BOD measures the amount of dissolved oxygen required by aerobic organisms to break down organic matter over 5 days, while COD measures the oxygen required to chemically oxidize organic compounds using a strong chemical oxidant. BOD uses a biological oxidation process that is slower but measures naturally degradable organics, while COD uses a chemical oxidation process that is faster but measures all organics including those not degraded biologically. COD values are typically higher than BOD and are used to measure pollution from industrial sources.

2. BIOLOGICAL OXYGEN DEMAND
(BOD)
WHAT IS BOD ?
Biological Oxygen Demand (BOD, also called Biochemical
Oxygen demand ) is the amount of dissolved oxygen
needed (i.e. demanded) by aerobic biological organisms to
break down organic material present in a given water
sample at certain temperature over specific time period .

3. Firstly remember few important points
1. Biochemical oxygen demand is a measure of the quantity
of the oxygen used by microorganisms (e.g.. Aerobic
bacteria ) in the oxidation of the organic matter .
2. Most of the bacteria in the aquatic water column are
aerobic . That means that they use oxygen to perform their
metabolic activities of decomposition.
3. If dissolved oxygen concentration drop below 5 parts per
million (ppm), fish will be unable to live for very long .

4.  To check quality of water
 Biologically oxidisable
substance (e.g.
carbohydrates )
increases the BOD of
water
 When BOD is high ,
dissolved oxygen
becomes low
 Greater the BOD greater
is the pollution .
 The demand of oxygen
is proportional to the
amount of organic waste
to be degraded
aerobically .

5. CHEMICAL OXYGEN DEMAND
(COD)
WHAT IS COD ?
Chemical oxygen demand is the amount of
oxygen needed to oxidize the organic matter
present in water. Chemical oxygen demand
testing is used to determine the amount of
oxidation that will occur and the amount of
organic matter in a water sample. Chemical
oxygen demand testing is also used to
determine the amount of inorganic chemicals in
a sample.

6. SIGNIFICANCE OF COD
 COD values are greater than BOD values .
 Pollution measuring factor
 Time period – 3 hours .
 COD values are not effected by toxins .
 Proposing standard for discharging domestic or
industrial effluents.

7. • Definition
• BOD: Biological Oxygen Demand (BOD, also called Biochemical Oxygen
demand ) is the amount of dissolved oxygen needed (i.e. demanded) by aerobic
biological organisms to break down organic material present in a given water sample
at certain temperature over specific time period
COD : Chemical oxygen demand is the amount of oxygen needed to oxidize the
organic matter present in water. Chemical oxygen demand testing is used to
determine the amount of oxidation that will occur and the amount of organic matter in
a water sample. Chemical oxygen demand testing is also used to determine the
amount of inorganic chemicals in a sample.

8. Decomposition
BOD: BOD is a biological oxidation process.
COD: COD is a chemical oxidation process.
Test Method
BOD: The common test method for BOD is
method 5210B.
COD: The common method for COD is method
410.4.
Test Procedure
BOD: BOD is determined by incubating a sealed
water under specific temperature sample for five
days and measuring the loss of oxygen from the
beginning of the test

9. COD: COD is determined by incubating a
closed water sample with a strong oxidant like
potassium dichromate (K2Cr2O7) in
combination with boiling sulfuric acid (H2SO4)
under specific temperature for a specified
period of time.
Time Taken for Determination
BOD: Five days are taken for the
determination of the BOD.
COD: COD measurement can be taken from
few days.
Permissible Limit of Test
BOD: The permissible limit of BOD is 30 mg/L.

10. COD: The permissible limit of COD is 250 to 500
ppm.
Values of Measurement
BOD: BOD value is lower than the COD value.
COD: COD value is always greater than the BOD
value. Therefore, the more organic material can be
oxidized by COD.
Oxidation Ability
BOD: Biological oxidation is capable of oxidizing
natural organic detritus and organic waste in the
water.
COD: Industrial sewage is only degraded by COD.
But, COD does not measure the oxygen
consumption of acetate