Process Analysis of Ultrafiltration Membranes in Water Treatment Applications (I)

The permeability of ultrafiltration membranes increases with increasing temperature. Generally, the viscosity of aqueous solutions decreases with temperature, thus reducing flow resistance and correspondingly increasing the permeability rate. The actual temperature of the supply solution at the work site should be considered in engineering design. Previously, ultrafiltration was used in water treatment and other industrial purification, concentration, and separation processes as a pretreatment or advanced treatment method. In widely used Water Treatment Processes, it is often used as a means of advanced purification. Due to the characteristics of hollow fiber ultrafiltration membranes, certain pretreatment requirements exist for the water supply. Suspended solids, colloids, microorganisms, and other impurities in the water can adhere to the membrane surface, causing fouling.

Because ultrafiltration membranes have a relatively high water flux, the concentration of impurities trapped on the membrane surface increases rapidly, resulting in concentration polarization. More seriously, some very fine particles can enter the membrane pores and clog the water channels. In addition, the viscous substances generated by microorganisms and their metabolic products in the water also adhere to the membrane surface. These factors all lead to a decrease in the permeability of the ultrafiltration membrane and changes in its separation performance. Simultaneously, there are certain limitations on the temperature, pH value, and concentration of ultraFiltration Water supply.

Therefore, appropriate pretreatment and water quality adjustment are necessary for ultrafiltration water supply to meet supply requirements, extend the lifespan of the ultrafiltration membrane, and reduce water treatment costs.

Microbial (Bacteria, Algae) Elimination

When water contains microorganisms, some of the trapped microorganisms may adhere to the pretreatment system, such as the media surface of multi-media filters, after entering the pretreatment system. When these microorganisms adhere to the ultrafiltration membrane surface, they grow and multiply, potentially completely clogging the micropores and even the hollow fiber lumen.

The presence of microorganisms is extremely harmful to hollow fiber ultrafiltration membranes. Removing bacteria and algae from the raw water is crucial. In water treatment projects, oxidants such as NaClO and O3 are typically added at concentrations of 1–5 mg/L. Ultraviolet sterilization can also be used. In the laboratory, hollow fiber ultrafiltration membrane modules can be sterilized by circulating hydrogen peroxide (H2O2) or potassium permanganate aqueous solution for 30–60 minutes.

Microbial treatment can only kill microorganisms, but it cannot remove them from the water; it only prevents their growth.

Reducing Influent Turbidity

When water contains suspended solids, colloids, microorganisms, and other impurities, it will produce a certain degree of turbidity. This turbidity obstructs the transmission of light, and this optical effect is related to the amount, size, and shape of the impurities. Turbidity is generally measured in degrees, with 1 mg/L SiO2 producing 1 degree of turbidity. A higher degree indicates a higher impurity content.

Different fields have different requirements for water supply turbidity. For example, for general domestic water, the turbidity should not exceed 5 degrees. Since turbidity measurement involves passing light through raw water and measuring the amount, color, and opacity of light reflected by particles in the water, the size, number, and shape of the particles all affect the measurement. The relationship between turbidity and suspended solids is random. Turbidity cannot reflect particles smaller than a few micrometers.

In membrane treatment, the precise microstructure of water filters traps particles at the molecular and even ionic levels, making turbidity an inaccurate indicator of water quality. To predict the tendency of raw water pollution, the SDI (Self-Containing Dissolved Intake) test was developed.

The SDI value is primarily used to detect the amount of colloids and suspended solids in water, and is an important indicator of the quality of the system's influent. The SDI value is generally determined by using a 0.45 μm microporous membrane under a constant water flow pressure of 0.21 MPa. First, the time t0 required to filter 500 ml of water sample is recorded. Then, under the same conditions, water is flowed for another 15 minutes, and the time t15 required to filter another 500 ml of water sample is recorded. The value is then calculated using the following formula: SDI = (1 - t0/t15) × 100/15. The magnitude of the SDI value in water roughly reflects the degree of colloidal pollution. Well water has an SDI < 3, surface water has an SDI above 5, and the SDI limit is 6.66… indicating that pretreatment is necessary.

Ultrafiltration technology is most effective at reducing SDI (Solid Density Index). Water treated by hollow fiber ultrafiltration membranes achieves an SDI of 0. However, when the SDI is too high, especially when larger particles severely foul the hollow fiber ultrafiltration membrane, pretreatment is necessary in the ultrafiltration process. This pretreatment can be achieved using quartz sand, activated carbon, or filters filled with multiple media. There is no fixed formula for the specific pretreatment method, as different water sources require different approaches.

For example, for tap water or groundwater with low turbidity, using a 5–10 μm precision filter (such as honeycomb, melt-blown, or PE sintered pipe filters) can generally reduce the turbidity to around 5. Before the precision filter, flocculants must be added and double or multi-layer media filters must be used. Generally, the filtration rate should not exceed 10 m/h, with 7–8 m/h being ideal. The slower the filtration rate, the better the quality of the filtered water.

Removal of Suspended Solids and Colloidal Matter

For impurities with a particle size larger than 5 μm, filters with a 5 μm filtration precision can be used for removal. However, for fine particles and colloids between 0.3 and 5 μm, conventional filtration techniques are difficult to use. Although ultrafiltration has an absolute removal effect on these particles and colloids, it is extremely harmful to hollow fiber ultrafiltration membranes. In particular, colloidal particles carry an electric charge and are aggregates of molecules and ions. The stability of colloids in water is mainly due to the mutual repulsion between colloidal particles with the same charge.

Adding a charged substance (flocculator) with the opposite charge to the colloidal particles to the raw water breaks the stability of the colloidal particles, neutralizing them and causing the dispersed colloidal particles to aggregate into large clumps, which can then be easily removed by filtration or sedimentation. Commonly used flocculants include inorganic electrolytes such as aluminum sulfate, polyaluminum chloride, ferrous sulfate, and ferric chloride. Organic flocculants include polyacrylamide, sodium polyacrylate, and polyimide. Because these high-molecular-weight polymers can neutralize the surface charge of colloidal particles, forming hydrogen bonds and "bridging," coagulation and sedimentation are completed quickly, resulting in significant water quality improvement. Therefore, in recent years, polymeric flocculants have shown a trend of replacing inorganic flocculants.

When adding flocculants, coagulant aids can be added, such as pH adjusters (lime and sodium carbonate), oxidants (chlorine and bleaching powder), stabilizing agents (water-soluble fiber), and adsorbents (polyacrylamide), to enhance the coagulation effect.

Flocculants are often prepared as aqueous solutions and added using metering pumps, or they can be directly introduced into the water treatment system using injectors installed on the water supply pipeline.

Removal of Soluble Organic Matter

Soluble organic matter cannot be completely removed by flocculation sedimentation, multi-media filtration, or ultrafiltration. Currently, oxidation or adsorption methods are commonly used.

(1) Oxidation Method: Oxidation using chlorine or sodium hypochlorite (NaClO) is effective in removing soluble organic matter. Ozone (O3) and potassium permanganate (KMnO4) are also good oxidants, but their cost is slightly higher.

(2) Adsorption Method: Activated carbon or macroporous adsorption resins can effectively remove soluble organic matter. However, for alcohols and phenols that are difficult to adsorb, oxidation is still necessary.