Oil and grease cannot be contained in the reverse osmosis feed water, because the presence of oil and grease in the raw water may cause chemical degradation of the aromatic polyamide active layer of the reverse osmosis membrane in the application process and cause degradation of membrane performance. At the same time, the adhesion of grease on the membrane surface is easier to make other pollutants in the water stay on the membrane surface, resulting in other pollution of the reverse osmosis membrane.

1、 Design of reverse osmosis pretreatment system for insoluble inorganic salts in water

      1. Ion exchange softening

      This process is often used when the system does not choose to add organic scale inhibitor, the hardness content of raw water is low and there is a certain content of barium and strontium ions. Generally speaking, this process is most widely used in the pretreatment system of small reverse osmosis units and the reverse osmosis purified water preparation system for drinking water purification.

      2. Lime softening auxiliary adding magnesium agent

      This process is often used in large reverse osmosis systems with high carbonate hardness and dissolved silica content in raw water. Generally speaking, this method can reduce the carbonate hardness of raw water to about 100mg / L, and at the same time, the content of dissolved silica in raw water can also be removed by about 50 ~ 60%. This process is mostly used in the treatment of surface water and industrial circulating water with poor water quality.

      3. Dosing scale inhibitor in feed water

      Due to its strong applicability to raw water and site conditions, easy automatic control and reliable operation, the process is widely used in large-scale reverse osmosis systems and systems with high insoluble inorganic content in raw water.

      At present, in the newly-built reverse osmosis system, the added scale inhibitors are mostly imported products. The common feature of this kind of scale inhibitor is that it is very convenient to dilute and add. The agent has high dispersion ability for a variety of insoluble substances in water. The agent manufacturer can even ensure that there is no scaling when the LSI or S & DSI index of R / O concentrated water system is up to + 2.5 ~ + 3.0. In addition, CaSO4 The saturation of SrSO4, BaSO4 and CaF2 can be expanded by 2.3, 8.0, 60 and 100 times respectively; And some scale inhibitors are compatible with adding flocculants in the pretreatment system.

      In the past, sodium hexametaphosphate, which was often used as a scale inhibitor in China, is gradually being replaced because of its disadvantages such as inconvenient dissolution, affected by temperature, instability and poor dispersion ability. In addition, the phosphate ion and phosphate scale generated after hydrolysis of sodium hexametaphosphate are likely to become the nutrient of microorganisms contained in raw water, thus promoting the reproduction of microorganisms in the reverse osmosis system, which is also one of the reasons why sodium hexametaphosphate is gradually abandoned by users. No matter which scale inhibitor is selected, special attention should be paid to the control of LSI and S & DSI values in the concentrated water system to ensure the safe operation of the system.

      Weak acid cation exchange dealkylation and softening: this method is mostly used in large reverse osmosis systems with high salt content and high alkalinity components (accounting for more than 70% of anion content). However, after this process, the pH value of the treated water is low (4 ~ 5), so the desalination rate of the reverse osmosis system is low due to the excessive increase of inorganic acid permeability of the reverse osmosis system; Even if the pH value of the treated water after decarburization is adjusted or the process without removing carbon dioxide is adopted, the desalination rate can not reach the original ideal level. Nevertheless, the process has been widely used under the water quality conditions of high salt content and high alkalinity.

2、 Design of reverse osmosis pretreatment system for raw water with high dissolved silicon content

      There are generally several methods for the design of reverse osmosis pretreatment system operating under such water source conditions:

      If the site conditions permit, adjust the feed water temperature to about 28 ~ 35 ℃ through the heat exchanger set in the system, so as to improve the solubility of silicic acid compounds in the water, and combine it with the process design to control the water recovery rate of the system to ensure that there is no silica gel scale formation in the operation process of the reverse osmosis system. This is a method often used in engineering. Under such conditions, it is generally necessary to control the content of silica in the reverse osmosis concentrated water system below 150mg / L.

The method of pre softening with lime and adding magnesium agent (siderite) was used to remove silicon. This method can remove more than 60% of silica dissolved in raw water. In addition, this process is troublesome in the actual operation of users. Therefore, this process is rarely used in small water treatment systems, but widely used in large reverse osmosis systems.

Add silicon dispersant. At present, due to the superior performance of imported silicon dispersant, this method has been widely used in large-scale reverse osmosis projects recently started in China. According to the technical documents and relevant information submitted by the reagent supplier, some even allow the content of silica in the reverse osmosis concentrated water system to reach about 240 ~ 290ppm.

