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Process Analysis of Ultrafiltration Membrane in Water Treatment Application


Ultrafiltration membrane technology in water treatment applications, the water permeability of UF membrane increases with the increase of temperature, and the viscosity of water solution decreases with temperature, which reduces the flow resistance and increases the water permeability rate accordingly. In engineering design, the actual temperature of the liquid supplied at the work site should be taken into account.


Ultrafiltration can be used as pretreatment in water treatment and other industrial purification, concentration and separation processes, as well as in advanced treatment of process. In the widely used water treatment process, it is often used as a means of deep purification. According to the characteristics of hollow fiber ultrafiltration membrane, there are certain requirements for water supply pretreatment. Because suspended solids, colloids, microorganisms and other impurities in water will adhere to the surface of the membrane, and the membrane will be polluted.


Because the water flux of UF membrane is relatively large, the concentration of the trapped impurities on the membrane surface increases rapidly, resulting in the so-called concentration polarization phenomenon. More seriously, some very small particles will enter the membrane hole and block the water channel. In addition, viscous substances produced by microorganisms and their metabolites in water also adhere to the membrane surface. These factors will lead to the decrease of water permeability and the change of separation performance of ultrafiltration membrane. At the same time, the temperature, PH value and concentration of UF water supply are also limited.


Therefore, the UF water supply must be properly pretreated and adjusted to meet the water supply requirements, so as to prolong the service life of UF membrane and reduce the cost of water treatment.


Killing of Microorganisms (Bacteria, Algae)


When microorganisms are present in water, some of the intercepted microorganisms may adhere to the surface of the pretreatment system, such as multi-media filters, after entering the pretreatment system. When adhered to the surface of ultrafiltration membrane, growth and reproduction may completely block the micropore, or even completely block the hollow fiber cavity.


The harmfulness of microorganisms to hollow fiber ultrafiltration membranes is extremely serious. The removal of bacteria and algae in raw water must be paid attention to. In water treatment engineering, NaClO, O3 and other oxidants are usually added, and the concentration is generally 1-5 mg/l. In addition, ultraviolet sterilization can also be used. The hollow fiber ultrafiltration membrane module was sterilized in the laboratory. It could be treated with hydrogen peroxide (H2O2) or potassium permanganate solution for 30 to 60 minutes.


Microbial killing treatment can only kill microorganisms, but it can not remove microorganisms from water, only prevent the growth of microorganisms.


Reducing turbidity of influent water


When water contains suspended substances, colloids, microorganisms and other impurities, it will produce a certain degree of turbidity, which will hinder the transmission of light. This optical effect is related to the amount, size and shape of impurities. The turbidity of water is usually measured by erosion, and the turbidity produced by 1 mg/l SiO 2 is defined as 1 degree. The greater the turbidity, the more impurities there are.


There are different requirements for water turbidity in different fields. For example, for general domestic water, turbidity should not be greater than 5 degrees. Because the measurement of turbidity is to measure the amount, color and opacity of light reflected by particulate matter in water through raw water, the size, quantity and shape of particulate matter affect the measurement, and the relationship between turbidity and suspended solid is random. For particles smaller than a few microns, turbidity cannot be reflected.


In the treatment of membrane process, it is obviously inaccurate to use turbidity to reflect the water quality because of the precise microstructures, the interception of molecular or even ionic particles. In order to predict the tendency of raw water pollution, SDI value test was developed.


SDI value is mainly used to detect the number of colloids and suspended particles in water, which is an important indicator to characterize the water quality of the system. The method to determine the SDI value is usually to use the microporous membrane with a pore size of 0.45 um under a constant flow pressure of 0.21 MPa. First, the time required for the start of the filtration of 500 ml water sample is recorded, then the time required for the filtration of 500 ml water sample is recorded for 15 minutes under the same conditions, and then the time required for the filtration of 500 ml water sample is recorded again. Then, the calculation is made according to the following formula: SDI=(1-t0/t15)*100/15.


The value of SDI in water can roughly reflect the degree of colloidal contamination. Well water SDI < 3, surface water SDI is above 5, SDI limit value is 6.66... That is to say, pretreatment is needed.


Ultrafiltration technology is the most effective way to reduce SDI value. SDI of water treated by hollow fiber ultrafiltration membrane is 0. However, when SDI is too large, especially the larger particles have serious pollution to hollow fiber ultrafiltration membrane. In ultrafiltration process, pretreatment must be carried out, i.e. quartz sand, activated carbon or filter with various filter materials. There is no fixed mode for which treatment process to be adopted. This is because the sources of water supply are different, so the pretreatment methods are also different.


For example, for tap water or groundwater with low turbidity, precise filters (such as honeycomb type, melt blown type and PE sintering pipe) with 5-10 microns can generally be reduced to about 5. Before the precise filter, flocculant must be added and double or multi-layer media filters must be placed to filter. Generally, the filtration speed is not more than 10m/h, and 7-8m/h is suitable. The slower the filtration speed is, the better the quality of the filtered water is.


