1. RO membrane separation technology

RO membrane separation technology is a membrane process in which small molecule solvents represented by water molecules pass through the RO membrane to achieve separation from impurities while overcoming osmotic pressure. The operating pressure is generally 1.5~10.5MPa, which can trap 1~10A of small molecule impurities. In water treatment, RO, as a key equipment, can remove more than 97% of dissolved inorganic substances, 99% of organic matter with relative molecular weight of 300 and above, more than 99% of various particles including bacteria, and 95% SiO2 from water.

However, the high operating cost in practical applications restricts the widespread promotion of RO membrane separation technology. On the one hand, due to the high operating pressure and high energy consumption of the RO system, and more importantly, the membrane pollution accompanying the entire operation process not only leads to further increase operating pressure, decrease the desalination rate, but even require frequent replacement of expensive RO membrane elements. When the RO system is operated under high pressure, the suspended solids (SS) in the influent water are easy to accumulate on the surface of the RO membrane to form a filter cake layer, and the dissolved organic matter may be adsorbed on the membrane surface to form a gel layer, and microorganisms or other colloidal substances will adhere to the membrane surface. Taking the water recovery rate of 50%~75% as an example, the salt ion content in the RO concentrated water is about 2~4 times that of the influent water, and the gel layer or filter cake layer generated on the surface of the membrane will greatly reduce the solubility volume of insoluble inorganic ions such as Ca2+ and Mg2+. It can be seen that for the deep treatment of sewage and wastewater, the RO system will face the interaction of multiple pollutions, and the operation and management difficulty will be further increased.

In order to give full play to the technical advantages of RO that can escape most of the monovalent salt ions and small molecule organic matter in raw water, the influent water must undergo strict pretreatment. In engineering, the turbidity of RO influent is generally controlled <1NTU, and the sludge pollution index (SDI) is <5. SDI is used to measure the content of colloids, sludge, iron-manganese oxides and humus in water. SDI<3 is generally considered to be very trace pollution, and SDI>5 is considered to be moderate pollution. In addition, during operation, it is necessary to adjust the pH of the influent water or add scale inhibitors to prevent scale pollution on the membrane surface.

2. Application of RO membrane separation technology in sewage and wastewater treatment

2.1 Application in the treatment of wastewater with high salinity

2.1.1 Mine influent water treatment

The high salinity wastewater represented by mine water is characterized by high salinity, especially underground water inflow, with an average salinity of more than 1000mg/L, containing a large amount of Ca2+, Mg2+, K+, Na+, Cl-, SO42-, HCO3- plasma, less organic components in SS, and COD less than 1.5mg/L. For mining areas with severe water scarcity, the use of RO technology for deep treatment has been widely promoted as production and domestic water.

Chen Wei et al. removed most of the SS in the water by adding flocculation, precipitation and rapid filtration to the mine water as pretreatment, ensuring that the turbidity of RO influent < 1NTU. The effluent was further treated by RO, the turbidity removal rate in the water was close to 100%, the desalination rate reached 96%, and the produced water reached the drinking water quality standard, with a treatment cost of about 5.17 yuan/m3.

Considering the high content of iron and manganese in the water of high-salinity mines, and the high concentration of Ca2+ and SO42- ions may form CaSO4 scale pollution on the surface of the RO membrane, Wang Xuhui et al. oxidized the Fe2+ in the water to Fe3+ through aeration. The pH of the water was adjusted by adding lime emulsion to the aeration tank, so that Ca2+ and Fe3+ formed CaCO3 and Fe(OH)3 precipitate, and then the added PAM coagulant and PAC flocculant were used to form a large floc of CaCO3 and Fe(OH)3, which were removed in the clarification tank. The Mn2+ in the effluent was further reduced to 0.04mg/L through the manganese sand filter. Ultrafiltration (UF) has a strong retention effect on macromolecular organic matter, pathogens and suspended solids, and usually treats SS in effluent

Cui Yuchuan et al. sorted out the engineering cases of influent water from high salinity mines treated by RO technology for drinking water: when the SS< in the raw water is 50mg/L, microflocculation and direct filtration can be used as the pretreatment of RO. When SS> is 50mg/L, flocculation, precipitation and filtration are used as RO pretreatment. When the Fe > is 0.3mg/L, it is necessary to consider using manganese sand filter for iron removal and filtration. when the organic matter content is high, chlorine oxidation, coagulation, precipitation, filtration, and activated carbon adsorption technology should be used for pretreatment; When the carbonate hardness is high, ion exchange and deCO2 technology should be added in the pretreatment process to prevent CaCO3 precipitation pollution on the surface of the RO film. For other insoluble salts, scale inhibitors can be added before RO water inlet; When the silicate content is high, lime milk or MgO can be added during the flocculation stage.

2.1.2 Wastewater treatment in the metallurgical industry

As a resource-based industry with high water consumption and high pollution, the iron and steel industry has accounted for 14% of the total water consumption of the national industry. The water quality composition of wastewater in the iron and steel industry is complex, and the indicators fluctuate greatly, especially the content of Ca2+, Mg2+, Fe2+, Mn2+, SO42-, F- and SiO2 is high.

Fang Zhonghai first used aeration tank to oxidize Fe2+ to Fe3+, and added NaClO to improve the oxidation capacity and sterilization effect of Fe2+ in the water body. The effluent is further removed by adding lime emulsion to adjust the pH, adding PAM and PAC for flocculation, and then precipitation, rapid filtration and activated carbon adsorption to further remove organic matter, residual chlorine, heavy metal ions, etc. in the water. After the effluent is treated by UF, it enters the RO system after adding reducing agent, scale inhibitor and acid. Among them, the purpose of adding NaSO3 reducing agent is to prevent the oxidation of aromatic polyamide membranes made of residual chlorine in water. In the end, the primary RO mainly removes most of the dissolved salts, colloids, organic matter, etc. in the water, and part of the produced water is used as process water for steel mills, and the other part is treated by the secondary reverse osmosis and ion exchange system after adding alkali for high-pressure boiler make-up water.

Chen Xiaoqing et al. jointly added powdered activated carbon and lime emulsion to the pretreatment clarifier to reduce 60%~70% of organic matter and oils and some high-valence ions such as Ca2+ and Ba2+ in metallurgical industrial wastewater, and remove nearly 90% of SS and colloidal substances in the water. It can effectively prevent the formation of CaSO4, BaSO4 and CaF2 precipitation pollution on the surface of RO membrane by high concentrations of SO42- and F (the highest content is -402mg/L and 3.96mg/L, respectively). It is worth noting that although activated carbon has a good adsorption effect on organic matter and SS, it is non-selective adsorption, and its high dosage and high price are bound to increase the cost of water treatment when used in the pre-treatment of SS-rich sewage and wastewater. Therefore, it is generally only used for engineering emergencies.

