Ultrafiltration Membrane Process in Water Treatment Applications – Pretreatment

Ultrafiltration is widely used in water treatment and other industrial purification, concentration, and separation processes. It can be employed as a pretreatment step or a more in-depth processing step in various process sequences. In common water treatment processes, ultrafiltration is often utilized as a means of deep purification. Due to the characteristics of hollow fiber ultrafiltration membranes, certain pretreatment requirements are necessary for water supply. Suspended solids, colloids, microorganisms, and other impurities in water can attach to the membrane surface, leading to fouling. The large water flux of ultrafiltration membranes can cause concentration polarization, where retained impurities accumulate on the membrane surface, and fine particles may even enter the membrane pores and block water channels. Additionally, microbial growth and the production of sticky substances from microbial metabolism can also lead to membrane fouling. These factors can result in reduced water permeability and changes in separation performance of ultrafiltration membranes. There are also specific requirements for factors such as ultrafiltration feedwater temperature, pH value, and concentration. Therefore, appropriate pretreatment and water quality adjustments are necessary to meet the water supply requirements, extend the lifespan of ultrafiltration membranes, and reduce water treatment costs.

1. Microbial Elimination

When water contains microorganisms, they may adhere to the pretreatment system, such as the medium surface of a multimedia filter. If these microorganisms attach and grow on the surface of ultrafiltration membranes, they can block the pores and negatively impact filtration. Effective measures must be taken to eliminate bacteria and algae. Common practices involve adding oxidants like NaClO or O3, typically at concentrations of 1-5 mg/l. UV disinfection is also an option. Sterilization treatments using hydrogen peroxide (H2O2) or potassium permanganate solution can be applied to hollow fiber ultrafiltration membrane components. Microbial killing treatments only eliminate microorganisms but do not remove them from the water.

2. Reduction of Inlet Turbidity

When water contains suspended solids, colloids, microorganisms, and other impurities, it can exhibit varying degrees of turbidity. Turbidity obstructs the passage of light through water and is influenced by the amount, size, and shape of impurities. Turbidity is often quantified in nephelometric turbidity units (NTU), and higher values indicate higher impurity levels. Different applications have different requirements for inlet water turbidity; for example, general domestic water should have turbidity below 5 NTU. Measuring turbidity is an indirect indicator of particle concentration and does not reflect the removal of particles smaller than a certain size. To predict the tendency of raw water contamination, the Silt Density Index (SDI) test has been developed. SDI values provide important information about the colloid and suspended particle content in water, which is crucial for assessing the quality of the feedwater.

3. Removal of Suspended and Colloidal Substance

For impurities with particle sizes above 5 μm, filters with a filtration precision of 5 μm can be used for removal. However, conventional filtration techniques struggle to eliminate fine particles and colloids ranging from 0.3 to 5 μm. Ultrafiltration is effective in removing these particles, but it also poses risks to the integrity of the ultrafiltration membranes. Colloidal particles, especially those carrying charges, can be stabilized in water due to repulsion between particles of the same charge. Coagulants with opposite charges are added to neutralize the charges, causing colloidal particles to aggregate and facilitating their removal through filtration or sedimentation. Inorganic coagulants like aluminum sulfate and ferric chloride are commonly used, as well as organic coagulants such as polyacrylamide. Recent trends show a shift towards using high-molecular-weight organic coagulants to replace inorganic ones due to their more efficient coagulation.

4. Removal of Soluble Organic Compounds

Soluble organic compounds cannot be completely removed using coagulation, sedimentation, multimedia filtration, or ultrafiltration. Oxidation or adsorption methods are commonly employed.

  1. Oxidation Method: Chlorine or sodium hypochlorite (NaClO) is used for oxidation to effectively remove soluble organic compounds. Ozone (O3) and potassium permanganate (KMnO4) are also effective oxidants but are slightly more costly.
  2. Adsorption Method: Activated carbon or macroporous adsorption resins can efficiently remove soluble organic compounds. Difficult-to-adsorb substances like alcohols and phenols may require oxidation.

5. Adjustment of Feedwater Quality

  1. Temperature Adjustment: The permeability of ultrafiltration membranes is directly influenced by temperature. The standardized water permeation rate of ultrafiltration membrane components is generally determined at 25°C. The water permeation rate increases proportionally with temperature, typically with a coefficient of about 0.02/1°C. Thus, raising the temperature by 1°C can lead to a 2.0% increase in water permeation rate. Therefore, measures can be taken to raise the temperature in cases of lower supply water temperatures (< 5°C) to enhance efficiency. Conversely, excessively high temperatures can also adversely affect membrane performance, requiring cooling measures to reduce the supply water temperature.
  2. pH Adjustment: Ultrafiltration membranes made from different materials have varying pH adaptability ranges. For instance, cellulose acetate membranes are suitable for pH 4-6, while membranes like PAN and PVDF can operate within a pH range of 2-12. If the inlet water pH exceeds these ranges, adjustment is necessary. Common pH adjusters include acids (HCl and H2SO4) and bases (NaOH). Inorganic salts can pass through ultrafiltration membranes without causing concentration polarization or scaling issues. Therefore, during the pretreatment process, the focus is on preventing colloid layer formation, membrane fouling, and clogging issues rather than the impact of inorganic salts on the membranes.

In conclusion, ultrafiltration is a powerful technique in water treatment, especially in pretreatment processes. Proper pretreatment is essential to ensure the longevity and efficiency of ultrafiltration membranes and to achieve high-quality treated water.

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Further reading

Purified UF Water Membrane

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