COD

Virtually eliminates hazardous metals waste disposal

Concerned about mercury, chromium or silver in waste streams? With the newManganese 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

Simplifies COD analysisIn less than 90 minutes, up to 25 COD tests can be completed. Like Hach’s 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 alsoincludes a Vacuum Pretreatment Device (VPD) and patent-pending Chloride Removal Cartridges, which eliminate the use of mercury-containing reagents for sample chloride removal. After preparingthe samples, simply digest them in the COD Reactor for one hour.

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 offersseveral 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

Manganese III Method for COD

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.