IntroductionCurrently, a complete desalination water preparation process system consists of three parts: pretreatment of raw water, pre-desalination by reverse osmosis, and secondary desalination by ion exchange or EDI.The reverse osmosis system has specific requirements for the pretreatment of raw water. Water sources are generally categorized into groundwater, surface water, municipal tap water, and urban recycled water. These water sources are influenced by various factors, and different geographical conditions and seasonal climates result in variations in the characteristics of the water sources and the impurities they contain. Consequently, the corresponding processes may also differ; For municipal tap water, since the water quality is pure, no clarification tanks are required, and the water can directly enter the mechanical filter for operation without the need for disinfection treatment; For other water sources, due to the complex composition of suspended solids, impurities, and salts in the water, appropriate pretreatment processes must be selected based on these conditions and the main design parameters of the reverse osmosis system, such as recovery rate, to reduce fouling and scaling of the reverse osmosis membranes, prevent a decrease in desalination rate and water production rate, especially given the current trends of dwindling water resources and deteriorating water quality. selecting the correct pretreatment system directly impacts the functionality of the entire water treatment system.As is well known, poor performance of reverse osmosis systems is often due to inadequate pretreatment system functionality. To ensure the normal operation of the reverse osmosis process, strict pretreatment of the raw water is essential. Introduction to Pretreatment ProcessesThe mainstream pretreatment processes currently in use are primarily composed of coagulation treatment equipment, mechanical filters, and ultrafiltration equipment. Coagulation treatment equipment specifically includes sedimentation tanks (clarification tanks) and washing filters; mechanical filters primarily include activated carbon filters and multimedia filters; while ultrafiltration serves as the final barrier to ensure that the water quality from pretreatment meets the operational requirements of the reverse osmosis equipment. Sedimentation Tank (Clarification Tank):Taking a mechanically stirred clarification tank as an example, this type of clarification tank combines the mixing chamber and reaction chamber into one unit. Raw water directly enters the first reaction chamber, where the stirring action of the mixer impeller promotes rapid contact and mixing between the incoming water, chemicals, and a large amount of recirculated sludge. Mechanical reactions occur in the first reaction chamber, and the sludge particles collide and adsorb with existing sludge particles in the recirculated sludge to form larger flocs. These flocs are then lifted by the impeller to the second reaction chamber, where they are diverted to the clarification zone for separation. Clear water rises and is discharged through the collection channel, while sludge is returned to the first reaction chamber from the lower part of the clarification zone. The sludge is scraped into the sludge concentration chamber by a sludge scraper and discharged through the bottom sludge discharge valve, achieving the effect of clarifying and separating the raw water. Under normal operation, the turbidity of the effluent is less than 10 mg/L.Gravity sand filtration:Here, we will use a gravity air-scrubbing filter as an example. This type of filter takes the clarified water treated by mechanical stirring and coagulation in the clarification tank and feeds it evenly into the upper chamber of the filter through the inlet pipe. The water then passes through the sand layer in the filtration zone, the water cap, and the collection chamber at the bottom of the perforated plate, undergoing gravity filtration from top to bottom. (The quartz sand filter layer is backwashed from bottom to top using clear water from the upper water tank of the filter tank. Simultaneously, a Roots blower is used to air-scrub the quartz sand filter layer, removing suspended solids and other impurities trapped in the sand layer to ensure water quality.) The filtered clear water enters the upper water tank of the water-separating chamber via a connecting pipe for storage. Once the water tank is full, the clear water overflows from the overflow of the upper water tank and is sent to the chemical water tank via the outlet pipe as feedwater for ultrafiltration and reverse osmosis desalination. A portion of it can also be sent to the domestic and fire protection water tank as domestic and fire protection water. Under normal operation, the turbidity of the effluent is maintained below 2 mg/L.Mechanical filter:Here, we will use an activated carbon filter as an example. This type of filter can adsorb residual chlorine that cannot be removed during preliminary pretreatment to prevent irreversible damage to the reverse osmosis membrane caused by oxidation. It also adsorbs small-molecule organic pollutants that leak from the previous pretreatment process. It has a significant adsorption and removal effect on odors, colloids, pigments, and heavy metal ions in water, and also reduces COD.Ultrafiltration equipment:  Ultrafiltration (UF) systems are physical filtration devices with a pore size of 0.01 μm. They utilize external pressure to retain colloids, particles, and substances with relatively high molecular weights in water. Controlling the SDI of UF permeate water to no more than 5 ensures the purity of UF permeate water, meeting the feedwater quality requirements for reverse osmosis systems. Precision security filter:Its primary function is to prevent damage to the membrane components in the reverse osmosis feed water. Based on operational mode, it can be divided into backwashable and non-backwashable types. Non-backwashable filter cartridges are disposable, resulting in higher operational costs but better performance. A small number of power plants use backwashable security filters, which are more complex to operate but have lower