Review of Pretreatment and Design for Reverse Osmosis Systems

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The information is sourced from the internet; the author is unknown. Review of Pretreatment and Design for Reverse Osmosis Systems 1-1. Classification of contaminants that may occur during the operation of pretreatment systems. Suspended solids: These contaminants are commonly found in surface water and wastewater sources, with particle diameters typically exceeding 1 micron. Such impurities can completely settle down when the water flow is not disturbed, and they can be easily filtered out by the fine sand filters and multi-media filters installed in the reverse osmosis system ； When a flocculant/coagulant dosing process is employed in the reverse osmosis pretreatment system, such impurities can also be adsorbed by the flocs formed by the flocculants, and subsequently removed by a multi-media filter or a sand filter. Colloidal pollutants: These types of pollutants are also commonly found in surface water and wastewater sources. Their particle diameter is usually less than 1 micron, and such impurities do not settle on their own even when there is no flow of water; they remain in a suspended state at all times. Such impurities may be monomeric compounds composed of organic or inorganic components, or they may be complex compounds made up of multiple types of compounds. Such as silicate compounds, iron-aluminum oxides, sulfides, tannic acids, humus, and so on. Biological pollutants: These types of pollutants are also commonly found in surface water or wastewater. When dealing with such water sources, pollution often begins to manifest itself in the membrane elements at the front end of the reverse osmosis system. When this occurs, the pressure at the front part of the R/O system increases rapidly; initially, the desalination efficiency of the reverse osmosis system may even improve as a result. However, as the membrane system continues to operate, biological contamination gradually spreads throughout the entire reverse osmosis system, leading to widespread membrane fouling. When biofouling occurs in membrane systems, it ultimately leads to a significant increase in operating pressure and a decrease in water production. Such pollutants are usually bacteria, biofilms, algae, and fungi. When designing a reverse osmosis process system, it is essential to pay attention to controlling the activity of the raw water. When the bacterial content in the raw water exceeds 1000 cfu/100mg, measures for removing these bacteria must be taken into consideration during the design phase. Organic pollutants: As we have mentioned, the components of organic substances in raw water are extremely complex, and it is very difficult to predict the degree of contamination and impact they have on reverse osmosis membrane elements. However, this parameter is the main pollutant that needs to be carefully considered for protection when designing reverse osmosis systems. The sources of organic pollutants can generally be divided into two categories: one is natural humic organic matter formed from the decay of animals and plants ； Another category is the organic substances resulting from industrial waste pollution. When organic matter contaminates reverse osmosis membranes, it tends to adhere firmly to the membrane surface, making cleaning difficult. Generally speaking, the consequences of organic pollutants on membrane systems are the most difficult to predict: some organic substances in water have almost no destructive effect on reverse osmosis membranes ； However, certain trace amounts of organic substances, once adsorbed on the membrane surface, can not only contaminate the reverse osmosis membrane but also cause its degradation and deterioration. Therefore, we must pay close attention to the organic substances present in raw water. Therefore, in principle, when designing an reverse osmosis system, if the TOC content of the raw water reaches 3 mg/l, corresponding removal measures must be considered within the system. Generally speaking, in reverse osmosis pretreatment systems for treating surface water and wastewater, it is necessary to remove or decompose and transform most of the organic pollutants as much as possible during pretreatment processes such as flocculation, clarification, and oxidation. If the water quality requirements still cannot be met after the above treatments, it is possible to consider using activated carbon adsorption filters, organic matter removers, or ultrafiltration equipment to further remove these impurities, thereby satisfying the requirements for water feed to the reverse osmosis system. 1-2. Design principles and overview of reverse osmosis pretreatment systems 1) Design of reverse osmosis pretreatment systems for insoluble inorganic salts present in water. Ion exchange softening: This process is often used when no organic scale inhibitors are added to the system, when the hardness of the raw water is low, and when the water source contains certain amounts of barium and strontium ions. Generally speaking, this process is currently most commonly used in the pretreatment systems of small-scale reverse osmosis units and in reverse osmosis water purification systems for drinking water. Addition of magnesium agents to assist in lime softening: This process is often employed in large-scale reverse osmosis systems where the carbonate hardness of the raw water and the concentration of dissolved silica are high. Generally speaking, this method can reduce the carbonate hardness of the raw water to around 100 mg/l, and at the same time, it is also possible to remove approximately 50–60% of the dissolved silica in the raw water. This process is commonly used in treating surface water with poor quality and industrial circulating water. Dosage of scale inhibitors in feed water: Due to its strong adaptability to raw water and site conditions, the ease of achieving automatic control, and the reliable operation of the system, this