Turbidity

What is turbidity?

Turbidity describes how cloudy water looks. Fine particles suspended in water scatter light, making the water hazy or muddy. The more particles, and the finer they are, the higher the turbidity. It is a measure of water clarity, not of how much the particles weigh: two samples with the same mass of suspended solids can have very different turbidity depending on particle size, shape and color.

How turbidity is measured

Turbidity is usually measured with a nephelometer, which shines a light through the sample and measures the light scattered at an angle, typically 90 degrees. Results are reported in nephelometric turbidity units (NTU).

Why turbidity matters

• Drinking water. Turbidity can shield microorganisms from disinfection and indicates how well treatment is working. EPA rules require treated water from conventional filtration plants to meet strict turbidity limits, such as 0.3 NTU or less in at least 95 percent of monthly samples.
• Industrial water. Suspended solids contribute to fouling and wear.
• Mining and mineral processing. The turbidity of thickener overflow shows how well fine particles are being settled and whether recycled water is clean enough for reuse. High turbidity often points to poor flocculation or problem clays.
• Discharge. Permits often limit the turbidity or suspended solids of water released to the environment.

Frequently asked questions