1、REVERSE OSMOSIS  
Reverse osmosis technology is a new high-tech membrane separation technology developed in the past 20 years. It uses reverse osmosis principle and reverse osmosis membrane with highly selective permeability, a portion of the feed water passes through the membrane perpendicular to the membrane to form desalting water, and the salts and colloidal substances in the water are concentrated on the surface of the membrane, while the remaining portion of the feed water carries the concentrated substances away in a direction parallel to the membrane, self-cleaning during operation. This method can remove 97-98% of inorganic salt, hard ions, organic matter and bacteria in water, and has the advantages of simple operation, low energy consumption and no pollution, has Been widely used in medicine, electronics, food, chemical and other industries. Reverse osmosis system is the core technology of the whole water station. Its main function is to desalinate the pre-treated water. The system includes high pressure pump, reverse osmosis device, reverse osmosis cleaning device. Reverse osmosis and high-pressure pump placed on the reverse osmosis membrane unit rack, is an integrated set of equipment. The complete set of equipment has all kinds of manual valves and all kinds of instrument interface, which is convenient for users to maintain and realize automatic operation of water station. The pre-treated water is pressurized by a high-pressure pump into a reverse osmosis device, where H2o and soluble ions, organic compounds, bacterial viruses and very small particles are separated by a reverse osmosis membrane. More than 97% of the soluble ions, organic matter, bacteria, virus and very small particles were discharged into the sewer with a small amount of concentrated water. 
2、 Problems and treatment of reverse osmosis 
With the wide application of reverse osmosis membrane technology in the field of water treatment, the problem of pollution and blockage of reverse osmosis membrane in operation has attracted more and more attention, and the research on the causes of reverse osmosis pollution and blockage has been deepened day by day. The key component of reverse osmosis system is reverse osmosis membrane. The key to ensure the normal operation of the reverse osmosis system is to keep the membrane and surface clean. The deposition of any substance on the surface will block the membrane and reduce the outlet flow and quality. In order to maintain the production of the factory, it is necessary to increase the inlet pressure, resulting in an increase in production costs. At this time, the membrane must be cleaned to remove dirt and blockage, otherwise the membrane will be damaged, and the frequent cleaning of the membrane will shorten its service life. Serious membrane pollution can only be replaced by membrane replacement. These increase the cost of the factory. In any case, prevention is much better than treatment. 
Effects and manifestations of membrane fouling: 
Increase the pressure of water supply. 
Increase the differential pressure at both ends of the reverse osmosis membrane. 
Reduce infiltration flow. 
Reduce water production quality and increase water production conductivity. 
Increase cleaning frequency and water production cost. 
The fouling of reverse osmosis was initially considered to be caused by the precipitation and scaling on the surface of the membrane after the concentration of calcium carbonate 
The initial dosing of reverse osmosis system was simple acid and inorganic scale inhibitor (such as sodium hexametaphosphate), which simply prevented the precipitation, deposition and scaling of calcium carbonate scale. It was later found that membrane fouling was complex. Even inorganic salt scale, there are also calcium sulfate, barium sulfate, strontium sulfate, calcium fluoride, etc. According to the chemical principle, this scaling should occur at the place with high salt concentration, that is, the end of reverse osmosis. However, at present, the fouling of most reverse osmosis systems is generally in the first stage. After analyzing the fouling on the membrane surface, it is found that there are colloids, microorganisms, particulate matter, etc. The main reason for the large pressure difference in the first section is the deposition of colloids and microorganisms on the membrane surface. With the deepening of understanding, it is found that membrane pollution can be divided into four categories: 
Particle pollution 
Microbial pollution 
Inorganic salt chemical pollution 
Colloid pollution 
The increase of the system pressure difference is caused by the blockage of the water flow channel. We study the nature of various sewage blockage 
The influence of flow channel is discussed. 
1. Particle contamination 
● cause: 
(1) Design and equipment shortage of pretreatment process. Due to the change of water source, pretreatment equipment and reverse osmosis equipment 
The pretreatment efficiency is too low due to different suppliers. 
(2) Leakage of equipment during pretreatment, especially 5 μ The filter is not well sealed, and fine sand particles, activated carbon powder and suspended crystal particles in water leak into the reverse osmosis system. 
(3) Problems in the operation of pretreatment process, such as inappropriate addition of flocculant and coagulant, and too high pretreatment flow rate. 
● performance: the pressure difference in the first section increases 
● solution: 
(1) Check the pretreatment process. 
(2) Check the flocculant, flow rate and filter material of the filter. 
(3) Check the differential pressure and installation of the security filter. 
(4)Cleaning. Once the particles enter the membrane, they will deposit at the intersection of the screen. This usually occurs at the inlet of the system, and it is difficult to clean them out by conventional cleaning. In this case, the current solution is to extract and replace the first membrane element at the inlet of a section, or replace the membrane at the inlet end to the end of the system and change its water inlet direction. Large flow and low pressure flushing shall be adopted. 
