Overview of the development of filter brick backwashing technology

Filter gas water distribution system

1.1.1 Overview

In the conventional water treatment process, filtration generally refers to the process of trapping suspended impurities in water with a granular filter material layer such as quartz sand, so that the water is clarified. The filter tank is usually placed after the sedimentation tank or clarification tank. The effect of filtration is not only to further reduce the turbidity of the water, but also organic matter, bacteria and even viruses in the water will be partially removed as the turbidity of the water decreases. As for the bacteria and viruses remaining in the filtered water, when they lose the protection or attachment of the turbid particles, they will be easily killed during the disinfection process after filtration, which creates good conditions for disinfection after filtration. In the purification process of drinking water, sometimes the sedimentation tank or clarification tank can be omitted, but filtration is indispensable. It is an important measure to ensure the sanitation and safety of drinking water.

There are many types of filter tanks, among them, the longest use history is the ordinary quick filter tank with quartz sand as the filter material. From a different technological perspective, a variety of other forms of fast filter have been developed on this basis. Among them, the V-type filter is a kind of gravity fast filter developed by the French Degremont company in the 1970s. It is named after the water inlet on both sides (or on one side) is designed into a V shape.

The filter in the water plant is an important structure in the filtration process, and the key to the stable and efficient operation of the filter is the regeneration of the filtration capacity of the filter layer. If the backwashing technology is better, the working state of the filter is always in the optimal condition, which can not only save energy and water, but also improve the water quality, increase the sewage interception capacity of the filter layer, extend the working cycle, and increase the water production. . The regeneration of the filtering capacity of the V-type filter adopts the advanced backwashing technology of air and water backwashing and surface sweeping. In the air flushing process of the V-type filter, because the air blower is used to press the air into the filter layer, the filtration performance of the filter is improved from the following aspects, as shown in Table 1.1:

I believe that as we pay more and more attention to the use of water resources, the popularity and application of V-type filters will become more and more widespread

1.2 Development of Backwash Technology

During the operation of the filter, the impurities removed from the feed water accumulate on the surface of the filter material and the pores between the particles. As the filter continues to operate, the impurities stored in the filter bed will cause the porosity of the filter bed Decrease, the amount of impurities that can be retained by the filter bed is continuously reduced. When the head loss increases to the maximum allowable head loss when the water flows through at a predetermined flow rate, or the penetration of suspended substances finally causes the filtered water quality to decline At this time, the filter tank will eventually be shut down. At this time, the filter tank needs to be backwashed to remove the trapped impurities and restore the operating capacity of the filter tank.

The so-called "backwashing" refers to the operation process of washing the filter layer with reverse water flow used to restore the normal work of the filter. It is to allow the filtered clean water to pass through the filter layer at high speed in reverse (from bottom to top) and be trapped in The suspended impurities on the surface of the filter material are washed down by the action of high-speed water flow and carried out of the filter layer by the water flow. The effect of backwashing will directly affect the filtering behavior. If the washing effect of the filter tank is not good, it will produce a series of harmful effects.

For the old-fashioned slow filter, most of the impurities removed from the water will accumulate on the surface of the filter bed. When the quality of the filtered water drops, the upper filter material is replaced with clean sand to restore the running capacity of the filter. The early cleaning method of the sand filter fast filter was to gently clean the sand particles, so as to leave an undamaged organic film on the surface of the filter bed. Based on the knowledge and understanding inherited from the filtration process, it is believed that this membrane is needed for effective filtration. The first sand filter in the United States is equipped with a ring-shaped stirring rake, which is used to stir the sand during the cleaning process to help separate impurities and sand. But the backwashing intensity of this type of filter is only 1.7 -3.4L/s m2. Naturally, the filter bed cannot be thoroughly cleaned under such low-speed and weak-intensity washing, and the effect is not ideal.

As early as the end of the nineteenth century in England, some people began to air-wash the soaked filter material and then backwash it with water. This can be regarded as the prototype of air-water backwashing. By 1900, the gravity fast filter of the Little Fall installation in New Jersey, USA used air agitation instead of rotating agitation rake, which is considered to be the earliest application of air-water backwash technology. Experiments conducted by Harvard University in the following years showed that air washing is required to break up the mud or mud lumps formed on the surface of the sand layer of the filter.

From 1903 to 1905, in the experimental research process of the Cincinnati Water Filtration Plant in the United States, the so-called "high-speed washing method" was adopted, that is, the backwashing intensity was 10-16L/sm2, and the rotating rake and air washing were abandoned. In this way, under high backwashing intensity, the entire filter layer is in a suspended state and the filter layer is in a fluidized state. At this time, the dirt adsorbed on the filter material particles is washed away by the mutual contact and collision between the filter materials in the rising water flow. Then the flushing water is taken out of the tank, so that the filter material is "cleaned". Because of its simple structure, convenient operation and management, and obvious washing effect, this method has been widely used in the United States, Japan and other countries for a long time. However, if this flushing method is adopted, the water flow must be sufficient to make the filter material swell and fluidize, that is, the apparent volume of the filter layer increases by at least 15% after expansion. Hublert and Herring in 1929 showed that according to the research results of high-intensity backwashing , 50% expansion rate of backwashing is appropriate, after that, high-speed backwashing is widely used in countries all over the world.

