Views: 0 Author: Site Editor Publish Time: 2025-12-16 Origin: Site
The water permeability of ultrafiltration (UF) membranes increases with rising temperature. Generally, the viscosity of aqueous solutions decreases as temperature rises, thereby reducing flow resistance and correspondingly enhancing water permeation rate. In engineering design, the actual temperature of the feed liquid at the job site must be taken into consideration.
In water treatment and other industrial purification, concentration, and separation processes, ultrafiltration can serve either as a pre-treatment step or a deep treatment process. It is widely employed as a deep purification method in water treatment technologies. Due to the characteristics of hollow fiber ultrafiltration membranes, specific pre-treatment requirements are imposed on the feed water. Suspended solids, colloids, microorganisms, and other impurities in water can adhere to the membrane surface, leading to membrane fouling.
Given the high water flux of UF membranes, the concentration of rejected impurities on the membrane surface increases rapidly, resulting in the so-called concentration polarization phenomenon. More critically, fine particles can penetrate membrane pores and block water channels. Additionally, viscous substances produced by microorganisms and their metabolites in water can also adhere to the membrane surface. These factors all contribute to the decline of UF membrane permeability and changes in separation performance. Meanwhile, there are specific limits on the temperature, pH value, and concentration of the feed water for ultrafiltration systems.
Therefore, appropriate pre-treatment and water quality adjustment must be conducted on the feed water to meet the specified requirements, so as to extend the service life of UF membranes and reduce water treatment costs.
When water contains microorganisms, some of them may be retained and adhere to the surfaces of pre-treatment system components (e.g., media in multi-media filters) after passing through the pre-treatment stage. If these microorganisms attach to the UF membrane surface and proliferate, they can completely block the membrane micropores and even clog the inner lumen of hollow fibers.
The presence of microorganisms poses a severe threat to hollow fiber UF membranes, so great importance must be attached to removing bacteria, algae, and other microorganisms from raw water. In water treatment projects, oxidants such as sodium hypochlorite (NaClO) and ozone (O₃) are commonly dosed at a concentration of 1–5 mg/L. Alternatively, ultraviolet (UV) sterilization can also be applied. For sterilizing hollow fiber UF membrane modules in laboratory settings, circulating treatment with hydrogen peroxide (H₂O₂) or potassium permanganate (KMnO₄) aqueous solution for 30–60 minutes is feasible.
It should be noted that microbial disinfection only kills microorganisms but does not remove them from water; it merely prevents microbial growth and reproduction.
Suspended solids, colloids, microorganisms, and other impurities in water can cause varying degrees of turbidity. Such turbidity hinders light transmission through water, and this optical effect is related to the quantity, size, and shape of the impurities. Turbidity is generally measured in nephelometric turbidity units (NTU), with a standard definition that 1 mg/L of silica (SiO₂) produces a turbidity of 1 NTU. Higher NTU values indicate higher levels of impurities in water.
Different fields have distinct requirements for feed water turbidity. For example, the turbidity of general domestic drinking water should not exceed 5 NTU. Turbidity measurement is based on detecting the light reflected by particles in water when light passes through the raw water sample. The measurement results are affected by particle size, quantity, and shape, and the relationship between turbidity and suspended solids is not fixed. Turbidity cannot accurately reflect the presence of particles smaller than a certain micron size.
In membrane-based water treatment processes, membranes feature precise microstructures capable of retaining molecular-level and even ionic-level particles. Thus, using turbidity to characterize water quality is clearly inaccurate. To predict the fouling potential of raw water, the Silt Density Index (SDI) test has been developed.
The SDI value is primarily used to measure the concentration of colloids, suspended solids, and other fine particles in water, serving as a key indicator for evaluating the feed water quality of membrane systems. The standard method for determining SDI involves the following steps:
The SDI value of water can roughly reflect the degree of colloidal contamination. Well water typically has an SDI value below 3, while surface water generally has an SDI value above 5. The theoretical maximum SDI value is 6.66. When the SDI value exceeds the acceptable limit, pre-treatment is mandatory.
Ultrafiltration technology is highly effective in reducing SDI values; water treated by hollow fiber UF membranes typically has an SDI value of 0. However, excessively high SDI values—especially those caused by large particles—can severely foul hollow fiber UF membranes. Therefore, pre-treatment is essential in ultrafiltration processes, usually involving filtration through quartz sand filters, activated carbon filters, or multi-media filters. There is no fixed pre-treatment process, as the selection depends on the source of the feed water.
