Process Analysis of Ultrafiltration Membranes in Water Treatment Applications

Process Analysis of Ultrafiltration Membranes in Water Treatment Applications

Process of Ultrafiltration Membranes in Water Treatment Applications

The water permeability of ultrafiltration membranes increases with rising temperatures. Generally, the viscosity of aqueous solutions decreases with temperature, thereby reducing flow resistance and correspondingly enhancing water permeation rates. Engineering designs should account for the actual temperature of the feed solution at the operational site.

Ultrafiltration serves as a pretreatment step or advanced treatment stage in water purification, industrial purification, concentration, and separation processes. Within water treatment applications, it frequently functions as an advanced purification method. Given the characteristics of hollow fiber ultrafiltration membranes, specific pretreatment requirements exist for feedwater. Suspended solids, colloids, microorganisms, and other impurities in water can adhere to the membrane surface, causing fouling.

Due to the relatively high water flux of ultrafiltration membranes, the concentration of retained impurities rapidly increases on the membrane surface, causing the so-called concentration polarization phenomenon. More critically, some very fine particles can enter the membrane pores and block the water channels. Additionally, microorganisms and their metabolic byproducts in water form viscous substances that adhere to the membrane surface. These factors collectively cause a decline in ultrafiltration membrane permeability and alterations in separation performance. Concurrently, ultrafiltration feedwater must meet specific requirements regarding temperature, pH, and concentration.

Therefore, appropriate pretreatment and water quality adjustment are essential for ultrafiltration feedwater to satisfy operational conditions, thereby extending membrane lifespan and reducing water treatment costs.

>>>>Microbial (Bacteria, Algae) Elimination

When water contains microorganisms, some retained by pretreatment systems (e.g., on the media surface of multimedia filters) may adhere to these systems. If these microorganisms attach to and proliferate on ultrafiltration membranes, they can completely block microporous channels or even clog the hollow fiber lumen entirely.

The presence of microorganisms poses extremely severe hazards to hollow fiber ultrafiltration membranes. Removing bacteria, algae, and other microorganisms from raw water is critical. Water treatment processes typically incorporate oxidizing agents like NaClO or O₃ at concentrations of 1–5 mg/L. UV sterilization is also viable. For sterilizing hollow fiber ultrafiltration membrane modules in laboratories, circulating hydrogen peroxide (H₂O₂) or potassium permanganate solutions for 30–60 minutes is effective.

Microbial inactivation treatment only kills microorganisms; it does not remove them from the water, merely preventing their proliferation.

>>>>Reducing Inlet Water Turbidity

When water contains suspended solids, colloids, microorganisms, and other impurities, it becomes turbid to varying degrees. This turbidity obstructs light transmission, an optical effect related to the quantity, size, and shape of the impurities. Water turbidity is typically measured in turbidity units, defined as 1 degree per 1 mg/L of SiO₂. Higher turbidity values indicate greater impurity content.

Different sectors impose varying turbidity requirements for water supplies. For instance, domestic water should not exceed 5 NTU. Turbidity measurement involves assessing light transmitted through raw water and the amount of light reflected by suspended particles, color, and opacity. Particle size, quantity, and shape all influence readings, making the relationship between turbidity and suspended solids random. Particles smaller than several micrometers are not reflected in turbidity measurements.

In membrane treatment, where precision microstructures retain particles at the molecular or even ionic level, using turbidity to assess water quality is clearly imprecise. To predict raw water contamination tendencies, the SDI value test was developed.

The SDI value primarily measures the quantity of colloidal and suspended particles in water, serving as a crucial indicator for characterizing system inlet water quality. The SDI value is typically determined using a 0.45μm pore size microporous membrane filter under a constant hydraulic pressure of 0.21MPa. First, record the time t0 required to filter 500ml of water sample from the start of flow. Then, continue flowing water under identical conditions for 15 minutes and record the time t15 required to filter another 500ml of water sample. Calculate the SDI value using the following formula: SDI = (1 - t0/t15) × 100 / 15

The SDI value in water roughly reflects the degree of colloidal contamination. Well water typically has an SDI < 3, while surface water often exceeds SDI 5. The SDI threshold is 6.66..., indicating the need for pretreatment.

Ultrafiltration technology is most effective at reducing SDI values. Water treated with hollow fiber ultrafiltration membranes achieves an SDI of 0. However, when SDI is excessively high—particularly due to larger particles causing severe fouling of hollow fiber membranes—pretreatment is essential in ultrafiltration processes. This involves filtration using quartz sand, activated carbon, or multi-media filters. There is no fixed treatment protocol, as water sources vary, necessitating different pretreatment approaches.

For instance, municipal or groundwater with low turbidity can typically be reduced to around 5 SDI using 5–10μm precision filters (e.g., honeycomb, melt-blown, or PE sintered tube filters). Prior to precision filtration, flocculants must be added, and dual-layer or multi-layer media filters must be employed. Under normal circumstances, the filtration rate should not exceed 10 m/h, with 7–8 m/h being optimal. Slower filtration rates yield higher-quality filtered water.
 

