Biochemical Oxygen Demand or alternatively termed as Biological Oxygen Demand (BOD) is

the amount of oxygen needed or demanded by aerobic microorganisms to break down the
organic matter present in a certain sample of water at a specific temperature and over a
given time period.

Water bodies have a certain amount of oxygen dissolved in it on which the aquatic life is
dependent for its respiratory needs. When there is organic matter present in the water
body, aerobic microbes use the dissolved oxygen in the water to break down the organic
substance thus reducing the oxygen available for aquatic life. The molecular oxygen present
in water is either a byproduct of photosynthesis occurring in aquatic plants or is
atmospheric oxygen in the dissolved state. The more the BOD of a water body or water
sample, the more it is polluted. The increase in organic matter can be due to environmental
factors but are mostly due to anthropogenic causes like pollution. BOD is used as an index
for measuring water quality. Determining organic matter present in a water body and its
effect on the ecosystem and aesthetics of the water body is an integral part of water quality
management.

Measuring Biochemical Oxygen Demand

Biochemical Oxygen Demand of a water sample is measured by a Bioassay procedure which
measures the oxygen consumed by the bacteria from the decomposition of the organic
matter over a period of five days at an incubation temperature of 20°C. BOD is expressed in
milligrams per litre of sample water. Although this is not a precise quantitative test, it is
widely used as an indication of the polluting potential of water. This test was given by
Sawyer and McCarty in 1978.

Typical Values of BOD and its Indication:

 Below 1 mg/L- Pristine water quality.

 2-8 mg/L- Moderately polluted water.

 Above 8mg/L- Severely polluted water.

Factors Affecting BOD

BOD of a Water Body is Affected by the Following Factors -

 Temperature

 pH value of the water

  The presence of certain kinds of microorganisms affects the growth of aerobic
bacterias.

 Type of inorganic substances in water.

 Amount and type of organic substance in water.

Significance of BOD
Biochemical Oxygen Demand finds its importance in various fields. These are -

 BOD finds its primary importance in sewage treatment plants. It gives the respiration
rate of sewage, sludge, soil, and garbage.

 It determines the rate of respiration in living beings.

 Measuring BOD gives the COD or Chemical Oxygen Demand of inorganic substances.

 It indicates the polluting potential of water.

 BOD is used in the medical and pharmaceutical industries to measure the oxygen
consumption of cell cultures.

Sources of BOD
Sources that increase the Biological Oxygen Demand of water are both natural and man-
made. Pollution is a major contributor to increasing the BOD of water bodies. A good
lifestyle is associated with an ample usage of water on a regular basis which results in a lot
of wastewater with organic content in it. With increasing industrialization, pollution is
increasing manifold. Factories have enormous wastewater being generated. Few industries
that have huge quantities of wastewater are paper mills, food processing plants, jute mills,
etc. The environmental factors contributing to increasing BOD include surface runoff,
floating debris, dead animals and plants, soil erosion, etc. There are few chemicals that
affect the BOD of drinking water. One of these is phosphate, which when present in high
amounts increases the BOD of water.

Usage of BOD in Sewage Treatment Plants

Biochemical Oxygen Demand is used in secondary sewage treatment or biological sewage
treatment. After the primary treatment in which the floating debris is removed by
sequential filtration and sedimentation, the primary effluent is passed to aeration tanks
where it is constantly agitated and the air is pumped into it. In aeration tanks, there are
vigorous growth heterotrophic microbes into flocs. Flocs are masses of bacteria associated
with fungal filaments.

These microbes consume the organic matter present in the primary effluent. The water is
treated until the BOD of the water is reduced. This is now called the activated sludge. This
effluent from aeration tanks goes into settling tanks for bacterial flocs to settle down and
after which they are treated with anaerobic microbes and physicochemical processes before
discharge in water bodies.

Effect of High BOD on the Aquatic Ecosystem

Increasing BOD has the same effect as the effects of dissolved depleting oxygen. When the
BOD of a water body increases significantly, aquatic life is adversely affected. The oxygen
used by aquatic organisms for respiration and metabolism is significantly reduced by the
microbes for breaking down of organic waste. This results in the death of fishes and aquatic
plants and complete disruption in the aquatic ecosystem. Oxygen concentration below
5ppm (parts per million) risks even low oxygen organisms like catfish and carps. The
freshwater fishes like Catla and rohu do not survive at these concentrations. The overall
aesthetic and beauty of the water body are damaged.

