Biochemical oxygen demand (BOD) AND Chemical Oxygen Demand PDF

1. B.O.D & C.O.D
( BIOCHEMICAL OXYGEN DEMAND)
AND
(CHEMICAL OXYGEN DEMAND )

2. DEFINATION ( B.O.D)
 Biochemical oxygen demand (BOD) is the amount
of dissolved oxygen needed by aerobic biological organisms
in a body of water to break down organic material present in
a given water sample at certain temperature over a specific
time period.
 The term also refers to a chemical procedure for
determining this amount. This is not a precise quantitative
test, although it is widely used as an indication of the organic
quality of water.[1]
 The BOD value is most commonly expressed in milligrams of
oxygen consumed per litre of sample during 5 days of
incubation at 20 °C and is often used as a robust surrogate of
the degree of organic pollution of water.

3. DEFINATION ( C.O.D)
 COD (Chemical Oxygen Demand) is the amount of
oxygen required to degenerate all poution in a
chemical way (by adding oxidising agents and
heating). In general with chemical destruction you
can remove more polution than with the biological
way.
 . 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.

4. DIFFERENCE
B.O.D
 BOD is only a measure-
ment of consumed oxygen
by aquatic microorganisms
to decompose or oxidize
organic matter.
> Although, some of the organic compounds, which can be broken
down by microorganisms, are countable for the biological oxygen
demand, they may not be encountered in measuring chemical
oxygen demand
C.O.D
 COD refers the
requirement of
dissolved oxygen for
the oxidation of organic
and inorganic
constituents both.

5. IMPORTANT POINTS
 BOD is similar in function to chemical oxygen demand (COD),
in that both measure the amount of organic compounds in
water. However, COD is less specific, since it measures
everything that can be chemically oxidized, rather than just
levels of biologically active organic matter.
 For many years, the strong oxidizing agent potassium
permanganate (KMnO4) was used for measuring chemical
oxygen demand. Measurements were called oxygen
consumedfrom permanganate, rather than the oxygen
demand of organic substances. Potassium permanganate's
effectiveness at oxidizing organic compounds varied widely,
and in many cases biochemical oxygen demand (BOD)
measurements were often much greater than results from
COD measurements. This indicated that potassium
permanganate was not able to effectively oxidize all organic
compounds in water, rendering it a relatively poor oxidizing
agent for determining COD.

6. CALCULATION (C.O.D)
 The following formula is used to calculate COD:
COD= 8000(b-s)n
sample volume
 where b is the volume of FAS used in the blank sample, s is the volume
of FAS in the original sample, and n is the normality of FAS. If milliliters
are used consistently for volume measurements, the result of the COD
calculation is given in mg/L.
 The COD can also be estimated from the concentration of oxidizable
compound in the sample, based on its stoichiometric reaction with
oxygen to yield CO2 (assume all C goes to CO2), H2O (assume all H
goes to H2O), and NH3 (assume all N goes to NH3), using the following
formula:
 COD = (C/FW)(RMO)(32)Where C = Concentration of oxidizable
compound in the sample,FW = Formula weight of the oxidizable
compound in the sample,RMO = Ratio of the # of moles of oxygen to # of
moles of oxidizable compound in their reaction to CO2, water, and
ammoniaFor example, if a sample has 500 wppm of phenol:
 C6H5OH + 7O2 → 6CO2 + 3H2OCOD = (500/94)(7)(32) = 1191 wppm

7. CALCULATION (B.O.D)
 BOD5 is calculated by: ( Dilution method )
unseeded : BOD5 = ( DO – D5)
P
seeded : BOD5 = ( DO-D5 ) – ( B0 – B5 )F
P
 where: DO is the dissolved oxygen (DO) of the diluted
solution after preparation (mg/l); D5 is the DO of the
diluted solution after 5 day incubation (mg/l); P is the
decimal dilution factor ; B0 is the DO of diluted seed
sample after preparation (mg/l) ; B5 is the DO of diluted
seed sample after 5 day incubation (mg/l) ; F is the ratio
of seed volume in dilution solution to seed volume in
BOD test on seed

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