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Control of reverse osmosis membrane contamination

Reverse osmosis membrane contamination

reverse osmosis membrane pollution refers to the absorption or deposition of the colloidal particles or solute particles on the surface or in the reverse osmosis membrane pore, is the reverse permeability membrane flux and separation characteristics.

The pollutants on the reverse osmosis membrane surface can be divided into three categories: organic pollutants, inorganic pollutants and bioactive pollutants.

A method for the control of reverse osmosis membrane pollution

Due to the complexity of the phenomenon of reverse osmosis membrane contamination, only general discussion of the methods to control pollution. Special treatment is required for each specific separation problem.

01 Preof prepretreatment

Pre-filtering

Remove particulate matter to prevent blocking the flow channel into damage reverse osmosis membrane, fasten and meters. As a rule, children larger than the component at 1 / 5 of the minimum flow channel size must be removed. Removal of large debris with natural settlement, grille (sieve) and water biological, crude sieve or hydraulic centrifugal filter can remove large particles, flotation to remove light levitates (e. g., plankton, microorganisms, etc.) and light flocculant, simple microfilter or multi-media filter to remove fine particles. Thick filters isolated particles and mechanical impurities with particle sizes larger than 10 μm, and microbiological ultrafiltration to remove E. coli and bacteria. In gas separation, solid particles, liquid water or liquid hydrocarbon, including cyclone separator, ultrafiltration technology combined with capillary condensation technology. In general, the water inlet the coil assembly shall be filtered at 20-50 μm while the hollow shell feed shall be filtered by 5 μm.

Remove the organic matter

Activated carbon filtration can remove trace oil and hydrocarbons, and ultraviolet light can also decompose organic matter and remove low molecular organic matter. UV oxidation is more appropriate than ion exchange and reverse osmosis. Ozone + ultraviolet treatment also has sterilization, gel micronization, colloidal silica oxidation and decomposition.

Fculculation

Colliids are charged particles <1 μm diameter, as not removed, can seriously affect the reverse osmosis membrane flux. The common removal method is for routine filtration after coagulation or flocculation. Commonused flocculants are FeCl3, alum or polyelectrolyte (polymer aluminum chloride), etc. The principle is to change the characteristics of suspended particles to affect the reverse osmosis membrane flux, which is to produce fluffy non-free flocculation to significantly reduce reverse osmosis membrane pollution; in the case of oil and water separation, human flocculation to the raw material liquid can not only improve the reverse osmosis membrane flux, but also improve the retention rate of reverse osmosis membrane, you can replace conventional microfilter ultrafiltration.

Adjust the pH values

When treating heavy metal ion containing waste water, alkaline substances may add the pH value of the solution or add sulfide or some other substances, so that the heavy metal ions form hydroxide precipitation or unsoluble sulfide or other substances and remove. If H2SO4 is added to adjust the pH value to about 6, remove the carbon dioxide from the seawater, and prevent the formation of calcium carbonate scale on the reverse osmosis membrane. After forming CaSO4 to avoid increasing SO42-concentration, pH was adjusted with HCI. NaOH, Na2CO2, BeCl2 lime can also be removed in the form of precipitation. Adding acid to the fermentation solution of methoxy cephalosporin C can stabilize the material viscosity and prevent bacterial contamination. For proteins, the regulation of pH values is important. Solution pH value has great influence on solubility, charge and configuration of protein in water. In general, when the protein has the lowest solubility, and deviates from the iselectric point, the solubility increases, and is charged. The highest protein adsorption at the isoelectric point and the lowest reverse osmosis membrane flux. Therefore, when separating, concentrating proteins or enzymes with the reverse osmosis membrane, the pH value is generally adjusted to stay away from the isoelectric point (without deactivating the protein), and choosing the appropriate reverse osmosis membrane can reduce the reverse osmosis membrane pollution.

Sterilization

To prevent biological pollution, when using chlorine or hypochlorite, ozone, formaldehyde, hydrogen peroxide, thick sodium bisulfite solution, isothiazolinone, etc., or ultraviolet, electronic fungizer, heat treatment, pay attention to the heat treatment temperature, especially for protein, to prevent superior protein degeneration.

