1. Causes of reverse osmosis membrane fouling 1.1 Damage to reverse osmosis membrane performance leading to membrane fouling
1. Polyester material reinforced non-woven fabric, approximately 120 μ m thick; 2. Porous intermediate support layer of polysulfone material, approximately 40 μ m thick; 3. Ultra thin separation layer of polyamide material, approximately 0.2 μ m thick. According to its performance structure, if the performance of the permeable membrane is damaged, there may be several reasons: 1. The maintenance of the new reverse osmosis membrane is not standardized; 2. If the maintenance meets the requirements and the storage time exceeds 1 year; 3. Under shutdown conditions, the maintenance of the reverse osmosis membrane is not standardized; 4. The ambient temperature is below 5 ℃; 5. The system operates under high voltage conditions; 6. Improper operation during shutdown. 1.2 Frequent changes in water quality can cause membrane fouling. The water quality of the raw water has changed compared to the design, increasing the pre-treatment load. Due to the increase in impurities such as inorganic matter, organic matter, microorganisms, particulate matter, and colloids in the influent, the probability of membrane fouling increases. 1.3 Failure to clean in a timely manner and incorrect cleaning methods can cause membrane fouling. During use, in addition to the normal degradation of membrane performance, failure to clean in a timely manner and incorrect cleaning methods are also important factors leading to severe membrane fouling. 1.4 Improper addition of chemicals in the use of composite polyamide film, due to poor residual chlorine resistance of polyamide film, improper addition of chlorine and other disinfectants during use, coupled with insufficient attention to microbial prevention by users, can easily lead to microbial contamination. 1.5 Membrane surface wear: If the membrane element is blocked by foreign objects or the membrane surface is worn (such as sand particles), the detection method should be used to detect the components in the system, find the damaged components, modify the pre-treatment, and replace the membrane element
2. Phenomenon of reverse osmosis membrane fouling
During the reverse osmosis operation, due to the selective permeability of the membrane, certain solutes accumulate near the membrane surface, resulting in membrane fouling and blockage.
There are several common signs of pollution blockage: one is biological blockage (symptoms gradually appear). Organic sediments are mainly composed of live or dead microorganisms, hydrocarbon derivatives, natural organic polymers, and all carbonaceous substances. Initially manifested as an increase in desalination rate, an increase in pressure drop, and a decrease in water production. Another issue is colloidal fouling (symptoms gradually appear). During the membrane separation process, the concentration of metal ions and changes in solution pH may be caused by the deposition of metal hydroxides (mainly represented by Fe (OH) 3), leading to fouling. Initially manifested as a slight decrease in desalination rate, gradually increasing, and finally increasing pressure drop and decreasing water production. In addition, during the operation of the reverse osmosis system, if there is a problem with the security filter, particulate matter can enter the system and cause membrane fouling. The initial manifestation is an increase in concentrated water flow rate, with little change in desalination rate in the early stages, a gradual decrease in water production, and a rapid increase in system pressure drop. Finally, chemical scaling (symptoms appear quickly) is also common. When the feedwater contains high levels of Ca2+, Mg2+, HCO3-, CO32-, SO42- ions, CaCO3, CaSO4, MgCO3 and other scales will deposit on the membrane surface. It is manifested as a decrease in desalination rate, especially in the last stage, and a decrease in water production. Membrane fouling is the main reason for the decrease in membrane permeation flow rate. The blockage of membrane pores and large molecular solutes leads to an increase in membrane filtration resistance; Solute adsorbs on the inner wall of the pore; The gel layer formed on the membrane surface increases the mass transfer resistance. The deposition of components in the membrane pores will cause a reduction or even blockage of the pores, which actually reduces the effective area of the membrane. The additional resistance generated by the fouling layer formed by the deposition of components on the membrane surface may be much greater than the resistance of the membrane itself, making the permeate flow rate independent of the permeability of the membrane itself [25]. This effect is irreversible, and the degree of pollution is related to the concentration, properties, pH value, ionic strength, charge composition, temperature, and operating pressure of the membrane material, retained solvent, and macromolecular solute in the solution. Severe pollution can cause a decrease of over 80% in membrane flux. During system operation, membrane fouling is a very tricky problem, which causes a significant decrease in the removal rate, permeability, and membrane flux of reverse osmosis devices. At the same time, it increases the operating pressure of each section, leading to an increase in operating costs and seriously affecting the service life of membranes and the development and utilization of reverse osmosis technology.
