The nephelometric turbidity unit (NTU) is the standard unit for measuring turbidity in water, based on the scattering of light by suspended particles. Lower NTU values indicate clearer water.
The nephelometric turbidity unit (NTU) quantifies turbidity by measuring the intensity of light scattered at a 90-degree angle from the incident beam as it passes through a water sample. A nephelometer directs a beam of light into the sample and detects the scattered light with a photodetector positioned perpendicular to the light source. The amount of light scattered is proportional to the concentration and characteristics of suspended particles such as clay, silt, organic matter, algae, and microorganisms. Drinking water standards in the United States require that treated water from conventional or direct filtration plants not exceed 1 NTU at any time and must be 0.3 NTU or lower in at least 95 percent of daily samples each month. Raw surface waters can have turbidity values ranging from less than 1 NTU in clear mountain streams to over 1,000 NTU in flood-laden rivers. Turbidity is a critical parameter because elevated levels can interfere with disinfection by shielding pathogens from UV light and chlorine, making it a surrogate for microbiological safety. The Formazin Nephelometric Unit (FNU) is an equivalent unit used in ISO standard measurements.