1. causes of reverse osmosis membrane pollution

one point one   The damage of the performance of RO membrane, which causes membrane pollution

1. Polyester reinforced non-woven fabric, ca. 120 μ M thick;

2. Polysulfone material porous intermediate support layer, ca. 40 μ M thick;

3. Ultra thin separation layer of polyamide material, about 0.2 μ M thick.

According to its performance structure, such as the performance damage of the membrane, there are several reasons:

1. The maintenance of new RO membrane is not standardized;

2. The storage time is more than 1 year when the maintenance meets the requirements;

3. Under the shutdown condition, the maintenance of reverse osmosis membrane is not standardized;

4. The ambient temperature is below 5 ℃;

5. The system operates under high pressure;

6. Improper operation when shutting down.

one point two   Membrane pollution caused by frequent changes in water quality

The raw water quality has a change with the design water quality, which makes the pretreatment load increase. Because the impurities such as inorganic matter, organic matter, microorganism, granular matter and colloid are increased in the water, the membrane pollution probability increases.

one point three   Membrane pollution caused by improper cleaning and improper cleaning methods

In the process of using, in addition to the normal attenuation of the performance of the membrane, the improper cleaning and cleaning methods are also an important factor leading to serious membrane pollution.

one point four   No proper dosing

In the use of the compound polyamide membrane, because of the poor residual chlorine resistance of the polyamide film, no disinfectant such as chlorine was correctly added in the use. In addition, the user paid insufficient attention to the prevention of microorganisms, which could easily lead to microbial pollution.

one point five   Wear of membrane surface

The membrane element is blocked by foreign matter or the surface of the membrane is worn (such as sand), in this case, the detection method shall be used to detect the components in the system, find the damaged components, modify the pretreatment and replace the membrane elements

2. the phenomenon of reverse osmosis membrane pollution

In the process of reverse osmosis, some solutes accumulate near the membrane surface due to the selective permeability of membrane, which leads to membrane fouling.

The common signs of pollution plugging are as follows: one is that the organic sediment is mainly living or dead microorganisms, hydrocarbon derivatives, natural organic polymers and all carbon containing substances. The initial performance was the increase of desalination rate, pressure drop and water production. The other is that the concentration of metal ions and the change of pH value of solution may be the deposition of metal hydroxide (mainly represented by Fe (OH) 3, which may cause pollution plugging during the membrane separation process. The initial performance was a slight decrease in desalination rate, and gradually increased, and finally the pressure drop and water production decreased. In addition, if there is a problem with the security filter in the operation of the RO system, the particles will enter the system and cause the particulate matter pollution blocking of 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 gradually decreases, and the pressure drop of the system increases rapidly. Finally, chemical scaling (symptoms appear soon) when the water supply contains higher ca2+, mg2+, hco3-, co32-, SO42 plasma, CaCO3, CaSO4, MgCO3 and other scales will be deposited on the membrane surface. The results show that the desalination rate is decreased, especially in the last section, and the water yield is decreased.

Membrane pollution is the main reason for the decrease of membrane permeation flow.

The filtration resistance of the membrane increases due to the pore and the blockage of macromolecular solute; Solute adsorbed on the inner wall of the pore; The gel layer on the film surface increases the mass transfer resistance. The deposition of components in the membrane pore will cause the decrease of the pore and even the blockage of the membrane, which in fact reduces the effective area of the membrane. The additional resistance of the polluted layer formed by the deposition of the component on the membrane surface may be much greater than that of the membrane itself, and the permeability flow is not related to the permeability of the membrane itself [25]. The influence is irreversible. The pollution degree is related to the concentration, property, pH value, ion 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 system operation, membrane pollution is a very difficult problem, which causes the removal rate, permeability and membrane flux of reverse osmosis device to decrease greatly, and increases the operating pressure of each section, and increases the operation and operation cost, which seriously affects the service life of membrane and the development and utilization of reverse osmosis technology.