operational costs. Security filters have become a primary concern for bacterial growth and organic matter deposition within the system.Issues Encountered During Operation and AdjustmentsAs mentioned earlier, the proper functioning of the pretreatment process directly impacts the overall operation of the reverse osmosis system. Currently, the primary issues faced by most power plants can be categorized as follows: improper dosing of coagulants and inadequate control of biocides.Control of coagulant dosage:Currently, most power plants use polyaluminum chloride (PAC) as the coagulant. The aluminum oxide concentration of crystalline PAC is 95%, and it needs to be prepared into a low-concentration PAC solution with an aluminum oxide concentration of approximately 10%. The dosage of the coagulant is administered using a diaphragm metering pump in a continuous manner; By adding the solution to the sedimentation tank (clarification tank), the flocculation effect in the clarification tank is controlled, ensuring that the effluent water quality from the clarification tank meets the standard, i.e., less than 10 mg/L; In actual operation, due to various factors, over-dosage may occur. If over-dosage occurs, it will increase the residual aluminum content in the water, and the generated colloidal substances are prone to deposition on the membrane surface, leading to colloidal contamination. These colloidal contaminants, due to their identical charges, are difficult to treat, thereby forming aluminum salts (if polyferric is used, it may be iron salts); This can reduce membrane flux and increase membrane pressure drop. Similarly, if the dosage is insufficient, it may affect the operation of the clarification tank, failing to ensure the water quality of the effluent, thereby increasing suspended solids in the effluent.Microbial control:Currently, most power plants primarily use surface water as their water source, which contains high levels of organic matter and microbial contamination. These contaminants can cause irreversible damage to reverse osmosis equipment, so it is essential to disinfect the raw water to control microbial growth and minimize the impact on reverse osmosis equipment. A certain power plant uses a Prominent CDVa8A chlorine dioxide generator. During spring and summer, due to high raw water flow rates and water temperatures, microbial and bacterial populations increase, necessitating regular disinfection and algicide treatment of the mechanically stirred clarification tank to control microbial and bacterial growth;In autumn and winter, due to lower raw water temperatures, microorganisms and bacteria in the water are less likely to survive, so disinfection and algicide treatment can be suspended. It is also necessary to control the residual chlorine concentration in the effluent from the mechanically stirred clarification tank between 0.1 and 0.3 mg/l. Excessive addition can lead to high residual chlorine levels in the water. If subsequent reducing agent addition is insufficient, it may cause oxidation of the reverse osmosis membrane, resulting in irreversible damage to the membrane.Activated carbon primarily adsorbs small organic molecules in water. Additionally, activated carbon typically removes 40% to 98% of COD in water. Therefore, installing an activated carbon filter before the ultrafiltration system is essential to meet the feedwater requirements of the ultrafiltration equipment. Since there may still be a small amount of microorganisms and bacteria in the water, and these microorganisms and bacteria can directly adhere to the filter cartridges of the precision security filter, thereby affecting the operation of the security filter, a disinfectant addition system is also installed before the ultrafiltration equipment. If disinfection treatment is performed in the pretreatment stage, the dosage of disinfectant can be reduced here, and the residual chlorine content at the ultrafiltration inlet can be controlled between 0.1 and 0.2 mg/L; If the water source for some small thermal power plants is municipal tap water, since municipal tap water has already undergone disinfection treatment and has good water quality, disinfectant addition can be discontinued. Considering that the ultrafiltration equipment's effluent contains residual chlorine, a reducing agent (sodium bisulfite) must be added at the reverse osmosis inlet to neutralize the residual chlorine's impact on reverse osmosis operation.Organic pollution:Reducing agent: Sodium bisulfite, a white monoclinic crystalline powder with a sulfur dioxide odor, has a purity of 99%, a relative density of 1.49, and is highly soluble in water. It decomposes easily when heated, is slightly soluble in ethanol, and forms an acidic solution in water. It has strong reducing properties but is easily oxidized or loses sulfur dioxide in air. It can be stored for an extended period in a sealed, dark, and dry environment; In sodium bisulfite aqueous solutions, exposure of the solution metering tank to direct sunlight should be avoided. Under sunlight, it reacts with oxygen in the air at any time. Additionally, the storage periods for sodium bisulfite solutions of different concentrations vary. The dosage of the reducing agent should be monitored using oxidation-reduction potential (ORP) to control the residual chlorine content in the water. However, excessive addition may lead to the growth of anaerobic bacteria on the membrane surface, forming new organic pollutants. Long-term operation may cause contamination of the reverse osmosis membrane, resulting in reduced water production and increased operating pressure differential, and frequent replacement of the security filter cartridge; Typically, the dosage is controlled at 3–5 ppm, with residual chlorine at the reverse osmosis inlet maintained between 0.05–0.1 mg/L, and the ORP at the reverse osmosis inlet between 200–300 mV; Excessive addition of reducing agents is a common phenomenon and one of the primary causes of reverse osmosis membrane fouling. Many power plant operators solely rely on ORP meters for dosing, which is not advisable. ORP meters are essentially potential meters, and their measurement data is lagging, easily influenced by factors such as solution temperature, pH, and the reversibility of chemical reactions; Additionally, the redox potential is related to oxygen partial pressure and is also influenced by pH (i.e., the redox potential is higher at lower pH and lower at higher pH). In actual operation, attention should be paid to the linear relationship between ORP redox potential, residual chlorine, conductivity, or pH; Furthermore, the flow rate required for the ORP meter is 8–10 L/h. During operation, due to the presence of a small amount of microorganisms in the water, they may gradually adsorb onto the measurement electrode. If not cleaned for an extended period, this can cause measurement errors. Therefore, after continuous operation for 30 days (or 60 days if the pretreatment water quality is good), the measurement electrode of the ORP meter should be cleaned to ensure the accuracy of the measurement results.