Chlorine removal

The reverse osmosis membrane is not resistant to chlorine, and NaHSO, deoxygenation is added before the raw water goes into the human reverse osmosis process.

Water removal

When separating methane from biogas, remove the water vapor in the biogas through the mist reverse osmosis membrane, because water will pass through the reverse osmosis membrane faster than methane.

Ion exchange

Polyvalent ions were removed with oho resin.

Add an ion concealment agent

Add scale inhibitors, such as sodium hexadyl phosphate, etc., to slow down the scaling speed, to avoid the formation of ions and organic macromolecules.

A stabilizer was added to nurture the peptide to prevent inactivation.

Reduce the viscosity

During the filtration of a high-viscosity solution, appropriate reagents can be added to reduce the solution viscosity and increase the shear rate, etc. Thus to improve the reverse osmosis membrane filtration performance.

Heat it up or cool it down

In gas separation, heating the raw gas to keep it away from the dew point to avoid water vapor condensation in the reverse osmosis membrane and prepare sterile air. Raw air needs to be heated to 30~35℃ and natural gas dewetting to 5-10℃. The pretreatment of raw material liquid must consider the characteristics of the system, and the system cannot be pretreatment if the nature cannot be changed.

02 Selection of reverse osmosis membrane materials

Hydrohydrophobic, charge of reverse permeability membrane will affect the size of the interaction between reverse osmosis membrane and solute, generally speaking, electrostatic interaction is easier to predict, but reverse hydrophobic measurement is difficult, especially for biological fermentation system, composition is extremely complex, must screen different objects, under different conditions, usually think that hydrophilic reverse osmosis membrane and reverse osmosis membrane material charge and solute charge of reverse osmosis membrane is more resistant to pollution. For example, the adsorption of several protein IgG by a polymer microporous reverse osmosis membrane is listed in the table below

To enhance the contamination resistance of the hydrophobic antipermeability membrane, the reverse osmosis membrane can be pretreated with a surface of the antipermeability membrane, covering the surface with a protective layer. This reduces the adsorption of the reverse osmosis membrane, but is readily detached since these surfactants are water-soluble and bonded by intermolecular weak force to the reverse osmosis membrane (Van Der Waals). To obtain permanent pollution resistance, reverse osmosis film surface modification is commonly used to attract hydrophilic groups, or by composite reverse osmosis membrane means to compound a layer of hydrophilic separation layer, or using cathode spray coating with a layer of carbon on the surface of ultrafiltration reverse osmosis film.

03 Selection of the RO membrane aperture or the trapped molecular weight

Theoretically, under the premise of retaining the desired particles or macromolecular solutes, the aperture or the reverse osmosis membrane with a little larger molecular weight should be selected to obtain a higher reverse osmosis membrane flux. However, it was found that the larger reverse permeability membrane aperture was chosen because of the higher pollution rate. The long-time reverse osmosis membrane flux decreases instead. This is because when the size of the material to be separated is similar to the reverse permeability membrane pore size, when the solvent passes through the pressure; when the reverse permeability membrane aperture is less than the particle or solute size, it is difficult to gather on the reverse osmosis membrane surface due to shear flow.

04 Structure selection of the reverse osmosis membrane

The usual principle is for the microfiltration of reverse osmosis membranes. Most use symmetric reverse osmosis membrane, but the researchers also attach great importance to the preparation of asymmetric microfilter reverse osmosis membrane, if the bacteria want to be collected. That is, concentration is needed, wrong flow filtration is adopted, the choice of asymmetric structure reverse osmosis membrane is more resistant to pollution, but attention should be paid to the appropriate structural reverse osmosis membrane should be selected according to different needs.

05 reverse osmosis membrane surface modification

The modification of the reverse osmosis membrane surface can be divided into physical and chemical modification. Physical modification refers to the separation properties of one or several of the reverse osmosis membrane

Small molecule compounds that do not have great impact, such as surfactants or soluble polymers, partially cover the adsorptive structure of the reverse osmosis membrane surface, forming a functional pre-coating layer on the surface of the reverse osmosis membrane, preventing the reverse osmosis membrane from acting with components in solution, and improving the antipollution performance of the reverse osmosis membrane.

