Biochemical oxygen demand
Dr Utpal Sharma
Department of Community Medicine, SMIMS
For the beginning…..
Water is “polluted” by many organic matter in its course of flow
When organic matter is present in a water supply, the bacteria
present in water will begin the process of breaking down this waste.
With this much of the available dissolved oxygen is consumed by
aerobic bacteria, robbing other aquatic organisms of the oxygen
they need to live.
Biological Oxygen Demand (BOD) is a measure of the oxygen used
by microorganisms to decompose this waste.
A large quantity of organic waste in the water supply, guarantees a
large number bacteria present to decompose this waste.
In this case, the demand for oxygen will be high (due to all the
bacteria) so the BOD level will be high.
As the waste is consumed or dispersed through the water, BOD
levels will begin to decline.
The amount of oxygen absorbed by a sample of sewage
during a specific period, generally 5 days at a specific
temperature, generally 20° C for the aerobic destruction of
the organic matter by living organisms.
Total BOD is of more significance to food webs than to water
Nitrates and phosphates in a body of water
can contribute to high BOD levels.
Nitrates and phosphates are plant nutrients
and can cause plant life and algae to grow
When plants grow quickly, they also die
This contributes to the organic waste in the
water, which is then decomposed by bacteria
resulting in a high BOD level.
Is Biochemical oxygen demand different from
Biological oxygen demand….???
Biochemical oxygen demand measures the molecular
oxygen utilized for the biochemical degradation of
organic material (carbonaceous demand) and…..
……the oxygen used to oxidize inorganic material
such as sulfides and ferrous ion.
It also may measure the amount of oxygen used to
oxidize reduced forms of nitrogen (nitrogenous
Biological oxygen demand only addresses the oxygen
used up by the bacteria to degrade organic
Difference between Aerobic and
Element Aerobic Decay
C CO2 CH4
N NO3 NH3
H H2O CH4, NH3, H2S, H2O
S SO4²- H2S
P PO4³- PH3
Biochemical oxygen demand takes into account of oxygen used up for
degradation of both carbonaceous and inorganic materials
Principle test for biodegradability of any sample and strength
of the waste so measures the amount of pollution.
Important parameter to assess the pollution of surface and
ground waters where contamination occurred due to disposal
of domestic and industrial effluents.
Allows calculation of the effect of the discharges on the
oxygen resources of the receiving water.
Measurement of BOD in raw (influent) and treated (effluent)
wastewaters is a standard practice to evaluate treatment
Data from BOD tests used for the development of engineering
criteria for the design of wastewater treatment plants.
Used in studies to measure the self-purification capacity of
Serves regulatory authorities as a means of checking on the
quality of effluents discharged to stream waters.
One of the most important method in sanitary analysis to
determine the polluting power, or strength of sewage, industrial
wastes or polluted water.
It serves as a measure of the amount of clean diluting water
required for the successful disposal of sewage by dilution.
And we expect…..
Any effluent to be discharged into natural bodies of water
should have BOD less than 30 mg/L.
Drinking water usually has a BOD of less than 1 mg/L.
But, when BOD value reaches 5 mg/L, the water is doubtful in
Items BOD standards(mg/L)
Most pristine rivers <1
Moderately polluted rivers 2-8
Ordinary domestic sewage 150- 200
Municipal sewage efficiently treated <20
Factors affecting BOD…
Elevated temperature decreases the level of DO of water
harming aquatic organisms like fish, amphibians and others
Increases the metabolic rate of aquatic animals and
enzyme activity, resulting in consumption of more food in a
High temperature limits oxygen dispersion into deeper waters,
contributing to anaerobic conditions.
This can lead to increased bacterial levels when there is
ample food supply.
Higher water temperature increases plant growth rates
This results in a shorter lifespan and species overpopulation
causing an ”algae bloom” which reduces oxygen levels.
Eutrophication occurs due to oversupply of nutrients, which
causes explosive growth of plants and algae.
when such organisms die, consume the oxygen in the body of
water, thereby creating the state of hypoxia.
