The presence of suspended material such as finely divided organic material, clay, silt, and other inorganic material in water is known as turbidity. Turbidity is tested by measuring the amount of light scattered by particles in the water. As the number of particles increases, more light is scattered and a higher turbidity reading is obtained. The measuring instrument is called a nephelometer, and the readings are expressed as nephelometric turbidity units (NTU) or turbidity units. Turbidity in excess of 5 NTU is easily detected in a glass of water and is usually objectionable for aesthetic reasons.

 Excessive turbidity is a problem for several reasons:

1. It protects microorganisms from chlorine and other disinfectants;

2. It acts as a food source for microorganisms, allowing them to survive and multiply;

3. It interferes with the maintenance of a chlorine residual;

4. It interferes with the test for coliform bacteria.

Clay or other inert suspended particles in drinking water drawn from groundwater sources may not adversely affect health, but water containing such particles may require treatment to make it aesthetically suitable for its intended use. Following a rainfall, variations in groundwater turbidity may be considered an indication of surface or other introduced pollution. Excessive turbidity must be removed by filtration.