2.2 Application in the treatment of refractory organic pollutants and wastewater

2.2.1 Treatment of printing and dyeing wastewater and petrochemical wastewater

In addition to a large number of dyes and slurries, printing and dyeing wastewater also contains inorganic salts, acids and alkalis. Its color intensity is as high as 4000 times, and it has the characteristics of large water volume, high content of organic pollutants, large changes in water quality, and poor biodegradability.

Qi Luqing et al. used O3-aerated biological filter for pretreatment, combined with UF+RO double membrane system to deeply treat printing and dyeing textile wastewater. In the treatment process, O3 was used to preliminarily degrade the refractory organic matter in the wastewater to improve its biodegradability, and then the biological filter was used for biodegradation and filtration, so that the COD in the water was reduced to 27.4mg/L and SS <5mg/L； The effluent is filtered through a multi-media filter, which is filled with a combination of quartz sand and manganese sand filter material, which can further adsorb and filter out impurities such as SS, colloidal bacteria, and viruses in the filtered water. The RO influent was stabilized at turbidity <0.4NTU and SDI between 0.4~1.5 after UF treatment. To avoid microbial contamination on the membrane surface, the system adds NaClO disinfectant before the UF is fed into the water. UF outlet high-pressure pump into the security filter, and the effluent plus scale inhibitor and NaSO3 reducing agent into RO. After this process, the pH of the final produced water is 7.4~7.9, the conductivity is 50~200μS/cm, the total hardness is 2~10mg/L, and the total alkalinity is 25~60mg/L, which meets the water quality standard of production and reuse water.

Petrochemical sewage has the characteristics of large fluctuations in water volume and water quality, complex pollutant composition, among which the oil content brought in the production can reach up to 30g/L, sulfide is close to 50mg/L, COD is about 1g/L, the mass concentration of various salts is close to 12g/L, and it also contains volatile phenols and other toxic and harmful substances.

Various forms of oil in sewage and wastewater are generally treated by gravity grease trap recovery and air flotation escape, which can reduce the oil concentration in effluent to less than 30mg/L. Wang Xiaoyang first used grease traps to remove most of the oil slick in petrochemical sewage; then adjust the pH of the sewage to 8~8.5, add catalyst and aeration to oxidize sulfide in the water to control the sulfide concentration in the effluent below 5mg/L; Air flotation removes suspended solids and emulsified oil in sewage; Then, in an anoxic and then aerobic environment, microorganisms were used to degrade organic matter and ammonia nitrogen in the water into CO2, water and N (i.e., 2A/O biological treatment process). After rapid filtration, UF and activated carbon adsorption further escape the SS and organic matter in the water, it enters the RO system. The salt concentration in the final treatment water was reduced to 500mg/L and used as production supplementary water.

The membrane bioreactor (MBR) process is to organically combine the membrane filtration process with the traditional activated sludge method, and directly add curtain microfiltration (MF) or UF membrane components to the activated sludge tank instead of the secondary sedimentation tank for slurry water separation. It has the advantages of large space for regulation of technical conditions such as sludge age and sludge concentration, high effluent quality standards, small equipment footprint, and easy integration. The use of MBR instead of biochemical treatment, multi-media filtration, and UF as RO pretreatment can greatly shorten the RO water treatment process.

A large textile dyeing and finishing enterprise in Shandong first used the hydrolysis acidification tank to anaerobic acidify and decompose the macromolecular organic matter in the printing and dyeing wastewater into small molecule organic matter with high biodegradability, and then relied on gravity to flow into the MBR biochemical tank, and after full adsorption, oxidation and degradation, and filtration, the COD in the wastewater was reduced to less than 110mg/L, and the SS could hardly be effectively detected, which basically met the requirements of RO influent water quality. The engineering design treatment scale is 10,000 m3/d, and the treatment cost of reused water is expected to be 3.92 yuan/m3.

It can be seen that compared with the UF+RO double-membrane method, the MBR+RO double-membrane method not only simplifies production management, but also has a relatively low water treatment cost, making it more suitable for the in-depth treatment of difficult-to-degrade organic wastewater.

2.2.2 Deep treatment of landfill leachate

The landfill leachate is mainly derived from landfill precipitation, and its pollutants are mainly derived from the decomposition of garbage and precipitation leaching by microorganisms, and the water quality is very complex and fluctuate, and the COD is much higher than that of urban sewage, up to 30000mg/L, and the biodegradability is poor. In addition, the leachate may also contain various metal ions such as Fe2+, Cd2+, Cr3+, Cu2+, and Zn2+. During the fermentation stage, the concentration of Fe2+ is even as high as 2000mg/L, and Ca2+ is as high as 4000mg/L.

Although the A/O two-stage biochemical treatment process has been widely used for the degradation and denitrification of organic matter in landfill leachate, the effluent effect is not stable. Therefore, Jiang Yanchao et al. used the combination of UF membrane and A/O to form an MBR process based on mechanical filtration to strengthen the removal rate of organic matter in landfill leachate. The effluent is separated by UF membrane cement and enters the nanofiltration (NF) system. The combined treatment process of MBR+NF+RO was formed by effectively separating organic matter with molecular weight of 200~2000 and some high-valence metal ions in MBR production water. The results show that the process has a good operation effect, and the effluent quality meets the pollution control standards of domestic waste landfills. During operation, the produced water can be discharged directly when the NF effluent meets the discharge requirements, otherwise it will continue to be treated using the subsequent RO system.

WANG et al. used the A/O-MBR+NF+RO process to treat landfill leachate and found that adding activated carbon to the A/O reactor can not only improve the system's removal effect on organic matter and heavy metals, but also reduce membrane pollution.

For highly polluted and difficult-to-degrade landfill leachate, after biochemical treatment and MBR degradation and filtration, a certain amount of free small molecule organic matter and highly polluting microbial metabolites still remain in the effluent.

