What is ntu in turbidity?

NTU stands for nephelometric turbidity unit, which is the unit used to measure the turbidity of a fluid or the presence of suspended particles in water. The higher the concentration of suspended matter in water, the dirtier it looks and the higher the turbidity.

NTU units are most commonly used in water purification plants. We prefer to use suspended solids (SS) measured in mg/l or ppm because we operate primarily in the wastewater treatment plant market, but there is a relationship between suspended solids and NTU.

According to the World Health Organization, the turbidity of drinking water should not exceed 5 NTU.

What does NTU measure?

Turbidity describes the cloudiness or loss of clarity caused by suspended particles such as clay, silt, microorganisms, algae and other particulate matter.

A nephelometer directs light through a water sample and measures the light scattered by suspended particles. The measured response is compared with a calibration standard and reported as NTU.

The result depends not only on the amount of particles present but also on characteristics such as particle size, shape and optical properties. For this reason, two water samples with the same suspended-solids concentration may produce different NTU readings.

What is the difference between turbidity units NTU, FNU, FTU, JTU and FAU?

NTU stands for Turbidimetric Turbidity Units and indicates that the instrument is measuring the scattered light of the sample at an angle of 90 degrees to the incident light.

FNU stands for Formazin Nephelometric Units and also indicates that the instrument is measuring scattered light from a sample at an angle of 90 degrees to the incident light. fnu is most commonly used in reference to ISO 7027 (European) turbidity method.

NTU is most commonly used in reference to USEPA Method 180.1 or the standard method for water and wastewater examination.
When formazin was originally adopted as the primary reference standard for turbidity, FTU units or formazin turbidity units were used. However, these units do not specify how the instrument measures the sample.

JTU or Jackson turbidity units are the historical units used when measurements were made visually using a Jackson candle turbidimeter. Pour water into the test tube until the flame is no longer visible below the test tube.

FAU or Formazin attenuation units indicate that the instrument is measuring the reduction of transmitted light in a sample at an angle of 180 degrees to the incident light. This type of measurement is typically performed in a spectrophotometer or colorimeter and is not considered a valid turbidity measurement by most regulatory agencies.

The turbidity units NTU, FNU, FTU and FAU are based on calibrations using the same formazin primary standards. Therefore, when measuring the formazin standards, the values will be the same for each unit, but the values on the samples may vary significantly.

What does an NTU reading mean in water treatment?

In water and wastewater treatment, turbidity is useful for monitoring changes in particle loading, filtration performance and treatment-process stability.

Low turbidity is particularly important before disinfection because suspended particles can reduce the effectiveness of chlorine or UV treatment. The World Health Organization recommends keeping turbidity as low as practical to support effective treatment and disinfection.

For industrial plants and WWTPs that require continuous monitoring rather than occasional laboratory measurements, online turbidity meters and sensors can provide real-time turbidity data for process monitoring and control.

Conclusion

NTU means Nephelometric Turbidity Unit and represents turbidity measured through light scattering rather than the direct mass of suspended solids.

The most important points are simple: higher NTU generally means higher turbidity; NTU and mg/L measure different properties; and turbidity results should always be interpreted according to the measurement method and the characteristics of the water being tested.

For continuous turbidity monitoring projects, request a quotation, send wastewater data for design, or contact us for an engineering solution.