The primary limiting factor is phosphorus which promotes
excessive plant growth and decay, favouring simple algae and
plankton, and causes a severe reduction in water quality.
Phosphate adheres tightly to soil, so it is mainly transported
Once into the lakes, phosphate is extracted into water is slow,
hence the difficulty of reversing the effects of eutrophication
The source of this excess phosphate are detergents,
industrial/domestic run-off, and fertilizers.
At higher levels of BOD….
At high BOD levels, organisms such as macro invertebrates
that are more tolerant of lower dissolved oxygen (i.e. leeches
and sludge worms) may appear and become numerous.
Organisms needing higher oxygen levels (i.e. caddisfly larvae
and mayfly nymphs) will NOT survive.
The basics of BOD test..
The first step is to obtain equal volumes of water from the area to be
Dilute each specimen with a known volume of distilled water which
has been thoroughly shaken to insure oxygen saturation.
Oxygen meter is used to determine the concentration of oxygen
within one of the vials.
The remaining vial is than sealed and placed in darkness and
tested for oxygen content five days later.
BOD is then determined by subtracting the second meter reading
from the first.
Water from an exceptionally clear lake might show a
BOD of less than 2 ml/L of water.
Raw sewage may give readings in the hundreds and
food processing wastes may be in the thousands.
Samples for BOD test can be either grab or composite.
Composite sample more preferred as it is more representative of
the wastestream over a period of time.
Samples should be taken at a point of well-mixed and proportional to
the amount of the flow.
Preservation of sample is not practical as biological activity will
continue after a sample has been taken
Testing should be started as quickly as possible, preferrrably <6hrs.
If not possible samples should be kept ≤ 4°C.
Do not allow samples to freeze.
Samples may be kept for no more than 48
hours before beginning the BOD test.
The BOD concentration in waste water is more than sample
due to biological activity.
So, it is necessary to dilute the sample before incubation to
bring the oxygen demand and supply into appropriate balance
If not diluted, microorganisms will use up the DO in the BOD
bottle before the five day incubation time is up.
There is no way of knowing at what point during the five days
the DO reached zero.
It is high quality organic free water .
The required volume of water is aerated with a supply of clean
compressed air for at least 12 hours.
It is stabilized by incubating it at 20ºC for at least 4 hours.
Samples with extreme pH values (>8.5 or <6.0) must be neutralized
to pH 7.0 prior to testing.
This is done by adding either or H2SO4 or NaOH
Any samples containing residual chlorine must be pretreated to
remove chlorine before the test is run.
This is done by adding sodium sulfite to the sample
The process of adding live bacteria to a sample.
Samples form sources like high temperature, extreme pH or having
heavy metals could kill or injure the microorganisms
The condition must be corrected and healthy active organisms
Completely fill two BOD bottles with dilution water.
Into additional BOD bottles, partially filled with dilution water,
carefully mix the proper volume of sample.
Add dilution water until the bottles are completely filled.
Stopper each bottle taking care to avoid trapping air bubbles
inside the bottles as the bottle stoppers are inserted.
Fill the top of each bottle neck around the stopper with dilution
Determine the initial DO content of each set of duplicate
bottles, including the dilution water blank
Place the remaining bottles in the incubator at 20°C and
incubate for five days.
At the end of exactly five days (+/-3 hours), test the DO
content of the incubated bottles.
Calculate the BOD for each dilution.
The dilution water blanks are used only to check the quality of
the dilution water.
If the quality of the water is good and free from impurities, the
depletion of DO should be less than 0.2 mg/L.
In any event, do not use the depletion obtained as a blank
Samples treated for chlorine should always be seeded
BOD, mg/L = (Initial DO - Final DO) x bottle volume)
Major interferences of BOD test are substances that inhibit the
growth of the microorganisms viz. chlorine, mineral acids, and heavy
Excessive nitrites can interfere with the BOD determination.
Growth of algae in the presence of light can increase the DO
Artifacts in BOD testing results from residues building up in the BOD
and dilution water bottles.
Variations in BOD test is subject to a number of factors temperature,
weather etc. results can vary widely from day to day, or even hour to