For instance, for tap water or groundwater with relatively low turbidity, a precision filter with a filtration rating of 5–10 μm (such as a wound filter, melt-blown filter, or PE sintered tube filter) can reduce the SDI value to around 5. Prior to precision filtration, coagulant dosing and double-layer or multi-layer media filtration are usually required. The filtration rate is generally maintained at no more than 10 m/h, with an optimal range of 7–8 m/h. Lower filtration rates typically yield higher quality filtered water.
Impurities with a particle size above 5 μm can be removed using filters with a 5 μm filtration rating. However, conventional filtration technologies struggle to remove fine particles and colloids in the size range of 0.3–5 μm. Although ultrafiltration can effectively retain these particles and colloids, their presence poses a severe threat to hollow fiber UF membranes. In particular, colloidal particles carry electric charges and exist as aggregates of molecules and ions. The stable dispersion of colloids in water is mainly attributed to the mutual repulsion between colloidal particles with the same charge.
To destabilize colloidal particles, charged substances (coagulants) with opposite polarity to the colloids are added to the raw water. This neutralizes the surface charge of colloidal particles, causing the dispersed colloidal particles to aggregate into larger flocs, which can then be easily removed through filtration or sedimentation.
Commonly used coagulants include:
In recent years, organic polymeric coagulants have increasingly replaced inorganic coagulants because they can neutralize the surface charge of colloidal particles, form hydrogen bonds, and create "bridges" between particles, enabling rapid coagulation and sedimentation and significantly improving water quality.
Coagulants can be used in conjunction with coagulant aids to enhance the coagulation effect. Common coagulant aids include pH adjusters (lime, sodium carbonate), oxidants (chlorine, bleaching powder), weighting agents (silica sand), and adsorbents (polyacrylamide).
Coagulants are usually prepared as aqueous solutions and dosed using metering pumps. Alternatively, they can be directly injected into the water treatment system via an injector installed on the feed water pipeline.
Dissolved organic matter cannot be completely removed by coagulation-sedimentation, multi-media filtration, or ultrafiltration alone. Currently, oxidation or adsorption methods are the most commonly employed techniques.
The water permeability of UF membranes is directly related to temperature. The rated water permeation rate of UF membrane modules is typically tested using pure water at 25°C. The water permeation rate of UF membranes is proportional to temperature, with a temperature coefficient of approximately 0.02/°C—that is, the water permeation rate increases by about 2.0% for every 1°C rise in temperature.
Therefore, when the feed water temperature is low (e.g., below 5°C), heating measures should be adopted to operate the system at a higher temperature and improve treatment efficiency. Conversely, excessively high temperatures can also adversely affect membrane performance, necessitating cooling measures to reduce the feed water temperature.
UF membranes made from different materials have varying pH tolerance ranges. For example, cellulose acetate (CA) membranes are suitable for use in the pH range of 4–6, while membranes made from polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) can operate within a broader pH range of 2–12. If the pH of the feed water exceeds the membrane's tolerance range, pH adjustment is required. Commonly used pH adjusters include acids (hydrochloric acid (HCl), sulfuric acid (H₂SO₄), etc.) and alkalis (sodium hydroxide (NaOH), etc.).
Since inorganic salts in the solution can pass through UF membranes, concentration polarization and scaling caused by inorganic salts are not concerns. Therefore, in the pre-treatment and water quality adjustment processes, the focus is on preventing the formation of colloidal layers, membrane fouling, and membrane clogging, rather than the impact of inorganic salts on the membrane.
Operating parameters are critical to the long-term, stable operation of ultrafiltration systems. Key operating parameters generally include: flow rate, pressure, pressure drop, concentrated water discharge, recovery rate, and temperature.
Flow rate refers to the linear velocity of the raw feed water flowing across the membrane surface, and it is a crucial operating parameter for ultrafiltration systems. Excessively high flow rates not only waste energy and cause excessive pressure drop but also accelerate the degradation of UF membrane separation performance. Conversely, excessively low flow rates increase the thickness of the boundary layer formed by rejected substances on the membrane surface, leading to concentration polarization, which impairs both water permeation rate and permeate quality. The optimal flow rate must be determined through experimental testing.
For hollow fiber UF membranes operated at a feed pressure below 0.2 MPa, the flow rate of internal-pressure membranes is only about 0.1 m/s, corresponding to a fully laminar flow regime. External-pressure membranes can achieve higher flow rates. For capillary-type UF membranes with a capillary diameter of 3 mm, the flow rate can be appropriately increased, which is beneficial for reducing the thickness of the concentration boundary layer.
Two key points must be noted:
Therefore, increasing the capillary diameter and appropriately raising the concentrated water discharge (reflux flow rate) can effectively improve the flow rate. This is particularly effective in enhancing the ultrafiltration rate during UF concentration processes, such as electrophoretic paint recovery. Within the allowable pressure range, maximizing the feed water flow rate to achieve the highest possible flow rate is conducive to ensuring the optimal performance of hollow fiber UF membranes.
