What is Turbidity?

Turbidity is usually suitable for water quality determination of natural water, drinking water, and some industrial water. The water sample for measuring turbidity should be tested as soon as possible, or it must be refrigerated at 4°C and tested within 24 hours. Before the measurement, the water sample should be shaken vigorously and returned to room temperature.

Suspended solids and colloids such as soil, silt, fine organic matter, inorganic matter, and plankton in water can make the water turbid and present certain turbidity. According to the water quality analysis, the turbidity formed by 1 mg SiO2 in 1L water is one Standard turbidity unit, referred to as 1 degree. Generally, the higher the turbidity, the cloudier the solution.

Because the water contains suspended and colloidal particles, the original colorless and transparent water are turbid, and the degree of turbidity is called turbidity. The unit of turbidity is expressed by "degree", which is equivalent to 1 mg. of SiO2 (or non-straight mg kaolin, diatomaceous earth) in 1 L of water, and the degree of turbidity produced is 1 degree or Jackson. The turbidity unit is JTU, 1JTU=1mg/L kaolin suspension. The turbidity displayed by modern instruments is the scattered turbidity unit NTU, also known as TU. 1NTU=1JTU. Recently, it is believed internationally that the reproducibility of the turbidity standard prepared with urotropine-hydrazine sulfate is better, and it is selected as the unified standard FTU of various countries. 1FTU=1JTU. 

Turbidity is an optical effect, which is the degree to which light is hindered when passing through a water layer, indicating the ability of the water layer to scatter and absorb light. It is not only related to the content of suspended solids but also related to the composition, particle size, shape, and reflective properties of the surface of impurities in water. Turbidity control is an important part of industrial water treatment and an important water quality indicator. According to different uses of water, there are different requirements for turbidity. 

The turbidity of drinking water should not exceed 1NTU; the turbidity of supplementary water for circulating cooling water treatment is required to be 2 to 5 degrees; the influent water (raw water) for desalinated water treatment is turbid The degree of turbidity should be less than 3 degrees; the manufacture of man-made fibers requires that the turbidity of water should be less than 0.3 degrees. Since the suspended and colloidal particles that constitute turbidity are generally stable and mostly negatively charged, they will not settle without chemical treatment. In industrial water treatment, the methods of coagulation, clarification, and filtration are mainly used to reduce the turbidity of water.

What are the units of turbidity?

Turbidity unit

1. FTU

2. NTU

3. JTU

4. EBC

1. FTU, turbidity, that is, the degree of turbidity of the water, is caused by traces of insoluble suspended matter and colloidal substances in the water. The measurement unit used by the ISO standard is FTU (turbidity unit), and FTU is consistent with NTU (turbidity measurement unit). . EBC unit used in the winemaking industry, 1FTU=4EBC.

2. Scattering Turbidity Units (NTU)

Polymerize a certain amount of hydrazine sulfate with hexamethylenediamine to form a white polymer, which is used as a turbidity standard solution, and is compared with the turbidity of water samples under certain conditions.

3. Candela Turbidity Units (JTU)

Compare the water sample with the standard solution prepared with diatomite (or kaolin) to determine the turbidity of the water sample. The turbidity produced by 1 mg of diatomite (or kaolin) with a certain particle size in 1L of distilled water is specified It is a unit of turbidity, referred to as "degree".

The specific conversion formula is as follows:

Turbidity Unit Conversion Table

JTU(degrees) FTU(NTU) SiO2(mg/l)

JTU (degrees) 1 19 2.5

FTU(NTU) 0.053 1 0.13

SiO2 (mg/l) 0.4 7.5 1

Why measure turbidity?

Turbidity refers to the degree of obstruction of suspended matter in water when light passes through.

Suspended solids in water are generally soil, sand, fine organic and inorganic matter, plankton, microorganisms, and colloidal substances.

The turbidity of water is not only related to the content of suspended substances in the water but also related to their size, shape, and refractive index.

Turbidity is a very important wooden indicator of natural water and water use, and it is also an important sign that water may be polluted. Turbid water will hinder the transmission of light into the water body, reduce the depth of the light-transmitting layer, and affect the survival of aquatic organisms and fish. It also affects the recreational use of water bodies, such as swimming and other water sports.

In the design and operation of waterworks, the determination of turbidity is also an important parameter in the selection and design of treatment equipment and an important control index for operation and dosage.

Sewage and industrial wastewater generally do not measure turbidity, but when wastewater is treated by chemical methods.

Sometimes the measurement of turbidity is also used to control the dosage of chemical agents. Some industrial water has certain requirements for turbidity. For example, in order to prevent scaling and blockage in cooling water, the turbidity should not exceed 50-100 degrees. Water should not exceed 2 to 5 degrees, textile, bleaching, and dyeing water should not exceed 5 degrees, semiconductor integrated circuit water turbidity should be 0, etc., these are to ensure the quality of the product.

What are the methods of measuring turbidity?

Nephelometric or radiometric determination

Turbidity can be measured by the nephelometric method or scattered light method. In my country, turbidimetry is generally used for determination, and the water sample is compared with the turbidity standard solution prepared with kaolin, and the turbidity is not high, and it is stipulated that 1 mg of silicon dioxide in one liter of distilled water is a turbidity unit. For different measurement methods or different standard substances, the obtained turbidity measurement values are not necessarily consistent. The level of turbidity generally cannot directly explain the pollution degree of water quality, but the increase of turbidity caused by human life and industrial domestic sewage indicates that the water quality has deteriorated.

Nephelometer measurement

Turbidity can also be measured with a nephelometer. A nephelometer sends light through a section of the sample and measures how much light is scattered by particles in the water at a 90° angle to the incident light. This scattered light measurement method is called the scattering method. Any true turbidity must be measured this way. The turbidity meter is suitable for both field and laboratory measurements, as well as continuous monitoring around the clock. Turbidimeters can be set up to sound an alarm when measured turbidity values exceed safety standards.

The turbidity meter is made according to the principle of light scattering or transmission by turbid liquid and is generally used for continuous automatic measurement of water turbidity.

Other methods

Turbidity can also be estimated by using a colorimeter or spectrophotometer to measure the degree of attenuation of transmitted light intensity caused by the obstruction of particulate matter in the sample. However, regulatory agencies do not recognize the validity of this method, nor does it meet the American Public Health Association's definition of turbidity.

Measurements using light transmittance are susceptible to interference such as color absorption or particle absorption. Furthermore, there was no correlation between light transmittance and results measured with scattered light measurements. Nonetheless, at some point colorimeter and spectrophotometer measurements can be used to determine large changes in turbidity in water treatment systems or process control.

The SUP-PTU300 online turbidity analyzer is a patent product with independent intellectual property rights for online monitoring of drinking water quality. It has the characteristics of ultra-low turbidity detection limit, high precision measurement, long-time maintenance-free equipment, water saving, and digital output. It supports remote monitoring of the cloud platforms, mobile phone data, and RS485-Modbus communication.

SUP-PTU300 turbidity meter adopts the 90° scattering detection principle and designs a unique photoelectric receiving structure, as well as an automatic temperature and light compensation method, which significantly improves the accuracy of turbidity detection. The sensor ARM7 built-in data processor and adopts an efficient digital filtering algorithm to avoid noise interference. At the same time, it adopts standard Modbus digital signal output and 4-20 mA analog output, which is convenient for users to access the computer monitoring system.