Hsi Crusher Parts,Crusher Wear Parts,Wear Parts Chrome,Crusher Chrome Wear Parts Wuyi Slon Machinery Co.,Ltd , https://www.slonmachinery.cn
1 review
In water treatment, especially in large-scale water treatment facilities, energy consumption has become a very important indicator. If membrane technology is to become one of the main technologies for large-scale water treatment facilities, it is necessary to reduce energy consumption. As a result, many membrane manufacturers are beginning to develop low-energy membrane filtration systems, so-called dead-end filtration or semi-dead filtration.
This system works like a coffee filter where solid suspended solids settle on the surface of the membrane. This part of the solid is usually called "dirt", as long as the water contains solid suspended matter, there will inevitably be "dirt". In order to ensure that the water production of the membrane remains unchanged, the membrane filtration pressure is inevitably increased, so the membrane needs to be cleaned in the opposite direction to the filtration after a period of operation, so sometimes we also call "half dead end filtration". The solids deposited on the surface of the membrane were washed out and the membrane returned to its original design. Although backwashing removes most of the membrane fouling in the system, sometimes a more efficient approach is required to thoroughly clean the membrane. Because many substances stick to the surface of the membrane, they cannot be removed by mechanical force alone. This part of the material is usually organic or microbial organic matter, which will block the pores of the membrane after a long period of operation. The clogging problem of the membrane should be referred to as "dirt", which is a situation that is undesirable in the course of operation. The plug can dissolve (for some small molecular organics) and pass through the membrane if its adhesion to the membrane surface is not very strong; or the membrane is trapped, for some microbial organisms, when they adhere to the membrane surface, they will further multiply . This membrane fouling is mainly removed by chemical cleaning and is also a reversible contamination. The real problem with membrane fouling is irreversible pollution that cannot be removed.
2 semi-dead end ultrafiltration technology
The development of the semi-dead filter technology in recent years is the core technology of XIGATM, which was developed based on the 8-inch semi-dead filter ultrafiltration membrane module. The core technology of XIGATM uses an 8-inch pressure vessel, which is the standard design for reverse osmosis. In each pressure vessel, multiple membrane modules can be placed. Each membrane module was 1.5 m long, capillary membrane, membrane filament inner diameter 0.8 or 1.5 mm, and membrane area per membrane group was 22 or 35 m2. The membrane filtration process is divided into three steps: filtration, backwashing, and chemical enhanced backwashing.
The key to the successful application of semi-dead filtering technology is to rationally design the three processes of filtration, backwashing and chemical strengthening backwashing, so that end users can obtain the lowest operating costs. Therefore, it is not necessary to keep the water discharge rate per unit membrane area as large as possible. Because backflushing does not require the addition of any chemicals and is carried out for a short period of time (usually 20 to 60 seconds), the cost of backwashing is much lower than chemically enhanced backwashing. We believe that backwashing is the preferred method of removing fouling from the surface of the membrane.
To explain this problem more clearly, the figure below shows the change in pressure across the membrane during system operation. In the figure, section A represents the filtration process, section B represents the backwashing process, and C short represents the chemically enhanced backwashing process (CEB).
During the process of A, the key indicators to be guaranteed for specific water quality are membrane flux and membrane filtration pressure. Therefore, if the frequency of backwashing and chemically enhanced backwashing is reduced, the flux of the membrane will be affected. At the same time, it will increase the investment of the system. Another method is to improve the quality of the incoming water by dosing or chemical pretreatment. This also increases investment and operating costs, so it is usually a trade-off between the two approaches.
For the B process, the membrane filtration pressure drop depends on the thickness of the scale layer on the membrane surface and the mechanical pressure during backwashing. Backwashing should be as adequate as possible to ensure adequate removal of backwashed dirt, which is an effective way to delay the frequency of chemically enhanced backwashing. In addition, there is a trade-off between the two processes of backwashing mechanical pressure (such as backwash water flow) and changing the scale thickness (adding pretreatment).
The C process, chemically enhanced backwash (CEB), only after the backwash has been subjected to backwashing, after the filtration pressure drop across the membrane has reached a predetermined value, or after a predetermined number of longer backwashes. The chemical cleaning agent used is a mixture of conventional chemicals, including sodium hypochlorite, hydrogen peroxide, hypochlorous acid, etc., which can be very easily disposed of.