Removal of Suspended Solids and Colloidal Matter

For impurities larger than 5μm in particle size, filters with a 5μm filtration precision can be used for removal. However, for fine particles and colloids ranging from 0.3 to 5μm, conventional filtration techniques struggle to effectively remove them. Although ultrafiltration provides absolute removal of these particles and colloids, it poses severe damage to hollow fiber ultrafiltration membranes. Colloidal particles, being charged aggregates of molecules and ions, remain stable in water primarily due to the mutual repulsion of colloidal particles with identical charges. Adding charged substances (flocculants) with opposite electrical properties to the raw water disrupts colloidal stability. This neutralizes charged colloidal particles, causing dispersed colloids to aggregate into larger flocs. These flocs can then be readily removed via filtration or sedimentation.

Common flocculants include inorganic electrolytes such as aluminum sulfate, polyaluminum chloride, ferrous sulfate, and ferric chloride. Organic flocculants encompass polyacrylamide, sodium polyacrylate, and polyethyleneimine. Organic flocculants, being high-molecular-weight polymers, neutralize colloidal surface charges, form hydrogen bonds and “bridging” structures, and accelerate coagulation and sedimentation within a short timeframe, significantly improving water quality. Consequently, polymeric flocculants have increasingly replaced inorganic flocculants in recent years.

Coagulation aids can be added concurrently with flocculants to enhance coagulation efficiency. These include pH adjusters (lime, sodium carbonate), oxidants (chlorine, bleaching powder), flocculants (water-soluble polymers), and adsorbents (polyacrylamide). Flocculants are typically prepared as aqueous solutions and dosed using metering pumps. Alternatively, they can be directly injected into the water treatment system via ejectors installed on the water supply pipeline.

>>>>Removal of Soluble Organic Matter

Soluble organic matter cannot be completely removed by flocculation sedimentation, multimedia filtration, or ultrafiltration. Currently, oxidation or adsorption methods are predominantly employed.

(1) Oxidation Method

Oxidation using chlorine or sodium hypochlorite (NaClO) demonstrates good efficacy in removing soluble organic matter. Ozone (O₃) and potassium permanganate (KMnO₄) are also effective oxidants, though their costs are slightly higher.

(2) Adsorption Method

Activated carbon or macroporous adsorption resins can effectively remove soluble organic matter. However, for difficult-to-adsorb compounds like alcohols and phenols, oxidation methods are still required.

>>>>Water Supply Quality Adjustment

【Adjusting Water Supply Temperature】The permeability performance of ultrafiltration membranes is directly related to temperature. The permeability rate specified for ultrafiltration membrane modules is typically tested using pure water at 25°C. The permeability rate of ultrafiltration membranes is proportional to temperature, with a temperature coefficient of approximately 0.02/1°C. This means that for every 1°C increase in temperature, the permeability rate increases by about 2.0%. Therefore, when the feedwater temperature is low (e.g., <5°C), heating measures can be employed to operate the system at a higher temperature and improve efficiency. However, excessively high temperatures are detrimental to the membrane, causing performance degradation. In such cases, cooling measures should be implemented to reduce the feedwater temperature.

【Adjusting Feedwater pH】

Ultrafiltration membranes made from different materials exhibit varying pH tolerance ranges. For instance, cellulose acetate membranes are suitable for pH=4–6, while membranes like PAN and PVDF can operate within pH=2–12. If the feedwater pH exceeds the operational range, adjustment is necessary. Commonly used pH regulators include acids (HCl and H₂SO₄) and alkalis (NaOH, etc.).

Since inorganic salts in the solution can permeate ultrafiltration membranes, issues like inorganic salt concentration polarization and scaling do not arise. Therefore, their impact on the membrane is generally not considered during pretreatment water quality adjustment. The primary focus is on preventing colloidal layer formation, membrane fouling, and blockage.

Accurate control and execution of operating parameters are critical for the long-term and stable operation of ultrafiltration systems. Key operating parameters typically include: flow rate, pressure, pressure drop, concentrate discharge rate, recovery ratio, and temperature.

A. Flow Rate:

Flow rate refers to the linear velocity of the feed solution (supply water) over the membrane surface, serving as a vital operating parameter in ultrafiltration systems. Excessively high flow rates not only waste energy and cause excessive pressure drop but also accelerate the degradation of ultrafiltration membrane performance. Conversely, low flow rates increase the thickness of the boundary layer formed by retained substances on the membrane surface, leading to concentration polarization. This phenomenon adversely affects both permeate flow rate and permeate quality. The optimal flow rate is determined experimentally.

For hollow fiber ultrafiltration membranes, when the feed pressure is maintained below 0.2 MPa, the flow velocity for internal pressure membranes is only 0.1 m/s, maintaining a fully laminar flow pattern. External pressure membranes can achieve higher flow velocities. For capillary-type ultrafiltration membranes, when the capillary diameter reaches 3 mm, the flow velocity can be appropriately increased, which is beneficial for reducing the concentration boundary layer.

