Turbidity
Turbidity refers to the degree of obstruction caused by a solution to the passage of light, including the scattering of light by suspended solids and the absorption of light by solute molecules. The turbidity of water is not only related to the content of suspended substances in the water, but also to their size, shape, and refractive index.
Turbidity unit
1. FTU, turbidity, refers to the degree of turbidity in water, caused by the presence of trace amounts of insoluble suspended substances and colloidal substances in the water. The measurement unit used in ISO standards is FTU (turbidity unit), which is consistent with NTU (turbidity measurement unit). The alcohol industry uses EBC units, with 1EBC ≈ 4FTU.
2. Scattered turbidity unit (NTU)
The spectrophotometric method is suitable for measuring natural water, drinking water, and high turbidity water, with a minimum detection turbidity of 3 degrees. The measured turbidity unit is NTU.
A certain amount of Hydrazine sulfate is polymerized with hexamethylene amine to form a white high molecular polymer, which is used as the turbidity Standard solution and compared with the turbidity of water sample under certain conditions.
3. Candlelight turbidity unit (JTU)
The turbidity unit of water samples measured by visual turbidimetry is JTU. Visual turbidimetry is suitable for low turbidity water such as drinking water and source water, with a minimum detection turbidity of 1 degree.
Compare the water sample with a standard solution prepared with diatomaceous earth (or white clay) to determine the turbidity of the water sample. It is specified that the turbidity generated by 1L distilled water containing 1mg of diatomaceous earth (or white clay) with a certain particle size is a turbidity unit, abbreviated as “degree”.
The specific conversion table is as follows:
Turbidity Unit Conversion Table
JTU (degree) FTU (NTU) SiO2 (mg/l)
JTU (degree) 1 19 2.5
FTU (NTU) 0.053 1 0.13
SiO2 (mg/l) 0.4 7.5 1
Determination method
Turbidimetric or radiometric determination
Turbidity can be measured by turbidimetry or scattered light method. In China, turbidimetric method is generally used to measure the turbidity of water sample and the turbidity Standard solution prepared with kaolin. The turbidity unit is 1 mg silica in 1 liter of distilled water. The turbidity measurement values obtained may not necessarily be consistent for different measurement methods or standards used. The level of turbidity generally cannot directly indicate the degree of pollution in water quality, but the increase in turbidity caused by human and industrial sewage indicates that the water quality has deteriorated.
Turbidity meter measurement
Turbidity can also be measured using a turbidity meter. The turbidimeter emits light that passes through a section of the sample and detects how much light is scattered by particles in the water from a 90 ° angle to the incident light. This method of measuring scattered light is called scattering method. Any true turbidity must be measured in this way. Turbidity meters are suitable for both field and laboratory measurements, as well as continuous monitoring 24/7. A turbidity meter can be set to emit an alarm when the measured turbidity value exceeds safety standards.
Turbidity meters are made based on the principle of scattering or transmitting light from turbid liquids, and are generally used for continuous and automatic measurement of water turbidity.
Other methods
Turbidity can also be estimated by measuring the degree of attenuation of transmitted light intensity caused by the obstruction of particles in the sample using a colorimeter or spectrophotometer. However, regulatory agencies do not recognize the effectiveness of this method, and it does not meet the definition of turbidity by the American Public Health Association.
The use of transmittance measurement is susceptible to interference such as color absorption or particle absorption. Moreover, there is no correlation between the transmittance and the results measured using scattered light measurement. However, at certain times, the measurement results of colorimeters and spectrophotometers can be used to measure significant changes in turbidity in water treatment systems or process control.