Turbidity and Coagulation

Measurement of turbidity is useful in deciding whether water from a particular source can be used without chemical coagulation. Turbidity measurement, before and after coagulation by different chemicals, is used to select the most effective and economical substance and to determine its optimum dose. 

In the removal of suspended solids from domestic and industrial wastes of frequently changing composition, turbidity measurement, being much quicker than suspended solids determination, is especially useful to adjust the optimum dosages of chemicals required. 

Since turbidity is caused by a wide variety of materials, the following arbitrary standard has been chosen to compare turbidity of different waters 1 unit turbidity is the turbidity produced by 1 part of fuller s earth in a million parts of distilled water (1 unit = 1 mg Si02/1, and the size, of particles of Si02 used must lie within certain specific limits—the particles should pass through a 200-mesh sieve). 

Standard suspensions of pure Si02 were used priginally to calibrate Jackson Candle Turbidimeter, which is now used for routine work and no standards are 

The instrumental method which, instead, is based on the measurement of the intensity of light scattered by turbidity using Nephelometry, gives different results which are reported in Nephelometric Turbidity Units (NTU). 

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Pure casein solutions do not curdle on boiling. Acids do not cause complete precipitation unless the temperature is raised to the boiling point. A slight excess of calcium hydroxide also causes turbidity and coagulation on warming. The turbid accumulation dissolves again when the temperature is lowered. Concentrated solutions of calcium and casein become covered with a film on boiling just as milk does. 

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