Pay more attention to the public account: environmental protection water treatment
1. Reverse osmosis membrane pollution
1. The performance of the reverse osmosis membrane is damaged, resulting in membrane pollution
(1) Polyester material reinforced non-woven fabric, about 120 μm thick, (2) Polysulfone material porous middle support layer, about 40 μm thick, (3) Polyamide material ultra-thin separation layer, about 0.2 μm thick.
According to its performance structure, such as permeable membrane performance damage may have the following reasons:
(1) The maintenance of the new reverse osmosis membrane is not standardized; (2) The storage time exceeds 1 year under the maintenance requirements; (3) The maintenance of the reverse osmosis membrane is not standardized under the state of shutdown; (4) The ambient temperature is below 5 °C; (5) The system is operated under high pressure; (6) The operation is improper when the machine is shut down.
2. Membrane pollution caused by frequent changes in water quality
The water quality of the raw water changes from the water quality at the time of design, which increases the pretreatment load, and increases the probability of membrane pollution due to the increase of impurities such as inorganic matter, organic matter, microorganisms, particles and colloids in the influent water.
3. Membrane pollution caused by untimely cleaning and incorrect cleaning methods
In the process of use, in addition to the normal attenuation of membrane performance, untimely cleaning and incorrect cleaning methods are also important factors leading to serious membrane pollution.
4. The agent was not dosed correctly
In the use of composite polyamide film, because of the poor residual chlorine resistance of polyamide film, chlorine and other disinfectants are not correctly added in use, and users do not pay enough attention to the prevention of microorganisms, which can easily lead to microbial pollution.
5. The surface of the membrane is worn
If the membrane element is blocked by foreign matter or the membrane surface is worn (such as sand, etc.), the detection method should be used to detect the components in the system, find the damaged components, modify the pretreatment, and replace the membrane elements
Second, the phenomenon of reverse osmosis membrane pollution
During reverse osmosis operation, due to the selective permeability of the membrane, some solutes accumulate near the membrane surface, resulting in membrane fouling.
Common signs of fouling are as follows: one is biological fouling (symptoms appear gradually) Organic sediments are mainly live or dead microorganisms, hydrocarbon derivatives, natural organic aggregates, and all carbonaceous substances. Initially manifested by an increase in desalination, an increase in pressure drop, and a decrease in water production. In addition, colloidal fouling (symptoms gradually appear) During the membrane separation, the concentration of metal ions and the change of PH value of the solution may be caused by the deposition of metal hydroxides (mainly represented by Fe(OH)3). Initially manifested by a slight decrease in the desalination rate, it gradually increases, and finally the pressure drop increases and the water yield decreases. In addition, during the operation of the particulate matter fouling reverse osmosis system, if there is a problem with the security filter, it will cause particulate matter to enter the system, resulting in particulate matter fouling of the membrane.
Initially, the brine flow rate increased, the desalination rate did not change much in the initial stage, the water yield gradually decreased, and the system pressure drop increased rapidly. When the feed water contains high levels of Ca2+, Mg2+, HCO3-, CO32-, SO42- and other scales, CaCO3, CaSO4, MgCO3 and other scales will be deposited on the surface of the membrane. This is manifested by a decrease in the rate of desalination, especially in the last section, and a decrease in water production.
Membrane fouling is the main cause of the decrease in membrane permeate flow. Including the membrane pores and macromolecular solute blockage, the membrane filtration resistance increases, the solute adsorbs on the inner wall of the pores, and the gel layer forms on the membrane surface to increase the mass transfer resistance. The deposition of components in the membrane pores will cause the membrane pores to decrease or even block, actually reducing the effective area of the membrane. The additional resistance created by the deposition of the components on the surface of the membrane may be much greater than the resistance of the membrane itself, making the permeation flow independent of the permeability of the membrane itself. This effect is irreversible, and the degree of contamination is related to the concentration and properties of the membrane material, solvent and macromolecular solute in the retained solution, the pH value of the solution, ionic strength, charge composition, temperature and operating pressure, etc., and the membrane flux can be reduced by more than 80% when the pollution is serious.