method is widely used in large-scale reverse osmosis systems as well as in systems where the raw water contains high levels of insoluble inorganic substances. In newly installed reverse osmosis systems, the scale inhibitors used are mostly imported products from abroad, such as MDC-150/220/200 from the American company ARGO, and PTP-0100/2000 from the American company KING LEE. The common feature of these scale inhibitors is that they are very easy to dilute and apply. They possess a high capacity to disperse various insoluble substances in water. Manufacturers of these inhibitors can even guarantee that scaling will not occur even when the LSI or S&DSI values in R/O concentrate systems are as high as +2.5 to +3.0. Additionally, the saturation levels of CaSO4, SrSO4, BaSO4, and CaF2 can be increased by 2.3 times, 8.0 times, 60 times, and 100 times respectively ； Moreover, some scale inhibitors are compatible with the flocculants used in the pretreatment system (for example, the MDC-150 scale inhibitor is compatible with the MPT150 type flocculant). Sodium hexametaphosphate, which was previously widely used as a scale inhibitor in China, is gradually being replaced due to its drawbacks such as difficulty in dissolution, sensitivity to temperature, lack of stability, and poor dispersion ability. Furthermore, the phosphate ions and phosphate scale generated upon the hydrolysis of sodium hexametaphosphate can likely serve as nutrients for the microorganisms present in the raw water, thereby promoting their growth within the reverse osmosis system. This is one of the reasons why sodium hexametaphosphate is gradually being abandoned by users. Regardless of the type of scale inhibitor chosen, special attention must be paid to controlling the LSI and S&DSI values in the concentrated water system during its use, to ensure the safe operation of the system. Weak-acid type cation exchange de-alkalization and softening: This method is commonly used in large-scale reverse osmosis systems where the salinity of the raw water is high and the alkalinity content is high (accounting for over 70% of the anion content). However, after this treatment process, the pH value of the treated water is low (4–5), which often results in an increased permeation of inorganic acids through the reverse osmosis system, thereby lowering its desalination efficiency ； Even if the pH of the treated water after decarburization is adjusted or a process that does not remove carbon dioxide is used, its desalination rate cannot reach the previously ideal level. Nevertheless, this process has been widely applied in water conditions with high salt content and high alkalinity. 2) Design of reverse osmosis pretreatment systems for raw water with high dissolved silicon content. For reverse osmosis pretreatment systems operating under such water conditions, there are generally several design approaches: When site conditions permit, the temperature of the feed water is adjusted to around 28–35°C using heat exchangers installed within the system, thereby increasing the solubility of silicate compounds in the water. This approach is combined with process designs aimed at controlling water recovery rates, in order to prevent the formation of silica scale during the operation of the reverse osmosis system. This is a method commonly used in engineering practices. Under such conditions, it is generally necessary to keep the silica content in the reverse osmosis concentrate system below 150 mg/l. Silicon removal is achieved by combining lime pre-softening with the addition of a magnesium agent (gypsum). This method can remove more than 60% of the silica dissolved in the raw water. Additionally, this process is rather complicated to operate in practice, which is why it is rarely used in small water treatment systems; instead, it is widely employed in large-scale reverse osmosis systems. Add a silicon dispersant. Currently, due to the superior properties of imported silicon dispersants, this method has been widely adopted in large-scale reverse osmosis projects that have been commissioned recently in China. In such systems, it is common to use chemicals of the MSI-300 type supplied by the American company ARGO, and the PTP-0100 type supplied by the American company KING LEE. Based on the technical documents and relevant information provided by the chemical suppliers, it is permissible, in some cases, for the silica content in the reverse osmosis concentrate system to reach around 240–290 PPM during use. However, for an reverse osmosis system designer, the maximum allowable level of silica in the reverse osmosis concentrate stream in a specific project should be determined ultimately based on the technical specifications permitted by the chemicals used, as well as the simulation results from software designed for calculating chemical dosing under the actual site conditions. 3) Design of a reverse osmosis pretreatment system for raw water containing metal oxides: A pre-oxidation process is incorporated into the pretreatment system, followed by processes such as coagulation, sedimentation, and sand filtration or manganese sand filtration to remove iron, manganese ions, and their compounds from the raw water. In pretreatment systems, the addition of a combined treatment process involving lime pre-softening along with coagulation, clarification, and sedimentation generally enables the removal of most metal oxides from the raw water. A pretreatment combination process using electrochemical coagulation, sedimentation, and multi-media filtration can also remove the vast majority of iron metal oxides from water. Add a chemical dispersant. It can effectively prevent inorganic salt scaling, while also preventing the deposition of a certain amount of metal oxides in the reverse osmosis membrane system. For example, the MDC-200 chemical dispersant from the American company ARGO can be added to the system. 