Biochemical oxygen demand(BOD) &Chemical oxygen demand(COD)

What is B.O.D. ?- Amount of oxygen required for bacteria to decompose organic matter. ( Biodegradable )

Why we go for the testing of B.O.D. in sewage ?- To determine what effect dirty water and how much bacteria and organic matter present in the sewage .

What will happen if sewage have high B.O.D. ?- Then sewage will be in anaerobic condition.

Why B.O.D. is usually lesser than C.O.D. ?-Because the bacteria present in sewage is only responsible for decompose the biodegradable matter. In normal sewage B.O.D. is two-third of C.O.D.

Stage of Decomposition of Organic matter

- 1st stage :-In 1st stage the carbonaceous matter are oxidized. The B.O.D. of sewage for this stage is about 90 % of total B.O.D.

- 2nd stage :- In the 2nd stage the oxidation of nitrogenous matter take place . It contain protein, fats , amino acids etc.

Why we are taking 5 days when performing B.O.D. test ?- Because 70% to 80% of organic matter will be oxidized with in 5 days. In India people developed this BOD test at 27 C in 3 days .

Limitation of BOD test

1) It is not prefer for industrial sewage .2) Time duration of the test is 5 days.3) pre-treatment is needed if the sample contain toxic waste.4) It is essential to have high concentration of active bacteria .

Some facts related to B.O.D.

-Oxidant is microorganism.

-For drinking water B.O.D. is less than 1mg/l.

-D.O. should at least 4mg/l at time of disposal.

-Any to be discharged in natural bodies of water should have BOD less than 20 mg/l.

B.O.D. curve

What is C.O.D. ?- Amount of oxygen required to chemically oxidize both matter to water and carbon dioxide.

Why we go for COD test ?- It take less time to perform the test as compare to BOD test.

Why COD is called theoretical demand ?

-It is impossible to know the detail of organic compound present in the sewage so, that it called theoretical demand.

which chemical oxidizing agent is used for COD test ? Why- Potassium dichromate .

-Cheap. -Completely oxidize all organic matter . - Reagent contain no hazardous metal. - Stable for room temperature.

Which type of sewage we prefer COD test ?- Industrial sewage which do not respond BOD test .

Advantage of COD test

- It take less time .

- COD test oxidize a wide range of chemical compound . Disadvantage of COD test

- It is inability to differentiate the biological oxidizable and biologically inert organic matter .

- some organic compound aren't oxidise completely which contain carbon compound.

Thank you

Prepared By:Masud alam Ansari