Two critical points must be emphasized: First, flow rate cannot be arbitrarily determined, as it depends on inlet pressure and feed solution flow rate. Second, for hollow fiber or capillary membranes, flow velocity is non-uniform across the inlet. When concentrate flow accounts for 10% of the feed solution, the outlet flow velocity is approximately 10% of the inlet velocity. Furthermore, increasing pressure enhances permeate water volume but contributes minimally to flow rate improvement. Therefore, increasing capillary diameter and appropriately raising condensate discharge rate (return flow) can enhance flow velocity. This is particularly effective in ultrafiltration concentration processes, such as during electrostatic paint recovery, where it significantly boosts ultrafiltration rates.

Within permissible pressure limits, increasing feedwater volume and selecting the maximum flow rate helps ensure optimal performance of hollow fiber ultrafiltration membranes.

B. Pressure and Pressure Drop:

The operating pressure range for hollow fiber ultrafiltration membranes is 0.1–0.6 MPa, generally referring to the working pressure typically used for treating solutions within the ultrafiltration definition domain. Separating substances with different molecular weights requires selecting ultrafiltration membranes with corresponding molecular weight cut-offs, resulting in varying operating pressures.

Typically, plastic-cased hollow fiber membranes with internal pressure have shell pressure ratings below 0.3 MPa, and the membrane fibers themselves also generally withstand pressures below 0.3 MPa. Therefore, the operating pressure should be kept below 0.2 MPa, with the pressure difference across the membrane not exceeding 0.1 MPa. External pressure hollow fiber ultrafiltration membranes can withstand pressures up to 0.6 MPa. However, for plastic-cased external pressure membrane modules, the operating pressure is also 0.2 MPa.

It must be noted that due to their larger diameter, internal pressure membranes are prone to flattening and breaking at bonding points when used as external pressure membranes, causing damage. Therefore, internal and external pressure membranes are not interchangeable. When ultrafiltrate requires a specific pressure for subsequent processes, stainless steel housing ultrafiltration membrane modules should be employed. These hollow fiber ultrafiltration membrane modules operate at pressures up to 0.6 MPa, delivering ultrafiltrate at pressures reaching 30 m water column (equivalent to 0.3 MPa). However, the pressure difference across the hollow fiber ultrafiltration membrane must not exceed 0.3 MPa.

When selecting operating pressure, besides considering the pressure resistance of the membrane and housing, one must account for the membrane's pressure compaction and fouling resistance. Higher pressure increases water permeability, but correspondingly, more retained substances accumulate on the membrane surface, increasing resistance and causing permeability rate decay. Additionally, particles entering the membrane micropores are prone to clogging channels. In summary, selecting a lower operating pressure whenever possible is beneficial for maximizing membrane performance.

The pressure drop in hollow fiber ultrafiltration membrane modules refers to the difference between the inlet pressure of the feed solution and the outlet pressure of the concentrate. This pressure drop is closely related to the feed water flow rate, flow velocity, and concentrate discharge volume. Particularly for internally pressurized hollow fiber or capillary-type ultrafiltration membranes, the flow velocity and pressure along the membrane surface gradually change in the direction of water flow. Higher feed water flow rate, higher flow velocity, and greater concentrate discharge volume all increase the pressure drop. This can prevent the downstream membrane surface from reaching the required operating pressure, thereby affecting the overall water production of the membrane module.

In practical applications, the pressure drop should be kept as low as possible. Over extended operation, fouling accumulation increases flow resistance, causing the pressure drop to rise. Cleaning and flushing the water circuit should be performed when the pressure drop exceeds the initial value by 0.05 MPa.

C. Recovery Rate and Concentrate Discharge:

In ultrafiltration systems, recovery rate and concentrate discharge are mutually constraining factors. Recovery rate refers to the ratio of permeate flow to feed flow, while concentrate discharge denotes the volume of water rejected by the membrane. Since feed flow equals the sum of concentrate and permeate, high concentrate discharge results in a low recovery rate. To ensure normal operation of the ultrafiltration system, minimum concentrate discharge and maximum recovery ratios for the modules should be specified.

In typical water treatment projects, the recovery ratio for hollow fiber ultrafiltration membrane modules ranges from approximately 50% to 90%. Selection is determined by multiple factors including feed composition and condition (i.e., the amount of substances that can be retained), the thickness of the fouling layer forming on the membrane surface, and its impact on permeate flow rate. In most cases, operation at a lower recovery rate is feasible. The concentrated solution can be discharged back into the feed system, and increasing the circulation volume reduces the fouling layer thickness, thereby enhancing permeate flux. This approach sometimes does not increase energy consumption per unit of water produced.

D. Operating Temperature:

The permeate flux of ultrafiltration membranes increases with rising temperature. Generally, the viscosity of aqueous solutions decreases with temperature, reducing flow resistance and correspondingly improving permeate flux. Engineering designs must account for the actual supply temperature at the site, particularly seasonal variations. Temperature regulation should be considered when temperatures are excessively low, as permeation rates may fluctuate by approximately 50% with temperature changes. Conversely, excessively high temperatures can also degrade membrane performance. Typically, hollow fiber ultrafiltration membranes should operate at 25±5°C. For higher temperature requirements, high-temperature-resistant membrane and housing materials should be selected.