[ Hongjie Water ] Advantages and disadvantages of ultrafiltration and activated carbon devices in the preparation of ultrapure water

Ultrapure water is water from which all conductive ions, colloids, organic matter, etc. have been removed, and its water resistivity is greater than 18 MΩ·cm (25°C). Ultrapure water systems generally use pretreatment technology, reverse osmosis technology, electric desalination technology, and post -treatment , and are also equipped with auxiliary treatments such as ultraviolet lamps and TOC devices.

Ultrapure water preparation process

The process of preparing ultrapure water is: tap water → pretreatment system → multi-stage high-pressure reverse osmosis system → EDI electric desalination system → TOC device → polishing mixed bed system → water point. In the preparation process, the water pump is used as the power source, and the water storage tanks at all levels must ensure that the system water pump can operate safely and stably. Adding filters of different treatment levels to individual parts can ensure the quality of the produced water.

At the same time, during the operation of the filter, adding various 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, etc. In winter, in some areas in the north with lower temperatures, a heat exchanger will be added at the inlet of the pretreatment system to make the tap water temperature meet the requirements of the equipment's water production process.

In addition, in the ultrapure water tank, it is very important to use nitrogen to isolate the water from the external medium, which can ensure that the prepared ultrapure water is not contaminated by the air for the second time.

Importance of Preprocessing

1. Water inlet requirements for RO membrane

In the process of preparing ultrapure water, the RO system is the most critical. The normal operation of the RO system can ensure the operation safety of the entire pure water equipment and the water quality of the produced water. In the process of preparing ultrapure water, the RO system has certain requirements for the incoming 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 (actually controlled at 0 mg/L);
• When dissolved oxygen > 5 mg/L, iron content < 0.05 mg/L;
• SiO2 in concentrated water <100 mg/L;
• LSI, pHb -pHs < 0;
• Ions such as Sr and Ba that easily form insoluble salts have Ipb <0.8 Ksp .

2. The role of the pretreatment system

Before entering the reverse osmosis system, the pretreatment system should be used to remove residual chlorine, large suspended particles and flocculent colloidal impurities, organic matter, oxides, organic matter, and heavy metals in the water to reduce the COD value and SDI value.

In addition, calcium, magnesium, barium, sulfate, silicate and carbonate ions in water can be combined with the above ions to form large molecular particles by adding antiscalants, and then discharged as concentrated water in the reverse osmosis device .

(1) Impact of abnormal system operation on RO membrane

From the relevant content of pretreatment, it can be known that most of the high molecular harmful substances will be removed through the pretreatment system before entering the RO membrane. If the pretreatment system is defective and many reverse osmosis water inlet indicators do not meet the standards, the reverse osmosis membrane element will be irreversibly damaged, thus greatly shortening the service life of the reverse osmosis membrane element. The factors that affect the service life of the reverse osmosis membrane element are as follows:

• RO reverse osmosis membrane scaling;
• RO reverse osmosis membrane is contaminated by metal oxides;
• Suspended matter blocks the RO reverse osmosis membrane;
• Colloids, organic matter and microorganisms are contaminated, resulting in increased COD in the effluent.

(2) Impact of system abnormalities on reverse osmosis ultrapure water systems

When the pretreatment system operates abnormally, it will affect the operation of the entire reverse osmosis ultrapure water system, specifically in the following three aspects:

• Reduce the water output and 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 chemicals.

How Ultrafiltration and Activated Carbon Systems Work

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

1. Working principle of ultrafiltration system

The working principle of the disc filter is to achieve surface filtration and deep filtration through compressed plastic discs . It is mainly used to remove large particles such as mud and sand in water. Ultrafiltration is a pressurized membrane separation technology, that is, under a certain pressure, small molecular solutes and solvents pass through a special membrane with a certain pore size, while large molecular solutes (molecular weight of 10,000 to 30,000) cannot pass through and remain on one side of the membrane, so that the large molecular substances are partially purified. The working principle of the ultrafiltration system is shown in Figure 1.

2. Working principle of activated carbon system

Multi-media filters use one or more filter media to allow water with high turbidity to pass through a certain thickness of granular or non-granular filter material under a certain pressure to effectively remove suspended impurities and clarify the water.

Commonly used filter materials include quartz sand, anthracite and manganese sand , etc. They are mainly used for water treatment and turbidity removal, softening water, pre-treatment of pure water, etc., and the SDI value can reach below 3. After the water filtered by the multi-media filter enters the activated carbon filter, because the activated carbon filter is equipped with filter material with quartz sand as the base bed and activated carbon as the filter material, the filter material activated carbon can remove impurities such as residual chlorine, organic matter and suspended matter in the water through its own gaps under the action of van der Waals force.

Advantages and disadvantages of ultrafiltration devices and activated carbon devices

In the process of using the device, these two types of pretreatment systems have their own advantages and disadvantages.

Ultrafiltration system

1. Advantages

The advantages of ultrafiltration system are:

①High concentration multiple, product recovery rate can reach more than 90%;

② The clarity and quality of the effluent water are good;

③Ultrafiltration system is used as reverse osmosis pretreatment equipment. Its application can greatly reduce the investment in reverse osmosis and extend the service life of RO membrane (life span is more than 3 years);

④The system has a high degree of automation, simple structure, low operation and maintenance costs, and can be backwashed and cleaned online;

⑤Small footprint.

2. Disadvantages

The disadvantages of ultrafiltration systems are:

① Since the pore size of the ultrafiltration membrane is between 0.002 and 0.1 um, and the volume of colloids in water is ≥0.1 um, the volume of latex is ≥0.5 um, the volume of fungi 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 water flow rate and the service life of the membrane; ② The initial investment is large. Generally, the price of an ultrafiltration system with the same water output is 2 to 3 times the sum of the multi-media plus activated carbon system;

③The ultrafiltration system has strict requirements on the water inlet pressure;

④ The machine cannot be shut down for a long time. If it is shut down for a long time, the membrane must be sealed with chemicals.

Activated carbon system

1. Advantages

The advantages of activated carbon systems are:

① Small initial investment; ② Stable water output and low requirements on system water inlet pressure.

2. Disadvantages

The disadvantages of activated carbon systems are:

①Occupies a large area.

② During the operation of the system, due to the participation of activated carbon and anthracite in physical and chemical reactions, natural consumption increases, adsorption fails, and the quality of produced water deteriorates, which in turn affects the reverse osmosis system.

③ The backwash cycle of the activated carbon filter is difficult to control and the time is relatively short. The activated carbon becomes powder and flows out with the water, seriously polluting the water quality and the next level equipment. If the backwash cycle is long, the activated carbon filter is prone to form a high pressure difference, causing system operation risks and affecting the water output.

④ The system has high operation and maintenance costs in the later stage, heavy workload, and short service life of the filter material (needs to be replaced once a year).

For more information, please visit the official website of Shenzhen Hongjie Water Technology Co., Ltd. www.execro.com , If you need any help, please call our company hotline 1 80 3800 0078 for free. We will serve you wholeheartedly.