Advantages and disadvantages of ultrafiltration and activated carbon devices in the preparation of ultrapure water

Advantages and disadvantages of ultrafiltration and activated carbon devices in the preparation of ultrapure water

Ultrapure water is conductive ions, colloids, organics and so on all removed water, the resistivity of its produced water is greater than 18 MΩ-cm (25 ℃). Ultrapure water system generally uses pretreatment technology, reverse osmosis technology, electric desalination technology and post-treatment, etc., at the same time, with UV lamps, TOC devices and other auxiliary treatment.

The process of preparing ultrapure water
The process of preparing ultrapure water is as follows: tap water → pretreatment system → multi-stage high-pressure reverse osmosis system → EDI electro-desalination system → TOC device → polished mixed-bed system → water point. In the preparation process, the water pump as a power source, water storage tanks at all levels to ensure that the system pump can be safe and stable operation. Individual parts of the increase in different treatment levels of filters, can ensure the quality of water production. At the same time, in the process of filter operation, add a variety of agents can adjust the pH value of the water, reduce the residual chlorine in the water, slow down the scaling rate of the RO membrane and so on. In winter, in some northern regions with low temperatures, the heat exchanger will be added at the inlet of the pretreatment system, so that the water temperature of the tap water to meet the requirements of the water production process. In addition, in the ultrapure water tank, with nitrogen to isolate the water and the external medium is very important, it can ensure that the preparation of ultrapure water is not air secondary pollution.

The importance of pretreatment system
1, RO membrane water requirements
In the process of preparing ultrapure water, RO system is the most critical, the normal operation of the RO system can ensure that the operation of the entire pure water equipment safety and water quality. In the process of preparing ultrapure water, RO system has certain requirements for the feed water, specifically the following points:
Silt density index (SDI value) <4.0;
Turbidity (NTU) < 1.0 100 T/h;
Organic matter content (COD) <1.5 mg/L;
Residual chlorine content <0.1 mg/L (actual control at 0 mg/L);
Iron content <0.05 mg/L when dissolved oxygen >5 mg/L;
SiO2<100 mg/L in concentrated water; LSI, pHb-pHs<0; Sr, Ba and other ions that are easy to form insoluble salts, Ipb<0.8 Ksp.

2, the role of pretreatment system
Before entering the reverse osmosis system, it is necessary to use the pretreatment system to remove residual chlorine, large particles of suspended solids and flocculent colloidal impurities, organic matter, oxides, organic matter, heavy metals in the water to reduce the COD value, SDI value.
In addition, ions such as calcium, magnesium, barium, sulfate, silicate, and carbonate in the water can be combined with the above ions by adding scale inhibitors to form macromolecular particles, which are then discharged in the reverse osmosis unit in the form of concentrated water.
(1) The impact of abnormal system operation on RO membrane
As can be seen from the relevant content of the pretreatment, before entering the RO membrane, the vast majority of macromolecular harmful substances will be removed through the pretreatment system. If the pretreatment system is defective, reverse osmosis feed water indicators many items are not up to standard, it will lead to reverse osmosis membrane elements by irreversible physical and chemical damage, thus greatly shortening the service life of reverse osmosis membrane elements. Factors affecting the service life of reverse osmosis membrane elements are as follows:
RO reverse osmosis membrane scaling;
RO reverse osmosis membrane contaminated by metal oxides;
Suspended matter plugging the RO reverse osmosis membrane;
Colloids, organic matter and microorganisms are contaminated, resulting in increased COD in the effluent.

(2) system abnormalities on the reverse osmosis ultrapure water system
When the pretreatment system runs abnormally, it will affect the operation of the entire reverse osmosis ultrapure water system, specifically in the following 3 aspects:
Reduce the amount of water produced and the quality of water produced by the reverse osmosis ultrapure water system;
Increase the water energy consumption and electric energy consumption for the operation of the reverse osmosis equipment;
Increase the operating cost of water treatment, including scale inhibitors and other water treatment chemicals.

