Analysis of fouling phenomena in reverse osmosis

Thread Content

Abstract: It explains the principle of reverse osmosis and analyzes reverse osmosis scaling from the perspective of reverse osmosis membranes. And measures to prevent scaling were proposed. Keywords: reverse osmosis, scaling, membrane. Introduction: Currently, there are four methods for producing pure water: distillation, electrodialysis, ion exchange, and reverse osmosis. And reverse osmosis is the most advanced, efficient, and energy-saving technology for producing pure water. Reverse osmosis is a water treatment process that developed rapidly in the 1960s. Currently, it is used in urban water supply, boiler make-up water, power plant boiler make-up water, industrial wastewater, seawater desalination, and the separation of solutes in various solutions. At present, thermal power generation remains the main source of electricity production in China. In thermal power generation, reverse osmosis water treatment technology is widely used. There is the reverse osmosis-based seawater desalination system at Huaneng Yuhuan Power Plant, the reverse osmosis water treatment system at Datang Gangu Power Plant, the reverse osmosis water treatment system at Guodian Handan Thermal Power Plant, and there is also Yangguang Power Generation Co., Ltd. in Yangquan, Shanxi Province, among others. 1 Principle of reverse osmosis: If fresh water and saltwater are separated by a semipermeable membrane that allows only water to pass through while preventing solutes from doing so, the water in the fresh water will move across the semipermeable membrane to the saltwater side. This phenomenon is known as osmosis. Therefore, during the osmosis process, the rise in the liquid level on the saline side generates pressure, which prevents further penetration of fresh water into the saline side. Finally, when the liquid level on the concentrated side is at a certain height above the liquid level on the dilute side, such that the pressure it generates is sufficient to counteract its tendency to permeate, the liquid level on the concentrated side stops rising. At this point, the amount of water that enters the concentrated solution through the semipermeable membrane is equal to the amount of water that leaves the concentrated solution through it, so they are in equilibrium. At equilibrium, the height difference H between the surface of saltwater and that of fresh water represents the osmotic pressure difference between these two solutions. If fresh water is replaced with pure water, this pressure difference represents the osmotic pressure of saltwater. Based on this principle, it is easy to infer that if a pressure higher than the osmotic pressure is applied on the concentrated water side, the pure water in the saline solution can be forced out, that is, the water in the saline solution will penetrate into the pure water. In this way, its direction of penetration is opposite to that of natural osmosis, which is the principle of reverse osmosis (see Figure 1). 2 Semipermeable membranes: The phenomenon of osmosis was discovered in the 18th century. Most commonly, people use animals for experiments. Animal membranes are not true semipermeable membranes; they have many disadvantages and cannot be used in industry. Therefore, the development of reverse osmosis technology depends on the manufacturing process of semi-permeable membranes. A good semipermeable membrane should possess the following characteristics: 1. High water permeability and high desalination efficiency ； 2 High mechanical strength ； 3 Resistant to acids, alkalis, and microbial attack ； 4 Long service life ； 5 It is easy to produce and has a low price. 2.1 Cellulose acetate membrane: This was the first practical synthetic membrane, developed in 1960. Now, its manufacturing method has been improved numerous times, giving the product advantages such as high water permeability, high desalination efficiency, and low cost. 2.2 Polyamide membranes: Before 1970, the membranes produced were mainly aliphatic polyamide membranes, such as nylon-66 and nylon-6; these membranes had very poor water permeability. Later, aromatic polyamide membranes were developed, which exhibit good water permeability, desalination efficiency (see Table 1), mechanical strength, and chemical stability. It can be used in a pH range of 4 to 10 (the long-term usage range is pH 5 to 9). Aromatic polyamide membranes are mainly made into hollow fibers. Table 1 Water permeability and desalination performance of polyamide membranes 2.3 Composite membranes The semipermeable membranes listed above are capable of performing osmosis due to their active layer on the surface. This activation layer only needs to be a very thin layer; if it is too thick, it does not aid in penetration, instead it leads to a decrease in water permeability and accelerates the rate at which flow rate declines over time. However, when manufacturing these membranes, it is difficult to make the activation layer thinner than 0.0001 mm; for this reason, composite membranes were developed. A composite membrane is a composite of two thin layers (see Figure 2); a porous support layer is first created on fabric, and then a polymerization reaction of the active layer is carried out on its surface. The support layer material can be polysulfone, while the activation layer can be polyurea. Composite membranes exhibit superior performance in terms of water permeability, desalination efficiency, and flow rate degradation. Their introduction **reduces the operating pressure in reverse osmosis processes, extends the lifespan of the membranes, and improves the economic viability of reverse osmosis systems. 