Water quality: Determination of turbidity (GB 13200-1991)” refers to the international standard ISO 7027-1984 “Water quality – Determination of turbidity”. This standard specifies two methods for determining turbidity in water. The first part is spectrophotometry, which is applicable to drinking water, natural water and high turbidity water, with a minimum detection turbidity of 3 degrees. The second part is visual turbidimetry, which is applicable to low turbidity water such as drinking water and source water, with a minimum detection turbidity of 1 degree. There should be no debris and easy-to-sink particles in the water. If the utensils used are not clean, or there are dissolved bubbles and colored substances in the water, it will interfere with the determination. At an appropriate temperature, hydrazine sulfate and hexamethylenetetramine polymerize to form a white high-molecular polymer, which is used as a turbidity standard solution and compared with the turbidity of the water sample under certain conditions.
Turbidity is usually applicable to the determination of natural water, drinking water and some industrial water quality. The water sample to be tested for turbidity should be tested as soon as possible, or must be refrigerated at 4°C and tested within 24 hours. Before testing, the water sample must be shaken vigorously and returned to room temperature.
The presence of suspended matter and colloids in water, such as mud, silt, fine organic matter, inorganic matter, plankton, etc., can make the water turbid and present a certain turbidity. In water quality analysis, it is stipulated that the turbidity formed by 1mg SiO2 in 1L of water is a standard turbidity unit, referred to as 1 degree. Generally, the higher the turbidity, the more turbid the solution.
Because the water contains suspended and colloidal particles, the originally colorless and transparent water becomes turbid. The degree of turbidity is called turbidity. The unit of turbidity is expressed in “degrees”, which is equivalent to 1L of water containing 1mg. SiO2 (or non-curved mg kaolin, diatomaceous earth), 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 turbidity standard prepared with hexamethylenetetramine-hydrazine sulfate has good reproducibility and is selected as the unified standard FTU of various countries. 1FTU=1JTU. Turbidity is an optical effect, which is the degree of obstruction of light when passing through the water layer, indicating the ability of the water layer to scatter and absorb light. It is not only related to the content of suspended matter, but also to the composition, particle size, shape and surface reflectivity of impurities in the water. Controlling turbidity is an important part of industrial water treatment and an important water quality indicator. According to the different uses of water, there are different requirements for turbidity. The turbidity of drinking water shall not exceed 1NTU; the turbidity of supplementary water for circulating cooling water treatment is required to be 2-5 degrees; the turbidity of inlet water (raw water) for desalted water treatment should be less than 3 degrees; the turbidity of water required for the manufacture of artificial fibers is less than 0.3 degrees. Since the suspended and colloidal particles that constitute turbidity are generally stable and mostly carry negative charges, they will not settle without chemical treatment. In industrial water treatment, coagulation, clarification and filtration are mainly used to reduce the turbidity of water.
One more thing to add is that as my country’s technical standards are aligned with international standards, the concept of “turbidity” and the unit of “degree” are basically no longer used in the water industry. Instead, the concept of “turbidity” and the unit of “NTU/FNU/FTU” are used instead.
Turbidimetric or scattered light method
Turbidity can be measured by turbidimetry or scattered light method. my country generally uses turbidimetry to measure turbidity. The water sample is compared with the turbidity standard solution prepared with kaolin. The turbidity is not high, and it is stipulated that one liter of distilled water contains 1 mg of silicon dioxide as one turbidity unit. The turbidity measurement values obtained by different measurement methods or different standards are not necessarily consistent. The level of turbidity generally cannot directly indicate the degree of water pollution, but the increase in turbidity caused by human and industrial sewage indicates that the water quality has deteriorated.
1. Colorimetric method. Colorimetry is one of the commonly used methods for measuring turbidity. It uses a colorimeter or spectrophotometer to determine turbidity by comparing the absorbance difference between the sample and the standard solution. This method is suitable for low turbidity samples (generally less than 100 NTU).
2. Scattering method. Scattering method is a method of determining turbidity by measuring the intensity of scattered light from particles. Common scattering methods include direct scattering method and indirect scattering method. Direct scattering method uses a light scattering instrument or scatterer to measure the intensity of scattered light. Indirect scattering method uses the relationship between the scattered light generated by particles and absorbance to obtain the turbidity value through absorbance measurement.
Turbidity can also be measured with a turbidity meter. The turbidity meter emits light, passes it through a section of the sample, and detects how much light is scattered by particles in the water from a direction 90° to the incident light. This scattered light measurement method is called scattering method. Any true turbidity must be measured in this way.
The significance of detecting turbidity:
1. In the water treatment process, measuring turbidity can help determine the purification effect. For example, during the coagulation and sedimentation process, turbidity changes can reflect the formation and removal of flocs. During the filtration process, turbidity can evaluate the removal efficiency of the filter element.
2. Control the water treatment process. Measuring turbidity can detect changes in water quality at any time, help adjust the parameters of the water treatment process, and maintain water quality within an appropriate range.
3. Predict water quality changes. By continuously detecting turbidity, the trend of water quality changes can be discovered in time, and measures can be taken in advance to prevent water quality deterioration.
Post time: Jul-18-2024