3. solutions

three point one   Improve pretreatment

For each membrane device, people want it to play its maximum role, hope to have the highest desalination rate, the maximum water permeability and the longest life as possible. To achieve the above three points, the water quality is very important, so the raw water entering the membrane device must be well pre-treatment. Reasonable pretreatment is very important for the long-term safe operation of reverse osmosis plant. With the pretreatment to meet the requirements of the water quality of the reverse osmosis water, the water production flow can be maintained stable; The desalination rate is maintained at a certain value for a long time; The water recovery rate of the product can be constant; The operating cost is the lowest; The membrane has a long service life. Specifically, the purpose of RO pretreatment is to: (1) prevent the surface pollution of membrane, that is, to prevent suspended impurities, microorganisms, colloidal substances, etc. from adhering to the membrane surface or to block the flow channel of membrane elements（ 2) Prevent scaling on the surface of the film. In the operation of reverse osmosis unit, some insoluble salt is deposited on the membrane surface due to the concentration of water, so the formation of these insoluble salts should be prevented（ 3) Ensure that the film is free from mechanical and chemical damage to give the film good performance and long enough time.

three point two   Cleaning the film

Although the feed liquid is pretreated, the surface of the membrane may also deposit and scale after long-term use, which will block the membrane hole and reduce the water production. Therefore, it is necessary to clean the polluted film regularly. But the RO membrane system can not wait until the pollution is serious before cleaning, which will increase the cleaning difficulty, and increase the cleaning steps and the longer the cleaning time. The cleaning time should be correctly grasped and the dirt should be removed in time.

Cleaning principle: understand the local water quality characteristics, carry out chemical analysis on pollutants, and select the best cleaning agent and cleaning method through the result analysis, and provide the basis for finding the best method under specific water supply conditions;

Cleaning conditions:

a.   The water volume of the product is 5% to 10% lower than that of normal.

b.   In order to maintain the water quantity of the product, the water supply pressure increases by 10% to 15% after the temperature is corrected.

c.   The conductivity of water quality (salt content increased) increased by 5% to 10%.

d.   The pressure drop of multi-stage RO system increases obviously through different sections.

Cleaning method: first, carry out system backwash; Then carry out negative pressure cleaning; Mechanical cleaning if necessary; Chemical cleaning shall be carried out again; If there is any condition, it can be cleaned by ultrasonic; Online electric field cleaning is a good method, which is expensive; Due to the good chemical cleaning effect, some of the other methods are not easy to achieve, and the names and usage methods of the agents provided by each supplier are different, but the principle is basically the same. If our company is using membrane cleaning agent MC2, ma10.

The cleaning procedure is as follows:

Cleaning single section system: ⑴ configure cleaning solution; (2) low flow input cleaning solution; (3) circulation; (4) immersion; (5) high flow pump circulation; (6) flushing; (7) restart the system.

For special pollutants cleaning: cleaning sulfate scale, cleaning carbonate scale, cleaning iron and manganese pollution, cleaning organic pollution, etc.

three point three   Proper maintenance of the membrane

Maintenance of new RO membrane elements are usually stored in sealed plastic bags after they are soaked in 1% NaHSO3 and 18% glycerine aqueous solution. Under the condition of plastic bag not breaking, the storage for about one 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 as far as possible before use. In the non production period, the maintenance of RO system is a more important problem. This can be done as follows.

1. System shutdown in a short period (1-3 days): before the shutdown, the system shall be washed with low pressure (0.2-0.4mpa), and the high flow (about equal to the water production of the system) for 14-16 minutes; Keep the normal natural flow and let the water flow into the thick channel.

2. System shutdown for more than one week (ambient temperature is above 5 ℃): before shutdown, low pressure (0.2-0.4mpa) shall be carried out for the system, and large flow (about equal to the water production of the system) shall be flushed for 14-16 minutes; Chemical cleaning shall be carried out according to the chemical cleaning method of the system in the operation manual of reverse osmosis system; After chemical cleaning, wash the reverse osmosis membrane; Prepare 0.5% formalin solution, and input it into the system at low pressure for 10 minutes; Close all system valves and seal them;

If the system is shut down for more than 10 days, formalin solution shall be replaced every 10 days.