In the backwashing process, whether it is the shear force of the water flow or the collision friction between the particles is generated by the water velocity gradient G value, the actual G value generated by the high-speed water backwashing is generally not high, usually caused by sand particles . The value is 300-400s-1, and the coal particle is 150-300s-1, which is a weak flushing method. In addition, the vortex action caused by the expansion of the filter layer will cause a part of the dense sludge layer formed on the surface of the filter material to be taken into the deep part of the filter layer and gradually form a large, hard mud ball. With the designed backwashing strength, the backwashing effect of the filter is not good, and as the washing time is prolonged, the mud ball phenomenon becomes more serious, and the water consumption of backwashing increases, thus forming a vicious circle. In addition, high-speed water backwashing is also easy to cause the loss of filter material in the filter, so in subsequent applications, surface washing is usually assisted. In 1908, the first set of surface washing equipment appeared at the OKlahom Water Company in California. At first, the auxiliary flushing method was to use mechanical stirring or hydraulic stirring to enhance the friction and collision effect of the surface filter material to achieve better results.

Since 1910, most of the quick filter tanks designed in the UK have been cleaned with air or rotating rake or surface washing and then backwashed with water. The exception is the Bamford built in 1948. Both the filter and a well-known filter built in Birmingham in 1958 used American-style high-speed water backwashing, but the latter later added air washing equipment to save washing water. However, in view of the lack of air distribution facilities The promotion and application of air-water backwashing technology has been affected for many reasons. Until the Fourth International Water Supply Conference held in Sweden, the use of long-handled filters as the air distribution device was proposed. , With the rise of deep-bed filters with uniform particle size, and the improvement of the quality of pneumatic and electric valves, the air-water backwashing technology of the filter has gradually gained popularity, especially in European countries such as Britain and France. In the 1970s, some scholars in the United States began to notice the different washing methods being used in other countries, and tried to evaluate the advantages and disadvantages of these methods. Among them, air-water backwashing is compared with single-water backwashing. It has obvious advantages in the full utilization of the filter, the improvement of the filtration rate, the extension of the filtration cycle and the elimination of the filter layer mud ball, so the research work on the problems related to the air-water backwashing of the filter has started. (Amirtharajah), Chris Bai (Cleasby) and others have achieved great results in this regard. With the continuous research and the deepening of their understanding, the United Kingdom, France and other European countries, as well as the former Soviet Union and Japan have all vigorously developed In order to update and improve the process design of the air-water backwashing filter, the technology of air-water backwashing in practical applications has become more mature. For example, the excellent patented product Aquazer 1 of the French Degremont company The V-type filter adopts a single uniform filter material, and realizes the automation of filtration and flushing through the application of a two-pole PLC programmable logic controller. The combination of air-water backwashing and surface sweeping can completely realize constant water level and constant speed filtration. Now It has been widely used in European countries as a relatively mature filter air-water backwashing technology, and many developing countries have also introduced this technology. At present, 14 cities in China have directly imported from France Of this type of filter, the largest Zibo Xincheng Water Plant has a designed capacity of 1 million m3/d.

Since the Fushun East Park Water Plant in the 1930s first adopted air-water backwashing technology, it has a history of more than 70 years. However, it has not been widely used because of the failure of gas distribution facilities and other reasons. Until the mid-1980s, China Municipal Engineering Southwest Design and Research Institute took the KI long-handled filter head imported from Zhuhai Gongbei Water Plant as the basis. After repeated experiments, the semi-circular long-handled filter head was developed; The application of uniform particle size filter material in ordinary fast filter, siphon filter, V-type filter; the improvement of the quality of pneumatic and electric valves, etc., has made the air-water backwash filter have a great development. According to incomplete statistics, more than 50 newly-built water plant filters in the country have adopted gas-water backwashing technology for more than ten years, and the daily water supply capacity has exceeded 6.5 million m3/d. Water treatment workers at home and abroad have also responded again. Air-water backwashing technology has been highly valued. In terms of the theory and experimental research on the air-water backwashing of the filter, a large number of scientific researchers have done a lot of fruitful work, including Professor Li Guibai from Harbin Jianzhu University, Professor Zhu Yuehai from Tongji University, and Professor Andingnian from Tianjin University. work. The Water Supply Committee of the Chinese Civil Engineering Society also held two special seminars on air-water backwashing technology in Guangzhou and Hangzhou in October 1992 and April 1993, respectively. Application experience in design, production and construction management, and put forward ideas for further improvement and perfection. The air-water backwashing technology is popularized and applied in terms of application and practice. There are ordinary fast filters that use long-handled filter heads to distribute water and air, two-valve filters that use electric valves to control drainage, and siphon filters that use pressure plate valves. There are various forms such as transforming the old pools with single water flushing into new pools with air-water backwashing, and there are examples of successful operations. Facts have proved that the V-type filter introduced in recent years has a good operating effect.