Removal of suspended and colloidal substances


For impurities with a particle size of more than 5 microns, a filter with a filtering accuracy of 5 microns can be used to remove them. However, for fine particles and colloids between 0.3 and 5 microns, it is difficult to remove them by the conventional filtering technology mentioned above. Although ultrafiltration has absolute removal effect on these particles and colloids, the harm to hollow fiber ultrafiltration membranes is extremely serious. Particularly, colloidal particles are charged, which are polymers of substance molecules and ions. Therefore, colloids can exist stably in water, mainly because colloidal particles with the same charge repel each other.


In order to break the stability of colloidal particles, charged substances (flocculants) contrary to the electricity of colloidal particles are added to raw water to make charged colloidal particles neutral and electrically neutral so that dispersed colloidal particles can be coagulated into large clusters, which can be easily removed by filtration or sedimentation. The common flocculants are inorganic electrolytes, such as aluminium sulfate, polyaluminium chloride, ferrous sulfate and ferric chloride.


Organic flocculants such as polyacrylamide, sodium polypropionate, polyethyleneimine, etc. The organic flocculant polymer can neutralize the surface charge of colloidal particles, form hydrogen bonds and "bridge" to complete the coagulation and sedimentation in a short time, thus greatly improving the water quality. Therefore, in recent years, polymer flocculants have the tendency to replace inorganic flocculants.


When flocculant is added, coagulant aids such as PH regulator lime, sodium carbonate, oxidant chlorine and bleaching powder, underwater reinforcer and adsorbent polyacrylamide can be added to improve the coagulation effect.


Flocculants are usually prepared into water solution, which can be added by metering pump or directly into water treatment system by ejector installed in water supply pipeline.


Removal of Soluble Organic Matter


Soluble organic matter can not be completely removed by flocculation sedimentation, multi-media filtration and ultrafiltration. At present, oxidation method or adsorption method are mostly used.


(1) The oxidation method using chlorine or sodium hypochlorite (NaClO) has better effect on removing soluble organic matter. In addition, ozone (O3) and potassium permanganate (KMnO4) are better oxidants, but the cost is slightly higher.


(2) Soluble organic matter can be effectively removed by using activated carbon or macroporous adsorption resin. However, for alcohols and phenols which are difficult to adsorb, oxidation still needs to be used.


Water quality adjustment of water supply


[Adjustment of water supply temperature]


Ultrafiltration membrane permeability performance is directly related to temperature. Ultrafiltration membrane module calibration permeation rate is generally measured by pure water at 25 ~C. The permeation rate of Ultrafiltration membrane is proportional to temperature, and the temperature coefficient is about 0.02/1 ~C, that is, the permeation rate increases by 2.0% for every 1 ~C increase in temperature. Therefore, when the temperature of water supply is low (e.g. < 5 C), some heating measures can be adopted to make it run at a higher temperature in order to improve work efficiency. However, when the temperature is too high, it is also harmful to the membrane, which will lead to the change of membrane performance. For this, cooling measures can be adopted to reduce the water supply temperature.


[Adjustment of PH Value of Water Supply]


Ultrafiltration membranes made of different materials have different adaptation ranges to PH values. For example, cellulose acetate is suitable for membranes with pH=4-6, PAN and PVDF, which can be used in the range of PH=2-12. If the influent exceeds the application range, it needs to be adjusted. At present, the commonly used PH regulators are acid (HCl and H2SO4) and alkali (NaOH, etc.).


Because inorganic salts in solution can pass through ultrafiltration membranes, there is no problem of concentration polarization and scaling of inorganic salts, so the influence of inorganic salts on membranes is generally not considered in the process of adjusting pretreatment water quality, but the problems of formation of colloid layer, fouling and blockage of membranes are mainly prevented.


2. It is very important for the long-term and stable operation of UF system to master and implement the operation parameters correctly. The operation parameters generally include flow rate, pressure, pressure drop, concentrated water discharge, recovery ratio and temperature.


A. Flow velocity:


Flow velocity refers to the linear velocity of raw liquid (water supply) flowing on the membrane surface. It is an important operating parameter in ultrafiltration system. When the flow rate is high, it not only wastes energy and produces excessive pressure drop, but also accelerates the degradation of the splitting performance of UF membranes. On the contrary, if the flow rate is small, the thickness of the boundary layer formed by the interceptor on the membrane surface increases, which leads to concentration polarization, which affects both the permeability rate and the permeability quality. The optimum flow rate is determined by experiments.


When the inlet pressure of hollow fiber ultrafiltration membrane is below 0.2 MPa, the flow rate of the inner pressure membrane is only 0.1 m/s, and the flow pattern of the flow rate is in a completely laminar state. Large flow rate can be obtained by external pressure film. When the diameter of the capillary reaches 3 mm, the flow rate of the capillary ultrafiltration membrane can be increased appropriately, which is beneficial to reducing the concentration boundary layer.


