Turbidity is a physical property of fluids that relates the transparency of the medium to the presence of suspended materials. It is caused by solid particles in suspension, such as clay and organic matter, which form colloids and interfere with the propagation of light, altering the optical properties of water. The American Public Health Association (APHA) defines Turbidity as "an expression of the optical property that causes absorption or scattering of light rather than direct transmission of light in straight lines through the sample".
Turbidity is measured in FNU (Formazine Nephelometric Unit), FTU (Formazine Turbidity Unit), NTU (Nephelometric Turbidity Unit) and others. It is the quantitative statement of this qualitative phenomenon.
History
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The first studies to measure Turbidity date back to 1900. Wipple and Jackson prepared standards with a known value in ppm of Silica. The measurement consisted of slowly adding a sample into the glass tube until the image of the candle flame, seen from the top of the tube, diffused into a uniform glow. The depth of the sample inside the tube was then read in ppm of silica and the unit was UTJ (Jackson's Turbidity Unit). |
Turbidity measurements
The complexity of the optical interactions between the incident light, the properties of the dissolved and suspended materials and the characteristics of the fluid directly influence the measurement of the Turbidity of a medium. As such, it cannot be measured directly using a single property of the fluid or the materials suspended in it, but rather evaluated by comparison with standards established by technical norms.
In 1926, formazin solutions began to be used, as they had homogeneity between the particles and were easily reproduced. Since then, various scales have been used to express Turbidity, such as:
- FTU (Formazine Turbidity Unit),
- NTU (Nephelometric Turbidity Unit),
- FAU (Formazine Attenuation Unit),
- FNU (Formazine Nephelometric Unit),
- UT (Turbidity Unit).
There are other units for measuring Turbidity, but NTU is currently the most widely used.
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| Formazine standards |
Nephelometry
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Turbidity analysis is an optical measure of the scattering of light by materials in suspension. When light passes through a sample, particles in the path alter the direction of the light, causing the light beam to be scattered. The measurement of this scattering at an angle of 90° is called Nephelometry. |
The importance of turbidity analysis and its impact
Turbidity is an important indicator in the study of water quality, especially water for human consumption. An increase in Turbidity can occur due to soil erosion, commercial exploitation of clay or minerals, and the discharge of industrial and domestic effluents. This increase can cause a number of environmental problems, as the solid particles that remain on the surface absorb heat from sunlight and increase the temperature, altering the natural conditions of rivers, lakes and oceans.
It also reduces the penetration of light into the water column, making it difficult for algae and submerged vegetation to photosynthesize. Turbidity control is extremely important when it comes to water for human consumption, as suspended particulate matter provides ideal conditions for the proliferation of pathogenic micro-organisms.
Turbidity in potable water
There are ordinances and standards that regulate the procedures for controlling and monitoring the quality of water for human consumption and its standard of potability. These guidelines state the maximum amount of turbidity allowed in water to guarantee its safe consumption. In Brazil, for example, the maximum Turbidity value allowed at the outlet of Water Treatment Plant filters varies between 0.1 NTU and 0.5 NTU, depending on the technique used and the microbiological conditions of the water source.
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