The effect of activated carbon (GAC or PAC) pretreatment on membrane filtration performance

The cross section of the ultrafiltration membrane has an asymmetric structure. It is generally composed of a dense layer with a thickness of less than 1 micron, which plays a sieving effect, and a support layer with a large thickness (usually 125 micrometers) with a sponge-like or finger-like porous structure. At present, the membrane modules commonly used in industrial production and daily life mainly include: tube type, plate and frame type, roll type and hollow fiber type. Hollow fiber membranes have three structures: internal pressure membrane (with dense layer inside), external pressure membrane (with dense layer outside) and bidirectional membrane (with dense layer inside and outside). In general, there are still defects such as fewer types of membranes, wide membrane pore size distribution, and unstable performance.

The removal effect and influencing factors of organic matter by ultrafiltration membrane

The molecular weight cut-off range of ultrafiltration membranes is generally 5000 to 10000 ODalton. A considerable part of the molecular weight of dissolved organics in natural waters is below this range, resulting in a poor interception effect of ultrafiltration membranes. In fact, the proportion of low-molecular-weight soluble organic matter in natural water is often larger.
The removal of organic matter by ultrafiltration membranes varies greatly in different situations. Some scholars use hollow fiber ultrafiltration membranes with a cut molecular weight of 100,000 Dalton to filter 20 different raw waters. The average removal rate of TOC is 18%, and the average removal rate of UV25 is 28%. Also to remove TOC in water, Laine et al. used terminal filtration to treat surface water, and the removal rate of TOC by ultrafiltration membrane was about 42%.
Therefore, finding a suitable way to reduce this difference as much as possible, and improving the processing efficiency of the ultrafiltration membrane is the key. Starting from the membrane aspect is to find new membrane materials or to modify the membrane; starting from the treatment process is to find a suitable combination of treatment processes and ultrafiltration membranes to achieve the optimal treatment effect.

Influence of pretreatment on membrane filtration performance

Suspended solids, colloidal impurities, bacteria (virus) in the water will adhere to the membrane surface during the filtration process, causing the membrane to be contaminated, and the trapped impurities will quickly produce concentration polarization on the surface of the membrane; at the same time, part of the water Small particles will enter the membrane pores and block the water channels, and the microorganisms and their metabolites in the water body will also adhere to the membrane surface. All of these will affect the filtration performance of the membrane, plus the many influencing factors mentioned above. Therefore, the membrane water supply must be properly pretreated to remove as much dissolved organic matter in the water as possible or change its state and reduce the membrane. Pollution, improve membrane filtration performance, extend service life and reduce water treatment costs. There are many pretreatment methods: biological pretreatment, ozone pretreatment, activated carbon (granular carbon GAC or powdered carbon PAC) pretreatment and coagulation pretreatment. Among them, three pretreatment methods of ozone, activated carbon (GAC or PAC) and coagulation are the most studied.

The influence of ozone pretreatment on membrane filtration performance

Japan’s SSawada et al. studied the effect of ozone pre-oxidation on the filtration performance of a PVDF microfiltration membrane with a pore size of 0.1 micron. The experiment uses 5.0 mg/l of humic acid and 10 mg/l of kaolin to prepare raw water, and then adds 4.5 mg/l to the water. The result shows that the permeable flux under this condition is twice that of the non-ozone pretreatment. In addition, when dosing, the filter resistance drops; when the dosing is stopped, the filter resistance rises. He believes that ozone can decompose the organic matter (humic acid) in the water that originally pollutes the membrane into more dispersed substances that are not easy to pollute the membrane, slow down membrane pollution, and make it easier to remove the pollution layer on the membrane surface during backwashing.
It can be seen that ozone pre-oxidation is beneficial to the improvement of membrane filtration performance. However, the byproducts of ozone oxidation (such as bromate) and the ozone resistance of membrane modules are issues that should be considered in this pretreatment method.

The effect of activated carbon (GAC or PAC) pretreatment on membrane filtration performance

Activated carbon (GAC or PAC) and other porous adsorbents with a large specific surface area are used to adsorb the soluble organic matter that can pollute the membrane in the raw water, so as to achieve the purpose of reducing the filtration resistance, increasing the permeation flux and the removal rate of organic matter. Tests show that the combination of powder activated carbon and membrane can effectively improve the removal effect of organic matter; powder activated carbon will not increase the membrane filtration resistance, and the membrane filtration resistance decreases with the increase of the powder activated carbon dosage.
James A. Nilisont and others used PAC as the pretreatment of NF, and the test results found that PAC could not effectively prevent membrane fouling. They believe that hydrophobic organic matter is the main cause of membrane fouling and reduced membrane flux. However, PAC can only effectively remove hydrophilic organic matter, and it has a very poor removal effect on hydrophobic organic matter. Therefore, PAC does not play a substantial role in membrane filtration. The effect of activated carbon on membrane filtration performance is that it absorbs a large amount of organic matter that can contaminate the membrane, reduces filtration resistance, and increases the permeable flux of the membrane; the downside is that it also has certain limitations in removing organic matter; at the same time; Activated carbon adsorbed on the surface of the membrane for a long time may breed microorganisms in the carbon particles and contaminate the membrane.

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