2.3 Application in high-quality reuse treatment of municipal sewage and wastewater

After secondary biochemical treatment, the content of organic matter in the wastewater is relatively low, usually BOD5<30mg/L, SS<30mg/L, but the biodegradability of the residual organic matter is poor, and the TDS is about 3000mg/L. In recent years, in order to alleviate the pressure caused by water shortages, high-quality reclaimed water has been produced both domestically and internationally by using municipal wastewater treated with secondary biochemical treatment as raw water for RO treatment systems. The UOSA wastewater treatment plant in Virginia, USA, uses a combination of "UF+RO+UV disinfection" to produce reclaimed water that is injected back into the ground to replenish groundwater. Singapore uses the "MF+RO+UV disinfection" process to prepare "new water" to supplement drinking water source water. At present, the world's largest reclaimed water plant, Kuwait Sulaibiya Water Plant uses the secondary treatment effluent of the urban sewage treatment plant as the water source, and uses the UF+RO double-membrane process to treat it to meet the standards of industrial reuse and farmland irrigation.

our country has also set up large-scale reclaimed water plants in Beijing, Tianjin, Hebei, Shandong and other places. Several reclaimed water plants in Tianjin Binhai New Area use the "UF(MF)+RO" double-membrane process to produce high-quality reclaimed water from municipal wastewater after secondary biochemical treatment as raw water, one part as boiler water for thermal power plants, and the other part as landscape river and domestic miscellaneous water. The TOC< < 1.3mg/L, NH3-N0.03mg/L, TN<0.1mg/L, and TP in the treated water were not detected, and the conductivity and turbidity were less than 30μS/cm and 0.12NTU, respectively, and the effluent quality reached the drinking water quality standard.

In short, RO, as a core technology, has been widely used in the deep treatment of various sewage and wastewater or high-quality reuse water treatment. In order to give full play to the technical advantages of RO, while minimizing membrane contamination and reducing water treatment costs, a series of combined processes have been developed for specific water quality. Normally, high concentrations of SS can be removed from water by traditional water treatment methods such as dosing coagulation, sedimentation and efficient filtration; NaClO, aeration oxidation combined with lime emulsion and manganese sand filter to reduce iron, manganese, calcium and silica salts in RO influent. O3 oxidation and A/O biochemical treatment to degrade refractory organic matter in water. The retention of fine SS, small molecule organic matter and high-valence ions was strengthened by MBR, UF and NF, and the UF+RO or MBR+RO double-membrane method, or even MBR+NF+RO three-membrane method was formed to ensure the stable operation of the RO system for the treatment of refractory organic wastewater.

3. Research status of RO membrane separation technology in sewage and wastewater treatment

3.1 Reveal of the formation mechanism of membrane pollution

The effective control of membrane pollution is directly related to the service life of RO membrane and the stability of the treatment process, and the study of pollution mechanism includes the analysis of organic pollutants and the revelation of the generation process.

3.1.1 Analysis of organic pollutants in RO influent

After biochemical treatment and MF or UF membrane interception of sewage and wastewater, a certain amount of soluble small molecule organic matter still remains in the produced water, of which microbial metabolites and secondary metabolites (SMPs) account for about 83% of the effluent TOC, mainly including polysaccharides, proteins, oils, nucleic acids, humic acids, antibiotics, cellular materials, etc. At present, the research on SMP contamination of RO membranes is relatively enthusiastic, mainly focusing on SMP concentration, molecular weight, hydrophilicity, chargeability and simulation of its contamination process on RO membranes.

Zhou Yuexi et al. divided the SMP in biochemically treated organic wastewater into four types: hydrophilic, hydrophobic acidic, hydrophobic and hydrophobic, and analyzed it with gel chromatography, infrared spectroscopy, three-dimensional fluorescence spectroscopy and ultraviolet spectroscopy to study the molecular weight distribution of hydrophilic polysaccharides, hydrophobic organic matter rich in aromatic groups, and weakly hydrophilic humic acid and fulvic acid. MOSHE found that 80% of SMPs have a molecular weight of less than 1k. ZHAO confirmed that the hydrophilic components in the recognizable SMP accounted for 62.9%~69.9%, and the neutral hydrophilic organic matter was dominated by polysaccharides, with low aromatic group content, while the protein content in charged hydrophilic organic matter was high.

3.1.2 Formation of RO membrane pollution

For the RO system in the sewage and wastewater treatment process, the influent not only contains SMP, residual microorganisms, refractory phenols, ketones, aldehydes, polycyclic aromatic hydrocarbons and other small molecule organic matter and some chlorine-substituted hydrocarbons disinfection by-products, but also heavy metal ions carried by the sewage and wastewater itself and high-valent metal ions such as Fe3+ and Al3+ brought in by flocculants, which will further increase the complexity of RO membrane pollution.

SERGEY et al. used alginate instead of hydrophilic organic matter to study the pollution, and found that Ca2+ could aggravate the contamination of alginate to RO membranes, while the effect of Mg2+ could be ignored. SANYUP et al. believe that the alginate gurocuronic acid (G segment) may form an egg-box structure with Ca2+, thus accumulating to form a dense glue-linked network structure. MO et al. used bovine serum protein (BAS) to simulate the proteins in sewage and investigated the effects of different ion contents under various pH conditions on their contamination. It was found that Ca2+ could densify the BAS pollutant layer on the RO surface under the condition of BAS isoelectric point, which was attributed to the fact that Ca2+ could form a bridge bond with the carboxyl group of amino acids. ANG et al. confirmed that when Ca2+ was present, the calcium alginate formed was colloidal with BAS, resulting in a dense gel layer on the surface of the RO membrane. According to the five-stage theory of microbial contamination process proposed by HAN, the surface roughening of RO membrane caused by inorganic substances is the basis for microbial contamination of RO membrane.

It can be seen that for the deep treatment of sewage and wastewater, it is difficult to study the organic pollution, inorganic pollution and microbial pollution of RO membrane separately. Once started, the synergy between them will promote the rapid intensification of RO membrane contamination. Only by delaying the formation of initial membrane pollution and understanding the root cause of the aggravation of membrane pollution can effective mitigation and control methods be established in design and operation management.

3.2 Research on methods for predicting the pollution of influent water in RO system

The effective prediction of the potential pollution of RO influent is the technical guarantee for the stable operation of the RO deep treatment system for sewage and wastewater.

3.2.1 SDI value

The SDI value was determined by passing the water sample to be measured at a pressure of 207kPa through a microporous filter membrane with a diameter of 47mm and a pore size of 0.45μm, and recording the time required for the initial filtration of the 500mL water sample T1, and after the time interval T (usually 15min), the time required to filter the 500mL water sample again was recorded T2, SDI=100×(1-T1/T2)/T. SDI value has been the main reference index in RO system design and operation management for many years due to its simple measurement method and good reproducibility. Usually the RO system requires SDI<3 (some RO membranes are relaxed to SDI<5).

