Which of the following units is used for measuring the turbidity of water?

(a) EAU Formazin Attenuation Units

(b) FNU- Formazin Nephelometric Unit

(c) NTU – Nephelometric Turbidity Units

(d) All of the above

(d) All of the above

All the units listed—EAU, FNU, and NTU—are used for measuring the turbidity of water, though they are applied in slightly different contexts. Turbidity is the measure of the cloudiness or haziness of a fluid caused by suspended solid particles.¹

• NTU (Nephelometric Turbidity Units): This is the most widely used unit for turbidity. It measures the amount of light scattered at a 90-degree angle from a light beam passed through the water sample.²
• FNU (Formazin Nephelometric Unit): This is essentially the same as NTU in measurement principle (light scattering at 90 degrees) but is the unit specified by European (ISO 7027) standards, which use an infrared light source. NTU is the unit used by the US EPA, which uses a white light source.³
• EAU (Formazin Attenuation Units): This unit measures the attenuation (or decrease) of a light beam as it passes straight through a water sample. It is less common than NTU or FNU for general water quality but is used in some applications.