In the application of ultrafiltration membrane in pure water treatment, it can be used as a pre-treatment of the process or as a deep treatment of the process. In widely used water treatment processes, it is often used as a means of deep purification. According to the characteristics of hollow fiber ultrafiltration membranes, there are certain pre-treatment requirements for water supply. Because suspended solids, colloids, microorganisms, and other impurities in water can attach to the membrane surface, causing membrane fouling. Due to the high water flux of the ultrafiltration membrane, the concentration of intercepted impurities on the membrane surface rapidly increases, resulting in the so-called concentration polarization phenomenon. More seriously, some very small particles will enter the membrane pores and block the water channel. In addition, microorganisms and their metabolites in water generate viscous substances that also adhere to the membrane surface. These factors can all lead to a decrease in the permeability of ultrafiltration membranes and changes in separation performance. At the same time, there are also certain requirements for the temperature, pH value, and concentration of ultrafiltration water supply. Therefore, appropriate pre-treatment and water quality adjustment must be carried out for ultrafiltration water supply to meet the water supply requirements, in order to extend the service life of the ultrafiltration membrane and reduce the cost of water treatment.
1、 Disinfection of microorganisms (bacteria, algae):
When microorganisms are present in water, some of the trapped microorganisms may adhere to the surface of the pre-treatment system, such as the medium of a multi-media filter, after entering the pre-treatment system. When adhering to the surface of the ultrafiltration membrane, growth and reproduction may completely block the micropores, or even the inner cavity of the hollow fibers. The presence of microorganisms poses a serious threat to hollow fiber ultrafiltration membranes. Attention must be paid to removing bacteria and algae and other microorganisms from the raw water. In water treatment engineering, oxidants such as NaClO and O3 are usually added with a concentration of 1-5mg/l. In addition, ultraviolet sterilization can also be used. Sterilization treatment of hollow fiber ultrafiltration membrane components in the laboratory can be carried out by circulating hydrogen peroxide (H2O2) or potassium permanganate aqueous solution for 30-60 minutes. Microbicidal treatment can only kill microorganisms, but it cannot remove microorganisms from water, only preventing their growth.
2、 Reduce inlet turbidity:
When water contains suspended solids, colloids, microorganisms, and other impurities, it can cause a certain degree of turbidity, which hinders the transmission of light. This optical effect is related to the amount, size, and shape of impurities. The turbidity of water is generally measured in terms of corrosion, and the turbidity generated by 1mg/lSiO2 is specified as 1 degree. The higher the degree, the more impurities it contains. There are different requirements for the turbidity of water supply in different fields, for example, for general domestic water, the turbidity should not exceed 5 degrees. Due to the fact that turbidity measurement involves measuring the amount of light, color, and opacity reflected by particulate matter in the water by passing light through the raw water, the size, quantity, and shape of the particles all affect the measurement. The relationship between turbidity and suspended solids is random. For particles smaller than a few microns, turbidity cannot be reflected.
3、 Removal of suspended and colloidal substances:
For particle size 5 μ For impurities above m, 5 can be selected μ Filter removal with m filtration accuracy, but for 0.3 to 5 μ The fine particles and colloids between m are difficult to remove using the conventional filtration techniques mentioned above. Although ultrafiltration has an absolute removal effect on these particles and colloids, the harm to hollow fiber ultrafiltration membranes is extremely serious. Especially colloidal particles carry charges, which are aggregates of material molecules and ions. Therefore, colloids can exist stably in water, mainly due to the mutual repulsion of colloidal particles with isotropic charges. Add a charged substance (flocculant) with opposite electrical properties to the colloidal particles in the raw water to break the stability of the colloidal particles, neutralize the charged colloidal particles into electrical neutrality, and cause the dispersed colloidal particles to condense into large clumps. Then, it can be easily removed by filtration or sedimentation. Commonly used flocculants include inorganic electrolytes, such as aluminum sulfate, polyaluminum chloride, ferrous sulfate, and iron chloride. Organic flocculants such as polyacrylamide, sodium polyacrylate, and polyethylene imine. Due to the ability of organic flocculants, high molecular polymers can neutralize the surface charges of colloidal particles, form hydrogen bonds, and "bridge" to complete coagulation and sedimentation in a short period of time, thereby greatly improving water quality. Therefore, in recent years, there has been a trend for high molecular flocculants to replace inorganic flocculants.
At the same time as adding flocculants, coagulants such as pH regulator lime, sodium carbonate, oxidant chlorine and bleaching powder, reinforcing agent water, and adsorbent polypropylene amide can be added to improve the coagulation effect.
4、 Removal of soluble organic matter:
Soluble organic matter cannot be completely removed through flocculation sedimentation, multi media filtration, and ultrafiltration. Currently, oxidation or adsorption methods are commonly used.
(1) The oxidation method utilizes chlorine or sodium hypochlorite (NaClO) for oxidation, which has a good effect on removing soluble organic matter. In addition, ozone (O3) and potassium permanganate (KMnO4) are also good oxidants, but the cost is slightly higher.
(2) The adsorption method utilizes activated carbon or macroporous adsorption resin to effectively remove soluble organic matter. However, for alcohols, phenols, etc. that are difficult to adsorb, oxidation method still needs to be used for treatment.
