CHEMICAL OXYGEN

DEMAND

Presented By :-RAJNEESH KUMAR GAUTAM

MTECH 1ST SEM (ENERGY

&ENVIRONMENT)

BABA SAHEB BHIM RAO AMBEDKAR

UNIVERSITY-LUCKNOW 226001

INTRODUCTIONThe organic matter present in sewage can be

measured in number of ways. Organic matter is

often assessed in terms of oxygen required to

completely oxidise the organic matter to CO2 ,

H20 and other oxidised species.

The oxygen required to oxidise the

organic matter present in a given waste water

can be theoretically calculated .If the organic

compounds and their concentration are known

the oxygen demand of the sample can be

accurately computed but it is impossible to

know the details of organic compounds Present

in raw water or waste water. The COD is

therefore determined by performing a laboratory

test On given water sample using a strong

oxidant like dichromate solution.

It is expressed in milligrams per liter (mg/L) also

referred to as ppm (parts per million), which

indicates the mass of oxygen consumed per liter

of solution.

DEFINITION

COD is the total amount of oxygen required to

chemically oxidize the bio degradable and non-

biodegradable organic matter.

HISTORY• Earlier potassium permanganate was used

as an oxidizing agent

• COD values were very much lower than that

of 5th day BOD

• It indicated that potassium permanganate

was not very effective in oxidizing all the

organic matter present

• After that other oxidizing agents like ceric

sulphate, potassium iodate and potassium

di-chromate were also used

HISTORY (cont.)• Potassium di chromate was found to be the

most effective due to

- Completely oxidize all organic matter

- Relatively cheap

- Easy to purify

COD TEST BY DICHROMATE PRINCIPLE

• Water sample is refluxed in strong acidic

solution with a known excess amount of

potassium dichromate.

• After digestion, the remaining unreduced

K2Cr2O7 is titrated with Ferrous Ammonium

Sulfate (FAS)to determine K2Cr2O7

consumed.

• This gives us the oxidizable organic matter in

terms of oxygen equivalent.

TEST PROCEDURE1. Wash 300 ml round bottom refluxing flask.

2. In refluxing flask put one spatula of HgSO4 +

10 ml sample + 5ml K2Cr2O7 + 15 ml

concentrated H2SO4.

3. Add small amount of silver sulphate

4. Shake well and reflux for 2 hr.

5. Cool and add little amount of distilled water

to the flask through the condenser

TEST PROCEDURE (CONT.)6. Titrate the solution in the flask against FAS

using Ferroin indicator

7. End point green color to reddish brown

NOTE: For blank, add 10 ml distilled water

instead of sample. Rest of the procedure is the

same.

CALCULATIONS• The COD in mg/l is determined by the

formula,

COD mg/l = (A-B)xNx8000

ml sample taken

A = ml of FAS required for blank.

B = ml of FAS requires for sample.

ADVANTAGES OF COD TEST

• COD result are available much sooner than

BOD test results.

• The COD test requires fewer manipulations

of the sample.

• The COD test oxidizes a wide range of

chemical compounds.

• It can be standardize more easily.

DISADVANTAGES OF COD TEST• The major disadvantage is that the results

are not directly applicable to 5-day BOD

results without correlation studies over a

long period of time.

• One more limitation of COD is it’s inability to

differentiate between biologically oxidizable

and biologically inert organic matter.

References

• Journal on industrial waste water treatment

by k.l moed

• Environmental engineering vol 1&2 by S.K

Garg

• Industrial waste water treatment by metcalf

& eddy