BIOCHEMICAL OXYGEN
DEMAND
• Imagine a leaf falling into a stream. The
leaf, which is composed of organic matter,
is readily degraded by a variety of
microorganisms inhabiting the stream.
• Aerobic (oxygen requiring) bacteria and
fungi use oxygen as they break down the
components of the leaf into simpler, more
stable end products such as carbon
dioxide, water, phosphate and nitrate.
• As oxygen is consumed by the organisms,
the level of dissolved oxygen in the stream
begins to decrease
• Water can hold only a limited supply of
dissolved oxygen and it comes from only two
sources- diffusion from the atmosphere at the
air/water interface, and as a byproduct of
photosynthesis.
• Photosynthetic organisms, such as plants and
algae, produce oxygen when there is a
sufficient light source. During times of
insufficient light, these same organisms
consume oxygen. These organisms are
responsible for the diurnal (daily) cycle of
dissolved oxygen levels in lakes and streams.
• Oxygen available to aquatic organisms is
found in the form of dissolved oxygen.
Oxygen gas is dissolved in a stream through
aeration, diffusion from the atmosphere, and
photosynthesis of aquatic plants and algae.
• Plants and animals in the stream consume
oxygen in order to produce energy through
respiration. In a healthy stream, oxygen is
replenished faster than it is used by aquatic
organisms.
• Biological Oxygen Demand (BOD) is a measure of the
oxygen used by microorganisms to decompose this
waste. If there is a large quantity of organic waste in the
water supply, there will also be a lot of bacteria present
working 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.
• 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 quickly.
• When plants grow quickly, they also die quickly.
This contributes to the organic waste in the water,
which is then decomposed by bacteria. This results
in a high BOD level.
• The temperature of the water can also contribute to high BOD
levels. For example, warmer water usually will have a higher
BOD level than colder water.
• As water temperature increases, the rate of photosynthesis by
algae and other plant life in the water also increases. When
this happens, plants grow faster and also die faster.
• When the plants die, they fall to the bottom where they are
decomposed by bacteria.
• The bacteria require oxygen for this process so the BOD is
high at this location.
• Therefore, increased water temperatures will speed up
bacterial decomposition and result in higher BOD levels.
• When BOD levels are high, dissolved oxygen (DO) levels
decrease because the oxygen that is available in the water is
being consumed by the bacteria. Since less dissolved oxygen
is available in the water, fish and other aquatic organisms may
not survive.
Biochemical Oxygen Demand (BOD)
•Biochemical Oxygen Demand, or BOD, is a
measure of the quantity of oxygen consumed by
microorganisms during the decomposition of
organic matter.
•BOD is the most commonly used parameter for
determining the oxygen demand on the receiving
water of a municipal or industrial discharge.
•BOD can also be used to evaluate the efficiency
of treatment processes, and is an indirect
measure of biodegradable organic compounds in
water.
• If the amount of decomposing organic
material is too high, dissolved oxygen
levels can be severely reduced.
• In a body of water with large amounts of
decaying organic material the dissolved
oxygen levels may drop by 90%—this
would represent a high BOD.
• In a mountain stream with low levels of
decaying organic material, the dissolved
oxygen levels may drop by only 10% or
20%—a low BOD.
• Organic materials, such as leaves, fallen
trees, fish carcasses, and animal waste,
end up in the water naturally and are
important in the recycling of nutrients
throughout the ecosystem.
• Organic materials that enter the water as a
result of human impact can be considered
sources of pollution.