However, in the operation of sewage and wastewater treatment projects, even if the RO influent water quality meets the current water quality requirements, there is still serious membrane pollution in the RO system.

Existing analysis suggests that the SDI value is based on the pore blocking mechanism to determine the degree of congestion of the 0.45μm microfiltration membrane pore by influent water, and the numerical deviation is large for the measurement of fine colloids and small molecule organic matter that are not mainly clogged. Therefore, a single SDI value cannot fully reflect the potential pollution characteristics of refractory organic wastewater after biochemical treatment. In addition, the size of SDI value is related to the pH of the water and the type of organic matter. Zhang Wei et al. found that natural humic acid organic matter showed high SDI values under high pH and low salt content. For water bodies containing iron colloids and organic macromolecules, the SDI value increases sharply with the increase of pH. In the study, the author also found that when the pH of the electronic wastewater pretreated by MBR is less than 4, the SDI value fully meets the requirements of RO influent water quality, but the subsequent RO system still produces serious pollution phenomena in operation.

3.2.2 Research on SDI substitution parameters

In response to the shortcomings of SDI test methods, SCHIPPERS et al. proposed a modified pollution index (MFI). The test method of MFI is the same as that of SDI, but the relationship curve between t/V and V is made according to the time required for the volume of filtered water V t, and the slope of the curve is the MFI value. Although the MFI value shows a good linear relationship with the RO membrane blockage caused by particulate pollutants, it can reflect the filter cake layer filtration mechanism well, but like the SDI value, it can only characterize the contamination of substances trapped by the 0.45μm microporous filter membrane. To this end, BOERLAGE et al. proposed the MIF-UF index by replacing the 0.45μm microfiltration membrane with a cut-off molecular weight of 13 k. In order to be closer to the RO membrane in terms of pore size to reflect the membrane contamination caused by the adsorption of small molecular weight organic matter, KHIRANI et al. proposed the MIF-NF index. KEEWOONG believed that the three groups of values of MIF-MF, MIF-UF and MIF-NF should be measured at the same time for comprehensive evaluation, which could more fully reflect the potential contamination of particulate matter, microcolloids and organic matter to the RO membrane.

In short, through continuous efforts, the current monitoring technology for the potential pollution of RO influent water quality has made great progress in terms of membrane pollution mechanism to pollutant monitoring scope. However, these measurement indicators still have certain deviations in guiding actual production. With the continuous maturity of Internet technology, big data and cloud platform technology, these technologies will have great advantages in the prediction of RO influent water pollution and the linkage of various treatment units.

3.3 RO concentrated water treatment

The RO treatment process concentrates pollutants in the influent water by nearly 3 times while obtaining about 70% of the high-quality reclaimed water, producing about 1/3 of the RO concentrated water. It has the characteristics of large water volume, high mineralization, poor biodegradability, and great potential environmental hazard. Because the treatment and disposal of RO concentrated water have not received enough attention, it has even become one of the obstacles hindering the large-scale application of RO technology in some areas.

Considering the low SS content, scale inhibitor and large residual pressure in RO concentrated water, in addition to partially mixing with RO influent water to improve water recovery in the project, it is often used as a rapid filtration device and UF backwash water, or mixed with raw water after simple treatment and re-entered the treatment system. This is bound to increase the scale and difficulty of sewage and wastewater treatment. At present, most of the research on RO concentrated water treatment focuses on advanced oxidation for organic matter removal and distillation concentration, forward osmosis and electrolysis for resource recovery.

Among them, membrane distillation (MD) technology combines membrane technology with distillation process, using hydrophobic microporous membrane as the medium, and using the effect of vapor pressure difference between the two sides of the membrane to make the volatile components in the material liquid pass through the membrane pores in the form of steam, so as to achieve separation. Compared with other separation processes, membrane distillation has the advantages of high separation efficiency, mild operating conditions, low requirements for the interaction between the membrane and the raw material liquid, and the mechanical properties of the membrane. However, high-quality hydrophobic membranes are still under development.

The technical advantages of electrolysis treatment of RO concentrated water are: 1) high salinity ensures good conductivity of water body, thereby reducing power consumption; 2) The high chloride content in water is conducive to the production of strong oxidants such as hypochlorite in the electrolysis process, which can enhance the oxidative degradation of organic matter. 3) It can treat ammonia nitrogen and refractory organic matter in water at the same time. Electrolysis is also considered one of the most promising RO concentrated water treatment technologies. In addition, combined electrolysis methods that combine ultraviolet irradiation with electrolysis or introduce ultrasound during electrolysis have also been proposed.

In short, for the treatment of RO concentrated water, how to efficiently degrade organic matter and reduce treatment energy consumption is a challenge at present.

4. Conclusion and prospects

At present, RO has become an indispensable core technology for the deep treatment of various sewage and wastewater or high-quality reuse water treatment, in order to give full play to its technical advantages to ensure the stable operation of the RO system and reduce the cost of water treatment, for the SS, easy to scale inorganic pollutants and refractory organic matter in raw water, the combined treatment process has also developed from the traditional flocculation, precipitation, rapid filtration + RO to UF+RO or MBR+RO double-membrane method, and even MBR+NF+RO three-membrane method.

The operation of the RO system is consistent with membrane contamination. With the continuous advancement of research methods in separation, purification and visualization, the root causes of membrane pollution from the aspects of microstructure and pollution formation mechanism, and the key factors of early formation and aggravation of RO membrane pollution can lay a theoretical foundation for effective mitigation and control of membrane pollution. Establishing a more scientific prediction method for the potential pollution of RO influent is the technical guarantee for the design and operation management of RO deep treatment sewage and wastewater process. In view of the fact that the SDI value cannot fully reflect the pollution of RO influent water quality (mainly biochemical treatment effluent is RO influent), the use of MIF combined with different pore size membranes to replace SDI value has been widely studied, and the introduction of Internet technology, big data and other technologies is worth looking forward to. In addition, the effective treatment and disposal methods of high salinity RO concentrated water need to be explored, and the challenges of efficient degradation of organic matter and low energy consumption treatment are its challenges. (Source: Urban Construction Management Office, Baoqing County, Heilongjiang Province, Beijing Key Laboratory of Material Waste Resource Utilization of Beijing United University Students) As a key means in the field of water treatment in the 21st century, membrane separation technology with reverse osmosis (RO) as the core provides an important technical guarantee for obtaining high-quality reclaimed water with its advantages of high efficiency, small footprint, high water quality, reliable operation, easy to realize automatic control and integration.

