In water treatment and other industrial purification, concentration and separation processes, ultrafiltration membranes can be used as pretreatment of the process, or as an advanced treatment of the process. It is often used as a means of deep purification in widely used water treatment equipment. According to the characteristics of hollow fiber ultrafiltration membrane modules, there are certain requirements for water supply pretreatment. Because suspended solids, colloids, microorganisms and other impurities in the water will adhere to the membrane surface, the membrane will be contaminated. Especially the ultrafiltration membrane has a relatively large water flux, and the concentration of the trapped impurities on the membrane surface increases rapidly, resulting in the so-called concentration polarization phenomenon. These factors will cause the decrease of the ultrafiltration membrane water permeability and the change of the separation performance. At the same time, there are also certain limits on the temperature, pH and concentration of the ultrafiltration water supply. Let us look at the following water inlet requirements for Ultrafiltration Membrane Modules:
1. Killing of microorganisms (bacteria, algae)
When the water contains microorganisms, after entering the pretreatment system, some of the trapped microorganisms may adhere to the pretreatment system. When it adheres to the surface of the ultrafiltration membrane, it grows and reproduces, which may completely block the micropores and even cause more serious problem. Therefore, the bacteria and algae in the raw water must be paid attention to. In water treatment projects, oxidants such as NaCIO and O3 are usually added, and the concentration is generally 1-5 mg/I. In addition, UV sterilization can also be used. It can use hydrogen peroxide (H2O2) or circulating treatment of potassium permanganate aqueous solution for 30-60min to sterilize hollow fiber ultrafiltration membrane modules in the laboratory. Anti-microbial treatment can only kill microorganisms, but it can not remove microorganisms from the water, but only prevents the growth of microorganisms.
2. Reduce UF Membrane Modules influent turbidity
When the water contains suspended solids, colloids, microorganisms and other impurities, it will make the water a certain degree of turbidity. The turbidity will hinder the transmission of the light, and this optical effect is related to the amount, size and shape of impurities. The turbidity of water is generally measured by corrosion degree, and the turbidity produced by 1mg/lSiO2 is 1 degree. The greater the degree, the more impurity content. There are different requirements for the turbidity of the water supply in different fields. Generally, the turbidity of the inlet water of the ultrafiltration membrane module should be less than 15NTU.
3. Water supply temperature control
The water permeability of the ultrafiltration membrane is directly related to the temperature. The calibrated water permeability of the ultrafiltration membrane module is generally tested with pure water at 25℃. The permeation rate of the ultrafiltration membrane is proportional to the temperature, and the temperature coefficient is about 0.02/1℃, that is, for every 1℃ increase in temperature, the permeation rate will increase by approximately 2.0％. Therefore, when the temperature of the water supply is lower than 5℃, some temperature rise measures can be adopted to make it run at a higher temperature to improve work efficiency. But when the temperature is higher than 45℃, it is also unfavorable to the membrane and will cause changes in membrane performance. For this, cooling measures can be used to reduce the temperature of the water supply.
4. Water supply PH value control
Ultrafiltration membranes made of different materials have different adaptation ranges for PH. For example, cellulose acetate is suitable for membranes such as PH=4-6, PAN and PVDF, etc. It can be used in the range of PH=2-12. If the water inflow exceeds the use range, it need to be adjusted. Currently commonly used PH regulators mainly include acids (HCI and H2SO4) and alkalis ( NaOH, etc). By the way, the inorganic salt in the solution can pass through the ultrafiltration membrane, and there is no problem of inorganic salt concentration polarization and scaling. Therefore, their impact on the membrane is generally not considered in the process of pretreatment water quality adjustment, and the focus is on the formation of colloidal layer, membrane fouling and clogging.
Correctly mastering and implementing the operating parameters is extremely important for the long-term and stable operation of the ultrafiltration system. The operating parameters generally mainly include: flow rate, pressure, pressure drop, concentrated water discharge, recovery ratio and working temperature. Therefore, we should meet the requirements of water inflow, extend the service life of ultrafiltration membranes, and ultimately reduce water treatment costs.