Turbidity Units Explained: JTU, NTU, FTU, FNU and EBC, and Why Plants Still Say Degrees

Turbidity is measured on natural water, drinking water and a range of industrial waters, and it is one of the few water quality parameters defined by comparison rather than by mass. A sample intended for turbidity measurement should be tested as soon as possible; if it cannot be, it needs refrigeration at 4 degrees Celsius and measurement within 24 hours, and it must be shaken vigorously and brought back to room temperature before reading.

What Turbidity Actually Is

Suspended and colloidal matter - soil, silt, fine organic and inorganic matter, plankton - makes water cloudy. That cloudiness is turbidity. It is an optical effect: the degree to which light is obstructed as it passes through a layer of water, expressing that layer's capacity to scatter and absorb light. It depends not only on how much suspended solids are present but also on their composition, particle size and shape, and on the reflective properties of their surfaces.

Water analysis defines one standard turbidity unit as the turbidity produced by 1 mg of SiO2 in 1 L of water, written as 1 degree. The cloudiness produced when 1 L of water contains 1 mg of SiO2, or an equivalent mass of kaolin or diatomite, is also called the Jackson unit. As a rule, the higher the turbidity, the cloudier the solution.

Four Units, One Quantity

The Jackson turbidity unit, JTU, is based on visual comparison against a standard prepared from diatomite or kaolin of defined particle size in distilled water. 1 JTU equals a kaolin suspension of 1 mg/L. Visual comparison suits low-turbidity water such as drinking water and source water, with a minimum detectable turbidity of 1 degree.

Modern instruments report in nephelometric turbidity units, NTU, also written TU, and 1 NTU equals 1 JTU. Spectrophotometry suits natural water, drinking water and high-turbidity water, with a minimum detectable turbidity of 3 degrees.

More recently, international practice settled on a standard prepared by polymerising a defined quantity of hydrazine sulfate with hexamethylenetetramine to form a white polymer. That standard reproduces better than kaolin and has been adopted across countries as FTU, where 1 FTU equals 1 JTU. The ISO standard uses FTU (formazin turbidity unit), and FTU is consistent with NTU. The brewing industry uses the EBC unit, where 1 EBC is approximately 4 FTU.

How It Is Measured

A turbidimeter emits a beam of light through a section of the sample and detects how much light the particles scatter at 90 degrees to the incident beam. That scattered-light approach is called nephelometry, and any true turbidity measurement must be made this way. Turbidimeters work in the field and the laboratory and can run round-the-clock continuous monitoring; they can also be configured to raise an alarm when measured turbidity exceeds the safety standard.

Turbidity can also be estimated with a colorimeter or spectrophotometer by measuring how far transmitted light intensity is attenuated by the particles in the sample. Regulators do not recognise this method - it does not conform to the American Public Health Association definition of turbidity. Transmittance readings are readily disturbed by colour absorption or particulate absorption, and there is no correlation between transmittance results and scattered-light results. Even so, colorimeter and spectrophotometer readings can serve inside a treatment system or process control to detect large changes in turbidity.

Different Uses, Different Limits

Controlling turbidity is an important part of industrial water treatment and an important water quality index, and the requirement depends entirely on the use. Drinking water must not exceed 1 NTU. Make-up water for circulating cooling water treatment is required to be 2 to 5 degrees. Feed water for demineralisation treatment should be below 3 degrees. Manufacture of synthetic fibres requires water turbidity below 0.3 degrees.

The practical difficulty is that the suspended and colloidal particles responsible for turbidity are generally stable and mostly carry a negative charge, so they will not settle without chemical treatment. In industrial water treatment, coagulation, clarification and filtration are the principal methods used to reduce it.

Two Things Worth Remembering

First, turbidity level alone does not directly indicate how polluted water is, but a rise in turbidity caused by domestic and industrial wastewater does indicate deteriorating water quality. Different measurement methods or different standard substances do not necessarily yield identical turbidity values, so comparing numbers across instruments requires care.

Second, as technical standards converged internationally, the water industry largely abandoned the concept of cloudiness and the unit of degrees in favour of turbidity and the units NTU, FNU and FTU. Because waterworks followed the older standard for a long time, engineers and technicians at some older plants still speak in degrees. That is a habit, not an error.

For anyone specifying treatment equipment, the practical takeaway is that the unit quoted on a datasheet tells you which method produced it. A figure in JTU came from visual comparison, one in NTU or FTU from scattered light, and one in EBC from a brewing standard roughly four times coarser. Comparing them without converting first is the most common source of confusion in turbidity specifications.