1. RO membrane separation technology

RO membrane separation technology is a membrane process in which small molecule solvents represented by water molecules pass through the RO membrane to achieve separation from impurities while overcoming osmotic pressure. The operating pressure is generally 1.5~10.5MPa, which can trap 1~10A of small molecule impurities. In water treatment, RO, as a key equipment, can remove more than 97% of dissolved inorganic substances, 99% of organic matter with relative molecular weight of 300 and above, more than 99% of various particles including bacteria, and 95% SiO2 from water.

However, the high operating cost in practical applications restricts the widespread promotion of RO membrane separation technology. On the one hand, due to the high operating pressure and high energy consumption of the RO system, and more importantly, the membrane pollution accompanying the entire operation process not only leads to further increase operating pressure, decrease the desalination rate, but even require frequent replacement of expensive RO membrane elements. When the RO system is operated under high pressure, the suspended solids (SS) in the influent water are easy to accumulate on the surface of the RO membrane to form a filter cake layer, and the dissolved organic matter may be adsorbed on the membrane surface to form a gel layer, and microorganisms or other colloidal substances will adhere to the membrane surface. Taking the water recovery rate of 50%~75% as an example, the salt ion content in the RO concentrated water is about 2~4 times that of the influent water, and the gel layer or filter cake layer generated on the surface of the membrane will greatly reduce the solubility volume of insoluble inorganic ions such as Ca2+ and Mg2+. It can be seen that for the deep treatment of sewage and wastewater, the RO system will face the interaction of multiple pollutions, and the operation and management difficulty will be further increased.

In order to give full play to the technical advantages of RO that can escape most of the monovalent salt ions and small molecule organic matter in raw water, the influent water must undergo strict pretreatment. In engineering, the turbidity of RO influent is generally controlled <1NTU, and the sludge pollution index (SDI) is <5. SDI is used to measure the content of colloids, sludge, iron-manganese oxides and humus in water. SDI<3 is generally considered to be very trace pollution, and SDI>5 is considered to be moderate pollution. In addition, during operation, it is necessary to adjust the pH of the influent water or add scale inhibitors to prevent scale pollution on the membrane surface.

2. Application of RO membrane separation technology in sewage and wastewater treatment

2.1 Application in the treatment of wastewater with high salinity

2.1.1 Mine influent water treatment

The high salinity wastewater represented by mine water is characterized by high salinity, especially underground water inflow, with an average salinity of more than 1000mg/L, containing a large amount of Ca2+, Mg2+, K+, Na+, Cl-, SO42-, HCO3- plasma, less organic components in SS, and COD less than 1.5mg/L. For mining areas with severe water scarcity, the use of RO technology for deep treatment has been widely promoted as production and domestic water.

Chen Wei et al. removed most of the SS in the water by adding flocculation, precipitation and rapid filtration to the mine water as pretreatment, ensuring that the turbidity of RO influent < 1NTU. The effluent was further treated by RO, the turbidity removal rate in the water was close to 100%, the desalination rate reached 96%, and the produced water reached the drinking water quality standard, with a treatment cost of about 5.17 yuan/m3.

Considering the high content of iron and manganese in the water of high-salinity mines, and the high concentration of Ca2+ and SO42- ions may form CaSO4 scale pollution on the surface of the RO membrane, Wang Xuhui et al. oxidized the Fe2+ in the water to Fe3+ through aeration. The pH of the water was adjusted by adding lime emulsion to the aeration tank, so that Ca2+ and Fe3+ formed CaCO3 and Fe(OH)3 precipitate, and then the added PAM coagulant and PAC flocculant were used to form a large floc of CaCO3 and Fe(OH)3, which were removed in the clarification tank. The Mn2+ in the effluent was further reduced to 0.04mg/L through the manganese sand filter. Ultrafiltration (UF) has a strong retention effect on macromolecular organic matter, pathogens and suspended solids, and usually treats SS in effluent

Cui Yuchuan et al. sorted out the engineering cases of influent water from high salinity mines treated by RO technology for drinking water: when the SS< in the raw water is 50mg/L, microflocculation and direct filtration can be used as the pretreatment of RO. When SS> is 50mg/L, flocculation, precipitation and filtration are used as RO pretreatment. When the Fe > is 0.3mg/L, it is necessary to consider using manganese sand filter for iron removal and filtration. when the organic matter content is high, chlorine oxidation, coagulation, precipitation, filtration, and activated carbon adsorption technology should be used for pretreatment; When the carbonate hardness is high, ion exchange and deCO2 technology should be added in the pretreatment process to prevent CaCO3 precipitation pollution on the surface of the RO film. For other insoluble salts, scale inhibitors can be added before RO water inlet; When the silicate content is high, lime milk or MgO can be added during the flocculation stage.

2.1.2 Wastewater treatment in the metallurgical industry

As a resource-based industry with high water consumption and high pollution, the iron and steel industry has accounted for 14% of the total water consumption of the national industry. The water quality composition of wastewater in the iron and steel industry is complex, and the indicators fluctuate greatly, especially the content of Ca2+, Mg2+, Fe2+, Mn2+, SO42-, F- and SiO2 is high.

Fang Zhonghai first used aeration tank to oxidize Fe2+ to Fe3+, and added NaClO to improve the oxidation capacity and sterilization effect of Fe2+ in the water body. The effluent is further removed by adding lime emulsion to adjust the pH, adding PAM and PAC for flocculation, and then precipitation, rapid filtration and activated carbon adsorption to further remove organic matter, residual chlorine, heavy metal ions, etc. in the water. After the effluent is treated by UF, it enters the RO system after adding reducing agent, scale inhibitor and acid. Among them, the purpose of adding NaSO3 reducing agent is to prevent the oxidation of aromatic polyamide membranes made of residual chlorine in water. In the end, the primary RO mainly removes most of the dissolved salts, colloids, organic matter, etc. in the water, and part of the produced water is used as process water for steel mills, and the other part is treated by the secondary reverse osmosis and ion exchange system after adding alkali for high-pressure boiler make-up water.

