Ultrafiltration technology is a kind of membrane separation technology that uses the pressure difference as the driving force, and utilizes the permeability of the membrane to separate ions, molecules and certain particles in water. The pore size of ultrafiltration membrane for water treatment ranges from 0.005 to 1 micron, filling the gap between microfiltration and nanofiltration.
Scholars at home and abroad have proposed that the ultrafiltration process actually exists in three aspects at the same time:
1. Solute adsorption on the membrane surface and within the microporous wall.
2. The particle size of the solute is similar to the pore size of the membrane, and the solute stays in the pore, causing clogging.
3. The particle size of the solute is larger than the pore size of the membrane, the solute is mechanically retained on the surface of the membrane to realize sieving.
Ultrafiltration process generally has two ways:Terminal filtration and wrong flow filtration. For low turbidity, good water quality raw water, generally use terminal filtration, which can greatly reduce the energy consumption of the process; for higher turbidity, pollution is more serious water, the use of staggered flow filtration, so as to avoid a large number of pollutants accumulated on the surface of the membrane, resulting in membrane contamination, reducing the filtration performance.
Ultrafiltration membrane structure and type of morphology
The cross-section of ultrafiltration membrane has an asymmetric structure. It generally consists of a dense layer with a thickness of <1 micron, which plays the role of sieving, and a support layer with a larger thickness (usually 125 microns) and a spongy or finger-like porous structure. At present, has been commonly used in industrial production and life of the membrane module: tube, plate and frame, roll and hollow fiber type and so on several kinds. Hollow fiber membrane and internal pressure membrane (dense layer within), external pressure membrane (dense layer outside) and two-way membrane (both inside and outside the dense layer) three structures. In general, there are still fewer varieties of membranes, wider distribution of membrane pore size and unstable performance and other defects.
Ultrafiltration membrane removal effect on organic matter and the factors affecting
Ultrafiltration membrane molecular weight cut-off range is generally 5000 ~ 10000ODalton, a large portion of the molecular weight of dissolved organic matter in natural water is lower than the range, resulting in the ultrafiltration membrane on the interception of its effect is very poor. In fact, the proportion of low molecular weight dissolved organic matter in natural water is often larger. The removal of organic matter by ultrafiltration membranes varies greatly from case to case. Some scholars cut molecular weight of 100,000 Dalton hollow fiber ultrafiltration membrane filtration of 20 different kinds of raw water, TOC removal rate of 18% on average, the average removal rate of UV25 for 28%. Also to remove TOC from water, Laine et al. treated surface water with terminal filtration, and the UF membranes removed about 42% of TOC. Therefore, it is critical to find ways to minimize this difference and increase the efficiency of the UF membranes. From the aspect of membrane, it is to look for new membrane materials or modify the membrane; from the aspect of treatment process, it is to look for suitable treatment process combined with ultrafiltration membrane, so as to achieve the effect of optimized treatment.
Impact of pretreatment on membrane filtration performance
Suspended solids in the water, colloidal impurities, bacteria, etc. in the filtration process will be attached to the membrane surface, so that the membrane is contaminated, the retained impurities will be rapidly produced on the surface of the membrane concentration polarization phenomenon; at the same time, some of the fine particles in the water will enter the membrane pores so that the water channel blockage, microorganisms and their metabolisms in the body of water will be adhered to the surface of the membrane. All of these will affect the membrane filtration performance, coupled with the many influencing factors mentioned earlier, therefore, the membrane water supply must be properly pre-treated, as much as possible to remove the dissolved organic matter in the water or to change the state in the state, reduce the membrane contamination, improve the membrane's filtration performance, prolong the service life and reduce the cost of water treatment. Pretreatment methods are many: biological pretreatment, ozone pretreatment, activated carbon pretreatment coagulation pretreatment. One of the most researched is ozone, activated carbon and coagulation three pretreatment methods.
Effect of ozone pretreatment on membrane filtration performance
The effect of ozone pre-oxidation on the filtration performance of PVDF microfiltration membrane with pore size of 0.1 micron. The test was carried out with 5.0mg/l humic acid and 10mg/l kaolin as raw water, and then 4.5mg/l of humic acid was added to the water, and it was found that the permeate flux in this condition was twice as much as that without ozone pretreatment. Moreover, the filtration resistance decreased when ozonation was applied; the filtration resistance increased when ozonation was stopped. Ozone can decompose the organic matter in the water that originally had a contaminating effect on the membrane into more dispersed substances that are not easy to contaminate the membrane, slowing down membrane contamination and making it easier to remove the contaminated layer on the membrane surface during backwashing. It can be seen that the ozone pre-oxidation of membrane filtration performance improvement is favorable. However, ozone oxidation by-products (e.g., bromate) and the ozone resistance of the membrane module are issues that should be considered with this pretreatment method.
Effect of activated carbon pretreatment on membrane filtration performance
The use of activated carbon and other porous adsorbents with a large specific surface area to adsorb dissolved organic matter in the raw water that can contaminate the membrane, to reduce the filtration resistance, improve water permeability flux and the removal rate of organic matter.
The effect of coagulation pretreatment on membrane filtration performance
Coagulation on the membrane filtration performance of the impact, so far there is not a conclusion. There are many reasons, on the one hand, people have not reached a consensus on the causes of membrane contamination and its substances; on the other hand, coagulation on the organic matter in water and the membrane surface of the charge of what effect is not yet clear.
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