In the operation of the system, the fouling of the membrane is a very difficult problem, which causes a significant decrease in the removal rate of the reverse osmosis device, the permeability and the membrane flux, and increases the operating pressure of each section, which increases the operation and operation costs, and seriously affects the service life of the membrane and the development and utilization of reverse osmosis technology.
Third, the solution
1. Improve pretreatment
For each set of membrane device, people want it to play the maximum role, hope to have the highest desalination rate, the maximum permeability and the longest possible life, to achieve the above three points, the quality of the water supply is crucial, 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 the reverse osmosis plant. With pretreatment that meets the requirements of reverse osmosis feed water quality, it is possible to ensure that the permeate flow rate remains constant, the desalination rate is maintained at a certain value for a long time, the product water recovery rate can be maintained, the operating costs are minimized, and the membrane service life is long. Specifically, reverse osmosis pretreatment is to achieve: (1) to prevent contamination on the membrane surface, that is, to prevent suspended impurities, microorganisms, colloidal substances, etc. from adhering to the membrane surface or blocking the water flow channel of the membrane element. (2) Prevent scaling on the surface of the film. In the operation of the reverse osmosis unit, due to the concentration of water, some insoluble salts are deposited on the surface of the membrane, so it is necessary to prevent the formation of these insoluble salts. (3) Ensure that the membrane is protected from mechanical and chemical damage, so that the membrane has good performance and long enough service time.
2. Clean the membrane
Although the material liquid has undergone various pretreatment measures, the surface of the film may also be deposited and scaled after long-term use, which will block the membrane pores and reduce the water production, so it is necessary to clean the polluted film regularly. However, the reverse osmosis membrane system cannot wait until the pollution is very serious before cleaning, which will increase the difficulty of cleaning, and also increase the cleaning steps and prolong the cleaning time. It is necessary to grasp the correct cleaning time and remove the dirt in time.
Cleaning principle:
Understand the characteristics of local water quality, conduct chemical analysis of pollutants, and analyze the results to select the best cleaning agent and cleaning method, and provide a basis for finding the best method under specific feed water conditions;
Cleaning conditions:
a. The amount of water in the product is reduced by 5%-10% compared with normal.
b. In order to maintain the amount of water in the product, the water supply pressure after the temperature correction is increased by 10%-15%.
c. Increase the conductivity (increase in salt content) of water quality by 5%-10%.
d. Multi-stage RO system, the pressure drop through different sections increases significantly.
Cleaning Method:
Carry out system backflushing first, then carry out negative pressure cleaning, if necessary, carry out mechanical cleaning, and then carry out chemical cleaning; ultrasonic cleaning if possible; on-line electric field cleaning is a good method, but it is expensive; because the chemical cleaning effect is better, the rest of the methods are not easy to achieve, and although the names and methods of use of the chemicals provided by various suppliers are different, their principles are roughly the same. For example, our company now uses membrane cleaning agents MC2 and MA10.
The cleaning steps are as follows:
Single-stage cleaning system: (1) Equipped with cleaning liquid, (2) Low flow input cleaning solution, (3) Circulation, (4) Soaking, (5) High flow water pump circulation, (6) Flushing, (7) Restart the system.
For special pollutants, cleaning sulfate scale, cleaning carbonate scale, cleaning iron and manganese pollution, cleaning organic pollution, etc.
3. Proper maintenance of the membrane
Maintenance of New Reverse Osmosis Membranes: New reverse osmosis membrane elements are typically soaked with 1% NaHSO3 and 18% glycerol in aqueous solution and stored in a sealed plastic bag. In the case that the plastic bag is not broken, it will not affect its life and performance if it is stored for about 1 year. When the plastic bag is opened, it should be used as soon as possible to avoid the adverse effects on the components due to the oxidation of NaHSO3 in the air. Therefore, the film should be opened as much as possible before use. During the non-production period, the maintenance of the reverse osmosis system is a more important issue. This can be done in the following ways.
