Pollution analysis and solution of reverse osmosis membrane

Reverse osmosis membrane pollution 1.1 the damage of reverse osmosis membrane performance results in membrane pollution 1 Polyester reinforced non-woven fabric, about 120 μ m thick; 2 Polysulfone porous intermediate support layer, about 40 μ m thick; 3 Polyamide ultra-thin separation layer, about 0.2 μ m thick According to its performance structure, if the performance of the membrane is damaged, there may be the following reasons: 1 The maintenance of the new reverse osmosis membrane is not standardized; 2 If the maintenance meets the requirements, the storage time is more than 1 year; 3 Under the shutdown state, the maintenance of the reverse osmosis membrane is not standardized; 4 The environment temperature is below 5 ℃; 5 The system operates under the high pressure state; 6 The operation is improper during the shutdown 1.2 frequent change of water quality results in the change of the original water quality of membrane pollution as compared with the design water quality, which increases the pre-treatment load Because of the increase of inorganic, organic, microbial, granular and colloidal impurities in the water, the probability of membrane pollution increases 1.3 untimely cleaning and incorrect cleaning methods lead to membrane pollution In addition to normal degradation of membrane performance, untimely cleaning and incorrect cleaning methods are also an important factor leading to serious membrane pollution 1.4 the composite polyamide membrane is not properly added in use, because the polyamide membrane has poor resistance to residual chlorine, does not correctly add disinfectants such as chlorine in use, and the user does not pay enough attention to the prevention of microorganisms, which is easy to cause microbial pollution 1.5 membrane surface wear: the membrane element is blocked by foreign matters or the membrane surface is worn (such as sand) In this case, the detection method should be used to detect the components in the system, find the damaged components, and modify the pretreatment, In the process of reverse osmosis operation, due to the selective permeability of membrane, some solutes accumulate near the membrane surface, resulting in membrane fouling Common signs of fouling are as follows: one is biological fouling (symptoms gradually appear) Organic sediments are mainly living or dead microorganisms, hydrocarbon derivatives, natural organic polymers and all carbon containing substances The initial performance is the rise of desalination rate, the rise of pressure drop and the decrease of water production In the process of membrane separation, the concentration of metal ions and the change of pH value of solution may be caused by the deposition of metal hydroxide (mainly represented by Fe (OH) 3) The initial performance is that the desalination rate slightly decreases, and gradually increases, then the pressure drop increases and the water production decreases In addition, during the operation of the reverse osmosis system, if there is a problem with the security filter, it will cause particles to enter the system, resulting in the fouling of particles in the membrane The initial performance is that the flow rate of concentrated water increases, the desalination rate does not change much in the initial stage, the water yield decreases gradually, and the system pressure drop increases rapidly After that, chemical scaling is also common (symptoms appear quickly) When the feed water contains high Ca2 +, Mg2 +, HCO3 -, CO32 -, SO42 - plasma, the scale such as CaCO3, CaSO4, MgCO3 will be deposited on the membrane surface Its performance is the decrease of desalination rate, especially in the later stage, and the decrease of water production Membrane fouling is the main reason for the decrease of membrane permeate flow Including membrane pore and macromolecule solute blockage, membrane filtration resistance increases; solute is adsorbed on the inner wall of the pore; the gel layer is formed on the surface of the membrane to increase the mass transfer resistance The deposition of components in the membrane pores will reduce or even block the membrane pores, actually reducing the effective area of the membrane The additional resistance produced by the fouling layer formed by the deposition of components on the membrane surface may be far greater than the resistance of the membrane itself, so that the permeability flow has nothing to do with the permeability of the membrane itself [25] This effect is irreversible The degree of pollution is related to the concentration, properties, pH value, ionic strength, charge composition, temperature and operating pressure of membrane materials, solvents in retention solution and macromolecular solutes When the pollution is serious, the membrane flux can be reduced by more than 80% In the operation of the system, the membrane pollution is a very difficult problem, which causes the removal rate, water permeability and membrane flux of the reverse osmosis device to decrease greatly, and at the same time increases the operating pressure of each section, which promotes the operation and operating cost to increase, which seriously affects the service life of the membrane and the development and utilization of the reverse osmosis technology Solution 3.1 perfect pretreatment for each set of membrane device, people want it to play a role in a large extent, hope to have a high desalination rate, a large amount of water permeability and as long as possible life, to achieve the above three points, water quality is essential, so the raw water entering the membrane device must have a good pretreatment Reasonable pretreatment is very important for the long-term safe operation of RO plant With pre-treatment to meet the requirements of reverse osmosis inlet water quality, it can ensure that the water production flow remains stable; the desalination rate remains at a certain value for a long time; the product water recovery rate can remain unchanged; the operating cost is low; the membrane service life is long, etc Specifically, the purpose of RO pretreatment is to: (1) prevent the membrane surface from pollution, that is to prevent suspended impurities, microorganisms, colloidal substances from adhering to the membrane surface or fouling the membrane element water channel (2) Prevent fouling on the membrane surface During the operation of reverse osmosis unit, due to the concentration of water, some insoluble salts are deposited on the membrane surface, so it is necessary to prevent the formation of these insoluble salts (3) Ensure that the membrane is free from mechanical and chemical damage, so that the membrane has good performance and long enough use time 3.2 cleaning the membrane