3. Application of ultrafiltration technology in the field of water treatment
Although ultrafiltration can have many application areas, large-scale water treatment usually focuses on the following aspects:
Drinking water supply terminal
Surface water treatment
Seawater treatment
Fluid reuse
3.1 drinking water treatment
As the quality requirements for drinking water become more stringent, water treatment companies are investing more and more effort to control the amount of microorganisms present in the water supply network. In order to do this, one method is to perform an expensive and frequent water quality test, or to provide a barrier to the entry of bacteria and viruses at the water supply terminal.
With the UF system, such a barrier can be built very conveniently. The ultrafiltration membrane can remove bacteria up to 6 logs, and the virus removal rate reaches 4 logs, so water plants and water users do not have to worry about bacteria and viruses. Since the quality of the drinking water itself is very high (the turbidity and suspended solids are very low), the membrane system at this time can be used with a high membrane flux of up to 135 liters per square meter. At the same time, the water inlet condition is higher, so the frequency of recoil and chemically enhanced backwashing can be very low, and the water production can reach 99%. If necessary, a secondary ultrafiltration system can be set up to further reuse the first stage backwash water.
3.2 Surface water treatment
UF systems are used in a large number of applications for surface water treatment, and the treated water is used for irrigation or as a reverse osmosis water to prepare industrial water.
In the Netherlands, there are more and more such factories. This technology provides a new way of using industrial water, that is, it is not necessary to purchase more and more expensive drinking water, but to use it near the surface water.
3.3 seawater desalination
The Middle East is the worst place for water scarcity. In order to solve this problem, the earliest people usually use distillation technology. From the 1860s, membrane technology was used to address water shortages in these countries. However, many reverse osmosis desalination systems face serious membrane fouling problems. Mainly because the traditional pretreatment method of reverse osmosis system can not provide reliable water quality. As a result, most desalination plants work far below their designed water output, and even some plants produce less than 30% of the original design.
The study of small desalination plants clearly shows that the ultrafiltration system can control the quality of seawater with great confidence and provide high quality water for reverse osmosis systems. Long-term tests have also shown that the effluent SDI value of the ultrafiltration system can be very well controlled below 2. These tests do not require any pre-treatment before the ultrafiltration system and are suitable for a variety of seawater qualities.
3.4 Wastewater reuse
Western countries have spent a lot of energy on wastewater treatment, and it is really unreasonable to simply discharge it through the drainage network to the surface water source. Once again, ultrafiltration offers an attractive solution for wastewater reuse because of its price advantage.
In fact, the wastewater discharged from urban sewage treatment plants and factories is a very good water resource for industrial water and even drinking water. This is technically achievable, but it is very difficult for Western users to believe in this practice. It is not so much a technical problem as it is a psychological problem. However, at Windhoek, Namibia, a water plant of 850 tons/hour is already being built, which uses membrane technology to reuse the effluent from the sewage treatment plant as drinking water.
4. in conclusion
Semi-dead end ultrafiltration is a technique that does not have to be doubted. It has a wide range of applications, some for small projects, but others, like the ones we mentioned above, are large and even very large. This technology is related to a problem that humanity must face if the world is still developing at the current rate. Drinkable water resources are an important part of everyone's life. Developing a technology to maintain drinking water resources is the only way to maintain human life and the only way to ensure that water will not be like oil in the next century.
Prospect of ultrafiltration membrane technology in the field of water treatment in the 21st century
In the last 10 years of the 20th century, the owners of water treatment facilities around the world began to change. (drinkable) Water supply has gradually changed from a large-scale, government-controlled operation to a privately owned, multi-country-participating business, and is seen as the next business opportunity of this century. As a result, there is a need for new water treatment technologies and reduced water treatment costs. This demand will inevitably lead to the rise of membrane technology. Since the 1960s, membrane technology originated from the reverse osmosis membrane of seawater desalination. The film technology has been developed very rapidly and is widely used in more and more fields. After both desalination and reverse osmosis, a series of looser permeable membranes have been developed, ranging from nanofiltration (loose reverse osmosis) to ultrafiltration (removal of bacteria and viruses) to microfiltration (removal of suspended solids). And any application has its own unique, specially designed membrane to meet the requirements. In the early days, most membrane filtration used the form of cross-flow filtration, that is, the liquid flows in a horizontal direction with the membrane surface. Such a filtration form can prevent the formation of "membrane scale", but only a small portion of the liquid can be actually filtered out. This form of filtration therefore results in very high energy consumption, which hinders the application of the membrane to large-scale water treatment facilities.