3. Solution 3.1 Improve Preprocessing
For each membrane device, people hope that it can maximize its effectiveness, achieve the highest desalination rate, maximum permeability, and longest lifespan. To achieve these three points, the quality of the supplied water is crucial. Therefore, the raw water entering the membrane device must have good pretreatment. Reasonable pretreatment is crucial for the long-term safe operation of reverse osmosis devices. With pre-treatment that meets the requirements of reverse osmosis inlet water quality, it is possible to ensure stable water production flow; The duration for which the desalination rate remains at a certain value; The product water recovery rate can remain unchanged; Minimum operating costs achieved; The membrane has a longer service life, etc. Specifically, the purpose of reverse osmosis pretreatment is to: (1) prevent contamination on the membrane surface, that is, to prevent suspended impurities, microorganisms, colloidal substances, etc. from adhering to the membrane surface or clogging the water flow channels of the membrane components. (2) Prevent scaling on the membrane surface. During the operation of the reverse osmosis device, due to the concentration of water, some insoluble salts deposit on the membrane surface, so it is necessary to prevent the generation of these insoluble salts. (3) Ensure that the membrane is protected from mechanical and chemical damage to provide good performance and sufficient service life. 3.2 Cleaning the membrane: Despite various pretreatment measures, the surface of the membrane may still deposit and scale after long-term use, causing blockage of membrane pores and a decrease in water production. Therefore, regular cleaning of contaminated membranes is necessary. But the reverse osmosis membrane system cannot be cleaned only after severe pollution, which will increase the difficulty of cleaning, increase the number of cleaning steps, and prolong the cleaning time. To grasp the cleaning timing correctly and remove dirt in a timely manner. Cleaning principle: Understand the local water quality characteristics, conduct chemical analysis on pollutants, select the best cleaning agent and cleaning method through result analysis, and provide a basis for finding the best method under specific water supply conditions; Cleaning conditions: a The water content of the product has decreased by 5% -10% compared to normal. b. To maintain the correct product water volume, the water supply pressure after temperature correction is increased by 10% -15%. c. By increasing the water conductivity (salt content) by 5% -10%. d. The multi-stage RO system experiences a significant increase in pressure drop across different stages. Cleaning method: First, perform a system backwash; Perform negative pressure cleaning again; If necessary, perform mechanical cleaning; Perform chemical cleaning again; If conditions permit, ultrasonic cleaning can be used; Online electric field cleaning is a good method, but it is expensive; Due to the good cleaning effect of chemical cleaning, some other methods are not easy to implement, and although the names and usage methods of the chemicals provided by various suppliers are not the same, their principles are generally the same. Our company currently uses membrane cleaning agents MC2 and MA10. The cleaning steps are as follows: Clean the single-stage system: ⑴ Prepare cleaning solution; ⑵ Low flow input cleaning solution; ⑶ Cycle; ⑷ Soak; ⑸ High flow water pump circulation; ⑹ Rinse; ⑺ Restart the system. Cleaning for special pollutants includes cleaning sulfate scale, cleaning carbonate scale, cleaning iron and manganese pollution, cleaning organic pollution, etc. 3.3 Proper maintenance of the membrane New reverse osmosis membrane maintenance New reverse osmosis membrane components are usually soaked in 1% NaHSO3 and 18% glycerol aqueous solution and stored in sealed plastic bags. Storing the plastic bag for about a year without breaking it will not affect its lifespan and performance. After the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the components caused by NaHSO3 oxidation in the air. Therefore, the film should be opened as much as possible before use. During non production periods, maintenance of the reverse osmosis system is a significant issue. The following method can be used. 1. Short term shutdown of the system (1-3 days): Before shutdown, the system should be flushed with low pressure (0.2-0.4MPa) and high flow (approximately equal to the system's water production) for 14-16 minutes; Maintain normal natural water flow and let the water flow into the thick channel. 