Process of ultrafiltration membrane in water treatment application

Pre-treatment

The ultrafiltration method can be used as the pretreatment of the process in the water treatment and other industrial purification, concentration and separation processes, and it can also be used as the advanced treatment of the process. 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 attach 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, resulting in 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 surface of the membrane. These factors will lead to a decrease in the water permeability of the ultrafiltration membrane and a change in the 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 media surface of a multi-media filter. When adhering 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 removing bacteria and algae in raw water. In water treatment projects, oxidants such as NaClO and O 3 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 (H 2 O 2) or potassium permanganate aqueous solution for 30 to 60 minutes. The microbe-killing treatment can only kill microbes, but it cannot remove microbes from the water, but only prevents the growth of microbes.

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/lSiO 2 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 the raw water to measure the amount of light reflected by the particles in the water, the color and opacity, the size, number and shape of the 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 membrane treatment, 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.
The 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 to characterize the quality of the influent water of the system. The method for determining the SDI value is generally to use a 0.45μm microporous filter membrane under a constant flow pressure of 0.21MPa hydraulic pressure, first record the time t0 required to filter the 500ml water sample after passing water, and then continue to pass the water under the same conditions 15min, again record the time t15 required to filter the 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 limit value of the SDI is 6.66…, that is, pretreatment is required.
Ultrafiltration technology is the most effective for 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, filtration with quartz sand, activated carbon or a filter equipped with a variety of filter materials. 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.
For example, for tap water or groundwater with lower turbidity, a precision filter (such as honeycomb type, melt blown type and PE sintered pipe, etc.) with a thickness of 5 to 10 μm can generally be reduced to about 5. Before the precision filter, it is necessary to add flocculant and place a double-layer or multi-layer media filter to filter. Generally, the filtration speed should not exceed 10m/h, preferably 7 ~ 8m/h, the slower the filtration speed , The better the quality of filtered water.

3. Removal of suspended solids and colloidal substances:

For impurities with a particle size of 5 μm or more, a filter with a filtration accuracy of 5 μm can be used to remove, but for fine particles and colloids between 0.3 to 5 μm, it is difficult to remove them using the above-mentioned conventional filtration technology. Although ultrafiltration can absolutely remove these particles and colloids, it is extremely harmful to hollow fiber ultrafiltration membranes. In particular, colloidal particles are charged, which is a polymer of material molecules and ions, so colloids can exist stably in water, mainly because colloidal particles of the same charge repel each other. Add a charged substance (flocculant) that has the opposite electrical properties of the colloidal particles to the raw water to break the stability of the colloidal particles, neutralize the charged colloidal particles to neutrality, and make the dispersed colloidal particles agglomerate into large agglomerates , And then can be easily removed by filtration or sedimentation. Commonly used flocculants are inorganic electrolytes, such as aluminum sulfate, polyaluminum chloride, ferrous sulfate and ferric chloride. Organic flocculants such as polyacrylamide, sodium polyacrylate, polyethyleneimine, etc. Since the organic flocculant polymer can neutralize the surface charge of the colloidal particles, form hydrogen bonds and “bridging”, the coagulation and sedimentation can be completed in a short time, thereby greatly improving the water quality, so in recent years, polymer flocculants have replaced The trend of inorganic flocculants.
When the flocculant is added, coagulant aids can be added, such as PH regulator lime, sodium carbonate, oxidizer chlorine and bleaching powder, reinforcing agent underwater class and adsorbent polyacrylamide, etc., to improve the coagulation effect.
The flocculant is often formulated into an aqueous solution and added by a metering pump, or it can be directly fed into the water treatment system using an ejector installed on the water supply pipeline.

4. Removal of soluble organic matter:

Soluble organic matter cannot be completely removed by flocculation sedimentation, multi-media filtration and ultrafiltration. At present, the oxidation method or the absorption method is mostly used.
(1) The oxidation method uses chlorine or sodium hypochlorite (NaClO) for oxidation, which has a better effect on removing soluble organic matter. In addition, ozone (O 3) and potassium permanganate (KMnO 4) are also better oxidants, but the cost is slightly higher.
(2) Adsorption method The use of activated carbon or macroporous adsorption resin can effectively remove soluble organic matter. However, oxidation treatment is still needed for alcohols and phenols that are difficult to adsorb.

5. Water quality adjustment:

(1) Adjustment of water supply temperature
The water permeability of the ultrafiltration membrane is directly related to the temperature. The calibrated water permeability of the ultrafiltration membrane module is generally tested with pure water at 25 ℃. The water permeability of the ultrafiltration membrane is proportional to the temperature, and the temperature coefficient is about It is 0.02/1 ℃, that is, for every 1 ℃ increase in temperature, the water permeation rate will increase by approximately 2.0%. Therefore, when the temperature of the water supply is low (such as <5 ℃), some kind of heating measures can be adopted to make it run at a higher temperature to improve work efficiency. However, when the temperature is too high, it is also unfavorable to the membrane and will cause changes in membrane performance. For this, cooling measures can be used to reduce the temperature of the water supply.
(2) Adjustment of PH value of water supply
Ultrafiltration membranes made of different materials have different adaptation ranges for pH. For example, cellulose acetate is suitable for membranes such as pH=4~6, PAN and PVDF membranes, and can be used in the range of pH=2~12. The scope of application needs to be adjusted. The commonly used PH regulators currently mainly include acids (HCl and H 2 SO 4) and alkalis (NaOH, etc.).
Since inorganic salts in the solution can pass through the ultrafiltration membrane, there is no problem of inorganic salt concentration polarization and fouling. Therefore, their influence on the membrane is generally not considered in the pretreatment water quality adjustment process, and the focus is on the colloidal layer. The problem of the generation, membrane fouling and clogging.

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