This document discusses biochemical oxygen demand (BOD), which is a measure of the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material in a body of water. It provides information on how BOD is affected by temperature, sources of organic pollution like fertilizers and detergents, and how BOD levels are interpreted to determine water quality. The standard test for BOD involves incubating a water sample for 5 days at 20°C and measuring the oxygen depletion, with higher depletion indicating higher BOD and more organic pollution in the water.

2. BOD is the amount of dissolved oxygen needed by
aerobic biological organism in a body of water to
breakdown organic material present in a given water
sample at certain temperature over a specific time
period
(Numerous 1974, Tchbanglous and Schroeder ,1985)
CH2O + O2 CO2 + H2O
Most of Bacteria in the aquatic columns are aerobic.
Escherichia coli, Bacillus subtilis, Vibrio cholera

3. The Relationship Between Temperature and Oxygen Solubility
Temperature
(degrees C)
Oxygen Solubility
(mg/L)
0 14.6
5 12.8
10 9.0
15 10.2
20 9.2
25 8.6
100 boiling 0
Higher the temperature of water higher will be the rate of
respiration. So, concentration of oxygen decreases.
Many Animal species can grow and reproduce normally when
dissolved oxygen level is ~ 5.0 mg/L.
Atmosphere contains 21% oxygen (210000
mg/dm3)

4. HYPOXIA: When dissolve oxygen content below 3.0 mg/L. Many Species
move elsewhere and immobile species may die
ANOXIA: When dissolve oxygen content below 0.5 mg/L. All aerobic
species will die

5. Sources of organic matter
Fertilizer contains Nitrate
contributes to high BOD

6. Phosphate present in Soap
and detergent that
enhances the growth of
algal blooms. As a result
depletion of oxygen occur.
High BOD and Low BOD:
In a body of water with large amount of decaying organic
material , the dissolved oxygen level may drop by 90 %,
this would represent High BOD
In a body of water with small amount of decaying organic
material , the dissolved oxygen level may drop by 10 %,
this would represent Low BOD

7. ANALYSIS OF BOD OF WATER
Use glass bottles having 60 mL or greater capacity. Take
samples of water.
Turn on the constant temperature chamber to allow the
controlled temperature to stabilize at 20°C ±1°C.
Record the DO level (ppm) of one immediately.
Place water sample in an incubator in complete darkness at
20 C for 5 days. Exclude all light to prevent possibility
of photosynthetic production of DO
If don't have an incubator, wrap the water sample bottle in
aluminum foil or black electrical tape and store in a dark
place at room temperature (20o C or 68 °F).

8. DILUTION OF SAMPLE
Most relatively unpolluted streams have a BOD5 that ranges from 1
to 8 mg/L
Dilution is necessary when the amount of DO consumed by
microorganisms is greater than the amount of DO available in the
air-saturated.
If the BOD5 value of a sample is less than 7 mg/L, sample dilution is
not needed.
The DO concentration after 5 days must be at least 1 mg/L and at
least 2 mg/L lower in concentration than the initial DO
(American Public Health Association and others, 1995).

9. BOD RANGE Millilitre of Sample Millilitre of Water
0-7 mg/L 300 0
6-21 mg/L 100 200
12-42 mg/L 50 250
30-105 mg/L 20 280
60-210 mg/L 10 290
[Adapted from Sawyer and McCarty, 1978. BOD5, 5-day biochemical oxygen
demand]
Recommended sample volumes

10. BOD of the dilution water is less than 0.2 mg/L.
Discard dilution water if there is any sign of
biological growth.
pH of the dilution water needs to be maintained in a
range suitable for bacterial growth
Bacterial growth is very good between 6.5 to 7.5
Sulfuric acid or sodium hydroxide may need to be
added to the dilution water to lower or raise the pH,
respectively.

11. CALCULATION:
The general equation for the determination of a BOD5
value is:
BOD = D1-D2/P
Where
D1 = initial DO of the sample,
D2 = final DO of the sample after 5 days, and
P = decimal volumetric fraction of sample used.
If 100 mL of sample are diluted to 300 mL, then P = 0.33.
Notice that if no dilution was necessary, P = 1.0 and the
BOD5 is determined by D1 - D2.
DO depletion of at least 2 mg/L, and there is no evidence
of toxicity

12. Nature BOD Reaction
* -dC/dt = kC, where C = concentration of oxidizable organic
matter at start of the time interval t, and k is the rate constant
for the reaction
First Order Reaction: Rate is proportional to the amount of
oxidizable organic matter remaining at any time

15. INTERPRETATION OF BOD LEVEL
BOD LEVEL STATUS
1-2 mg/L CLEAN WATER
3-5 mg/L MODERATELY CLEAN
6-9 mg/L POLLUTED WATER
> 10 mg/L VERY POOR QUALITY
Generally, when BOD levels are high, there is a decline in
DO levels. This is because the demand for oxygen by the
bacteria is high and they are taking that oxygen from the
oxygen dissolved in the water

16. The standard oxidation (or incubation) test period for BOD is 5 days
at 20 degrees Celsius (°C) (BOD5).
Twenty days is considered, by convention, adequate time for a
complete biochemical oxidation of organic matter in a water sample,
BOD results represent approximate stream oxygen demands because
the laboratory environment does not reproduce ambient stream
conditions such as temperature, sunlight, biological populations, and
water movement
The samples must be seeded with a population of microorganisms to
produce an oxygen demand
Interferences present in a water sample can inhibit biochemical
oxidation and interfere in BOD process.
Copper, lead, chromium, mercury, and arsenic are examples of
Interferences

18. In the USA, it has been estimated that each year 7.1 million suffer
from a mild to moderate infections, from severe waterborne
diseases, in estimated 12,000 deaths a year