4) Design of a reverse osmosis pretreatment system for raw water containing natural organic substances. In this pretreatment system, a combined treatment process of lime pre-softening, coagulation, and clarification is employed, followed by treatment through multi-media filtration and fine sand filtration to remove the adsorbed natural humic organic substances from the raw water. This process is widely used in large-scale reverse osmosis pretreatment systems for the desalination and purification of surface water and recycled water. An activated carbon adsorption and filtration process is installed in the pretreatment system to remove the remaining organic matter from the raw water. This process is frequently used in small and medium-sized reverse osmosis pretreatment systems, especially in pure water production and domestic drinking water purification systems. An organic matter cleaner process has been added to the pretreatment system to more thoroughly remove organic matter from the raw water. This process is commonly used in the ultra-pure water systems of the electronics industry, as well as in reverse osmosis pretreatment systems whose raw water comes from rivers, lakes, etc., where the organic content in the water is relatively high or the composition of the water is complex. Microfilters (0.2 μm) and ultrafilters (with a molecular weight cutoff of 6000–20000) are used as pretreatment devices to remove organic substances; this process is frequently employed in small-scale reverse osmosis systems. In a pretreatment system, using nanofiltration membrane separation equipment as the pretreatment unit for a reverse osmosis system allows for the removal of organic substances with a molecular weight of over 200, as well as microorganisms, viruses, and heat sources. It is widely used in secondary seawater desalination systems, ultra-pure water production systems using surface water as a source, and domestic drinking water purification systems. 5) Design for reverse osmosis pretreatment of surface water containing particles and colloids as the raw water: A lime pre-softening process is incorporated into the pretreatment system, with a small amount of sodium aluminate added in the clarifier to enhance the clarification effect. Before pre-treatment processes such as multi-media filtration or fine sand filtration, a series of pre-treatment steps including coagulant/coagulant aid addition, sedimentation, and clarification are added. Before the reverse osmosis membrane separation system, microfiltration or ultrafiltration pretreatment equipment is installed to remove such pollutants from the raw water. Reverse osmosis membrane systems do not allow particles larger than 5 micrometers to pass through, as such particles in the feed water can damage the ultra-thin barrier layer of the reverse osmosis membranes during operation, thereby increasing the salt permeation rate of the membranes and reducing the system’s desalination efficiency. 6) Design of a reverse osmosis pretreatment system for situations where the raw water contains bacteria and microorganisms, or where microorganisms have already begun to grow in the system. Allowable non-oxidizing chemical disinfectants should be added intermittently to the reverse osmosis feedwater system, such as MBC881 and MBC2881 from ARGO Company in the United States, or other non-oxidizing chemical disinfectants with equivalent performance. H`/@o$Y;^ Add an ultraviolet disinfection process to the reverse osmosis pretreatment system. Add microfiltration or ultrafiltration processes to the pretreatment system. Optimize the structural design of the reverse osmosis system to reduce areas where microorganisms can thrive ； For example, when designing the piping of reverse osmosis systems, efforts should be made to minimize dead zones in the water flow ； Furthermore, when designing the system’s connection pipes, efforts should be made to avoid designs and connections of the high-low-high type that can cause localized water accumulation in certain pipes during shutdowns ； Install discharge valves, etc., at the appropriate lower section of the pipeline. When treating lake, river, and seawater, a process of adding copper sulfate (0.1PPM) is incorporated into the pretreatment system to control the growth of microorganisms and algae as well as prevent pollution. Generally speaking, reverse osmosis systems require the total bacterial count in the raw water to be kept below 10,000 cfu/ml. 7) Design of a reverse osmosis pretreatment system for raw water that is of slightly poor quality, such as city tap water or self-sourced water. The pretreatment system should include a device for metered addition of a reducing agent (sodium bisulfite), or an activated carbon adsorption filter, in order to remove any residual free chlorine from the water and prevent degradation of the surface active layer of the reverse osmosis membranes due to the prolonged presence of oxidizing substances in the water. Generally, activated carbon filters are installed in small reverse osmosis systems, while in large systems, the addition of reducing agents is usually considered as part of the pretreatment process. When selecting the type of scale inhibitor to be added to the system, it is necessary to consider whether the selected scale inhibitor is compatible with the flocculants and coagulants that have been added to the water supply earlier on. Historically, in order to ensure effective coagulation, water treatment plants that relied on surface water as a source, as well as systems that used their own water sources, have generally opted for the use of cationic flocculants. Therefore, when selecting scale inhibitors for reverse osmosis systems, it is essential to pay attention to the compatibility of these chemicals. If cationic flocculants are used in the pretreatment process of the raw water, then the use of anionic scale inhibitors in the subsequent reverse osmosis system must be avoided at all costs ； If it cannot be avoided, the addition of anionic scale inhibitors in subsequent processing steps may react with the cationic