Most reverse osmosis membranes are made from hydrophobic materials, except for cellulose, chitosan, and polyphovinyl alcohol. And the water often contains some organic matter, easy to be adsorbed in the hydrophobic reverse osmosis membrane surface or pores, forming irreversible pollution. General proteins are more readily adsorbed and difficult to remove on hydrophobic than on hydrophilic reverse osmosis membranes. To prevent these contaminants, it is often hydrophilic reverse osmosis membrane surface or make it self-cleaning, photocatalytic or photodegradable.

In order to obtain permanent anti-pollution characteristics, the following three chemical modification methods are often used: 1 Prepare the composite reverse osmosis film; @ Lead the hydrophilic or hydrophobic group on the reverse osmosis film surface: 3 Add some substances in the human reverse osmosis film liquid, such as mixing, so that it is evenly distributed on the inside and outside surface of the reverse osmosis film to change the surface performance of the reverse permeability film and improve the pollution resistance of the reverse osmosis film.

06 Assembly structure selection

The degree of contamination of the reverse osmosis membrane decreases with concentrated polarization. The ence polarization can be reduced by increasing the mass transfer coefficient and using lower flux osmosis membrane contamination. When the content of the suspended material in the material fluid is low and the product is in the penetrating fluid, it is clarified by microfiltration or ultrafiltration, then there is more room for selecting the component structure. However, if the intercept is a product and is highly concentrated, then the component structure should be carefully selected. Generally speaking. Components with mesh as material flow channel, such as rolling assembly, should not be used due to the reverse osmosis film blockage; capillary and thin flow channel assembly design can make fluid flow at high speed. The large shear force is conducive to reduce the deposition of particles or macromolecular solutes in the reverse osmosis membrane, which can reduce concentrated differential polarization or avoid gel layer formation.

07 Control of the salt concentration in the solution

Inorganic salt is through two ways to have a significant impact on the reverse osmosis membrane, one is the inorganic salt and some inorganic salt complex will directly deposit in the reverse osmosis membrane surface or reverse osmosis membrane pore, or make the reverse osmosis membrane adsorption and aggravate the reverse osmosis membrane pollution: second, the inorganic salt changed the solution ion strength, affect the protein solubility, structure and suspension state, change the density of the deposition layer, thus affecting the reverse osmosis membrane flux. The addition of NaC1 increases the protein adsorption of the reverse osmosis membrane, but the reverse osmosis membrane flux increases with the increased surface of NaC1 plus human volume, because NaC1 changes the protein configuration and suspension state to form a relatively loose "gel layer".

08 Control of the solution temperature

The effect of temperature on the reverse osmosis membrane pollution is not very clear. According to the general law, the solution temperature increases, its viscosity decreases, and the reverse permeability membrane flux should be improved. But for some protein solutions, the temperature increases and the reverse osmosis membrane flux decreases, because their solubility decreases at higher temperatures.

09 Control of solute concentration, material flow rate and pressure

The effect of pressure on the reverse osmosis membrane flux is often associated with the material flow rate when separating or concentrating the protein or other macromolecular solutes by ultrafiltration techniques. When operating conditions, such as flow velocity, are certain, and before differential polarization is not apparent (low pressure region), the reverse permeability membrane flux approximately linearly with pressure. The pressure increases after differential polarization works.The reverse permeability membrane flux increases and the thick differential polarization is serious, making the reverse permeability membrane flux increases with the increasing pressure. When the pressure rises to a certain value, the thick differential polarization brings the solute concentration of the reverse osmosis membrane surface to the limit concentration, the solute begins to precipitate on the reverse osmosis membrane surface and form a "gel layer", this pressure is called the critical pressure. At this point, the effect of the "gel layer" resistance determines the RO membrane flux, which is almost pressure-dependent. Therefore, when the solute concentration is certain, the appropriate pressure (below the critical pressure) and the material and liquid flow rate should be selected to avoid the formation of the "gel layer", and the optimal reverse osmosis membrane flux can be obtained.