Masud alam Ansari

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

3.  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 .

4. 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.

5. 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.

6. 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 .

7. 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.

8. B.O.D. curve

9. 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.

10. 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 .

11. 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.

12. Prepared By:
Masud alam Ansari