3.1 Scaling analysis 3.1 Treatment of raw water To prevent clogging of the reverse osmosis units, the raw water must be pre-treated to remove suspended solids and reduce its turbidity ； In addition, sterilization should also be carried out to prevent microorganisms from growing inside the reverse osmosis system. 3.1.1 Suspended solids The amount of suspended solids is an indicator of water quality; if there is a high level of such substances and they are not filtered in time, they will deposit on the reverse osmosis membrane, thereby reducing its water permeability. This causes the membrane pores to become blocked and prevent proper functioning. So the suspended solids are filtered out before water reaches the surface of the membrane. 3.1.2 Turbidity Turbidity is a measure of the degree to which suspended particles in water hinder the passage of light. In other words, due to the presence of insoluble substances in the water, some of the light passing through the water sample is absorbed or scattered, preventing it from passing through in a straight line. Therefore, turbidity is an optical property of water samples. If the turbidity of water is too high, it indicates that there are many insoluble pollutants in the water; if these pollutants enter the pores of the reverse osmosis membrane, they will also block those pores (with pore sizes ranging from <5nm to 40nm), thereby rendering the reverse osmosis membrane ineffective. 3.1.3 Microorganisms There are many types of microorganisms, which are mainly divided into anaerobic and aerobic organisms. Anaerobic microorganisms oxidize ammonia or organic ammonia in water into nitrogen along with nitrate or nitrite compounds in higher oxidation states. These compounds possess strong oxidizing properties; under the action of oxidants, the aromatic amides that make up the membrane get oxidized, causing the pore size of the membrane to change and the long polymer chains to break. As a result, the membrane loses its ability to allow permeation and becomes ineffective, resulting in economic losses for producers. At present, membranes are imported from abroad, with few domestic manufacturers producing them, and they are also expensive. Additionally, the excessive growth of certain microorganisms can attach to the surface of the reverse osmosis membrane, physically blocking its pores as well. 3.1.4 Pollution Index According to the SDGJ2----85 standards issued by the Ministry of Water Resources and Electric Power, the requirements for the water quality of water entering reverse osmosis systems are specified in the table. The pollution index (FI) is an artificial indicator used to indicate the degree to which water quality is affected by suspended impurities. The method of measurement is as follows: Under a certain pressure, water is continuously passed through a small ultrafilter (with a pore size of 0.00045 mm). The time required to allow 500 mL of water to flow out at the start of the process (t0) is recorded. After 15 minutes of water flow, the time required to allow another 500 mL of water to flow out is measured again and recorded as t0; after another 15 minutes, the time required to allow 500 mL of water to flow out is measured once more and recorded as t15. Based on this, the pollution index (FI) can be calculated using Equation 1: Equation 1. This method essentially serves to determine the degree of clogging of the ultrafilter by suspended solids in water. 3.2 Cleaning: After long-term operation, the reverse osmosis unit needs to be cleaned; otherwise, scaling will occur and the water production rate will decrease. Chemical agents can be used for cleaning. The agents used for cleaning include dilute HCl solutions, as well as chelating agents such as citric acid, sodium bisulfite, and sodium hexametaphosphate; the choice can be made depending on the specific circumstances. Ordinary dilute HCl solutions have a high degree of ionization and react rapidly; a pH value greater than 2 can degrade the performance of the membrane. These polyaromatic amides undergo structural deformation when exposed to strong acids and strong bases. Thus, it affected its production efficiency. Citric acid (C6H8O7) is a medium-strength acid; its structure is chiral, which gives it chelating properties and enables it to provide excellent scale-removal effects. Buffer solutions formed by the complexation of sodium bisulfite (NaHSO3) and sodium hexametaphosphate can also remove scale, but sodium ions may remain in such solutions; since the membrane has a negative charge, sodium ions will adsorb onto its surface, which is also detrimental to the membrane. 3.3 Operating pressure: For reverse osmosis to take place, the applied pressure must ensure that the pressure difference across the membrane (△p) is greater than its osmotic pressure difference (△). The effective pressure for carrying out reverse osmosis is the difference between △p and △. The flux of water produced through reverse osmosis can be expressed using Equation 2. In Equation 2, F represents the flux of water produced by reverse osmosis, in units of m3/(m2·h); W represents the permeability coefficient, in units of m3/(m2·h·MPa); △P represents the pressure difference across the membrane, that is, the pressure applied on the saline side, in MPa; and △ represents the osmotic pressure difference across the membrane, also in MPa. The choice of operating pressure determines the concentration of the feed solution, as it is related to the osmotic pressure difference △. Furthermore, it also depends on the water permeability of the membrane and the water recovery rate. Generally, increasing the operating pressure raises the water production rate, but excessive pressure can reduce the water permeation rate due to stress on the membrane. After the membrane is compacted, its water permeability decreases, which reduces its efficiency and leads to economic losses in production. 