Continuous sand filters are also called active sand filters. In 1974, Hans F. Larsson and Ulf Hjelmner of the Swedish family business Axel Johnson began to study continuous sand filters, and proposed the idea of cleaning the adsorption saturated filter material separately, and solved the problem of effective cleaning of the filter material and preventing the filtered water from interacting with each other. The two key problems of backwash water mixing, the equipment was successfully developed and patented in 1976. In September 1977, the American Parkson company was authorized to produce and sell continuous sand filters in North America and was registered as "DynaSand". At the same time, Parkson applied for 8 continuous filtration technology patents from 1981 to 1997. Because the continuous filtration technology has the advantages of economy, high efficiency and wide range of applicable water quality, it has aroused great interest of researchers in France, the Netherlands, Turkey, Denmark, Finland, Canada, Japan and other countries. After the introduction of continuous filtration equipment from Parkson, the company passed Different forms of improvements were made to the water distribution method, sand washing method, and filter material lifting method, and a variety of different types of continuous sand filters were produced. The scope of application has also expanded from the initial drinking water treatment to the sewage treatment of food, textile, metallurgy, papermaking, metal processing, glass polishing, chemical industry and other industries with high suspended solids content, as well as the advanced treatment of the secondary effluent of urban sewage.
The firstly developed continuous sand filter mainly uses internal sand extraction, which is called internal circulation continuous sand filter. In order to facilitate the observation of the lifting condition of the filter material in the continuous sand filter, the filter material riser can also be moved outside the sand filter. , The so-called continuous sand filter for extracorporeal circulation. Figure 1 is a schematic diagram of the structure of Parkson's DynaSand internal circulation continuous sand filter. The filtration process is as follows: the raw water added with coagulant flows downward through the water inlet pipe (A), and enters the radial water distributor (C) through the water inlet annulus flow channel (B). The water flows out of the orifice of the water distribution branch pipe and then passes through the sand filter layer, flows through the filter bed from bottom to top, and the filter sand is circulated and cleaned from top to bottom in the filter bed. The water and sand are in a countercurrent state in the filter. , Enhance the retention effect of filter sand. When the suspended matter in the raw water passes through the sand filter layer from bottom to top, it is intercepted by the filter material. After the filtered water rises and flows through the filtered water weir (J), it enters the reservoir through the outlet pipe (E). The backwashing process is: pass a small amount of compressed air into the bottom of the air sand lift pipe (F), and the dirty sand at the bottom of the filter will rise together through the air lift effect, and the sand, mud, water flow and air in the air sand lift pipe are upward. During the flow, the shear force of the water and air flow and the friction between the particles are used to produce a short-term but strong backwashing process to complete a cleaning. The lifted mixture falls from the top air sand extraction pipe (G) into the sand washing tank (H), where air, sewage, and sand are separated, the sand washing water is discharged through the backwashing water outlet pipe (L), and the filter material enters downwards A sand washer with a staggered loop and labyrinth structure (I). After a small amount of upwardly flowing filtered water enters the sand washer, the flow rate will increase, so that the downwardly moving filter material will be in a rotating and tumbling state, so that the filter material can be thoroughly backwashed. Under the action of the water level difference between the filtered water and the backwash water, the air lift water in the sand lifting pipe and the washing water in the sand washer are discharged from the filter tank together. The washed filter material slowly moves downward under the action of gravity, thus starting a new round of filtering process.
The measurement of zeta potential confirms that the mechanism of continuous filtration is contact flocculation based on surface energy and van der Waals force. Changing the zeta potential of microflocs creates conditions for adhesion, so continuous filtration belongs to the category of microflocculation deep filtration. Although there is a trend towards surface filtration in the later stage of filtration, it is not the main one. The adsorption bridging between the filter material or its sediment and the filter microfloc is the main mechanism of continuous filtration and adhesion.
In order to solve the problem of increased infrastructure investment caused by the combined use of multiple steel sand filters, Parkson has developed a continuous sand filter combination device in the form of concrete. Multiple sand lifting and sand washing devices are placed in the same filter. It solves the problem of water limitation applicable to large-scale water plants, saves steel materials, and makes the equipment simple and easy to operate and control. In the mid-1990s, Sweden, France and other countries studied the nitrogen and phosphorus removal performance of continuous sand filters, and achieved better results through the improvement of sand filters. During the test, the height of the sand filter was increased to 6m, and the thickness of the filter material layer reached 4.5m. Anaerobic, anoxic and local aerobic zones were formed in the filter layer, so that the nitrification, denitrification and phosphorus removal process can be completed in the equipment. The improved continuous sand filter can remove suspended solids and nitrogen and phosphorus at the same time. It has good promotion and use value, and it is believed that it will be promoted and used all over the world.
At present, the active sand filters on the market can be divided into three types: ordinary active sand filters, biological active sand filters and denitrification active sand filters according to their structure and function. Ordinary active sand filters mainly remove suspended solids (SS) and phosphorus in sewage; biological active sand filters can not only remove suspended solids (SS) in sewage, but also effectively remove COD, BOD and other organic substances in sewage. Pollutants; denitrification activated sand filters are used to remove HN4+-N in sewage.
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