The methods of water treatment include physical treatment and chemical treatmentWith the continuous improvement of human life, the problem of eutrophication of water bodies with nutrients such as ammonia nitrogen and phosphorus, and the gradual improvement of sewage discharge standards by the Environmental Protection Bureau, the traditional "primary treatment" and "secondary treatment" water treatment technology and equipment that have been used for many years are no longer suitable for the treatment requirements of high turbidity and high concentration sewage today. Moreover, the treatment process is long, the system is large, and a large amount of odor is emitted. In order for operators to meet the new emission standards, they need to invest heavily in expanding the existing sewage treatment system, increasing the land occupation, high cost of sewage treatment equipment, and high post maintenance costs. However, traditional sewage deep treatment and recycling technology systems (such as activated carbon filtration, microporous filtration, permeable membrane purification, etc.) have high investment and high post maintenance and operation costs, which are difficult for too many operators to afford.Water treatment includes two types: sewage treatment and drinking water treatment. In some places, sewage treatment is further divided into two types, namely sewage treatment and reclaimed water reuse. The commonly used water treatment agents include polyaluminum chloride, polyaluminum ferric chloride, basic aluminum chloride, polyacrylamide, activated carbon, and various filter media.The processing process of raw water (raw water) to meet the water quality requirements of finished water (domestic water, production water, or dischargeable wastewater).When processing wastewater, it is called wastewater treatment. The purpose of wastewater treatment is for the discharge (discharge into water bodies or land) or reuse (see wastewater treatment, wastewater reuse) of wastewater.Impurities in water and treatment methods: Impurities in water include carried coarse substances, suspended solids, colloids, and dissolved substances. Coarse substances such as floating aquatic plants, garbage, large aquatic organisms, gravel in wastewater, and large debris in rivers. In water supply engineering, coarse impurities are removed by the facilities of the water intake structure and are not included in the scope of water treatment.As for domestic water (or urban public water supply), raw water taken from high-quality water sources (well water or well protected water supply dedicated reservoirs) only needs to be disinfected to become finished water; Raw water from general rivers or lakes must first be removed of sediment and other impurities that can cause turbidity, and then disinfected; Raw water with severe pollution also needs to remove pollutants such as organic matter; Raw water containing iron and manganese (such as some well water) requires removal of iron and manganese. Domestic water can meet the water quality requirements of general industrial water, but industrial water sometimes requires further processing, such as softening, desalination, etc.By adopting a reasonable water treatment process and combining it with deep water treatment, the treated water can meet the standards of GB5084-1992 and CECS61-94 for water recovery, and can be recycled for a long time, saving a lot of water resources.The commonly used sewage treatment technologies include biochemical methods, such as Activated Sludge Process, Fixed Biofilm Processes, Combined Biological Processes, etc; Physical and chemical methods, such as Granular Media Filtration, Activated Carbon Adsorption, Chemical Precipitation, Membrane Processes, etc; Natural treatment methods, such as Stabilization Ponds, Aerated or Facultative Lagoons, Constructed Wetlands, Chemical Color Coarse Resin Treatment, and Nanofiltration Membrane Separation Principle.The principle of dissolution and diffusion: The permeate dissolves in the membrane and diffuses and transfers along its driving force gradient, forming a chemical equilibrium between phases on the surface of the nanofiltration membrane. The transfer form is: energy=concentration o mobility o driving force, so that a substance must overcome the osmotic pressure when passing through the membrane.The reason why the nanofiltration process has ion selectivity is that there are negative charged groups on or in the nanofiltration membrane, which interact with each other through electrostatic forces to hinder the permeation of multivalent ions. The possible charge density of nanofiltration membrane is 0.5-2meq/g.Separation principle of nanofiltration membraneNanofiltration membrane is mainly used to remove solute particles with a diameter of about 1 nanometer (nm) and a cut-off molecular weight of 100-1000. It is mainly used in the field of drinking water to remove trihalomethanes intermediates, odors, chromaticity, pesticides, synthetic detergents, soluble organic matter, hardness components such as Ca and Mg, and evaporation residues.Processing technologyThe purified water treatment process depends on the quality of the raw water.If it is ordinary surface water, it needs to be sterilized and flocculants added before entering the above process.

