Research hotspot and progress of membrane separation technology in water treatment



introduction

In the process of research and development of water treatment technology, membrane technology is known as the water treatment technology of the 21st century. Although the development time is short, its development is very rapid, and it has been widely promoted and applied in the field of water treatment, and its application prospects in the field of water treatment are also very considerable. As an important application field in the downstream of the membrane industry, water treatment has become an important direction for the development of membrane materials. Microfiltration membrane is mainly used in sewage, wastewater treatment and other fields; Ultrafiltration membrane technology is widely used in sewage, waste water treatment and reuse and water supply and purification. Nanofiltration membrane technology is mainly used in pure water, softened water, non-ionized water, seawater desalination and so on. Reverse osmosis membrane technology can be used in seawater desalination and other fields. In summary, the application of membrane technology in water treatment mainly includes the following aspects: sewage treatment and reuse, water supply purification, seawater desalination.

1 Membrane separation technology characteristics

Membrane separation technology mainly refers to the selective separation of mixtures of molecules with different particle sizes through the semi-permeable membrane at the molecular level, which is a more effective sewage treatment method. In the process of practical application, it has the following characteristics: First, the separation effect is good. This treatment method can effectively treat the molecules in the water without producing other substances, and can realize the separation of nanoscale substances, such as organic matter, bacteria, viruses, etc., which can be separated and has strong pertinancy. Second, the operation is simple. At present, most of the membrane separation equipment is equipped with a central control system, which can achieve one-click operation without additional operation, reducing the difficulty of technical processing, and generally does not require maintenance, and has strong reliability. Third, save energy. In terms of the particle size of the molecules in the sewage, phase transformation does not occur during the membrane separation process, which can save energy to the greatest extent, so this treatment is usually carried out in a normal temperature environment, and the energy loss is relatively small. Fourth, the cost is relatively low. In the process of sewage treatment, the membrane separation technology does not need to add any agents, and only needs to rely on the filter membrane to complete the classification, which can reduce the cost required for separation to the maximum extent, avoid secondary pollution, and has high application value.

2 Application of membrane separation technology in water treatment

2.1 Adsorption pretreatment

Adsorption pretreatment is a method that uses adsorbent with high dispersion or large porosity and large specific surface area to adsorb dissolved organic matter in water, and then intercepts adsorbent particles through filter membrane to remove organic matter in water and alleviate membrane pollution. Adsorption is one of the main measures to reduce organic pollution of filter membrane. At present, the commonly used in the pretreatment of membrane fouling of adsorption material with powder activated carbon (powderedactivatedcarbon, PAC), resin, thermalization alumina particles ((heatedaluminumoxideparticles, HAOPs), iron oxide nanoparticles, silica particles and polysulfone colloids, among which PAC is one of the typical representatives of adsorbents because of its developed void structure and good adsorption performance. PAC has a high removal rate of low molecular weight organic pollutants that are difficult to be removed by coagulation and ultrafiltration. Chi et al studied the effect of humic acid molecular weight (MW) on membrane pollution, and the results showed that low molecular weight organic matter can be effectively adsorbed and removed, and will not enter the membrane pore and cause membrane pollution. PAC pretreatment was also proved to have good removal effect on decontamination byproducts. In addition, submicron activated carbon (SSPAC) has a high selective adsorption capacity of biomacromolecules, and its pre-coating on the membrane surface can almost completely prevent the increase of the transmembrane pressure (TMP) caused by microbial extracellular organics. However, due to the different hydrophobicity and raw water characteristics of the membrane, a large number of experiments have concluded that PAC will aggravate membrane pollution or have no effect on membrane flux. In short, PAC as a membrane technology pretreatment is still worthy of further study. Although the adsorbent can enhance the removal of organic matter and slow down the pollution rate of ultrafiltration membrane, the adsorption pretreatment needs to set up a separate adsorption treatment unit, which has problems of large footprint and high infrastructure investment cost.

