Turbidity describes the scattering of light by particles in water and is a key quality parameter in water treatment.

It is usually measured in NTU (Nephelometric Turbidity Units), FTU (Formazin Turbidity Units) or FNU (Formazin Nephelometric Units).

 All are based on light scattering on formazin standard solutions, but differ depending on the measurement method and standard:

NTU is used according to US standards (EPA), FNU according to ISO 7027 with infrared light, FTU is a more general, less commonly used term.

Turbidity measurement is crucial in drinking water treatment, for monitoring filtration and flocculation processes, as well as in wastewater treatment and environmental monitoring. It provides indications of suspended solids, microorganisms or organic contamination and helps to ensure the effectiveness of disinfection processes.