Two problems must be pointed out. One is that the flow rate can not be determined arbitrarily. The inlet pressure is related to the flow rate of raw liquid. The other is that for hollow fibers or capillary membranes, the flow rate at the inlet end is not uniform. When the concentration water flow is 10% of the raw liquid, the flow rate at the outlet end is approximately 10% of that at the inlet end. In addition, the increase of pressure increases the flow rate through the water, and the increase of convection speed is minimal. Therefore, increasing the diameter of capillary and properly increasing the discharge of concentrated water (reflux) can improve the flow rate, especially in the process of ultrafiltration concentration, such as the recovery of electrophoretic paint.


In the allowable pressure range, increasing the water supply and choosing the highest flow rate are conducive to ensuring the performance of hollow fiber ultrafiltration membrane.


B. Pressure and pressure drop:


The working pressure of hollow fiber ultrafiltration membrane ranges from 0.1 MPa to 0.6 MPa, which is generally referred to as the working pressure used to treat solution in the definition domain of ultrafiltration. Ultrafiltration membranes with different molecular weights need to be selected for separating substances with different molecular weights, and the operating pressures are also different. In general, the compressive strength of plastic shell hollow fiber inner pressure film is less than 0.3 MPa, and the compressive strength of hollow fiber is generally less than 0.3 MPa, so the working pressure should be less than 0.2 MPa, and the pressure difference between the two sides of the film should be less than 0.1 MPa. The compressive strength of externally pressurized hollow fiber ultrafiltration membranes can reach 0.6 MPa, but the working pressure of externally pressurized plastic shell membranes is also 0.2 MPa. It must be pointed out that because of the large diameter of the internal pressure film, when used as the external pressure film, it is easy to flatten and cut off at the bond, causing damage, so the internal and external pressure film can not be used universally.


Stainless steel shell ultrafiltration membrane module should be used when the pressure of ultrafiltration fluid is required for the next process. The pressure of hollow fiber ultrafiltration membrane module reaches 0.6 MPa, and the pressure of providing ultrafiltration fluid can reach 30 m water column, i.e. 0.3 MPa, but the pressure difference between inside and outside of hollow fiber ultrafiltration membrane must be maintained not more than 0.3 MPa.


In choosing the working pressure, besides the compressive strength of the membrane and the shell, the tightness of the membrane and the anti-fouling ability of the membrane must be considered. The higher the pressure is, the greater the permeability is, the more the retained material accumulates on the membrane surface, and the greater the resistance is, the attenuation of the permeability rate will be caused. In addition, the particles entering the membrane micropore are easy to block the channel. In a word, if possible, it is advantageous to choose a lower working pressure to give full play to the performance of the membrane.


The pressure drop of hollow fiber ultrafiltration membrane module refers to the difference between the pressure at the inlet of raw liquid and the pressure at the outlet of concentrated liquid. Pressure drop is closely related to water supply, flow rate and concentrated water discharge. Especially for internal pressure hollow fiber or capillary ultrafiltration membranes, the velocity and pressure on the membrane surface change gradually along the flow direction. The larger the water supply, flow rate and concentrated water discharge, the greater the pressure drop. The pressure on the downstream membrane surface can not meet the required working pressure. The total water production of membrane modules will be affected to some extent. In practical application, the pressure drop should be controlled as far as possible and not too large. With the prolongation of operation time, the resistance of water flow increases due to the accumulation of dirt, so that the pressure drop increases. When the pressure drop is higher than the initial value of 0.05 MPa, the water should be cleaned and dredged.


C. Recovery Ratio and Concentrated Water Discharge:


In the ultrafiltration system, the recovery ratio and the discharge of concentrated water are a pair of mutual restrictive factors. Recovery ratio refers to the ratio of water flow through the membrane to water supply. Concentrated water discharge refers to the amount of water discharged through the membrane. Because the supply of water equals the sum of concentrated water and permeable water, if the discharge of concentrated water is large, the recovery is relatively small. In order to ensure the normal operation of ultrafiltration system, the minimum concentration water discharge and maximum recovery ratio of components should be specified.


In general water treatment projects, the recovery ratio of hollow fiber ultrafiltration membrane module is about 50-90%. The selection is based on the composition and state of the feed liquid, that is, the amount of substances that can be intercepted, the thickness of the fouling layer formed on the surface of the membrane, and the influence on the permeability of water, and other factors to determine the recovery ratio. In most cases, a smaller recovery ratio operation can also be used, while the concentrated liquid is discharged back into the raw liquid system, and the thickness of the fouling layer is reduced by increasing the circulation volume, thus increasing the permeability rate, sometimes without increasing the energy consumption per unit water yield.


D. Working temperature:


The permeability of ultrafiltration membrane increases with the increase of temperature, and the viscosity of water solution decreases with the increase of temperature, which reduces the flow resistance and increases the permeability rate accordingly. In engineering design, the actual temperature of the liquid supplied at the work site should be taken into account.


Especially for seasonal changes, when the temperature is too low, temperature adjustment should be taken into account. Otherwise, with the change of temperature, the permeability may change by about 50%. In addition, too high temperature will also affect the performance of the membrane. Usually, the working temperature of hollow fiber ultrafiltration membrane should be 25 +5 C, and high temperature resistant membrane material and shell material can be selected when working at higher temperature.


Shanghai Morui International Trading Co., Ltd