      However, for a reverse osmosis system designer, the maximum allowable content of silica in the reverse osmosis concentrated water system in a specific project shall be finally determined according to the technical indicators allowed for the specific dosing and the simulation results of the dosing calculation software meeting the site conditions.

3、 Design of reverse osmosis pretreatment system for raw water containing metal oxides

      The pre oxidation process of raw water is set in the pretreatment system, and then the iron, manganese ions and their compounds in raw water are removed through coagulation, sedimentation, sand filtration or manganese sand filtration.

      In the pretreatment system, the combined treatment process of lime pre softening, coagulation, clarification and sedimentation can generally remove most of the metal oxides in the raw water.

      The combined pretreatment process of electrochemical coagulation, sedimentation and multi-media filtration can also remove most iron metal oxides in water.

      Add chemical dispersant. It can not only effectively prevent the scaling of inorganic salts, but also prevent the deposition of a certain amount of metal oxides in the reverse osmosis membrane system.

4、 Design of reverse osmosis pretreatment system for raw water containing natural organic matter

      In the pretreatment system, the combined treatment process of lime pre softening, coagulation and clarification is set, and then treated through the process of multi-media filtration and fine sand filtration to remove the adsorbed natural humus organic matter in the raw water. The process is widely used in large-scale reverse osmosis pretreatment system for desalination and purification of surface water and circulating water.

      Activated carbon adsorption filtration process is set in the pretreatment system to remove the remaining organic matter in the raw water. This process is often used in small and medium-sized reverse osmosis pretreatment, especially in purified water production and drinking water purification system.

      The organic cleaner process is added in the pretreatment system to completely remove the organic matter in the raw water. This process is mostly used in ultrapure water system of electronic industry and reverse osmosis pretreatment system where the raw water is river, river and lake water with relatively high organic content or complex composition.

      Place the microfilter (0.2 μ m) And ultrafilter (with retained molecular weight of 6000 ~ 20000) are used as pretreatment equipment for removing organic matter. This process is often used in small reverse osmosis systems.

In the pretreatment system, nanofiltration membrane separation equipment is used as the pretreatment equipment of reverse osmosis system, which can remove organic matter, microorganism, virus and heat source with molecular weight above 200. It is widely used in secondary seawater desalination system, ultra pure water preparation system with surface water as water source and domestic drinking water purification system.

5、 The design of reverse osmosis pretreatment for raw water is surface water containing particles and colloids

      Lime pre softening process is set in the pretreatment system, and a small amount of sodium aluminate is added to the clarifier to increase the clarification effect.

      Before pretreatment processes such as multi-media filtration or fine sand filtration, combined pretreatment processes such as adding coagulant / coagulant aid, sedimentation and clarification shall be added.

      Before the reverse osmosis membrane separation system, microfiltration or ultrafiltration pretreatment equipment shall be set to remove such pollutants in raw water.

      The reverse osmosis membrane system does not allow particles larger than 5 microns to enter, because the particles in the reverse osmosis feed water will slip and damage the ultra-thin barrier layer of the reverse osmosis membrane during the operation of the system, resulting in the increase of the salt transmittance of the membrane system and the decrease of the desalination rate of the system.

6、 Design of reverse osmosis pretreatment system for raw water containing bacteria and microorganisms or existing microorganisms

      The allowed non oxidizing chemical bactericide is intermittently added to the reverse osmosis water supply system, and the ultraviolet disinfection process is added to the reverse osmosis pretreatment system.

      Add microfiltration or ultrafiltration process in the pretreatment system.

      Optimize the structural design of reverse osmosis device to reduce the breeding zone of microorganisms; For example, in the piping design of reverse osmosis device, the dead angle of water flow shall be minimized; For another example, when designing the system connection pipeline, try to reduce the design and connection in the form of high low high that causes local ponding of some pipelines during shutdown; Drain valve shall be added at the lower section of appropriate pipeline.

      When treating lakes, rivers and seawater, the process of adding copper sulfate (0.1ppm) to the pretreatment system is designed to control the growth and pollution of microorganisms and algae.

      Generally speaking, the reverse osmosis system requires that the total number of bacteria in raw water should be controlled below 10000 CFU / ml.

7、 Design of reverse osmosis treatment system for urban tap water or self provided water source with slightly poor raw water

      In the pretreatment system, it is considered to set the metering dosing device of reducing agent (sodium bisulfite) or activated carbon adsorption filter to eliminate the remaining free chlorine in the feed water and prevent the degradation of the performance of the surface active layer of the reverse osmosis membrane due to the long-term existence of oxidizing substances in the water. Generally speaking, activated carbon filters are selected in small reverse osmosis systems, while the dosing of reducing agent in pretreatment system is generally considered in large systems.