Chen Xiaoqing et al. jointly added powdered activated carbon and lime emulsion to the pretreatment clarifier to reduce 60%~70% of organic matter and oils and some high-valence ions such as Ca2+ and Ba2+ in metallurgical industrial wastewater, and remove nearly 90% of SS and colloidal substances in the water. It can effectively prevent the formation of CaSO4, BaSO4 and CaF2 precipitation pollution on the surface of RO membrane by high concentrations of SO42- and F (the highest content is -402mg/L and 3.96mg/L, respectively). It is worth noting that although activated carbon has a good adsorption effect on organic matter and SS, it is non-selective adsorption, and its high dosage and high price are bound to increase the cost of water treatment when used in the pre-treatment of SS-rich sewage and wastewater. Therefore, it is generally only used for engineering emergencies.

2.2 Application in the treatment of refractory organic pollutants and wastewater

2.2.1 Treatment of printing and dyeing wastewater and petrochemical wastewater

In addition to a large number of dyes and slurries, printing and dyeing wastewater also contains inorganic salts, acids and alkalis. Its color intensity is as high as 4000 times, and it has the characteristics of large water volume, high content of organic pollutants, large changes in water quality, and poor biodegradability.

Qi Luqing et al. used O3-aerated biological filter for pretreatment, combined with UF+RO double membrane system to deeply treat printing and dyeing textile wastewater. In the treatment process, O3 was used to preliminarily degrade the refractory organic matter in the wastewater to improve its biodegradability, and then the biological filter was used for biodegradation and filtration, so that the COD in the water was reduced to 27.4mg/L and SS <5mg/L； The effluent is filtered through a multi-media filter, which is filled with a combination of quartz sand and manganese sand filter material, which can further adsorb and filter out impurities such as SS, colloidal bacteria, and viruses in the filtered water. The RO influent was stabilized at turbidity <0.4NTU and SDI between 0.4~1.5 after UF treatment. To avoid microbial contamination on the membrane surface, the system adds NaClO disinfectant before the UF is fed into the water. UF outlet high-pressure pump into the security filter, and the effluent plus scale inhibitor and NaSO3 reducing agent into RO. After this process, the pH of the final produced water is 7.4~7.9, the conductivity is 50~200μS/cm, the total hardness is 2~10mg/L, and the total alkalinity is 25~60mg/L, which meets the water quality standard of production and reuse water.

Petrochemical sewage has the characteristics of large fluctuations in water volume and water quality, complex pollutant composition, among which the oil content brought in the production can reach up to 30g/L, sulfide is close to 50mg/L, COD is about 1g/L, the mass concentration of various salts is close to 12g/L, and it also contains volatile phenols and other toxic and harmful substances.

Various forms of oil in sewage and wastewater are generally treated by gravity grease trap recovery and air flotation escape, which can reduce the oil concentration in effluent to less than 30mg/L. Wang Xiaoyang first used grease traps to remove most of the oil slick in petrochemical sewage; then adjust the pH of the sewage to 8~8.5, add catalyst and aeration to oxidize sulfide in the water to control the sulfide concentration in the effluent below 5mg/L; Air flotation removes suspended solids and emulsified oil in sewage; Then, in an anoxic and then aerobic environment, microorganisms were used to degrade organic matter and ammonia nitrogen in the water into CO2, water and N (i.e., 2A/O biological treatment process). After rapid filtration, UF and activated carbon adsorption further escape the SS and organic matter in the water, it enters the RO system. The salt concentration in the final treatment water was reduced to 500mg/L and used as production supplementary water.

The membrane bioreactor (MBR) process is to organically combine the membrane filtration process with the traditional activated sludge method, and directly add curtain microfiltration (MF) or UF membrane components to the activated sludge tank instead of the secondary sedimentation tank for slurry water separation. It has the advantages of large space for regulation of technical conditions such as sludge age and sludge concentration, high effluent quality standards, small equipment footprint, and easy integration. The use of MBR instead of biochemical treatment, multi-media filtration, and UF as RO pretreatment can greatly shorten the RO water treatment process.

A large textile dyeing and finishing enterprise in Shandong first used the hydrolysis acidification tank to anaerobic acidify and decompose the macromolecular organic matter in the printing and dyeing wastewater into small molecule organic matter with high biodegradability, and then relied on gravity to flow into the MBR biochemical tank, and after full adsorption, oxidation and degradation, and filtration, the COD in the wastewater was reduced to less than 110mg/L, and the SS could hardly be effectively detected, which basically met the requirements of RO influent water quality. The engineering design treatment scale is 10,000 m3/d, and the treatment cost of reused water is expected to be 3.92 yuan/m3.

It can be seen that compared with the UF+RO double-membrane method, the MBR+RO double-membrane method not only simplifies production management, but also has a relatively low water treatment cost, making it more suitable for the in-depth treatment of difficult-to-degrade organic wastewater.

2.2.2 Deep treatment of landfill leachate

The landfill leachate is mainly derived from landfill precipitation, and its pollutants are mainly derived from the decomposition of garbage and precipitation leaching by microorganisms, and the water quality is very complex and fluctuate, and the COD is much higher than that of urban sewage, up to 30000mg/L, and the biodegradability is poor. In addition, the leachate may also contain various metal ions such as Fe2+, Cd2+, Cr3+, Cu2+, and Zn2+. During the fermentation stage, the concentration of Fe2+ is even as high as 2000mg/L, and Ca2+ is as high as 4000mg/L.

Although the A/O two-stage biochemical treatment process has been widely used for the degradation and denitrification of organic matter in landfill leachate, the effluent effect is not stable. Therefore, Jiang Yanchao et al. used the combination of UF membrane and A/O to form an MBR process based on mechanical filtration to strengthen the removal rate of organic matter in landfill leachate. The effluent is separated by UF membrane cement and enters the nanofiltration (NF) system. The combined treatment process of MBR+NF+RO was formed by effectively separating organic matter with molecular weight of 200~2000 and some high-valence metal ions in MBR production water. The results show that the process has a good operation effect, and the effluent quality meets the pollution control standards of domestic waste landfills. During operation, the produced water can be discharged directly when the NF effluent meets the discharge requirements, otherwise it will continue to be treated using the subsequent RO system.

WANG et al. used the A/O-MBR+NF+RO process to treat landfill leachate and found that adding activated carbon to the A/O reactor can not only improve the system's removal effect on organic matter and heavy metals, but also reduce membrane pollution.

For highly polluted and difficult-to-degrade landfill leachate, after biochemical treatment and MBR degradation and filtration, a certain amount of free small molecule organic matter and highly polluting microbial metabolites still remain in the effluent.