flocculants still present in the filtered water. The reaction products formed as a result of these additives will deposit on the membrane surface in the form of colloidal compounds, thereby contaminating the reverse osmosis membrane. Currently, some foreign chemical manufacturers in the market are able to provide organic flocculants that are compatible with their scale inhibitors. For example, the MPT150 flocculant supplied by the American company ARGO is compatible with its MDC-150 scale inhibitor, which makes the use of such systems particularly safe. 8) Design of a reverse osmosis pretreatment system for raw water that is in a reduced state (oxygen-deficient), and contains divalent iron, manganese, hydrogen sulfide, and ammonium salts. When treating raw water in a reduced state containing iron and manganese ions using a reverse osmosis system, designers must pay extra attention to preventing membrane fouling caused by the formation of iron and manganese oxides. This is because after the raw water undergoes pretreatment oxidation – that is, when the oxygen content in the water is above 5PPM – ferrous and manganese ions transform into a sol of insoluble hydroxides. Under normal circumstances, such pollutants can be removed through a combination of processes such as coagulation, sedimentation, and media filtration. However, in actual reverse osmosis water treatment projects, there are often many cases of iron causing fouling in the reverse osmosis membrane systems. Years of engineering experience have shown that when the pH value of the raw water is above 7.7, iron contamination of the reverse osmosis feed water can occur even if the iron content is 0.1 PPM and the SDI value is below 5. This is because the oxidation rate of iron is closely related to factors such as the iron content, the concentration of dissolved oxygen in the water, and the pH value; therefore, it is important to control the iron ion content in the raw water within the pretreatment system. Engineering practice has shown that, under normal conditions, when the pH value of the raw water is low, the allowable concentration of iron ions in the reverse osmosis feed water can be slightly higher. When the pH value of the raw water is less than 6.0, the dissolved oxygen content is less than 0.5 ppm, and the iron content in the raw water is below 4 ppm, iron contamination in the reverse osmosis membrane system is practically impossible. When the dissolved oxygen content in the raw water ranges from 0.5 to 5 ppm and the pH value is between 6.0 and 7.0, the safe allowable concentration of iron ions in the water should be below 0.5 ppm. When the dissolved oxygen content in the raw water is above 5 ppm and the pH value is greater than 7.7, the safe allowable concentration of iron ions in the reverse osmosis feed water is only 0.05 ppm. Furthermore, when using oxidation to treat raw water containing iron, do not employ chlorination, as the colloidal iron formed upon the chlorination of iron in the water is difficult to remove, thereby contaminating the reverse osmosis membrane. Hydrogen sulfide in groundwater can be removed through chlorination and oxidation methods, but the actual effectiveness of these methods is closely related to the pH value of the water being treated. When the pH of the raw water is below 6.4, chlorination of the raw water can convert hydrogen sulfide into sulfuric acid, which then remains in the water ； However, when the pH value of the raw water is higher than 6.4, during the chlorination of the raw water, a portion of hydrogen sulfide is oxidized to colloidal sulfur. Engineering practice has shown that at a pH of 7–10, the two reaction components each account for approximately 50%. However, once colloidal sulfur forms in the raw water system, it is very difficult to remove, and it causes significant contamination of the reverse osmosis membranes; therefore, special caution must be exercised in practical reverse osmosis applications. Additionally, most of the hydrogen sulfide in the raw water can be removed using degassing or air stripping methods before it enters the reverse osmosis system. Currently, significant progress and development have been made in technologies for removing hydrogen sulfide. The SulfaClean product provided by the American company SulfaTreat can effectively remove hydrogen sulfide from raw water, and it is simple to use and safe. This treatment method differs from other methods in that it does not convert hydrogen sulfide into other sulfides; instead, it extracts hydrogen sulfide from the filtered water stream, without producing any by-products that dissolve in the water. Sulfaclean is not a chemical additive and requires no maintenance; its consumption is solely dependent on the hydrogen sulfide level in water. It can adapt to changes in hydrogen sulfide levels without any adjustment needed, and it provides excellent removal efficiency – almost no hydrogen sulfide remains in the treated water. Therefore, this process is very safe for use in reverse osmosis systems. 9) Design of a reverse osmosis pretreatment system for raw water that may contain trace amounts of oil and fats. The reverse osmosis feed water must be free of oil and fats, as their presence in the raw water can cause chemical degradation of the aromatic polyamide active layer of the reverse osmosis membrane over time, leading to a decline in membrane performance. Additionally, the attachment of oils and fats to the membrane surface facilitates the retention of other pollutants in the water on that surface, thereby causing further contamination of the reverse osmosis membrane. When designing reverse osmosis systems, if the content of oils and fats in the feed water is above 0.1PPM, processes such as oil-water separation, chemical coagulation, activated carbon adsorption filtration, or ultrafiltration membrane separation should be selected to remove them, depending on the specific circumstances. Note: This article is from the Internet, and the author is unknown. :