3. The ambient temperature is below 5 ℃: before the shutdown, the system shall be washed with low pressure (0.2-0.4mpa), and the high flow (about equal to the water yield of the system) for 14-16 minutes; In the condition, the ambient temperature can be raised to above 5 ℃, and then the system maintenance can be carried out according to the method of 1; If the ambient temperature is increased unconditionally, the low pressure (0.1MPa) and the flow rate is 1/3 of the water produced by the system for long flow to prevent the reverse osmosis membrane from freezing and ensure the operation of each angel system for 2 hours; According to the method of 2) and 3) in 1, after cleaning the RO membrane, remove the RO membrane and move it to a place with an ambient temperature greater than 5 ℃, soak it in the prepared 0.5% formalin solution, and turn over once every two days. The water in the system pipeline shall be drained clean to prevent the system from damage caused by icing.

three point four   Avoid membrane operation at high pressure

The system has residual gas in the start and stop, so that the system can operate under high pressure. The pressure gauge before and after the filter in the system is used to monitor the pressure drop of filter element, while the primary and final pressure gauges are used to monitor the pressure drop of RO membrane assembly. Adjust the inlet valve and concentrated water valve to ensure the operating pressure and recovery rate. If the flow rate or total flow of water in operation decreases, or the difference between primary and intermediate pressure is larger than that in the initial stage of operation (based on the data of new RO membrane components in initial operation), the system shall be washed or cleaned to ensure the performance and safety of membrane components.

1. After the equipment is drained, when the equipment is re operated, the gas is not discharged and the pressure is increased rapidly. The remaining air shall be drained under the pressure of the system before gradually boosting the operation.

2. When the joint between the pretreatment equipment and the high-pressure pump is not well sealed or the water leaks (especially the water leakage of the micron filter and its rear pipeline), when the water supply of pretreatment is not enough, if the micron filter is blocked, some air will be sucked in due to vacuum in the place where the sealing is not good. The micron filter shall be cleaned or replaced to ensure that the pipeline is not leaking.

3. Whether the operation of each operating pump is normal, whether the flow is the same as the specified value, and compared with the pump operation curve to determine the operating pressure.

three point five   Pay attention to the operation when shutting down

1. The rapid depressurization was not thoroughly washed when the power was shut down. Because the concentration of inorganic salt on the thick water side of membrane is higher than that of raw water, it is easy to scale and pollute the membrane. When preparing to shut down, step down to about 3bar and wash with pre-treatment water for 14-16 minutes.

2. When preparing to shut down, chemical reagent will be added to the film and the film shell, which will cause membrane pollution and affect the service life of the film. The addition shall be stopped.

The company produces:

Pure water equipment for industry, pure water equipment for glass cleaning, pure water equipment for water-based coating, pure water equipment for electroplating industry, pure water equipment for electronic chemical industry, pure water equipment for paper industry, grade I RO pure water equipment, grade II RO pure water equipment, ultrafiltration equipment system, mixed bed system, ultra pure water equipment for laboratory, softened water treatment equipment, EDI ultra pure water equipment Deionized water equipment, reclaimed water reuse equipment, pharmaceutical purified water, sewage treatment equipment, medical water system, pharmaceutical water system, purified water equipment, purified water equipment, water for injection equipment, pure water equipment for food industry, pure water equipment for cosmetics industry, pure water equipment for biopharmaceutical industry, electronics industry, electric power energy, medical equipment   Boiler softened water, desalted water / iron and manganese removal equipment, surface treatment industrial pure water, electronic chip industry, semiconductor industry, lithium battery ultra pure water industry, new energy industry, new materials, integrated circuit board industry, power plant ultra pure water equipment, photovoltaic semiconductor ultra pure water equipment, display screen ultrasonic cleaning ultra pure water equipment, vehicle urea ultra pure water equipment, EDI system Manufacturers of mixed bed system, purified water machine and pure water equipment