Ultrafiltration system and activated carbon system working principle

In the actual production process, the common pretreatment systems are ultrafiltration system and activated carbon system. Ultrafiltration system is composed of disc filter and ultrafiltration device; activated carbon system is composed of multi-media filter and activated carbon filter.

1、The working principle of ultrafiltration system
The working principle of disc filter is to realize surface filtration and deep filtration by pressing plastic discs. It is mainly used to remove large particulate matter such as sediment from water. Ultrafiltration is a pressurized membrane separation technology, i.e., under a certain pressure, it makes the small molecule solutes and solvents pass through the special film with a certain pore size, while the macromolecule solutes (molecular weight of 10,000-30,000) cannot pass through it, and stay on one side of the membrane, so that the macromolecule substances can be partially purified. The working principle of the ultrafiltration system is shown in Figure 1.
2、The working principle of activated carbon system
Multi-media filter is the use of one or more filtration media, under a certain pressure, so that the turbidity of the water through a certain thickness of granular or non-granular filtration material to effectively remove suspended impurities, so that the water is clarified. Commonly used filter media are quartz sand, anthracite and manganese sand, etc., they are mainly used for water treatment to remove turbidity, water softening, pure water, such as the front pre-treatment, SDI value up to 3 or less. The water filtered out with multi-media filter and then enter the activated carbon filter, because the activated carbon filter is equipped with quartz sand as the base bed, activated carbon as the filter media, the filter material activated carbon through its own voids in the Van der Waals force, you can remove the residual chlorine in the water, organics, and suspended solids and other impurities.

Advantages and disadvantages of ultrafiltration systems and activated carbon devices

There are advantages and disadvantages to both types of pretreatment systems when using the devices.

Ultrafiltration system
1, the advantages of the advantages of the ultrafiltration system are:
① high concentration multiplier, product recovery rate of up to 90% or more;
② the clarity and quality of the effluent water is better;
③ ultrafiltration system as a reverse osmosis pretreatment equipment, the application of which can greatly reduce the investment in reverse osmosis, to extend the service life of the RO membrane (life expectancy of more than 3 years);
(iv) The system has high degree of automation, simple structure, low operation and maintenance cost, and can be backwashed and drug washed online;
⑤ small footprint.

Disadvantages
The disadvantages of ultrafiltration system are:
① As the microporous diameter of ultrafiltration membrane is between 0.002～0.1 um, and the colloid volume in water is ≥0.1 um, latex volume is ≥0.5 um, bacterial volume is ≥0.2 um, and the particle volume is ≥5 um, with a wide range of filtration, therefore, during the operation of the system, some parts are easily clogged, which affects the water flow rate and the service life of the membrane;
② large upfront investment, generally the same amount of water produced by the ultrafiltration system price is multi-media plus activated carbon system and the sum of 2 to 3 times;
③ ultrafiltration system on the feed water pressure requirements are more stringent;
④ can not be shut down for a long time, if a long period of downtime, the membrane must be sealed with chemicals.

Activated carbon system
1, the advantages of activated carbon system are:
① small initial investment;
② Stable water output, less demand for system water pressure.

Disadvantages
The disadvantages of activated carbon system are:
① large footprint.
② In the process of system operation, due to the activated carbon, anthracite involved in physical and chemical reactions, resulting in increased natural consumption, adsorption failure, so that the quality of produced water deterioration, which in turn affects the reverse osmosis system.
③ Activated carbon filter backwash cycle is not easy to control, and the time is relatively short. Activated carbon becomes powder and flows out with the water, seriously polluting the water quality and the next level of equipment. If the backwashing cycle is long, the activated carbon filter is easy to form a high-pressure difference, triggering system operation risks and affecting the water output.
④ The system has high operation and maintenance cost in the later stage, large workload, and short service life of filter media (need to be replaced once a year).