The two basic characteristics that determine the strength of sewage are chemical oxygen demand (COD) and biological oxygen demand (BOD). Thus, the oxygen-demanding capacity of the wastewater is tested using both metrics.
This article explains the fundamental characteristics of COD and BOD, as well as their fundamental distinctions.
What is Biological Oxygen Demand (BOD)?
The amount of oxygen required by the sewage's microorganisms, to decompose biodegradable material in an aerobic environment is known as BOD. This is the metric that is most frequently used to assess the municipal water's quality and organic strength.
BOD test
The typical BOD test quantifies the quantity of oxygen needed by the microorganisms, to break down the biodegradable material in the wastewater sample, over the course of 5 days in an aerobic environment at 20 degrees Celsius. It is expressed as mg/l.
What is Chemical Oxygen Demand (COD)?
The volume of oxygen needed for the chemical oxidation of organic and inorganic compounds found in wastewater, using oxidising agents like potassium permanganate and potassium dichromate. Without the need of any additional machinery, chemical oxidation of organic matter is sped up when COD is present.
COD Test
The sample water is taken in a closed container and exposed to a potent oxidant, such as potassium dichromate and sulphuric acid for the prescribed amount of time and temperature, to measure COD.
The amount of oxygen required is the COD, which is used by both the inorganic and organic components in the wastewater sample. The mass of oxygen consumed divided by the volume of the solution is how the chemical oxygen demand is determined. It is expressed as mg/l.
What is the difference between BOD and COD?
BOD measures the amount of oxygen needed by aerobic organisms to break down organic material, while COD measures the amount of oxygen needed for chemical reactions to break down, both organic and inorganic components found in wastewater. As a result, COD is always greater than BOD.
The table below lists the key distinctions between BOD and COD:
PARAMETER |
BOD |
COD |
Producing agents |
Organisms that are aerobic produce BOD. |
Chemical reagents carry out COD. |
Test |
BOD is determined by incubating a sealed water sample for 5 days, at 20 degrees Celsius. The amount of oxygen utilised by aerobic organisms, is determined by the drop in dissolved oxygen. |
In the COD test, the water sample is incubated for a predetermined amount of time, at a predetermined temperature with a potent oxidant, in addition to boiling sulphuric acid. |
Value |
Value of BOD is less than COD |
COD value exceeds BOD |
Measurement |
BOD value can be determined in 5-days. |
COD measurements can be made in few hours. |
Utilisation |
BOD is used to oxidise organic waste, and naturally occurring organic materials in water. |
COD has the ability to break down industrial sewage. The amount of oxygen absorbed by the acetates contained in the water sample is not measured by COD. |
Application |
It is employed to handle waste loadings in STPs.
|
It is used to determine how much oxidizable contaminants, are present in water bodies.
It gives a measurement of the impact of an effluent on a body of water. |
Characteristics of domestic wastewater in terms of BOD and COD
Domestic wastewater can be categorised as weak, medium, strong, or extremely strong, based on the strength of BOD and COD that are present.
Classification |
BOD (mg/l ) |
COD (mg/l ) |
Weak |
<200 |
<400 |
Medium |
350 |
700 |
Strong |
500 |
1000 |
Very Strong |
>750 |
>1500 |
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