2.3 Application in high-quality reuse treatment of municipal sewage and wastewater

After secondary biochemical treatment, the content of organic matter in the wastewater is relatively low, usually BOD5<30mg/L, SS<30mg/L, but the biodegradability of the residual organic matter is poor, and the TDS is about 3000mg/L. In recent years, in order to alleviate the pressure caused by water shortages, high-quality reclaimed water has been produced both domestically and internationally by using municipal wastewater treated with secondary biochemical treatment as raw water for RO treatment systems. The UOSA wastewater treatment plant in Virginia, USA, uses a combination of "UF+RO+UV disinfection" to produce reclaimed water that is injected back into the ground to replenish groundwater. Singapore uses the "MF+RO+UV disinfection" process to prepare "new water" to supplement drinking water source water. At present, the world's largest reclaimed water plant, Kuwait Sulaibiya Water Plant uses the secondary treatment effluent of the urban sewage treatment plant as the water source, and uses the UF+RO double-membrane process to treat it to meet the standards of industrial reuse and farmland irrigation.

our country has also set up large-scale reclaimed water plants in Beijing, Tianjin, Hebei, Shandong and other places. Several reclaimed water plants in Tianjin Binhai New Area use the "UF(MF)+RO" double-membrane process to produce high-quality reclaimed water from municipal wastewater after secondary biochemical treatment as raw water, one part as boiler water for thermal power plants, and the other part as landscape river and domestic miscellaneous water. The TOC< < 1.3mg/L, NH3-N0.03mg/L, TN<0.1mg/L, and TP in the treated water were not detected, and the conductivity and turbidity were less than 30μS/cm and 0.12NTU, respectively, and the effluent quality reached the drinking water quality standard.

In short, RO, as a core technology, has been widely used in the deep treatment of various sewage and wastewater or high-quality reuse water treatment. In order to give full play to the technical advantages of RO, while minimizing membrane contamination and reducing water treatment costs, a series of combined processes have been developed for specific water quality. Normally, high concentrations of SS can be removed from water by traditional water treatment methods such as dosing coagulation, sedimentation and efficient filtration; NaClO, aeration oxidation combined with lime emulsion and manganese sand filter to reduce iron, manganese, calcium and silica salts in RO influent. O3 oxidation and A/O biochemical treatment to degrade refractory organic matter in water. The retention of fine SS, small molecule organic matter and high-valence ions was strengthened by MBR, UF and NF, and the UF+RO or MBR+RO double-membrane method, or even MBR+NF+RO three-membrane method was formed to ensure the stable operation of the RO system for the treatment of refractory organic wastewater.

3. Research status of RO membrane separation technology in sewage and wastewater treatment

3.1 Reveal of the formation mechanism of membrane pollution

The effective control of membrane pollution is directly related to the service life of RO membrane and the stability of the treatment process, and the study of pollution mechanism includes the analysis of organic pollutants and the revelation of the generation process.

3.1.1 Analysis of organic pollutants in RO influent

After biochemical treatment and MF or UF membrane interception of sewage and wastewater, a certain amount of soluble small molecule organic matter still remains in the produced water, of which microbial metabolites and secondary metabolites (SMPs) account for about 83% of the effluent TOC, mainly including polysaccharides, proteins, oils, nucleic acids, humic acids, antibiotics, cellular materials, etc. At present, the research on SMP contamination of RO membranes is relatively enthusiastic, mainly focusing on SMP concentration, molecular weight, hydrophilicity, chargeability and simulation of its contamination process on RO membranes.

Zhou Yuexi et al. divided the SMP in biochemically treated organic wastewater into four types: hydrophilic, hydrophobic acidic, hydrophobic and hydrophobic, and analyzed it with gel chromatography, infrared spectroscopy, three-dimensional fluorescence spectroscopy and ultraviolet spectroscopy to study the molecular weight distribution of hydrophilic polysaccharides, hydrophobic organic matter rich in aromatic groups, and weakly hydrophilic humic acid and fulvic acid. MOSHE found that 80% of SMPs have a molecular weight of less than 1k. ZHAO confirmed that the hydrophilic components in the recognizable SMP accounted for 62.9%~69.9%, and the neutral hydrophilic organic matter was dominated by polysaccharides, with low aromatic group content, while the protein content in charged hydrophilic organic matter was high.

3.1.2 Formation of RO membrane pollution

For the RO system in the sewage and wastewater treatment process, the influent not only contains SMP, residual microorganisms, refractory phenols, ketones, aldehydes, polycyclic aromatic hydrocarbons and other small molecule organic matter and some chlorine-substituted hydrocarbons disinfection by-products, but also heavy metal ions carried by the sewage and wastewater itself and high-valent metal ions such as Fe3+ and Al3+ brought in by flocculants, which will further increase the complexity of RO membrane pollution.

SERGEY et al. used alginate instead of hydrophilic organic matter to study the pollution, and found that Ca2+ could aggravate the contamination of alginate to RO membranes, while the effect of Mg2+ could be ignored. SANYUP et al. believe that the alginate gurocuronic acid (G segment) may form an egg-box structure with Ca2+, thus accumulating to form a dense glue-linked network structure. MO et al. used bovine serum protein (BAS) to simulate the proteins in sewage and investigated the effects of different ion contents under various pH conditions on their contamination. It was found that Ca2+ could densify the BAS pollutant layer on the RO surface under the condition of BAS isoelectric point, which was attributed to the fact that Ca2+ could form a bridge bond with the carboxyl group of amino acids. ANG et al. confirmed that when Ca2+ was present, the calcium alginate formed was colloidal with BAS, resulting in a dense gel layer on the surface of the RO membrane. According to the five-stage theory of microbial contamination process proposed by HAN, the surface roughening of RO membrane caused by inorganic substances is the basis for microbial contamination of RO membrane.

It can be seen that for the deep treatment of sewage and wastewater, it is difficult to study the organic pollution, inorganic pollution and microbial pollution of RO membrane separately. Once started, the synergy between them will promote the rapid intensification of RO membrane contamination. Only by delaying the formation of initial membrane pollution and understanding the root cause of the aggravation of membrane pollution can effective mitigation and control methods be established in design and operation management.

3.2 Research on methods for predicting the pollution of influent water in RO system

The effective prediction of the potential pollution of RO influent is the technical guarantee for the stable operation of the RO deep treatment system for sewage and wastewater.