Pollution and Its Effects on Biochemical Oxygen Demand

With increasing pollution and urbanization, the water quality of the water bodies is
significantly reducing. Water quality management is essential for the correct ecological
functions. Urbanization leads to the production of much larger quantities of sewage. The
number of sewage treatment plants was not enough for treating these large quantities of
sewage. Untreated sewage was often discharged directly into water bodies which caused
massive pollution and an increase in BOD of the water bodies. This also led to an increase in
water-borne diseases like cholera, dysentery, jaundice, etc.

This increasing amount of BOD and pollution led to extreme pollution of the two main river
bodies in India, Ganga, and Yamuna. The Ministry of Environment and Forests initiated the
Ganga Action Plan in 1985 and the Yamuna Action Plan in 1993, to save these major rivers of
the country. These plans initiated the building of a large number of sewage treatment plants
to facilitate the discharge of only treated sewage in rivers.

Methods to Reduce BOD in Water

The biological oxygen demand of the water can be reduced by the following methods
  Advanced Oxidation Processes (AOP) by using H2O2/ UV, O3/UV, Fenton's reagent (
H2O2+FeSO4), etc.

 Coagulation using alum or cationic polymers

 Flocculation (e.g. chitosan, isinglass, polyelectrolyte) and sedimentation.

 Adsorption using activated charcoal.

 Electro flocculation.

 Using the up-flow anaerobic sludge blanket reactor(UASB).

 Reverse osmosis.

 Dissolved air floatation technique.

Water is a basic need for all life forms to exist on earth. Therefore it is extremely important
for us to conserve water and not pollute it. Polluting water directly harms every living
creature be it mankind or the other forms of life. Rising biochemical oxygen demand levels
in water are harming the ecosystem of the water bodies which indirectly is harming the
whole ecology of the whole biosphere as a whole. It is our duty to keep these ecosystems
alive. The other forms of life have the same amount of rights on environmental resources as
human beings. We must keep our greed away and strive towards conservation of water
bodies and reduction of BOD levels in them.

The measurement of the amount of dissolved oxygen consumed by aerobic microorganisms

while decomposing organic matter in stream water is known as Biochemical Oxygen
Demand.

Affecting Factors
Mentioned below are the affecting factors of biological chemical demand:-

 Biological oxygen demand affects the amount of dissolved oxygen in streams and
rivers. The rate of oxygen consumption is affected by: pH, temperature, various
kinds of microorganisms, and organic and inorganic materials.

 Less oxygen is consumed by higher forms of aquatic life. The consequences of the
high BOD are the same as those for low dissolved oxygen.

Uses
Various uses of biological oxygen demand have been mentioned as follows :
  Biological oxygen demand is used in studies for measuring self purification capacity
of the streams.

 It is an important method in sanitary analysis in determining industrial waste, the

strength of sewage, and polluted water.

 It also serves as a source through which the quality of effluents discharged into the
stream water can be checked.

Importance
Let's discuss about the environmental significance of biological oxygen demand:
 Biological oxygen demand is important for water quality because it provides an index
to assess the effect discharged wastewater will have on our environment.
 BOD is used extensively for treating wastewater, as decomposition of organic waste
by microorganisms is commonly used for treatment.
 Data obtained from the BOD test is used for designing the wastewater treatment
plants.

COD

The chemical oxygen requirement (COD) is a metric for measuring the consistency of water
and wastewater. The COD test is often used to track the quality of water treatment plants.
This test is based on the fact that, under acidic conditions, a good oxidising agent can fully
oxidise almost any organic compound to carbon dioxide. The COD is the quantity of oxygen
used to convert organic water pollutants to inorganic end products by chemical oxidation.
Under acidic conditions, COD is often tested using a heavy oxidant (e.g. potassium
dichromate, potassium iodate, potassium permanganate). The oxidant is applied to the
sample in a quantity that is considered to be excessive. After the oxidation process is
complete, the amount of oxidant left in the solution is measured to determine the
concentration of organics in the sample. Titration of an indicator solution is commonly used
to do this. The mass of oxygen absorbed per litre of solution is measured in milligram per
litre of COD.
The COD test takes just a few hours, while the Biochemical (or aological) Oxygen Demand
(BOD) test takes five days. It detects all organic toxins, including non-biodegradable ones.
For each particular study, there is a relationship between BOD and COD, but it must be
defined empirically. The findings of a COD evaluation will then be used to estimate a
sample's BOD. Toxic compounds (such as heavy metals and cyanides) in the samples to be
tested have little effect on the oxidants used in the COD test, unlike the BOD test. As a
result, the COD test will be used to determine the intensity of wastes that are too harmful
to be measured using the BOD method.

Note:
The chemical oxygen requirement is a measure of the amount of oxygen that can be
absorbed by reactions in a measured solution of environmental chemistry. It's usually
measured in mass of oxygen absorbed per amount of solution, which is measured in mg /
litre in SI units.