The typical operating pressure range for hollow fiber UF membranes is 0.1–0.6 MPa, which refers to the pressure commonly used for treating solutions within the standard ultrafiltration application scope. Membranes with different molecular weight cut-offs (MWCO) are selected for separating substances of different molecular weights, and the corresponding operating pressures also vary.
Generally, for hollow fiber internal-pressure membrane modules with plastic housings, the pressure resistance of the housing is less than 0.3 MPa, and the pressure resistance of the hollow fibers themselves is also typically below 0.3 MPa. Thus, the operating pressure should be maintained below 0.2 MPa, and the transmembrane pressure difference (TMP) should not exceed 0.1 MPa. External-pressure hollow fiber UF membranes can withstand pressures up to 0.6 MPa, but for external-pressure membrane modules with plastic housings, the recommended operating pressure is still 0.2 MPa.
It is important to note that internal-pressure membranes have a relatively large diameter and can be easily flattened or even fractured at the bonding points when used as external-pressure membranes. Therefore, internal-pressure and external-pressure membranes are not interchangeable.
When the permeate water requires a certain pressure for subsequent processes, UF membrane modules with stainless steel housings should be adopted. Such hollow fiber UF membrane modules can operate at a pressure of up to 0.6 MPa and deliver permeate water at a pressure of 30 meters of water column (equivalent to 0.3 MPa). However, the transmembrane pressure difference (TMP) must be kept below 0.3 MPa at all times.
In selecting the operating pressure, in addition to considering the pressure resistance of the membrane and housing, membrane compaction and fouling resistance must also be taken into account. Higher pressures result in higher water permeation rates, but they also lead to increased accumulation of rejected substances on the membrane surface, higher flow resistance, and subsequent decline in water permeation rate. Furthermore, fine particles are more likely to penetrate membrane micropores and block water channels. In summary, selecting a lower operating pressure whenever possible is beneficial for maximizing UF membrane performance.
The pressure drop of a hollow fiber UF membrane module refers to the pressure difference between the raw water inlet and the concentrated water outlet. Pressure drop is closely related to feed water flow rate, linear velocity, and concentrated water discharge. Especially for internal-pressure hollow fiber or capillary UF membranes, the flow velocity and pressure along the membrane surface change gradually in the direction of water flow. Higher feed water flow rates, linear velocities, and concentrated water discharge rates result in larger pressure drops, which can cause insufficient operating pressure at the downstream membrane surface and reduce the overall water production of the module.
In practical applications, the pressure drop should be controlled within a reasonable range. As the system operates over time, fouling accumulation increases flow resistance, leading to a rise in pressure drop. When the pressure drop exceeds the initial value by 0.05 MPa, cleaning should be performed promptly to unclog the water channels.
In ultrafiltration systems, the recovery rate and concentrated water discharge are mutually constrained parameters. The recovery rate is defined as the ratio of permeate water flow rate to feed water flow rate, while concentrated water discharge refers to the volume of water discharged without passing through the membrane. Since feed water flow rate equals the sum of concentrated water flow rate and permeate water flow rate, higher concentrated water discharge corresponds to a lower recovery rate.
To ensure the stable operation of ultrafiltration systems, minimum concentrated water discharge and maximum recovery rate limits must be specified for each module. In general water treatment projects, the recovery rate of hollow fiber UF membrane modules ranges from 50% to 90%. The specific value is determined by multiple factors, including the composition and properties of the feed water, the thickness of the fouling layer formed on the membrane surface, and the impact of fouling on water permeation rate.
In many cases, the system can be operated at a lower recovery rate, with the concentrated water recycled back to the raw water system. Increasing the circulation flow rate helps reduce the thickness of the fouling layer, thereby improving water permeation rate—sometimes without increasing the energy consumption per unit of water production.
The water permeability of UF membranes increases with rising temperature. Generally, the viscosity of aqueous solutions decreases as temperature rises, thereby reducing flow resistance and correspondingly enhancing water permeation rate. In engineering design, the actual temperature of the feed liquid at the job site must be taken into consideration.
Seasonal temperature variations are a particular concern. When the temperature is too low, temperature adjustment measures should be implemented; otherwise, the water permeation rate can fluctuate by up to 50% with temperature changes. Excessively high temperatures can also compromise membrane performance. Under normal circumstances, the operating temperature of hollow fiber UF membranes should be maintained at 25±5°C. For applications requiring higher operating temperatures, membrane materials and housing materials with high-temperature resistance should be selected.