2. System shutdown for more than a week (ambient temperature above 5 ℃): Before shutdown, the system should be flushed with low pressure (0.2-0.4MPa) and high flow (approximately equal to the system's water production) for 14-16 minutes; Perform chemical cleaning according to the method described in the reverse osmosis system operation manual for system chemical cleaning; After chemical cleaning, rinse the reverse osmosis membrane thoroughly; Prepare a 0.5% formalin solution, input it into the system at low pressure, and circulate for 10 minutes; Close all valves of the system and seal them; If the system is shut down for more than 10 days, the formalin solution must be replaced every 10 days. 3. Environmental temperature below 5 ℃: Before shutdown, the system should be flushed with low pressure (0.2-0.4MPa) and high flow (approximately equal to the system's water production) for 14-16 minutes; In places where conditions permit, the ambient temperature can be raised to above 5 ℃, and then the system maintenance can be carried out according to method 1; If the ambient temperature is raised unconditionally, then: low pressure (0.1MPa), water with a flow rate of 1/3 of the system's water production is used for long flow to prevent the reverse osmosis membrane from freezing and ensure that the system runs for 2 hours every day; After cleaning the reverse osmosis membrane according to methods 1, 2), and 3), remove it and move it to an environment with a temperature greater than 5 ℃. Soak it in a prepared 0.5% formalin solution and flip it every two days. The water in the system pipeline should be drained completely to prevent damage to the system due to freezing. 3.4 Avoid residual gas accumulation during the start-up and shutdown of the membrane operating under high pressure, so that the system operates under high pressure. The pressure gauges before and after the filter in the system are used to monitor the pressure drop of the filter element, while the primary and final pressure gauges are used to monitor the pressure drop of the RO membrane components. Adjust the inlet valve and concentrate valve to ensure operating pressure and recovery rate. If the water flow rate or total flow rate decreases during operation, or if the pressure difference between the primary and intermediate stages increases significantly compared to the initial operation (based on the data of the new reverse osmosis membrane module), the system needs to be flushed or cleaned to ensure the performance and safety of the membrane module. 1. After the equipment is emptied, when it is restarted, the gas is quickly pressurized before it is fully discharged. The remaining air should be exhausted under the pressure of the system before gradually increasing the pressure for operation. 2. When the joint between the pre-treatment equipment and the high-pressure pump is poorly sealed or leaks water (especially when the micro filter and its subsequent pipeline leak), and the pre-treatment water supply is not sufficient, such as when the micro filter is blocked, some air will be sucked in due to vacuum in the poorly sealed area. The micro filter should be cleaned or replaced to ensure that the pipeline does not leak. 3. Check whether the operation of each pump is normal, whether the flow rate is the same as the specified value, and compare it with the pump operation curve to determine the operating pressure. 3.5 Pay attention to the operation when shutting down: 1. Rapid pressure reduction during shutdown without thorough flushing. Due to the higher concentration of inorganic salts on the concentrated water side of the membrane compared to the raw water, it is prone to scaling and fouling of the membrane. When preparing to shut down, gradually reduce the pressure to around 3 bar and rinse with pre treated water for 14-16 minutes. 2. When preparing to shut down, adding chemical reagents can cause the agents to remain in the membrane and membrane shell, leading to membrane fouling and affecting the lifespan of the membrane. The addition should be stopped.