3.4 Temperature: Since membranes are organic materials, they also follow the principle of thermal expansion and contraction; as the temperature rises, they expand. However, the hydrolysis rate of organic membranes is related to temperature; the higher the temperature, the easier it is for them to hydrolyze. Therefore, the water permeability of the membrane increases as the water temperature rises; however, this also accelerates the hydrolysis rate of the membrane and softens the organic membrane, making it easier to compress. Therefore, for organic membranes, it is generally advisable to keep the temperature within the range of about 20–30°C. 3.5 Concentration polarization: During the reverse osmosis process, as water continuously passes through the membrane, a concentration difference arises between the saline solution near the membrane surface and the inlet saline solution; the concentration of the solution on the membrane surface is higher, and this is what is known as concentration polarization. As a large amount of water passes through the membrane surface, the concentration **increases**, resulting in a supersaturated solution; some salts with low solubility, such as CaSO4 and MgSO4, gradually precipitate as crystals. At first, these salts were just tiny single crystals without any crystal nuclei, so they could not grow. It can only deposit on the surface of the membrane or reach a state of dissolution equilibrium in the solution. As the concentration of the solution on the membrane’s surface increases and a certain equilibrium is reached, crystal nuclei form; these nuclei then start to grow, gradually taking on a planar or spiral shape. If the external temperature is suitable and there is not much dissolving material, the crystals will gradually grow larger. The formation of hard scale on the surface of the membrane can block it, thereby **reducing the water production efficiency**. 4 Prevention of scaling 4.1 Pretreatment of raw water Removal of suspended solids, microorganisms, colloids, calcium sulfate, algae, bacteria, oxidants, residual chlorine, as well as oils or fats, organic substances, and iron-organic complexes from the water. Metal oxides such as those resulting from corrosion of iron, copper, and aluminum also need to be removed. The quality of the incoming water has a significant impact on the lifespan and performance of RO elements and IX resins. 4.2 Turbidity and pollution index: The pollution index must be measured before the water enters the reverse osmosis system. 4.3 Use of scale inhibitors: Classification and principles of scale inhibitors. Commonly used scale inhibitors in water treatment include polyphosphates, organic phosphates, phosphonocarboxylic acids, organic phosphonates, and polycarboxylic acids. 4.3.1 Polyphosphates The commonly used polyphosphates are sodium tripolyphosphate and sodium hexametaphosphate; these produce long-chain anions in water that can easily adsorb onto tiny calcium carbonate particles. Moreover, these anions can readily displace carbonate ions, thereby preventing the precipitation of calcium carbonate. 4.3.2 Organic phosphonic acids – with the systematic names HEDP and EDTMP – are highly effective in suppressing the precipitation or formation of calcium carbonate, hydrated iron oxide, or calcium sulfate. 4.3.3 Organic phosphonates Organic phosphonates are effective in inhibiting calcium sulfate scale, but less effective in inhibiting calcium carbonate scale. It has low toxicity and is easily hydrolyzed. 4.3.4 Polycarboxylic acids: Polycarboxylic acid compounds exhibit excellent scale-inhibiting effects on calcium carbonate scale, and only very small amounts are required. Commonly used ones include polyacrylic acid and hydrolyzed maleic anhydride. Scale inhibitors have a certain degree of reducibility. It can remove excess oxidants from water, which increases the Ksp of certain salts such as CaSO4 and MgSO4 by several times, thereby shifting the dissolution equilibrium in the direction of dissolution. It can also dissolve crystals, making it difficult for them to grow and thereby preventing scaling on the surface of the membrane. This effectively protects the reverse osmosis membrane, and thereby protects the entire reverse osmosis system. 4.4 Shutdown during operation: When reverse osmosis is not in use for a short period of time, it should be cleaned every 4 hours if the water temperature is between 20–30°C, and every 8 hours if the water temperature is below 20°C. While the system is shut down, it is advisable to keep adding scale inhibitors continuously. To prevent concentrated water or residual salts from having time to accumulate and crystallize during shutdown. 4.5 Operating Pressure: In a reverse osmosis system that has been properly calibrated, the frequency of the high-pressure pump is fixed; without the approval of a professional calibration service, the frequency and inlet pressure must not be changed arbitrarily. To prevent the membrane from being compressed. 4.6 Temperature: The temperature of the water also has an impact on reverse osmosis; generally, it is appropriate for the temperature of raw water to be between 20–30°C. 5 Conclusion This paper was completed under the careful guidance of Director Qiao Wanmou of Datang Gangu Power Plant. Here, I would like to express my highest respect and sincere gratitude to Manager Qiao Wanmou! References: 1. New Concepts in Water Purification. Zhang Yajie, Gu Zenan, Wang Weiyi, et al. Beijing: China Architecture & Building Press. 1982. 2. Yao Jixian, et al. Development of LLY High-Efficiency Filters. Water Treatment Equipment Technology. 1990. 3. Edited by East China Architectural Design and Research Institute. Water Supply and Drainage Design Manual. Volume 4: Industrial Water Treatment. Beijing: China Architecture & Building Press, 1986