2.Microbial contamination 
● cause: 
Microbial contamination is common in the operation of reverse osmosis. This is related to the water source. The microorganism in surface water is relatively high. For the groundwater system, the general method is to pump the groundwater into a large pool first, and then send it to the water treatment station. The pool is an important place for bacterial breeding. Even if there is a sterilization system, due to different types of bacteria, a common agent can not meet the requirements of time, and the bacteria in the system will produce drug resistance over time. 
On the other hand, the sterilization efficiency of commonly used fungicides such as NaClO will be reduced due to the increase of pH, and the sterilization effect is not ideal. Especially, the TC value is not considered in the design, making the sterilization system a virtual device. When microorganisms breed on the membrane surface, they will form biological colonies, and the surface will secrete a mucosal layer at the same time. This mucosal layer will adhere to colloidal suspended solids and inorganic scale particles in water. These colloidal suspended solids provide nutrient source and protective layer for the breeding of microorganisms and form composite scale clusters. Once this happens, improving the turbulence on the membrane surface can not effectively clean the membrane surface. This layer is formed by microorganisms, and the growth of microorganisms on the membrane surface can form a self-supporting multi-layer structure with both surface biological mucosa and self-supporting multi-layer structure. Bactericides are difficult to penetrate the mucosal layer, and bactericides that penetrate the mucosal layer can only kill a layer of microorganisms on its surface. The killed microorganisms form a protein layer, which protects the microorganisms in the lower layer and is difficult to kill completely. If alkali and formaldehyde are used for cleaning, the pressure difference of the membrane can be restored, but after water inflow, the microorganism can restore the breeding conditions, and the pressure difference will rise again. Once microbial contamination occurs on the membrane surface, it is difficult to clean it thoroughly. Microbial reproduction on the membrane surface is active, and biological pollution can be formed in the whole reverse osmosis membrane. 
● performance: 
With the increase of the pressure difference between the first stage and the second stage, the effluent conductivity decreases or increases, and the water yield decreases rapidly. 
● prevention and solution: 
(1) Strictly manage the inflow and cut off the access of microorganisms. 
(2) Change the design of pretreatment system. 
(3) The pretreatment system can change the bactericide regularly, and some efficient bactericides, such as ClO2, can be added by impact. 
(4) Add special bactericides for reverse osmosis, such as riak-781 and riak-787. 
(5) Cleaning. Due to the multi-layer characteristics of microbial pollution, for example, the heavily polluted system must be cleaned repeatedly, stripped layer by layer and killed layer by layer, and the temperature and concentration of the cleaning solution must be appropriately increased. 
3. Organic and colloidal contamination 
● cause: 
In the process of reverse osmosis water production, colloidal particles have an osmotic pressure with the flow direction of water, which forces the colloid 
It is pulled near the membrane surface for enrichment, forming colloid collision and growth, which is adsorbed on the membrane surface and becomes a sewage blockage. Colloid exists in both groundwater and surface water, but the state and quantity are different, so colloid pollution is the most common pollution of reverse osmosis. 
● performance: 
The first stage differential pressure increases and the water yield decreases. 
●membrane manufacturer's solution to colloid contamination: 
(1) Anionic scale inhibitor with high dispersion shall be used. 
(2) Try to reduce the roughness of the membrane surface and use pollution-resistant membrane. 
(3) Under the allowable feed water flow of the membrane, the water inflow of the membrane shall be increased as much as possible to ensure sufficient turbulence. If the funds permit, it is best to design with timed automatic low-pressure flushing. 
(4) Select appropriate anionic high-efficiency dispersant, and conduct low-pressure flushing regularly during operation, especially during shutdown. If the requirements for water recovery are not high, low recovery can be used for operation. Note that the addition amount of scale inhibitor is different when operating with different recovery rates. 
(5) Use a suitable cleaning agent for the membrane. 
4. Chemical pollution 
● cause: 
Chemical pollution is generally inorganic salt scale. Due to the change of water quality, the dosage of scale inhibitor dispersant is not adjusted in time, or the recovery rate is excessively improved, so that the salt in concentrated water crystallizes and precipitates and adheres to the membrane surface. At the same time, the occurrence of pollution will also cause chemical pollution. This is because the water flow channel of the film is not smooth, which will cause the local salt to be high, exceeding the KSP value. 
● performance: 
The second stage differential pressure increases, and the water yield and desalination rate decrease. 
● solution: 
(1) Add high-performance scale inhibitor dispersant. 
(2) Check the full analysis of influent, determine the type and saturation value of scale, and adjust the dosage. 
(3) Use a suitable cleaning agent for the membrane. 
All kinds of pollution mentioned above affect each other. As long as one kind of pollution occurs, it will cause other pollution and faults. Therefore, users should comprehensively do a good job in the operation and maintenance of reverse osmosis. For reverse osmosis system, in any case, prevention in advance is much better than post-treatment. Understanding the causes of pollution blockage is helpful to our prevention and application.