You don't often think that water bodies contain oxygen, but water does contain
a small amount of dissolved oxygen. A small amount, but it is essential for life
in the water. Biological oxygen demand (BOD) generally represents how
much oxygen is needed to break down organic matter in water.
Biological Oxygen Demand (BOD) and Water
Biochemical oxygen demand (BOD) represents the amount of oxygen consumed by bacteria
and other microorganisms while they decompose organic matter under aerobic (oxygen is
present) conditions at a specified temperature.
When you look at water in a lake the one thing you don't see is oxygen. In a way, we think that water
is the opposite of air, but the common lake or stream does contain small amounts of oxygen, in the
form of dissolved oxygen. Although the amount of dissolved oxygen is small, up to about ten
molecules of oxygen per million of water, it is a crucial component of natural water bodies; the
presence of a sufficient concentration of dissolved oxygen is critical to maintaining the aquatic life
and aesthetic quality of streams and lakes.
The presence of a sufficient concentration of dissolved oxygen is critical to maintaining the aquatic
life and aesthetic quality of streams and lakes. Determining how organic matter affects the
concentration of dissolved oxygen (DO) in a stream or lake is integral to water- quality management.
The decay of organic matter in water is measured as biochemical or chemical oxygen demand.
Oxygen demand is a measure of the amount of oxidizable substances in a water sample that can
lower DO concentrations.
Certain environmental stresses (hot summer temperatures) and other human-induced factors
(introduction of excess fertilizers to a water body) can lessen the amount of dissolved oxygen in a
water body, resulting in stresses on the local aquatic life. One water analysis that is utilized in
order to better understand the effect of bacteria and other microorganisms on the amount of
oxygen they consume as they decompose organic matter under aerobic (oxygen is present)
is the measure of biochemical oxygen demand (BOD).
Determining how organic matter affects the concentration of dissolved oxygen in a stream or lake is
integral to water-quality management. BOD is a measure of the amount of oxygen required to
remove waste organic matter from water in the process of decomposition by aerobic bacteria
(those bacteria that live only in an environment containing oxygen). The waste organic matter
is stabilized or made unobjectionable through its decomposition by living bacterial organisms which
need oxygen to do their work. BOD is used, often in wastewater-treatment plants, as an index of
the degree of organic pollution in water.
What is biochemical oxygen demand and why is it
important?
Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms
in decomposing organic matter in stream water. BOD also measures the chemical oxidation of
inorganic matter (i.e., the extraction of oxygen from water via chemical reaction). A test is used to
measure the amount of oxygen consumed by these organisms during a specified period of time
(usually 5 days at 20 C). The rate of oxygen consumption in a stream is affected by a number of
variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic
and inorganic material in the water.
BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the
more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms
of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen:
aquatic organisms become stressed, suffocate, and die.
Sources of BOD include leaves and woody debris; dead plants and animals; animal manure; effluents
from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants;
failing septic systems; and urban stormwater runoff.
The chemical oxygen demand
The chemical oxygen demand (COD) is a measure of water and wastewater quality. The COD test is
often used to monitor water treatment plant efficiency. This test is based on the fact that a strong oxidizing
agent, under acidic conditions, can fully oxidize almost any organic compound to carbon dioxide. The
COD is the amount of oxygen consumed to chemically oxidize organic water contaminants to inorganic
end products.
The COD is often measured using a strong oxidant (e.g. potassium dichromate, potassium iodate,
potassium permanganate) under acidic conditions. A known excess amount of the oxidant is added to the
sample. Once oxidation is complete, the concentration of organics in the sample is calculated by
measuring the amount of oxidant remaining in the solution. This is usually done by titration, using an
indicator solution. COD is expressed in mg/L, which indicates the mass of oxygen consumed per liter of
solution.
The COD test only requires 2-3 hours, while the Biochemical (or Biological) Oxygen Demand (BOD) test
requires 5 days. It measures all organic contaminants, including those that are not biodegradable. There
is a relationship between BOD and COD for each specific sample, but it must be established empirically.
COD test results can then be used to estimate the BOD of a given sample. Unlike for the BOD test, toxic
compounds (such as heavy metals and cyanides) in the samples to be analyzed do not have an effect on
the oxidants used in the COD test. Therefore, the COD test can be used to measure the strength of
wastes that are too toxic for the BOD test. Some organic molecules (e.g., benzene, pyridine) are relatively
resistant to dichromate oxidation and may give a falsely low COD.
NOTE
COD or Chemical Oxygen Demand is the total measurement of all chemicals (organics
& in-organics) in the water / waste water;
BOD is a measure of, the amount of oxygen that require for the bacteria to degrade the
organic components present in water / waste water.
The ratio of BOD/COD is about; COD is higher than that of BOD; maximum of up to 4
times in medium scale industries; but it varies based on the industrial process and
nature of the raw materials used;
