Biochemical Oxygen Demand (BOD) measures the amount of dissolved oxygen required by aerobic microorganisms to decompose organic material in water over a specified time at a set temperature, often used to assess water quality and organic pollution levels. The BOD testing process involves measuring the change in dissolved oxygen in water samples, with a focus on both the carbonaceous and nitrogenous stages of decomposition. Additionally, Chemical Oxygen Demand (COD) provides a broader measure of organic compounds in water, reflecting total oxidizable materials and usually yielding higher values than BOD.

1. L-16 and 17
Biochemical Oxygen Demand
(BOD)
Environmental Engineering-II

2. Definition
• Biochemical oxygen demand or
B.O.D. is the amount of dissolved
oxygen needed by aerobic biological
organisms in a body of water ororganisms in a body of water or
wastewater sample to break down
organic material present in a given
water or wastewater sample at certain
temperature (200C) over a specific
time period (5 days).

3. • Used to assess the relative strength of
a waste
• The amount of oxygen required to
stabilize a waste if discharged to a
surface water.surface water.
• This is not a precise quantitative test,
although it is widely used as an
indication of the organic quality of
water.

4. • The BOD value is most commonly
expressed in milligrams of oxygen
consumed per litre of sample duringconsumed per litre of sample during
5 days of incubation at 20 °C and
is often used as a indicator of the
degree of organic pollution of
water.

5. Background
• Most natural waters and wastewaters
contain small quantities of organic
compounds.
• Aquatic microorganisms have evolved
to use some of these compounds asto use some of these compounds as
food.
• Microorganisms living in oxygenated
waters use dissolved oxygen to convert
the organic compounds into energy for
growth and reproduction.

6. • Populations of these
microorganisms tend to increase
in proportion to the amount of
food available.
• This microbial metabolism
creates an oxygen demand
proportional to the amount of
organic compounds useful as
food.

7. • Under some circumstances,
microbial metabolism can consume
dissolved oxygen faster than
atmospheric oxygen can dissolve into
the water.the water.
• Fish and aquatic animals and plants
may die when oxygen is depleted by
microbial metabolism

8. Types of BOD
• There are two stages of
decomposition in the BOD test:
i. a carbonaceous stage and
ii. a nitrogenous stage.ii. a nitrogenous stage.

9. • The carbonaceous stage, or first stage,
represents that portion of oxygen demand
involved in the conversion of organic
carbon to carbon dioxide.
• The nitrogenous stage, or second stage,• The nitrogenous stage, or second stage,
represents a combined carbonaceous plus
nitrogenous demand, when organic
nitrogen, ammonia, and nitrite are
converted to nitrate. Nitrogenous oxygen
demand generally begins after about 6
days.

11. TEST
• The test for biochemical oxygen
demand (BOD) is a bioassay
procedure that measures the oxygen
consumed by bacteria from the
decomposition of organic matter.decomposition of organic matter.
• The change in DO concentration is
measured over a given period of time
in water samples at a specified
temperature.

12. • In a nutshell, BOD gives a measure
on the impact of a waste(water) on
the oxygen content of a receiving
System(stream/river/lake).
• Wastes are broken down by• Wastes are broken down by
microbial organisms (frequently
referred to as “bugs” or
“microorganisms”), and the bugs, in
turn, require oxygen for this
monumental effort.

13. • Thus, in order for this test to "work",
you need
(1) a food source (Organic matter),
(2) a nice population of bugs,(2) a nice population of bugs,
(3) available oxygen (DO) to drive
the bugs, and
(4) a system which provides a
hospitable environment for the
bugs (incubation temp.).

15. Procedure
A. Dilution
• Dilution Factor. The dilution factor, DF,
is the ratio of the final volume (e.g., for
the bottle method, the volume of the BOD
bottle, usually 300 mL; for the graduated
cylinder method, the volume of thecylinder method, the volume of the
cylinder, usually 1,000 mL) to the volume
of sample therein. DF for the bottle
method = Volume of Diluted
Mixture/Volume of Sample in Mixture.

18. B. DO fixation
• 1. Slowly siphon three portions of aerated
dilution water into three separate BOD
bottles. Avoid adding atmospheric O2 to
dilution water.
2. To two of the three BOD bottles, add 12. To two of the three BOD bottles, add 1
ml MnS04 solution, followed by 1 ml
alkali-iodide-azide reagent. Submerge
pipette tips in sample when adding
reagents. Rinse tips well between uses.

20. 3. Stopper carefully to exclude air
bubbles; mix by inverting bottle
several times.
4. When precipitate has settled to about
half the bottle volume, carefullyhalf the bottle volume, carefully
remove the stopper and add 1.0 ml
conc. sulfuric acid. Re-stopper and
mix by gentle inversion until the
iodine is uniformly distributed
throughout the bottle.

21. C. DO Measurement
5. Transfer 203 ml of sample into conical
flask and titrate with 0.0250N sodium
thiosulfate to a pale straw color.
6. Add 1-2 ml of starch solution and
continue to titrate to first disappearancecontinue to titrate to first disappearance
of the blue color. (200 ml of original
dilution water is equal to 203 ml of
dilution water plus reagents.)
•

22. • Determine the initial
concentration of dissolved
oxygen in one bottle of the
mixture of sample and dilution
water (DO ), and in one of thewater (DOinitial), and in one of the
bottles containing only dilution
water.
• Place the other bottles in the
incubator

23. • Incubate the blank dilution water and the
diluted samples for 5 days in the dark at
20 °C.
• After 5 days, remove the bottles, fix the
DO and measure the DO (DOfinal)DO and measure the DO (DOfinal)
• Calculate BOD5
• BOD5 (mg/L) = {DOinitial (mg/L) of first
bottle - DOfinal (mg/L) of second bottle}
x dilution factor

24. BOD
incubator
and BOD
bottle

25. BOD
monitors

26. BOD
monitors

27. Chemical Oxygen Demand (COD)
• Like BOD, COD provides a measure
of the amount of organic compounds
in water. The difference is that COD
is less specific since it measuresis less specific since it measures
everything that can be chemically
oxidized rather than just levels of
bio-degradable organic matter.

28. • COD also is different in that it reflects the
oxidation based on a specific chemical
oxidant (dichromate).
• COD value is always greater than
BOD for every wastewater sample.BOD for every wastewater sample.

29. Objective Questions
1. COD value is always _____________than
BOD value.
2. First stage BOD is also known as
___________________________.
3. Second stage BOD is also known as3. Second stage BOD is also known as
_______________________.
4. For BOD test sample is incubated at ____0C.
5. ______ is indicator of the degree of organic
pollution of water.
6. ______test is basis for BOD test.

30. Theory Questions
Q1. Define the following
i. BOD
ii. Dilution factor
iii. COD
Q2. Explain ‘BOD test’.