Reverse osmosis membrane fouling
1.1 Damage to the performance of the reverse osmosis membrane, resulting in membrane fouling
1. Polyester material reinforced non-woven fabric, about 120μm thick;
2. A porous intermediate support layer of polysulfone material, about 40 μm thick;
3. Ultra-thin separation layer of polyamide material, about 0.2μm thick.
According to its performance structure, such as permeable membrane membrane performance damage may have the following reasons:
1. The maintenance of the new reverse osmosis membrane is not standardized;
2, the maintenance meets the requirements, the storage time is more than 1 year;
3. Under the condition of outage, the maintenance of reverse osmosis membrane is not standardized;
4. The ambient temperature is below 5 °C;
5. The system operates under high pressure;
6. Improper operation when shutting down.
1.2 Water quality changes frequently and cause membrane fouling
The quality of raw water is different from that of the design, which increases the pretreatment load. As the influent contains more impurities such as inorganic substances, organic matter, microorganisms, granules and colloids, the probability of membrane fouling increases.
1.3 If the cleaning is not timely and the cleaning method is incorrect, the membrane will be contaminated.
In the process of use, in addition to the normal attenuation of the film, the cleaning is not timely and the cleaning method is not correct, which is also an important factor leading to serious membrane fouling.
1.4 Did not properly add pharmacy
In the use of the composite polyamide film, since the polyamide film is poor in residual chlorine resistance, the disinfectant such as chlorine is not properly added during use, and the user's attention to the prevention of microorganisms is insufficient, which tends to cause microbial contamination.
1.5 film surface wear
The membrane element is blocked by foreign matter or the surface of the membrane is subject to wear (such as sand, etc.). In this case, the components in the system are detected by the detection method, the damaged component is found, the pretreatment is modified, and the membrane component is replaced.
Reverse osmosis membrane pollution
During the reverse osmosis operation, due to the selective permeability of the membrane, some solute accumulates in the vicinity of the membrane surface, thereby causing membrane fouling.
Common signs of fouling are as follows: one is biological fouling (symptoms gradually appear). Organic deposits are mainly living or dead microorganisms, hydrocarbon derivatives, natural organic polymers, and all carbonaceous materials. Initially it showed an increase in salt rejection, an increase in pressure drop and a decrease in water production. Then there is the colloidal fouling (symptoms gradually appear). During the membrane separation process, the concentration of metal ions and the change of the pH value of the solution may be caused by the deposition of metal hydroxide (mainly represented by Fe(OH)3). Blocking. Initially it showed a slight decrease in the salt rejection rate and gradually increased, and finally the pressure drop increased and the water production decreased. There is also a particulate matter blocking reverse osmosis system in the process of operation, if there is a problem with the security filter, it will cause particles to enter the system, causing fouling of the membrane particles.
Initially, the flow rate of concentrated water increased, the desalination rate did not change much in the initial stage, the water production decreased gradually, and the system pressure drop increased rapidly. Finally, there is chemical fouling (symptoms appear soon). When the feed water contains high Ca2+, Mg2+, HCO3-, CO32-, SO42- plasma, CaCO3, CaSO4, MgCO3 and other scales are deposited on the surface of the membrane. It is characterized by a decrease in the rate of desalination, especially in the last paragraph, and a decrease in water production.
Membrane fouling is the main cause of the decrease in membrane permeate flow.
The pores including the membrane and the solute of the macromolecular solute cause the membrane filtration resistance to increase; the solute adsorbs on the inner wall of the pore; the gel layer on the membrane surface increases the mass transfer resistance. The deposition of components in the pores of the membrane will result in a reduction or even blockage of the pores of the membrane, effectively reducing the effective area of ​​the membrane. The additional resistance of the contaminated layer formed by the deposition of components on the surface of the membrane may be much greater than the resistance of the membrane itself, leaving the permeate flow independent of the permeability of the membrane itself [25]. This effect is irreversible, and the degree of contamination is related to the concentration of the solvent in the membrane material, the retentate, and the concentration and properties of the macromolecular solute, the pH of the solution, the ionic strength, the charge composition, the temperature, and the operating pressure. The flux has dropped by more than 80%.
In the system operation, membrane fouling is a very difficult problem. Its appearance has caused the reverse osmosis device removal rate and water permeability, membrane flux to drop greatly, and increased the operating pressure of each segment, prompting operation and operation. The increase in cost seriously affects the service life of membranes and the development and utilization of reverse osmosis technology.
Solution
3.1 Improve pretreatment
For each membrane device, people hope that it will play its maximum role. It is hoped that there will be a high salt rejection rate, the largest water permeability and the longest life. To achieve the above three points, the water quality is crucial. Therefore, the raw water entering the membrane unit must have good pretreatment. Reasonable pretreatment is very important for the long-term safe operation of the reverse osmosis unit. With the pretreatment that meets the requirements of reverse osmosis influent water quality, it can ensure that the water production flow rate remains stable; the salt rejection rate is maintained at a certain value for a long time; the product water recovery rate can be unchanged; the operating cost is the lowest; the membrane service life Longer wait. Specifically, the reverse osmosis pretreatment is to achieve: (1) preventing contamination on the surface of the membrane, that is, preventing suspended impurities, microorganisms, colloidal substances, etc. from adhering to the surface of the membrane or fouling the water flow passage of the membrane element. (2) Prevent scaling on the surface of the film. In the operation of the reverse osmosis device, due to the concentration of water, some insoluble salts are deposited on the surface of the membrane, so the formation of these insoluble salts is prevented. (3) Ensure that the membrane is protected from mechanical and chemical damage to give the membrane good performance and long enough time to use.