3.2.1 SDI value

The SDI value was determined by passing the water sample to be measured at a pressure of 207kPa through a microporous filter membrane with a diameter of 47mm and a pore size of 0.45μm, and recording the time required for the initial filtration of the 500mL water sample T1, and after the time interval T (usually 15min), the time required to filter the 500mL water sample again was recorded T2, SDI=100×(1-T1/T2)/T. SDI value has been the main reference index in RO system design and operation management for many years due to its simple measurement method and good reproducibility. Usually the RO system requires SDI<3 (some RO membranes are relaxed to SDI<5).

However, in the operation of sewage and wastewater treatment projects, even if the RO influent water quality meets the current water quality requirements, there is still serious membrane pollution in the RO system.

Existing analysis suggests that the SDI value is based on the pore blocking mechanism to determine the degree of congestion of the 0.45μm microfiltration membrane pore by influent water, and the numerical deviation is large for the measurement of fine colloids and small molecule organic matter that are not mainly clogged. Therefore, a single SDI value cannot fully reflect the potential pollution characteristics of refractory organic wastewater after biochemical treatment. In addition, the size of SDI value is related to the pH of the water and the type of organic matter. Zhang Wei et al. found that natural humic acid organic matter showed high SDI values under high pH and low salt content. For water bodies containing iron colloids and organic macromolecules, the SDI value increases sharply with the increase of pH. In the study, the author also found that when the pH of the electronic wastewater pretreated by MBR is less than 4, the SDI value fully meets the requirements of RO influent water quality, but the subsequent RO system still produces serious pollution phenomena in operation.

3.2.2 Research on SDI substitution parameters

In response to the shortcomings of SDI test methods, SCHIPPERS et al. proposed a modified pollution index (MFI). The test method of MFI is the same as that of SDI, but the relationship curve between t/V and V is made according to the time required for the volume of filtered water V t, and the slope of the curve is the MFI value. Although the MFI value shows a good linear relationship with the RO membrane blockage caused by particulate pollutants, it can reflect the filter cake layer filtration mechanism well, but like the SDI value, it can only characterize the contamination of substances trapped by the 0.45μm microporous filter membrane. To this end, BOERLAGE et al. proposed the MIF-UF index by replacing the 0.45μm microfiltration membrane with a cut-off molecular weight of 13 k. In order to be closer to the RO membrane in terms of pore size to reflect the membrane contamination caused by the adsorption of small molecular weight organic matter, KHIRANI et al. proposed the MIF-NF index. KEEWOONG believed that the three groups of values of MIF-MF, MIF-UF and MIF-NF should be measured at the same time for comprehensive evaluation, which could more fully reflect the potential contamination of particulate matter, microcolloids and organic matter to the RO membrane.

In short, through continuous efforts, the current monitoring technology for the potential pollution of RO influent water quality has made great progress in terms of membrane pollution mechanism to pollutant monitoring scope. However, these measurement indicators still have certain deviations in guiding actual production. With the continuous maturity of Internet technology, big data and cloud platform technology, these technologies will have great advantages in the prediction of RO influent water pollution and the linkage of various treatment units.

3.3 RO concentrated water treatment

The RO treatment process concentrates pollutants in the influent water by nearly 3 times while obtaining about 70% of the high-quality reclaimed water, producing about 1/3 of the RO concentrated water. It has the characteristics of large water volume, high mineralization, poor biodegradability, and great potential environmental hazard. Because the treatment and disposal of RO concentrated water have not received enough attention, it has even become one of the obstacles hindering the large-scale application of RO technology in some areas.

Considering the low SS content, scale inhibitor and large residual pressure in RO concentrated water, in addition to partially mixing with RO influent water to improve water recovery in the project, it is often used as a rapid filtration device and UF backwash water, or mixed with raw water after simple treatment and re-entered the treatment system. This is bound to increase the scale and difficulty of sewage and wastewater treatment. At present, most of the research on RO concentrated water treatment focuses on advanced oxidation for organic matter removal and distillation concentration, forward osmosis and electrolysis for resource recovery.

Among them, membrane distillation (MD) technology combines membrane technology with distillation process, using hydrophobic microporous membrane as the medium, and using the effect of vapor pressure difference between the two sides of the membrane to make the volatile components in the material liquid pass through the membrane pores in the form of steam, so as to achieve separation. Compared with other separation processes, membrane distillation has the advantages of high separation efficiency, mild operating conditions, low requirements for the interaction between the membrane and the raw material liquid, and the mechanical properties of the membrane. However, high-quality hydrophobic membranes are still under development.

The technical advantages of electrolysis treatment of RO concentrated water are: 1) high salinity ensures good conductivity of water body, thereby reducing power consumption; 2) The high chloride content in water is conducive to the production of strong oxidants such as hypochlorite in the electrolysis process, which can enhance the oxidative degradation of organic matter. 3) It can treat ammonia nitrogen and refractory organic matter in water at the same time. Electrolysis is also considered one of the most promising RO concentrated water treatment technologies. In addition, combined electrolysis methods that combine ultraviolet irradiation with electrolysis or introduce ultrasound during electrolysis have also been proposed.

In short, for the treatment of RO concentrated water, how to efficiently degrade organic matter and reduce treatment energy consumption is a challenge at present.

4. Conclusion and prospects

At present, RO has become an indispensable core technology for the deep treatment of various sewage and wastewater or high-quality reuse water treatment, in order to give full play to its technical advantages to ensure the stable operation of the RO system and reduce the cost of water treatment, for the SS, easy to scale inorganic pollutants and refractory organic matter in raw water, the combined treatment process has also developed from the traditional flocculation, precipitation, rapid filtration + RO to UF+RO or MBR+RO double-membrane method, and even MBR+NF+RO three-membrane method.

The operation of the RO system is consistent with membrane contamination. With the continuous advancement of research methods in separation, purification and visualization, the root causes of membrane pollution from the aspects of microstructure and pollution formation mechanism, and the key factors of early formation and aggravation of RO membrane pollution can lay a theoretical foundation for effective mitigation and control of membrane pollution. Establishing a more scientific prediction method for the potential pollution of RO influent is the technical guarantee for the design and operation management of RO deep treatment sewage and wastewater process. In view of the fact that the SDI value cannot fully reflect the pollution of RO influent water quality (mainly biochemical treatment effluent is RO influent), the use of MIF combined with different pore size membranes to replace SDI value has been widely studied, and the introduction of Internet technology, big data and other technologies is worth looking forward to. In addition, the effective treatment and disposal methods of high salinity RO concentrated water need to be explored, and the challenges of efficient degradation of organic matter and low energy consumption treatment are its challenges.