What Is Turbidity? A Detailed Explanation of Wastewater Treatment Terminology
Turbidity is generally used in the water quality determination of natural water, drinking water and certain industrial waters. A water sample intended for turbidity measurement should be tested as soon as possible, or it must be refrigerated at 4 degrees Celsius and measured within 24 hours; before measurement the sample must be shaken vigorously and allowed to return to room temperature.
Suspended and colloidal matter in water, including soil, silt, fine organic matter, inorganic matter and plankton, can all make water cloudy and give it a certain turbidity. Water analysis stipulates that the turbidity produced by 1 mg of SiO2 in 1 L of water constitutes one standard turbidity unit, abbreviated as 1 degree. In general, the higher the turbidity, the cloudier the solution. [1]
Because water contains suspended and colloidal particles, water that was originally colourless and transparent becomes cloudy, and the degree of that cloudiness is called turbidity. The unit of turbidity is expressed in degrees: the cloudiness produced when 1 L of water contains 1 mg of SiO2 (or an equivalent mass of kaolin or diatomite) is defined as 1 degree, also called the Jackson unit. The turbidity unit is JTU, where 1 JTU equals a kaolin suspension of 1 mg/L. Modern instruments display turbidity in nephelometric turbidity units (NTU), also written as TU, and 1 NTU equals 1 JTU. More recently, international practice has held that a turbidity standard prepared from hexamethylenetetramine and hydrazine sulfate offers better reproducibility, and it has been adopted as the unified standard FTU across countries, where 1 FTU equals 1 JTU. Turbidity is an optical effect: it is 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 the content of suspended solids but also on the composition, particle size and shape of the impurities in the water and the reflective properties of their surfaces. Controlling turbidity is an important part of industrial water treatment and an important water quality index. Different water uses carry different turbidity requirements: the turbidity of 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 (raw water) for demineralisation treatment should be below 3 degrees; and the manufacture of synthetic fibres requires water turbidity below 0.3 degrees. Because the suspended and colloidal particles that create turbidity are generally stable and mostly carry a negative charge, they will not settle without chemical treatment. In industrial water treatment, coagulation, clarification and filtration are the principal methods used to reduce water turbidity. [2]
One further note: as China's technical standards have converged with international standards, the water industry has essentially abandoned the concept of cloudiness and the unit of degrees in favour of the concept of turbidity and the units NTU, FNU and FTU. Since waterworks long followed the older standard, however, do not be surprised if engineers and technicians at some older waterworks still speak in degrees. [3]
FTU(NTU) 0.053 1 0.13
Turbidity can also be measured with a turbidimeter. The turbidimeter emits a beam of light, passes it through a section of the sample, and detects how much light is scattered by the particles in the water at 90 degrees to the incident beam. This scattered-light measurement approach is called nephelometry, and any true turbidity must be measured in this way. Turbidimeters are suitable for both field and laboratory measurement as well as for round-the-clock continuous monitoring. A turbidimeter can be configured to raise an alarm when the measured turbidity exceeds the safety standard. [5]
The turbidimeter is built on the principle that a turbid liquid scatters or transmits light, and it is generally used for continuous automatic determination of turbidity in water bodies.
Turbidity can also be estimated by using a colorimeter or spectrophotometer to measure how far the transmitted light intensity is attenuated by the obstruction of particles in the sample. Regulatory bodies, however, do not recognise the validity of this method, and it does not conform to the American Public Health Association's definition of turbidity.
Measurement based on transmittance is readily disturbed by interference such as colour absorption or particulate absorption. Moreover, there is no correlation between transmittance results and results obtained by scattered-light measurement. Even so, colorimeter and spectrophotometer readings can at times be used within a water treatment system or process control to detect large changes in turbidity. [5]
1. FTU. Turbidity is the degree of cloudiness of water, caused by trace amounts of insoluble suspended matter and colloidal matter in the water. The measurement unit used in the ISO standard is FTU (formazin turbidity unit), and FTU is consistent with NTU (nephelometric turbidity unit). The brewing industry uses the EBC unit, where 1 EBC is approximately 4 FTU.
2. Nephelometric turbidity unit (NTU)
Spectrophotometry is suitable for measuring natural water, drinking water and high-turbidity water, with a minimum detectable turbidity of 3 degrees. The turbidity measured is expressed in NTU. [6]
A defined quantity of hydrazine sulfate is polymerised with hexamethylenetetramine to form a white polymer, which serves as the turbidity standard solution and is compared with the turbidity of the water sample under defined conditions. [7]
3. Jackson turbidity unit (JTU)
The turbidity of a water sample determined by visual comparison is expressed in JTU. Visual comparison is suitable for low-turbidity water such as drinking water and source water, with a minimum detectable turbidity of 1 degree. [6]
The water sample is compared with a standard solution prepared from diatomite (or kaolin) to determine its turbidity. The turbidity produced by 1 mg of diatomite (or kaolin) of a defined particle size in 1 L of distilled water is defined as one turbidity unit, abbreviated as a degree. [6]
JTU (degree) FTU (NTU) SiO2 (mg/l)
JTU (degree) 1 19 2.5
Turbidity can be determined by comparison methods or by scattered-light methods. China generally uses the comparison method, in which the water sample is compared against a turbidity standard solution prepared from kaolin; precision is not high, and one turbidity unit is defined as 1 mg of silicon dioxide per litre of distilled water. Different measurement methods or different standard substances do not necessarily yield identical turbidity values. The level of turbidity alone cannot directly indicate how polluted water is, but a rise in turbidity caused by domestic and industrial wastewater does indicate deteriorating water quality. [4]
SiO2(mg/l) 0.4 7.5 1
