BOD measures the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material in water, while COD measures the amount of oxygen required to chemically oxidize organic compounds. COD is generally higher than BOD because it measures oxidation of all organic compounds, whereas BOD only measures biologically degradable compounds. Both are used to assess water quality, but COD provides a faster and more accurate measurement than BOD. The ratio of COD to BOD can also indicate the toxicity of wastewater.

1. What is Chemical Oxygen Demand (COD) and
Biochemical Oxygen Demand (BOD)?
Biochemical Oxygen Demand (BOD):
BOD (Biochemical Oxygen Demand), also often referred to as biological oxygen demand, is a
test performed to measure the potential of wastewater and other waters to deplete the oxygen
level of receiving waters. BOD (Biochemical oxygen demand) - The amount of oxygen required
by micro-organisms to degrade the organic matter and can be calculated as BOD of diluted and
Undiluted samples. The BOD values depends on the dissolved organic matter in the waste water
samples. More the organic matter more the demand of oxygen by microbes to degrade it. Or
BOD is a measurement of organics that can be oxidized (utilize as a food source) by
microorganisms while COD is a measurement of organics that can be oxidized by chemicals
(oxidizing agents) that why COD is higher than BOD in most of the cases.
In other words, the BOD test is performed to determine what effect dirty water, containing
bacteria and organic materials, will have on animal and plant life when released into a stream or
lake. When there is an abundance of bacteria and organic materials, the bacteria will take in
oxygen in order to breakdown these molecules. If bacteria are taking in large amounts of oxygen,
this will have a detrimental effect on the surrounding ecosystem. On the contrary, when there are
low levels of organic waste in the water, there are fewer bacteria present, the BOD will be lower
and the dissolved oxygen levels higher.
Chemical Oxygen Demand (COD):
COD analysis is a measurement of the oxygen-depletion capacity of a water sample contaminated
with organic waste matter. Specifically, it measures the equivalent amount of oxygen required to
chemically oxidize organic compounds in water.
COD is used as a general indicator of water quality and is an integral part of all water quality
management programs. Additionally, COD is often used to estimate BOD (Biochemical Oxygen
Demand) as a strong correlation exists between COD and BOD, however COD is a much faster,
more accurate test.
The most common COD method is the wet chemistry method. This involves a two hour
digestion at high heat under acidic conditions in which potassium dichromate acts as the oxidant
for any organic material present in a water sample. Silver sulfate is present as the catalyst and
mercuric sulfate acts to complex out any interfering chloride. Following the digestion, the extent
of oxidation is measured through indirect measurement of oxygen demand via electrons
consumed in the reduction of Cr6+ to Cr3+. This can be done by titration or spectrophotometry.
Total organic carbon (TOC) is the amount of carbon bound in an organic compound and is
often used as a non-specific indicator of water quality or cleanliness of pharmaceutical
manufacturing equipment. TOC may also refer to the amount of organic carbon in a geological

2. formation, particularly the source rock for a petroleum play; 2% is a rough minimum. For marine
surface sediments, average TOC content is 0.5 wt% in the deep ocean, and 2wt% along the
eastern margins.
DO
Dissolved Oxygen
The amount of Dissolved Oxygen, or DO, in water is expressed as a concentration. A concentration is the amount
of in weight of a particular substance per a given volume of liquid. The DO concentration in a stream is the mass
of the oxygen gas present, in milligrams per liter of water. Milligrams per liter, or mg/L, can also be expressed as
parts per million, or ppm.
The concentration of dissolved oxygen in a stream is affected by many factors:
• Temperature: Oxygen is more easily dissolved in cold water.
• Flow: Oxygen concentrations vary with the volume and velocity of water flowing in a stream. Faster
flowing white water areas tend to be more oxygen rich because more oxygen enters the water from the
atmosphere in those areas than in slower, stagnant areas.
• Aquatic Plants: The presence of aquatic plants in a stream affects the dissolved oxygen concentration.
Green plants release oxygen into the water during photosynthesis. Photosynthesis occurs during the day
when the sun is out and ceases at night. Thus in streams with significant populations of algae and other
aquatic plants, the dissolved oxygen concentration may fluctuated daily, reaching its highest levels in the
late afternoon. Because plants, like animals, also
The only relation between COD & BOD is - COD > BOD.
For most industrial process waste water, community sewage, drains and even some hetrogeneous
sources, COD/BOD ratio has been found to be in a defined range and hence, COD result gives
more or less BOD value range for such identifiable waste water.
COD is required to be done before BOD is most cases where the waste/water sample is from
unknown source as fixing dilution is critical for BOD.
BOD(5 d, 20C) can be replaced by BOD (3 d, 27 C) as the values are more or less similar and
yield faster result.
the ratio is important for wastewater containing mixture of domestic and industrial wastewater.
HIgh BOD/COD ratio indicates that toxicity is less. Low ratio indicate that toxicity is more. BOD
indicates that wastewater has biodegradable content and so difference means non-biodegradable
or toxic components are more. Therefore for industrial wastewaters, both COD and BOD are
measured