Turbidity of water - Definition and Measurement | Environment

Turbidity is one of the important physical characteristics of water. It is the measure of relative clarity of water samples. The higher the intensity of scattered light, the higher the turbidity.

Material that causes water to be turbid include clay, silt, very tiny inorganic and organic matter, algae, dissolved colored organic compounds, and plankton and other microscopic organisms

Turbidity is mainly due to water losing its transparency because of suspended materials like clay, silt or some other finally divided organic and inorganic matter. 


Presence of bacteria,fungi,algae and protozoa also contribute to the turbidity.


  • It is mainly expressed in ppm (or) mg/l and measured in silica scale .

  • Permissible limit for drinking water : 5 to 10 ppm.


How turbidity is measured?


Conventional methods or optical methods : direction of light and measurement direction are the same and parallel to each other.


In the optical method all the turbidity metres use 1mg of finely divided silica in 1 litre of distilled water as the standard unit of turbidity.


Turbidity rod


It is the easiest and field method. We have a graduated scale directly related to the standard unit of turbidity.

Jackson turbidity meter


Measured in JTU (jackson turbidity unit). It is useful in the range 25ppm to 1000ppm. It can not be used for measuring turbidity of drinking water.


Baylis turbidity meter


We compare two samples , a sample of known turbidity is used as a reference ,blue cobalt glass plates are used in the apparatus. It can be used for measuring turbidity in drinking water. Modern turbidity meters measure the scattered light.


Modern method


In modern methods light is measured at 90 deg or scattered light is measured . This can measure very low turbidity ranges. These are called nephelometers and measured in NTU. Sometimes formazin polymer is used when the unit changes to FTU. As per is code turbidity should be less than 1 NTU.

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