Application of ultrafiltration membrane in water treatment

1. Pre-treatment

Ultrafiltration method can be used as the pretreatment of the process and the advanced treatment of the process in the water treatment and other industrial purification, concentration and separation processes. In the widely used water treatment process, it is often used as a means of deep purification. According to the characteristics of hollow fiber ultrafiltration membranes, there are certain requirements for water supply pretreatment. Because suspended solids, colloids, microorganisms and other impurities in the water will adhere to the surface of the membrane, the membrane will be contaminated. Due to the relatively large water flux of the ultrafiltration membrane, the concentration of the trapped impurities on the membrane surface increases rapidly to produce the so-called concentration polarization phenomenon. What is more serious is that some very small particles will enter the membrane pores and block the water channels. In addition, the viscous substances produced by microorganisms and their metabolites in the water will also adhere to the membrane surface. These factors will lead to the decrease of ultrafiltration membrane water permeability and the change of separation performance. At the same time, there are also certain limits on the temperature, pH and concentration of the ultrafiltration water supply. Therefore, proper pretreatment and adjustment of water quality must be carried out for the ultrafiltration water supply to meet the water supply requirements, so as to extend the service life of the ultrafiltration membrane and reduce the cost of water treatment.

1. Killing of microorganisms (bacteria, algae):

When the water contains microorganisms, after entering the pre-treatment system, some of the trapped microorganisms may adhere to the pre-treatment system, such as the surface of the media of a multi-media filter. When it adheres to the surface of the ultrafiltration membrane, it grows and reproduces, which may completely block the micropores or even completely block the cavity of the hollow fiber. The existence of microorganisms is extremely harmful to the hollow fiber ultrafiltration membrane. Attention must be paid to the removal of bacteria and algae in raw water. In water treatment projects, oxidants such as NaClO and O3 are usually added, and the concentration is generally 1 to 5 mg/l. In addition, UV sterilization can also be used. The hollow fiber ultrafiltration membrane module is sterilized in the laboratory, and it can be treated with hydrogen peroxide (H2O2) or potassium permanganate aqueous solution for 30 to 60 minutes. The anti-microbial treatment can only kill microorganisms, but it cannot remove microorganisms from the water, but only prevents the growth of microorganisms.

2. Reduce influent turbidity:

When the water contains suspended matter, colloids, microorganisms and other impurities, it will cause the water to produce a certain degree of turbidity. The turbidity will hinder the transmission of light. This optical effect is related to the amount, size and shape of the impurities. The turbidity of water is measured by corrosion degree, and the turbidity produced by 1mg/lSiO2 is 1 degree. The higher the degree, the more impurity content. There are different requirements for water supply turbidity in different fields. For example, for general domestic water, the turbidity should not be greater than 5 degrees. Since turbidity is measured by passing light through raw water to measure the amount of light reflected by particles in the water, its color, and opacity, the size, number and shape of particles all affect the measurement, and the relationship between turbidity and suspended solids is random. For particles smaller than a few microns, turbidity cannot be reflected.

In the membrane process, the precise microstructure traps molecular or even ion-level particles, and it is obviously imprecise to use turbidity to reflect water quality. In order to predict the tendency of raw water pollution, an SDI value test was developed.

SDI value is mainly used to detect the amount of particles such as colloids and suspended solids in the water, and is an important indicator of the water quality of the system. The method for determining the SDI value is generally to use a 0.45μm microporous filter membrane under a constant water pressure of 0.21MPa hydraulic pressure, first record the time t0 required to filter 500ml of water through the water, and then continue to pass water for 15min under the same conditions , Again record the time t15 required to filter 500ml water sample, and then calculate it according to the following formula:

SDI=(1-t0/t15)×100/15

The value of SDI in water roughly reflects the degree of colloidal pollution. The SDI of the well water is less than 3, the SDI of the surface water is above 5, and the SDI limit value is 6.66…, which requires pretreatment.

Ultrafiltration technology is the most effective in reducing the SDI value. The SDI of the water treated by the hollow fiber ultrafiltration membrane is 0, but when the SDI is too large, especially the larger particles will seriously pollute the hollow fiber ultrafiltration membrane. In the process, pretreatment must be carried out, that is, quartz sand, activated carbon or filters equipped with various filter materials are used for filtration. As for the treatment process, there is no fixed mode. This is because the source of water supply is different, so the pretreatment method is also different. Vary.

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