3.2 Cleaning the membrane
Although the liquid is subjected to various pretreatment measures, deposition and scaling may occur on the surface of the membrane after long-term use, and the pores of the membrane are clogged, and the amount of water produced is reduced. Therefore, regular cleaning of the contaminated membrane is necessary. However, the reverse osmosis membrane system cannot be cleaned until the pollution is severe, which will increase the difficulty of cleaning, and also increase the number of cleaning steps and prolong the cleaning time. To properly grasp the timing of cleaning, remove dirt in time.
Cleaning principle: understanding the local water quality characteristics, chemical analysis of the pollutants, and analyzing the results to select the good cleaning agent and cleaning method to provide a basis for finding a good method for the specific water supply conditions;
Cleaning conditions:
a. The product water volume is 5%-10% lower than normal.
b. In order to maintain the product water quantity, the water supply pressure after the temperature is corrected increases by 10%-15%.
c. Increase the conductivity of water quality (increased salt content) by 5%-10%.
d. Multi-stage RO system, the pressure drop through different sections is significantly increased.
Cleaning method: first carry out system backflushing; then carry out vacuum cleaning; if necessary, mechanical cleaning; then chemical cleaning; conditional ultrasonic cleaning; online electric field cleaning is a good method, it is expensive Because the chemical cleaning effect is better, the other methods are not easy to achieve, and the names and methods of use provided by various suppliers are not the same, but the principle is basically the same. For example, our company now uses membrane cleaning agents MC2 and MA10.
The cleaning steps are as follows:
Cleaning single-stage system: (1) configuration cleaning solution; (2) low flow input cleaning solution; (3) circulation; (4) immersion; (5) high flow pump circulation; (6) rinsing; (7) restart system.
For the cleaning of special pollutants: cleaning sulphate scale, cleaning carbonate scale, cleaning iron and manganese pollution, cleaning organic matter pollution, etc.
3.3 Proper maintenance of the membrane
Maintenance of new reverse osmosis membranes New reverse osmosis membrane elements are typically impregnated with 1% NaHSO3 and 18% aqueous glycerol solution and stored in sealed plastic bags. In the case that the plastic bag is not broken, 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 oxidation of NaHSO3 in the air. Therefore, the film should be opened as much as possible before use. In the non-production period, the maintenance of reverse osmosis system is a relatively important issue. It can be done as follows.
1. The system is shut down in a short period of time (1-3 days): Before the shutdown, the system is firstly subjected to low pressure (0.2-0.4 MPa), large flow (about equal to the system's water production), and the time is 14~16 minutes; The usual natural water flow allows water to flow into the thick water channel.
2. The system is out of service for more than one week (the ambient temperature is above 5 °C): Before the shutdown, the system is firstly subjected to low pressure (0.2-0.4 MPa), large flow (about equal to the system's water production), and the time is 14~16 minutes. Chemical cleaning according to the system chemical cleaning method in the reverse osmosis system operation manual; after chemical cleaning, rinse the reverse osmosis membrane; prepare 0.5% formalin solution, low pressure input system, cycle for 10 minutes; close all The valve of the system is sealed;
If the system is out of service for more than 10 days, the formalin solution must be replaced every 10 days.
3, the ambient temperature is below 5 °C: before the shutdown, the system is first low pressure (0.2-0.4MPa), large flow (about equal to the system's water production), the time is 14~16 minutes; in the place where conditions permit, The ambient temperature can be raised to above 5 °C, and then the system maintenance is carried out according to the method of 1; if the ambient temperature is raised unconditionally, the pressure is low pressure (0.1 MPa), and the flow rate is 1/3 of the water production of the system. Long-flow to prevent the reverse osmosis membrane from being frozen, and to ensure that the system is operated for 2 hours per day; after the reverse osmosis membrane is cleaned according to the methods 2, 2) and 3), the reverse osmosis membrane is taken out and moved to ambient temperature. Where it is greater than 5 ° C, immerse in a prepared 0.5% formalin solution, once every two days, the water in the system piping should be drained to prevent system damage caused by icing.
3.4 Avoid film running under high pressure
The system has residual gas remaining during start-stop, allowing the system to operate at high pressure. The pressure gauges on the front and rear of the filter in the system are 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 inlet valve and the concentrate valve to ensure operating pressure and recovery. If the running water flow or total flow rate drops, or if the differential pressure between the primary and intermediate stages is greater than the initial pressure difference (as the initial operation of the new reverse osmosis membrane module data), the system needs to be flushed or cleaned. To ensure the performance of the membrane module is safe.
1. After the equipment is emptied, when the gas is re-run, the gas will not be exhausted and the pressure will be boosted quickly. The remaining air should be drained under the pressure of the system and then gradually boosted.
2. When the joint between the pretreatment equipment and the high pressure pump is not well sealed or leaks (especially the micron filter and the pipeline behind it), when the pretreatment water supply is not sufficient, such as the micro filter is clogged, Where the seal is not good, the vacuum will suck in some of the air. The micron filter should be cleaned or replaced to ensure that the tubing is not leaking.
3. Whether the operation of each running pump is normal, whether the flow rate is the same as the specified value, and compared with the pump running curve to determine the operating pressure.
3.5 Pay attention to the operation when shutting down
1. The quick step-down at shutdown is not thoroughly flushed. Since the concentration of the inorganic salt on the concentrated water side of the membrane is higher than that of the raw water, it is easy to foul and contaminate the membrane. When preparing to shut down, gradually reduce the pressure to about 3bar and rinse with pretreated water for 14 to 16 minutes.
2. When preparing to shut down, adding chemical reagents will cause the drug to stay in the membrane and membrane shell, causing membrane fouling and affecting the service life of the membrane. Should stop adding.
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