Updated: Jul 22, 2021
Turbidity is defined as the degree to which light is scattered by particles suspended in a liquid. The measured turbidity depends on the wavelength of light and the angle at which the detector is positioned. In this post I want to explain the different turbidity units of measure: NTU, FNU, FTU, FAU, and JTU. Each one of these turbidity units are based on calibrations using the same formazin primary standards. Therefore, when a formazin standard is measured, the value for each of these units will be the same, however the value on samples may differ significantly.
NTU: Nephelometric Turbidity Unit that signifies the turbidity meter is measuring scattered light from the sample at a 90-degree angle from the incident light. NTU is most often used when referencing the USEPA Method 180.1 or Standard Methods For the Examination of Water and Wastewater.
FNU: Formazin Nephelometric Unit also signifies the instrument is measuring scattered light from the sample at a 90-degree angle from the incident light. FNU is most often used when referencing the ISO 7027 (European) turbidity method.
FTU: When formazin was initially adopted as the primary reference standard for turbidity, units of FTU or Formazin Turbidity Units were used. The units of FTU do not specify how the instrument measures the sample.
FAU: Formazin Attenuation Unit that signifies the instrument is measuring the decrease in transmitted light through the sample at an angle of 180 degrees to the incident light. This type of measurement is often made in a spectrophotometer or colorimeter and is not considered a valid turbidity measurement by most regulatory agencies.
JTU: The Jackson Turbidity Unit is a historical unit used when measurements were made visually using a Jackson Candle Turbidimeter. Water was poured into a tube until a flame underneath the tube could no longer be distinguished.
The Turbidity Meter I Use
I use a portable turbidity meter from Hach and they offer two meters that give you the choice of measuring turbidity in units of NTU or FNU. I have used both meters, and though NTU is by far the most common unit of measurement for turbidity in the United States, I actually use the meter from Hach that measures turbidity in FNU units. My reason for using the FNU meter is that I have found the LED bulb to be more "robust" or less likely to break from the constant shipping I have to do with my lab equipment as I visit different work sites. The Hach NTU meter uses a tungsten filament bulb and that bulb has broken from shipping. Since switching to the FNU - LED bulb - turbidity meter I have had no issues. Just so you know, I have shared this view with Hach and they do not agree with me that one bulb is more or less robust than the other.
The Hach handheld NTU meter is shown below, identical in every way to the FNU meter that I use. Additional images, copied from Hach's website (www.hach.com),are shown at the bottom of this post.
2100Q Portable Turbidimeter (NTU)
Accuracy: ± 2 % of reading plus stray light
Range: 0 ‒ 1,000 NTU
Light source: Tungsten filament lamp
2100Q IS Portable Turbidimeter (FNU)
Accuracy: ± 2 % of reading plus stray light
Range: 0 ‒ 1,000 FNU
Light Source: LED
Hach states that each of these portable meters require a 15 mL sample volume. Yet, if you go to their website to order replacement sample cells you will be ordering 10-mL cells!?!
Turbidity Meter with Storage Case
In the photo below you see the turbidity meter being used on top of the storage case made for this meter. I use this case and recommend it be purchased along with the meter. It's a great way to store the meter, whether your travel, go out in the field, or only use the meter in a lab environment.
So What Are Turbidity Measurements Good For?
This is certainly a key question! Because the turbidity measurement is so easy and fast, I use this parameter as a surrogate measurement for any number of things: TSS (of course), COD, effectiveness of a chemical (addition of hydrogen peroxide to a sample), etc. To be clear, in every case, these are only relative measures, comparing a control (untreated) sample to a treated sample. This means the value of the turbidity measurement, used in this way, is of limited value. I use it more as a guide in my testing than for any other reason.
Metcalf & Eddy states that, "In general, there is no relationship between turbidity and the concentration of total suspended solids in untreated wastewater. There is, however, a reasonable relationship between turbidity and total suspended solids for the settled and filtered secondary effluent from the activated sludge process. The general form of the relationship is as follows:
The specific value of the conversion factor will vary for each treatment plant, depending primarily on the operation of the biological treatment process. The conversion factors for settled secondary clarifier effluent and for secondary effluent filtered with a granular medium depth filter will typically vary from 2.3 to 2.4 and 1.3 to 1.6, respectively."
Source: Metcalf & Eddy. "Wastewater Engineering: Treatment and Reuse." Fourth Edition. New York: McGraw-Hill, 2003.