Reverse osmosis equipment pipeline material selection, to consider the internal and external issues, the internal is the corrosion of water quality on the pipeline, the external is the operating environment. Reverse osmosis equipment operating environment to determine the external corrosion of the pipeline, if the operating environment is better, the general use of surface coating (such as paint or galvanized, etc.) can be. Water quality corrosion of the pipeline to consider residual chlorine, chemicals, PH value, temperature, etc..Stainless steel piping The basic advantage of stainless steel is that it has good resistance to general corrosion, stainless steel rarely produces current corrosion and stress corrosion damage, but stainless steel is prone to pitting and crevice corrosion, pitting corrosion represents the metal is subjected to localized erosion, resulting in sunken holes in its surface, and chlorine ions attack the bare metal to form pitting corrosion if the chromium oxide passivation layer is destroyed. Crevice corrosion is associated with pitting caused by a small handful of still water in small holes, on gasket surfaces, around deposits, and in cracks under screws. In order to avoid the occurrence of pitting and crevice corrosion of the membrane system high-pressure piping, it is recommended as follows: General water sources, you can choose 304 stainless steel: 1, the raw water salt content of 2-5000PPM, it is recommended to use the 316 stainless steel with a carbon content of less than 0.08%; 2, the raw water salt content of 5-7000PPM, it is recommended to use the 316L stainless steel with a carbon content of less than 0.03%; 3, the raw water Salt content in 7-30000PPM, it is recommended to use 904L stainless steel with molybdenum content of 4%-5%.316 stainless steel is after 304, the second widely used steel, mainly used in the food industry and surgical equipment, adding molybdenum to obtain a special structure of corrosion resistance. SS316 is commonly used in nuclear fuel recovery devices.UPVC pipe UPVC pipe characteristics are as follows: 1, strong corrosion resistance: compared with the general cast iron pipe, galvanized pipe, PVC pipe has a strong corrosion resistance, can resist strong acid, strong alkali, will not rust and scale. Do not worry about the use of “red water” phenomenon.2, small fluid resistance: PVC pipe wall is very smooth. Its surface roughness coefficient is only 0.009, fluid resistance is very small, will not excessively reduce water pressure.3, high mechanical strength: water with UPVC pipe has good water pressure, impact resistance, tensile strength, at room temperature can be 110 atmospheric pressure for 1 hour without rupture.4, health and non-toxic: water supply UPVC pipe with a unique green environmentally friendly lead-free formulation system instead of the traditional composite lead salt formulation system, which will not damage the water quality, affect human health.5, light weight, easy to install and construct: PVC pipe density of one-fifth of the general cast iron, easy to move the loading and unloading. And the use of special adhesive paste or elastic sealing joints, installation and construction is simple and fast.6, the temperature performance of the water supply pipe: water supply UPVC pipe temperature range, room temperature is good. Temperature is low, brittleness increases, unfavorable installation and construction; temperature is high, tensile strength decreases, water pressure resistance performance decreases.7, good watertight: 15 minutes after the adhesive bonding, bonding strength can reach 12.5 / C or more, after many years of use will not have a significant decline, and the elastic seals at the joints of the sealing ring, but also with the PVC pipe has a considerable life, no aging leakage concerns.