(1) The system is out of operation in a short period of time (1-3 days): before the shutdown, the system is washed with low pressure (0.2-0.4MPa) and high flow (about equal to the water production of the system) for 14~16 minutes, and the normal natural water flow is maintained to let the water flow into the concentrated water channel.
(2) The system is out of operation for more than one week (the ambient temperature is above 5 °C): before the shutdown, the system is carried out with low pressure (0.2-0.4MPa) and large flow (about equal to the water production of the system (flushing, the time is 14~16 minutes; chemical cleaning is carried out in accordance with the method of chemical cleaning of the system in the operation manual of the reverse osmosis system; after the chemical cleaning is completed, the reverse osmosis membrane is rinsed; 0.5% formalin solution is prepared and input into the system at low pressure. Cycle for 10 minutes, close the valves of all systems and mothball, and replace formalin solution every 10 days if the system is out of service for more than 10 days.
(3) the ambient temperature is below 5 °C: before the shutdown, the system is washed with low pressure (0.2-0.4MPa), large flow (about equal to the water production of the system), the time is 14~16 minutes; in the place where conditions permit, the ambient temperature can be raised to more than 5 °C, and then the system is maintained according to the method of 1; if the ambient temperature is raised without conditions, then: low pressure (0.1MPa), the flow rate is 1/3 of the water production of the system Long flow, in order to prevent the reverse osmosis membrane from being frozen and damaged, and ensure that the system runs for 2 hours a day; according to the method of 1 (2) and (3), after cleaning the reverse osmosis membrane, the reverse osmosis membrane is taken out, moved to the place where the ambient temperature is greater than 5 °C, soaked in the prepared 0.5% formalin solution, turned over every two days, and the water in the system pipeline should be discharged cleanly to prevent damage to the system caused by freezing.
4. Avoid membrane operation under high pressure
There is residual gas retention in the system during start-stop, so that the system operates under high pressure. In the system, pressure gauges are used to monitor the pressure drop of the filter element, and primary and final pressure gauges are used to monitor the pressure drop of the RO membrane module. Adjust the inlet valve and brine valve to ensure operating pressure and recovery. If the flow rate or total flow rate of produced water decreases during operation, or the pressure difference between the primary and intermediate stages increases greatly compared with the pressure difference at the beginning of operation (based on the data of the new reverse osmosis membrane module in the initial operation), the system needs to be flushed or cleaned to ensure the safety and integrity of the membrane module.
(1) After the equipment is emptied, when it is re-run, the gas is quickly boosted and operated without exhaustion. The remaining air should be exhausted under the pressure of the system, and then the pressure should be gradually increased.
(2) When the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaks (especially the leakage of the micron filter and the pipeline behind it), when the pretreatment water supply is not sufficient, such as the micron filter is blocked, part of the air will be sucked in due to the vacuum in the place where the sealing is not good. The micron filter should be cleaned or replaced to ensure that the pipeline does not leak.
(3) Whether the operation of each running pump is normal, whether the flow rate is the same as the specified value, and compare with the pump operation curve to determine the operating pressure.
5. Pay attention to the operation when the machine is turned off
(1) Rapid depressurization during shutdown without thorough flushing. Because the concentration of inorganic salts on the concentrated side of the membrane is higher than that of the raw water, it is easy to scale and pollute the membrane. When ready to shut down, gradually reduce the pressure to about 3bar and rinse with pretreated water for 14~16 minutes.
(2) When preparing to shut down, adding chemical reagents will make the agent stay in the membrane and membrane shell, cause membrane pollution, and affect the service life of the membrane. Dosing should be stopped.