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RSFs are not typically described as biological filters, or the biological processes are regarded as limited and secondary to the straining processes. Some studies have suggested that a combination with preozonation is required for an RSF to evolve into a biological rapid sand filter (BSF) . However, it is most likely that RSF will be colonized with a microbial community, irrespective of the presence of a preceding oxidation step. The operational conditions to some extent dictate the biological processes in the filters. For example, the high filtration rate (low contact time) implies that less time is available for any biological process. As a result, such a filter favors the development of bacterial species that grow rapidly on easily available BOM, while complex organic compounds may not be removed biologically. The small sand grains provide a tremendous surface area for colonization. Biomass concentrations in RSF can vary considerably, ranging in one broad study between 20 and 2000 ng ATP cm−3 (Table 1). However, knowledge about the required biomass start-up times and presence of specific microbial communities in RSF systems is severely limited. The regular backwashing of RSF has three obvious implications for the filter biology: (1) no permanent vertical biomass gradient develops in the filter, meaning a rather homogeneous distribution of the biomass concentration and composition in the filters; (2) no real schmutzdecke develops, which results in limited retention capabilities for pathogens; and (3) the concentration of protozoa will be limited if the backwash cycle is faster than the reproductive cycle of the organisms . Backwashing might reduce the biomass concentration in RSFs by as much as 20%, although the impact of backwashing on filter performance is not regarded as significant 
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Sand filters are widely used in water purification and remove suspended matter by a completely different mechanism. Instead of the water passing through small orifices through which particles cannot pass, it runs through a bed of filter medium, typically 0.75 mm sand 750 mm deep. The orifices between such sand particles are relatively large, but dirt is adsorbed onto the large surface area presented by the medium. The pressure loss rises as the dirt builds up and the filter must be cleaned when it reaches about 3 m WC, otherwise the dirt can be pushed right through the filter
Filter backwashing normally needs low-pressure compressed air and a flow of filtered water about ten times the rated filter throughput. These backwashing arrangements are critical, and providing the large flow of backwash water, as well as drainage for its disposal, can often create difficulties. Given good backwash arrangements, and on a water low in suspended matter, sand filters are simple, reliable, cheap and have low operating costs
Sand filters vary in sophistication. A simple filter will remove most particles down to 5 μm. Multi-media filters which use sand and anthracite, and possibly a third medium, in discrete layers, can yield very efficient filtration down to 2 μm. Granular activated carbon can be used instead of sand to add some measure of organic removal to the filtration process. The quality produced by any filter depends largely on the efficiency of the backwash. Sand filters in some form provide a satisfactory solution for the majority of water-filtration problems
Sand filters are typically designed to give 24 hrs or more between backwashing. Trouble follows quickly if for some reason filter runs become short, because then the filtered water used for backwashing uses up a large percentage of the filtered water and the net output of water falls sharply
Rapid sand filter (RSF) evolved at end of 19th century in the United States of America. It became popular in 1920s because it required lesser necessary facilities with respect to SSF. Unlike slow sand filters, RSF involves only physical process because of absence of biological layer (biofilm) on filter media. Coarse-grained sand and gravels efficiently remove suspended solid by straining and adsorption. RSF must be aided with pretreatment (sedimentation and flocculation) and posttreatment (disinfection) steps to remove pathogens and prevent fouling. It requires lesser area for construction as compared to SSF for treatment of unit volume of water. RSF is constructed in a rectangular tank usually made up of concrete. Three to five layers of graded gravel are installed at the bottom of tank over a network of drainage pipes placed on the floor. Filter media that is coarse sand with a diameter ranging from 0.4 to 0.6 mm is filled over gravel layer. As coarse sand provides larger void as compared with fine sand of SSF, RSF achieves a higher rate of filtration. Gravel layer prevents sand from being drained out during filtration. Also, it facilitates even distribution of water through filtration media during backwash. Top of the RSF is either open for supernatant water (gravity filter) or closed (pressure filter) (O'Connor and O'Connor, 2002)
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Granite Crushing Project Background At the end of 2016, ZENITH cooperated with a building aggregate company from Quzhou, Zhejiang. The customer decided to invest a special granite crushing production line. Until now, the construction of the production line has come to an end. Compared with previous production lines, what differences does this production line have?
Customer Profile The customer is an influential, government-supported green enterprise in local areas. To offer aggregates for local railway construction, the customer decided to build this production line. Because the production line is located in the industrial park, the project is required to be pollution- and noise-free and to meet the national stipulated standards. After multiple comparisons among different machine manufacturers, the customer chose to cooperate with ZENITH. Granite Crushers The first section: Apron feeder, C6X145 Hydraulic Jaw Crusher The second section: HST315 Medium-Crushing Cone Crusher, HST315 Fine-Crushing Cone Crusher, VSI5X Impact Crusher, S5X Heavy Vibrating Screen, fine sand recycle system, sewage treatment system and belt conveyor
Given the position of raw materials and layout of production base, the production line is divided into two sections. The first section, located in granite quarry, is coarse-crushing system. After being crushed coarsely, the materials are sent into the second section in industrial park--- crushing and screening system. The raw materials in granite quarry are sent into the industrial park for piling and then the piled materials undergo crushing and screening of two-sectional cone crushers. Then, VSI5X impact crusher is used to adjust granularity of finished aggregates. The finished machine-made sand is produced through wet process. To keep grading and avoid sewage disposal, the production line is equipped with sand-washing recycle system and sewage treatment system
Granite Crushing Project Advantage 1. Environmental-friendly design --- zero-pollution emission The production line designed by ZENITH is equipped with closed plant and sewage treatment system, avoiding noise and water pollution. Meanwhile, wet-process production avoids dust pollution. 2. Customized scheme --- professional sectional design Sectional design not only meets the requirement of processing materials in granite quarry but also achieve aggregate production in the industrial park. 3. High-quality aggregate output--- high investment return Core equipment and project scheme were offered by professional aggregate machine manufacturer --- ZENITH. The quality of equipment is reliable and the professional technologies guarantee the quality of finished products. Under such a market background that prices of aggregate are soaring, considerable investment return can be expected when the production line is put into use. 4. Multiple sources of raw materials --- turning wastes into treasures This project can use solid wastes like waste stone peeled off from mines, semi-finished products from aggregate plants and waste stone scraps as the raw materials to produce high-quality building aggregates, achieving recycle of wastes and adding the profits
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