How to measure turbidity of water

Turbidity, which is due to the presence of suspended matter in a finely divided state, can be interpreted as a lack of clearness or brilliance in a water. Turbidity should not be confused with color since a water may have a dark color but still be clear and not turbid. Clay, silt finely divided organic and inorganic matter, and microscopic organisms can all cause turbidity.

Turbidity is a measure of relative light transmission through a sample. Light not transmitted may  be either obstructed or reflected by the particles in the sample. Although closely allied with suspended matter, which is the absolute quantity of matter in a water sample that can be removed by filtration, turbidity is not synonymous with suspended matter.

The turbidity of industrial water, especially boiler and cooling system feedwaters, should be as low as possible. The small particles in suspension will concentrate in the boiler water and may settle out in the form of a heavy sludge or “mud”. Turbidity in boiler feedwater also limits the cycles of concentration that can be carried in the boiler. Finely divided particles can be responsible for a foaming and priming condition in the boiler.

Turbid makeup water to cooling water systems may cause plugging and overheating where solids settle out on heat exchange surfaces. Corrosive action may also be increased under such deposits which hinder penetration of corrosion inhibitors.

Although turbidity may be partially removed by settling, it is usually necessary to use coagulation and filtration methods to eliminate it from industrial water.

Theory of Test

Turbidity may be measured with the use of either a photometer or nephelometer. Units of turbidity are based upon comparison of the turbidity of the sample to the turbidity of standards. Two common units in use for turbidity are FTU (Formazin Turbidity Unit) and NTU (Nephelometer Turbidity Unit). A photometer measures the obstruction of light passed through the sample, usually before and after filtration. A nephelometer measures the intensity of light scattered by the particles 90o to the incident light. In either case, a working curve of instrument response versus standard turbidity is obtained, or the instrument itself is calibrated on a direct reading scale. The turbidity of the sample is then measured.