Although the feed solution has been pretreated, deposition and scaling may occur on the membrane surface after long-term use, which will block the membrane pore and reduce the water yield Therefore, it is necessary to clean the polluted membrane regularly However, the RO membrane system can not be cleaned until the pollution is very serious, which will increase the cleaning difficulty, increase the cleaning steps and extend the cleaning time It is necessary to grasp the cleaning opportunity correctly and remove the dirt in time Cleaning principle: understand the characteristics of local water quality, carry out chemical analysis of pollutants, and select the best cleaning agent and cleaning method through result analysis, so as to provide basis for finding the best method under specific water supply conditions; cleaning conditions: a the product water volume is 5% - 10% lower than that under normal conditions b In order to keep the product water volume positive, the water supply pressure after temperature correction is increased by 10% - 15% c The conductivity (salt content increase) of permeated water increased by 5% - 10% d In the multi-stage RO system, the pressure drop through different sections increases obviously Cleaning method: first carry out system recoil; then carry out negative pressure cleaning; if necessary, carry out mechanical cleaning; then carry out chemical cleaning; if possible, ultrasonic cleaning; online electric field cleaning is a good method, which is expensive; because the chemical cleaning effect is relatively good, some other methods are not easy to achieve, and although the name and user of the agent provided by each supplier The method is different, but its principle is roughly the same For example, our company now uses membrane cleaning agents MC2 and ma10 The cleaning steps are as follows: cleaning single-stage system: ⑴ configure the cleaning solution; ⑵ input the cleaning solution with low flow; ⑶ circulate; ⑵ soak; ⑤ circulate with high flow water pump; ⑥ flush; and (7) 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 maintain the membrane properly New RO membrane elements are usually soaked in 1% NaHSO3 and 18% glycerin aqueous solution and stored in sealed plastic bags In the case of unbroken plastic bags, storage for about 1 year will not affect its life and performance When the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the components due to the oxidation of NaHSO3 in the air Therefore, the membrane should be opened before use In the non production period, the maintenance of reverse osmosis system is a more important problem This can be done as follows 1 Short term shutdown of the system (1-3 days): before the shutdown, the system shall be flushed with low pressure (0.2-0.4mpa) and large flow (about equal to the water yield of the system) for 14-16 minutes; the normal natural water flow shall be maintained to let the water flow into the concentrated water channel 2 Shutdown of the system for more than one week (ambient temperature is above 5 ℃): before shutdown, the system shall be flushed with low pressure (0.2-0.4mpa) and large flow (about equal to the water yield of the system) for 14-16 minutes; chemical cleaning shall be carried out according to the relevant system chemical cleaning methods in the operating instructions of the reverse osmosis system; after chemical cleaning, the reverse osmosis membrane shall be flushed; 0.5% formalin shall be prepared Solution, low-pressure input system, circulation for 10 minutes; close all system valves, seal; if the system is stopped for more than 10 days, change the formalin solution every 10 days 3 The ambient temperature is below 5 ℃: before shutdown, flush the system with low pressure (0.2-0.4mpa) and large flow (about equal to the water yield of the system) for 14-16 minutes; if conditions permit, increase the ambient temperature above 5 ℃, and then carry out system maintenance according to the method of 1; if the ambient temperature is increased unconditionally, then: low pressure (0.1MPa) and flow is the water yield of the system 1 / 3 of the amount of water will flow for a long time 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 the method of 1, 2 and 3), take out the reverse osmosis membrane, move it to the place where the ambient temperature is higher than 5 ℃, soak it in the prepared 0.5% formalin solution, turn it over every two days, and drain the water in the system pipeline, To prevent damage to the system due to icing 3.4 avoid residual gas in the system when the membrane operates under high pressure, so that the system operates under high pressure The pressure gauge before and after the filter in the system is used to monitor the pressure drop of the filter element, and the primary and final pressure gauges are used to monitor the pressure drop of the RO membrane module Adjust the water inlet valve and concentrated water valve to ensure the operation pressure and recovery rate If the flow or total flow of water in operation decreases, or the pressure difference between the primary and intermediate stage increases more than that in the early stage of operation (based on the data of new RO membrane components in initial operation), the system shall be flushed or cleaned to ensure the performance and safety of membrane components 1 After the equipment is drained, when it is re operated, the gas is not exhausted and the pressure is increased rapidly After exhausting the remaining air under the pressure of the system, gradually boost the pressure for operation 2 When the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaking (especially the micro filter and the pipeline behind it) when the pretreatment water supply is not sufficient, for example, the micro filter is blocked, and some air will be absorbed due to the vacuum in the place with poor sealing The micron filter shall be cleaned or replaced to ensure that the pipeline does not leak 3 Check whether the operation of each operating pump is normal, whether the flow is the same as the specified value, and compare with the pump operation curve to determine the operation pressure 3.5 pay attention to the operation during shutdown 1 The rapid decompression during shutdown did not conduct thorough flushing Because the concentration of inorganic salt in the concentrated water side of the membrane is higher than that in the raw water, it is easy to scale and pollute the membrane When preparing to shut down, gradually reduce the pressure to about 3bar and wash with pre-treatment water for 14-16 minutes 2 In preparation for shutdown, adding chemical reagent will make the reagent stay in the membrane and membrane shell, causing membrane pollution and affecting the service life of the membrane Dosing shall be stopped.