2.2 Reverse Osmosis

Reverse osmosis technology is applied to the advanced treatment of domestic sewage, which can separate and remove the organic substances and salts contained in the water to ensure that the water quality meets the water standard. In the process of practical application, reverse osmosis membrane has the characteristics of high selection and high permeability, and the selective membrane is a membrane treatment method that the operating pressure is higher than that of solution osmosis. Although this method itself has a better effect of purifying water, but affected by technical factors, the application effect of reverse osmosis technology in China can not be compared with Western developed countries, and it still needs to be further improved in the future, making the application of reverse osmosis equipment more efficient. In California, the United States has set up a sewage plant, and has a daily sewage treatment capacity of 37.8m3 reverse osmosis equipment, so that domestic sewage can be better treated, and it is re-applied to the water supply system.

2.3 Chemical wastewater

In order to prevent the environmental impact of chemical wastewater discharge, it is necessary to treat it before discharge. The combination of nanofiltration osmosis and reverse osmosis can effectively treat industrial wastewater containing ammonia molybdate, and the recovery rate of molybdenum ions in water can exceed 96%, which can not only effectively recover molybdenum ions, but also achieve efficient purification of wastewater. At the same time, the membrane separation technology is green and pollution-free, and is very suitable for application in the treatment of industrial wastewater, with specific pretreatment methods, recovery of reusable substances, wastewater purification and resource recovery and other treatment goals. Treatment of drinking water, the use of membrane separation treatment technology has a good effect. At present, people's living standards are gradually improving, so the water quality requirements for drinking water are higher. Compared with the traditional treatment process, the application of membrane separation can effectively remove suspended substances and bacteria in water and improve the purification effect of drinking water. Ultrafiltration, microfiltration and nanofiltration processes can be used in the treatment process to achieve micron level particles in drinking water. At the same time, it can also remove some particles that do not reach the micron level, and remove dissolved gases, inorganic substances and pesticides in water to improve the quality of drinking water.

2.4 Introduction and application of microfiltration membrane technology

Compared with other membrane technologies, microfiltration membrane technology is the first proposed and applied, and the most widely used membrane technology, which is mainly used in the pre-treatment and primary treatment of drinking water production, and can effectively remove solid particles, suspended solids, bacteria and other substances in raw water. In the application of microfiltration membrane technology for drinking water treatment, in order to achieve a good treatment effect, it is often necessary to combine microfiltration membrane technology with other membrane technologies and other water treatment methods to jointly treat water, such as ultrafiltration membrane, nanofiltration membrane, adsorbent, coagulant, etc. On the basis of microfiltration membrane technology, some researchers have tried to use different coagulants to remove humic acid in drinking water and different coagulants to study the membrane pollution, and obtained the coagulant conditions under the optimal effluent quality of drinking water and the type and dosage of coagulants under the condition of minimal membrane pollution. Some researchers take river water as test objects, using different particle size activated carbon to treat river water samples, and the test found that the pollution caused by large particle size activated carbon is more serious than that of small particle size activated carbon.

2.5 Chemical precipitation pretreatment

Chemical precipitation method is a pretreatment method by adding chemical agents to transform the dissolved substances that may cause membrane pollution into difficult soluble substances and precipitate out. Amaral et al. used chemical precipitation as a microfilter-nanofiltration (MF-NF) pretreatment process to treat stable landfill leachate. This process has been proved to be effective in removing organic matter and inorganic matter in waste liquid, and the results show that precipitation as a microfiltration pretreatment can ensure low membrane pollution. Cao et al. used Ruth filter cake filtration theory to analyze the flux decline of Ca2+ in the filtration process of formation of suspension in alginate solution, and the results showed that the addition of Ca2+ reduced membrane contamination. In addition, the mixed precipitation-nanofiltration method was used to recover phosphorus and remove heavy metals from industrial phosphoric acid wastewater, and good purification and recovery efficiency was obtained. It can be seen that chemical precipitation pretreatment can effectively alleviate membrane pollution, and this method has the advantages of low input cost and simple operation, but it still has problems such as low treatment efficiency and secondary pollution to the environment caused by the treated solution.

peroration

In short, the efficient treatment of wastewater can control environmental pollution, relevant industries should attach great importance to the use of membrane separation process of wastewater treatment process, recycle useful substances, desalinate seawater, improve water quality, relevant personnel need to comprehensively analyze this technology, combined with water treatment needs, reasonable choice of separation technology, efficient water treatment.