turbidity

Turbidity is the cloudiness or haziness of a liquid caused by the presence of suspended solids or colloidal particles that scatter and absorb light. It is a measure of how opaque a sample is and is commonly used as an indicator of water quality, though it is not a direct measure of chemical or biological contamination.

Measurement and units are typically based on light scattering. Turbidity is commonly reported in nephelometric turbidity units (NTU) or Formazin nephelometric units (FNU), determined by nephelometry, which gauges the amount of light scattered at a 90-degree angle from a light source. Calibration usually uses Formazin standards.

Causes include inorganic particles such as clay, silt, and sediment, organic matter, algae, plankton, microorganisms, and industrial pollutants. Turbidity can result from natural processes like erosion and runoff, or from disturbances such as dredging, storm events, or resuspension of sediments.

Implications vary by context. In environmental water, higher turbidity reduces light penetration, affects photosynthesis in aquatic plants, and can alter habitat. In drinking water, turbidity can indicate the presence of particulate-associated microorganisms and can interfere with disinfection, protecting microbes from disinfectants. It is also used as an indicator of treatment performance and source-water quality.

Control and monitoring often involve pretreatment and treatment steps such as coagulation, flocculation, sedimentation, and filtration, along with source-water protection. Regulatory guidelines generally favor very low turbidity in finished drinking water, with village, national, or international standards setting target values (often around 1 NTU or lower), though actual limits vary by jurisdiction. Turbidity is typically measured online or in routine laboratory analyses to assess water quality and treatment effectiveness.