      When selecting the type of scale inhibitor to be added to the system, consider whether the added scale inhibitor is compatible with the flocculant and coagulant added in the early stage of water supply.

      Generally, historically, in order to ensure the coagulation effect, cationic flocculants were used in waterworks and self provided water source systems that generally use surface water as water source. Therefore, when selecting scale inhibitors for reverse osmosis system, we must pay attention to the compatibility of drugs. Once cationic flocculants are used in the pretreatment of raw water, Anionic scale inhibitors should be avoided in the subsequent reverse osmosis system; If it cannot be avoided, the addition of anionic scale inhibitor in the subsequent process may react with the remaining cationic flocculant in the filtered water, and the reactants formed due to the addition of the agent will be deposited on the membrane surface in the form of colloidal compounds, thus polluting the reverse osmosis membrane. At present, some foreign pharmaceutical manufacturers in the market can generally provide organic flocculants compatible with their scale inhibitors, so the system is particularly safe to use.

8、 The reverse osmosis pretreatment system is designed for raw water in reducing state (anoxic) and containing divalent iron, manganese, hydrogen sulfide and ammonia salt

      When the reverse osmosis system is used to treat the raw water in the reduced state and containing iron and manganese ions, the designer should pay more attention to prevent the membrane pollution caused by the formation of iron and manganese oxides. This is because after the raw water is treated by pretreatment oxidation process - that is, when the oxygen content in the water is more than 5ppm, divalent iron and manganese ions will become the sol of insoluble hydroxide, although generally, such pollutants can be removed by combined processes such as coagulation, sedimentation and medium filtration.

      However, in the actual reverse osmosis water treatment project, there are often many cases of iron fouling in the reverse osmosis membrane system. Years of engineering practice shows that when the pH value of raw water is above 7.7, even if the iron content in reverse osmosis feed water is 0.1ppm and the SDI test value is less than 5, iron membrane pollution may occur, because the oxidation rate of iron is closely related to factors such as iron content, concentration of dissolved oxygen in water and pH value, Therefore, in the pretreatment system, attention should be paid to the control of iron ion content in raw water.

      Engineering practice shows that: generally, when the pH value of raw water is low, the allowable content of iron ion in reverse osmosis feed water can be slightly higher: when the pH value of raw water is less than 6.0, the dissolved oxygen content is less than 0.5ppm, and the iron content of raw water is less than 4ppm, iron pollution is basically impossible in reverse osmosis membrane system; When the dissolved oxygen content of raw water is between 0.5-5ppm and pH is 6.0-7.0, the safe allowable content of iron ion in water shall be less than 0.5ppm; When the dissolved oxygen content of raw water is more than 5ppm and pH > 7.7, the safe allowable concentration of iron ion in reverse osmosis feed water is only 0.05ppm.

      In addition, do not use chlorination process when treating iron containing groundwater for oxidation treatment of raw water, because the colloidal iron formed when iron in water is chlorinated is difficult to remove, thus polluting the reverse osmosis membrane.

      Hydrogen sulfide in groundwater can be removed by chlorination and oxidation, but the actual effect of this method is closely related to the pH value of the treated water source. When the pH of raw water is lower than 6.4, chlorination of raw water can convert hydrogen sulfide into sulfuric acid and exist in water; However, when the pH value of raw water is higher than 6.4, part of hydrogen sulfide will be oxidized into colloidal sulfur during chlorination of raw water.

      Engineering practice shows that when pH is 7 ~ 10, the two reaction components account for about 50% respectively. However, once there is colloidal sulfur in the raw water system, it is very difficult to remove, and its pollution to the reverse osmosis membrane is large, so it should be particularly cautious in the actual reverse osmosis engineering application.

In addition, before the raw water enters the reverse osmosis system, most of the hydrogen sulfide in the raw water can be removed by degassing or stripping.

9、 The design of reverse osmosis pretreatment system for raw water may contain trace oil and grease

      Oil and grease cannot be contained in the reverse osmosis feed water, because the presence of oil and grease in the raw water may cause chemical degradation of the aromatic polyamide active layer of the reverse osmosis membrane in the application process and cause degradation of membrane performance. At the same time, the adhesion of grease on the membrane surface is easier to make other pollutants in the water stay on the membrane surface, resulting in other pollution of the reverse osmosis membrane.

      In the design of reverse osmosis system, when the content of oil and grease in feed water is more than 0.1ppm, oil-water separation, chemical coagulation, activated carbon adsorption filtration or ultrafiltration membrane separation shall be selected according to the specific situation.