Ultra pure water is water from which all conductive ions, colloids, organic matter, etc. have been removed, and its water resistance rate is greater than 18 M Ω· cm (25 ℃). Ultra pure water systems generally use pre-treatment technology, reverse osmosis technology, electric desalination technology, and post-treatment, and are equipped with auxiliary treatments such as ultraviolet lamps and TOC devices. The process of preparing ultrapure water is as follows: tap water → pretreatment system → multi-stage high-pressure reverse osmosis system → EDI electric desalination system → TOC device → polishing mixed bed system → water point. During the preparation process, the water pump serves as the power source, and all levels of water storage tanks must ensure that the system water pump can operate safely and stably. Adding filters of different treatment levels to certain parts can ensure the quality of the produced water. At the same time, adding various chemicals during the operation of the filter can adjust the pH value of the water, reduce residual chlorine in the water, and slow down the scaling rate of the RO membrane. In winter, in some areas with lower temperatures in the north, heat exchangers are added at the inlet of the pre-treatment system to ensure that the water temperature of the tap water meets the requirements of the equipment's water production process. In addition, it is very important to use nitrogen to isolate the water from the external medium in the ultra pure water tank, as it can ensure that the prepared ultra pure water is not contaminated by air for the second time. The Importance of Pre treatment System 1. RO Membrane Inlet Requirements: In the process of preparing ultrapure water, the RO system is the most critical. The normal operation of the RO system can ensure the safe operation of the entire pure water equipment and the quality of the produced water. In the process of preparing ultrapure water, the RO system has certain requirements for the inlet water, including the following: sludge density index (SDI value)<4.0; Turbidity (NTU)<1.0 100 T/h; Organic content (COD)<1.5 mg/L; Residual chlorine content<0.1 mg/L (actually controlled at 0 mg/L); When the dissolved oxygen is greater than 5 mg/L, the iron content is less than 0.05 mg/L; SiO2<100 mg/L in concentrated water; LSI，pHb-pHs<0； Ions such as Sr and Ba are prone to form insoluble salts, with Ipb<0.8 Ksp. 2. Before entering the reverse osmosis system, the pre-treatment system is used to remove residual chlorine, large suspended particles, flocculent colloidal impurities, organic matter, oxides, organic matter, and heavy metals from the water, in order to reduce COD and SDI values. In addition, ions such as calcium, magnesium, barium, sulfate, silicate, and carbonate in water can be combined with scale inhibitors to form large molecular particles, which are then discharged in the form of concentrated water in a reverse osmosis device. (1) The impact of abnormal system operation on the RO membrane is known from the relevant content of pretreatment. Before entering the RO membrane, the vast majority of harmful substances in polymers will be removed through the pretreatment system. If the pre-treatment system has defects and many of the reverse osmosis inlet indicators do not meet the standards, it will cause irreversible physical and chemical damage to the reverse osmosis membrane components, greatly shortening their service life. There are several factors that affect the service life of reverse osmosis membrane components, including scaling of RO reverse osmosis membranes; RO reverse osmosis membrane is contaminated by metal oxides; Suspended solids blocking RO reverse osmosis membrane; Colloids, organic matter, and microorganisms are contaminated, leading to an increase in COD in the effluent. (2) The impact of system abnormalities on the reverse osmosis ultrapure water system When the pre-treatment system operates abnormally, it will affect the operation of the entire reverse osmosis ultrapure water system, specifically manifested in the following three aspects: reducing the water production and water quality of the reverse osmosis ultrapure water system; Increase the water and electricity consumption of reverse osmosis equipment operation; Increase the operating costs of water treatment, including scale inhibitors and other water treatment agents.
The working principles of ultrafiltration system and activated carbon system. In actual production processes, common pretreatment systems include ultrafiltration system and activated carbon system. The ultrafiltration system consists of a disc filter and an ultrafiltration device; The activated carbon system consists of a multi-media filter and an activated carbon filter. 1. The working principle of the ultrafiltration system is that the disc filter achieves surface filtration and deep filtration through a compressed plastic disc. It is mainly used to remove large particles such as sediment from water. Ultrafiltration is a pressurized membrane separation technology that allows small molecule solutes and solvents to pass through a specially designed membrane with a certain pore size under a certain pressure, while large molecule solutes (with a molecular weight of 10000 to 30000) cannot pass through and remain on one side of the membrane, thereby partially purifying the large molecule substances. The working principle of the ultrafiltration system is shown in Figure 1. 2. The working principle of activated carbon system: Multi media filter is to use one or several filtering media to effectively remove suspended impurities from water with high turbidity through a certain thickness of granular or non granular filtering materials under a certain pressure, making the water clear. The commonly used filter media include quartz sand, smokeless coal, and manganese sand, which are mainly used for water treatment turbidity removal, pre-treatment of softened water and pure water, etc. The SDI value can reach below 3. After being filtered through a multi-media filter, the water enters the activated carbon filter. The activated carbon filter is equipped with a filter material based on quartz sand and activated carbon as the filter material. The activated carbon filter material can remove impurities such as residual chlorine, organic matter, and suspended solids in the water through its own pores under the action of van der Waals force. The advantages and disadvantages of ultrafiltration devices and activated carbon devices are different in the process of using these two types of pretreatment systems. The advantages of ultrafiltration system are: ① high concentration factor, product recovery rate can reach over 90%; ② The clarity and quality of the effluent water are good; ③ As a pre-treatment equipment for reverse osmosis, the application of ultrafiltration system can greatly reduce the investment in reverse osmosis and extend the service life of RO membrane (with a lifespan of more than 3 years); ④ The system has a high degree of automation, simple structure, low operation and maintenance costs, and can be backwashed and medicated online; ⑤ Small footprint. 2. The disadvantages of ultrafiltration systems are: ① Due to the pore size of ultrafiltration membranes ranging from 0.002 to 0.1 um, while the volume of colloids in water is ≥ 0.1 um, the volume of latex is ≥ 0.5 um, the volume of bacteria is ≥ 0.2 um, and the volume of particles is ≥ 5 um, the filtration range is wide. Therefore, during the operation of the system, some components are easily blocked, affecting the effluent flow rate and membrane service life; ② The initial investment is relatively large, and the price of ultrafiltration systems with the same water production is generally 2-3 times the sum of multi-media and activated carbon systems The ultrafiltration system has strict requirements for the inlet pressure; ④ Cannot be shut down for a long time. If it is shut down for a long time, a medication sealing film must be used.
The advantages of activated carbon system are: ① small initial investment; ② Stable water output with minimal requirements for system inlet pressure. 2. The disadvantages of activated carbon systems are: ① large footprint. ② During the operation of the system, the participation of activated carbon and anthracite in physical and chemical reactions leads to increased natural consumption, adsorption failure, and deterioration of water quality, which in turn affects the reverse osmosis system The backwash cycle of activated carbon filters is difficult to control and the time is relatively short. Activated carbon becomes powder and flows out with water, seriously polluting water quality and downstream equipment. If the backwash cycle is long, the activated carbon filter is prone to forming a high pressure difference, causing system operation risks and affecting water output The later operation and maintenance costs of the system are high, the workload is heavy, and the service life of the filter material is short (it needs to be replaced once a year).
