Sand filter removal efficiencies

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

Sand filters vary in sophistication. A simple filter will remove most particles down to 5 pm. Multi-media filters which use sand and anthracite, and possibly a third medium, in discrete layers, can yield very efficient filtration down to 2 pm. 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 usually provide efficient SS removal for suspended matter above 30 x particle size, but they may need to be periodically sanitized using chlorine, formaldehyde, or similar substances (especially filters with very fine grain media). It is not uncommon for filters to contain microbial slimes and therefore act as a source of microbiological reinfection. 

In this context it is the separation of solids from water by forcing the water through a porous filter media. The objective is typically to reduce the level of TDS in the water and often to reduce both the size of the particle remaining and the turbidity of the water. Filtration efficiency and quality is a function of many variable factors, although filtration is usually carried out at relatively low velocities, where velocity and pressure drop are directly related to each other. Typically a sand filter will remove a high percentage of particles above a diameter of 20 to 30 pm, whereas dual or multimedia filtration is required to remove particles down to a diameter of 10 to 20... 

No plant-specific identification number was available for this facility. The wastewater from Plant B contains pollutants from both metals processing and finishing operations. It is treated by precipitation-settling followed by filtration with a rapid sand filter. A clarifier is used to remove much of the solids load. Table 5.14 summarizes the data on pollutant removal efficiency at Plant B. 

This is physical operation for removal of suspended solids by filtration through a bed of sand and gravel. TSS removal efficiency is 50-90% depending on the type of filter media used and the filtration rate. 

Clasen, J. (1997). Efficiency control of particle removal by rapid sand filters in treatment plants fed with reservoir water A snrvey of different methods. Water Science Technol., Proc. 1997 1st lAWQ-lWSA Joint Specialist Conf. on Reservoir Manage, and Water Supply—An Integrated Syst., May 19-23, Prague, Czech Republic, 37, 2, 19-26. Elsevier Science Ltd., Exeter, England. 

In the mid-1990s, two spiral-wound nanofiltration plants were installed for the treatment of effluent from a paper mill. In both cases, the nanofiltration systems were installed to remove color, organic carbon, and dissolved solids from effluents for reuse or for further processing. Both plants had a very efficient pretreatment before the NF spiral-wound modules to prevent plugging of the filtration elements. For instance, the pretreatment included settling at several stages with chemicals, a sand filter, a backwashable screen filter, and a bag filter (5 pm) [106]. Neither of the plant is in operation today (P. Eriksson, personal communication, 2005). 

Slow Sand Filter. Fine grains of sand are used which leads to efficient suspended particle removal, but slow filtration rates. Relatively shallow beds are employed and biological activity on the surface is common. The initial sand layers become clogged first and cleaning is by removal and replacement of these layers. The deposit formed on the surfece is sometimes called schmutzdecke . 

The effective unification of the bubble-film extractor and sand filter (or other filtration device) into joint water treatment system, which is able to act as emergent water purifier, must be based on the corresponding relationships in order to achieve the desirable parameters. These relationships are the functions of driving forces, rate constants of the removal of admixtures, dimensions of reaction spaces in the water treatment equipment, etc. They allow to predict the efficiency of water purification by direct-flow and recirculation modes of water treatment. 

Generally, with coalescers, the overall cost of contaminant removal can be low, even when compared with other less efficient methods, like salt driers, electrostatic separators and sand filters. 

Removal of the fine sand has reduced the stopping power or breakthrough efficiency. Filter efficiency is also reduced. Filter efficiency has been directly related to the media particle surface area. The number of particles in a given volume of a dual-media filter has been reduced. The average size of the particles has also increased. As the filtered material is relatively loosely held in the coal layer, the materials are dislodged and will likely penetrate the sand layer and appear in the filter effluent if a flow change or applied solids surge occurs. 

Some of these are the influent oil content, the influent suspended solids content, the desired effluent quality, temperature, space, and dependability of treatment. An advantage that mixed media provides is that the size and quantity of each filter material can be varied to meet the requirement. As an example, the typical mixed-media filter bed is 30 in. deep and utilizes 60-pcrccnt anthracite coal of 1.0 mm size, 30-pcicenl sand of 2.6 S.G. and 0.5 mm size, and 10-percent garnet sand of 4.2 S.G. and 0.25 mm size. Such a filter will provide very efficient removal of suspended solids over 10 to 15 microns in size without the use of chemicals. With aluminum sulfate or other coagulants, much smaller particles can be removed. Any oil associated with these suspended solids is also removed. We believe that the removal of oil with mixed-media filters can be attributed partially to mechanical straining and partially to adsorption on the media. 

The most common process used for this is depth filtration through a bed of sand or similar material charged in a vertical vessel. The incoming water flows from top to bottom. To improve the efficiency of such filters, two or more layers of media with various particle sizes are used. Coarse and less dense material such as anthracite is located at the top of the bed, whereas finer and denser particles of sand are placed at the bottom. Such multimedia filters can remove most particles larger than 10-20 pm. Periodically the filter bed is back-washed by reversing the flow direction (from bottom to top) and by increasing the flow rate. During backwash, the captured particles are removed and sent to drain, whereas heavier particles of the filter bed remain in the vessel and settle back at the end of the cycle. 

SPILL CLEAN-UP Dry sand or earth should be spread on the leak, or spill area bulk liquid may also be absorbed with fly ash or cement powder cleanup of areas contaminated wit soot should involve dry vacuuming of surfaces with a vacuum cleaning system equipped with a high efficiency particulate (HEPA) filter after preliminary cleanup, wash surfaces with alkaline of nonionic synthetic detergents in water clean nonporous electrical and mechanical equipment with organic solvents remove all sources of ignition. 

SPILL CLEAN-UP Use water spray to cool and disperse vapors absorb as much as possible with noncombustible materials such as dry earth or sand high-efficiency particulate arrestor or charcoal filters can be used to minimize amount of carcinogen in exhausted air remove all sources of ignition. 

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