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Turbidity is an important water quality indicator for almost any treatment application. Turbidity represents the presence of dispersed, suspended solids-particles not in true solution and often includes silt, clay, algae and other microorganisms, organic matter and minute particles.
Suspended solids obstruct the transmittance of light through a water sample and impart a qualitative characteristic, known as turbidity, to water. Turbidity is not a direct measure of suspended particles in water. Instead, it is a measure of the scattering effect such particles have on light.
The way in which suspended particles scatter light is very complex. Particle size, shape, and nature all effect scattering. In addition, colored solutions adsorb light and can affect turbidity. Very small particles (0.2 microns) scatter light equally in the forward and backward direction. Larger particles (1 micron) scatter light primarily in the forward direction. Forward scattering is intensified as the concentration of suspended solids increases.
Turbidity is determined electronically by the apparatus shown in Figure 1. Light from a tungsten filament (usually) passes through the sample to be measured. Light scattered at an angle of 90 degrees is measured. Ninety degrees is chosen because it is very sensitive to light scatter.
Dividing scattered light intensity by that transmitted in a forward direction compensates for the affect of sample color. An instrument of this type is known as a ratio turbidimeter.
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