Filtration sand filters

Filtration is employed when the suspended soUds concentration is less than 100 mg/L and high effluent clarity is required. Finely dispersed suspended soUds require the addition of a coagulant prior to filtration. Filters most commonly used in wastewater treatment are a dual media (anthrafUt and sand) or a moving bed or continuous-backwash sand filter. Performance data for the tertiary filtration of municipal and industrial wastewater are shown in Table 10. 

Filter Cleaners. Grease and oils from bathers can affect filtration. Degreasers are employed to clean diatomaceous earth (DE) and sand filters. They can be surfactant or enzyme based. 

The growth of community water supply systems in the United States started in the early 1800s. By 1860, over 400, and by the turn of the century over 3000 major water systems had been built to serve major cities and towns. Many older plants were equipped with slow sand filters. In the mid 1890s, the Louisville Water Company introduced the technologies of coagulation with rapid sand filtration. 

Porosity constitutes a important criterion in a description based on straining. Porosity is determined by the formula V /Vc, in which V c is the total or apparent volume limitated by the filter wall and is the free volume between the particles. The porosity of a filter layer changes as a function of the operation time of the filters. The grains become thicker because of the adherence of material removed from the water, whether by straining or by some other fixative mechanism of particles on the filtering sand. Simultaneously the interstices between the grains diminish in size. This effect assists the filtration process, in particular for slow sand filters, where a deposit is formed as a skin or layer of slime that has settled on the... 

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

Belfort, Georges "Evaluation of a Rapid Sand Filter", Filtration Experiment, Rensselaer Polytechnic Institute, 1990. 

Insoluble suspended matter either picked up from the atmosphere or formed by deposition and corrosion within the system, together with slimes will, if not removed, cause blocking and abrasion problems. The build-up of such material can be controlled by side stream filtration, in which about 2-5 per cent of the circulating water flow is filtered continuously. A sand filter is commonly used for this type of duty. 

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. 

Another common problem with sand filters is clogging on the surface of the sand media, resulting in short runs and high head loss. This often is due to the finest grains of sand rising to the top of the media bed. If the problem persists, the easiest solution may be to replace the top few inches of sand with a slightly coarser grain so that some depth filtration occurs. 

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. 

Granular bed filters are used in porcelain enameling wastewater treatment to remove residual solids from clarifier effluent (sedimentation effluent or flotation effluent). Filtration polishes the effluent and reduces suspended solids and insoluble precipitated metals to very low levels. Fine sand and coal are media commonly utilized in granular bed filtration. The filter is backwashed after becoming loaded with solids and the backwash is returned to the treatment plant influent for removal of solids in the clarification step.10-12... 

In the second type of filtration, depth or deep-bed filtration, the particles penetrate into the pores of the filter medium, where impacts between the particles and the surface of the medium are largely responsible for their removal and retention. This configuration is commonly used for the removal of fine particles from very dilute suspensions, where the recovery of the particles is not of primary importance. Typical examples here include air and water filtration. The filter bed gradually becomes clogged with particles, and its resistance to flow eventually reaches an unacceptably high level. For continued operation, it is therefore necessary to remove the accumulated solids, and it is important that this can be readily achieved. For this reason, the filter commonly consists of a bed of particulate solids, such as sand, which can be cleaned by back-flushing, often accompanied by... 

Some fixed filtration systems are relatively inexpensive. For example, constructed wetlands and sand filters are generally the most successful methods of polishing the treated wastewater effluent from lagoons. These systems have also been used with more traditional, engineered primary treatment technologies such as septic tanks and primary clarifiers. In such constructed wetlands, the system utilizes the roots of plants to provide substrate for the growth of attached bacteria, which utilize the nutrients present in the effluents and for the transfer of oxygen. 

RAPID Sand Filtration. A comparison between raw water and sand-filtered water is shown in Figure 3. Three main fractions are typically recovered with Sephadex G25 from low-TOC surface waters. These fractions will be referred to as Gl, G3, and G5, which correspond to the fraction numbers by order of elution. The apparent MWs for these fractions are as follows >5000 daltons for Gl, 1000-5000 daltons for G3, and <1000 daltons for G5. During RSF, G5 exhibits a decrease in TOC, fluorescence, and UV260, whereas Gl shows a decrease only in TOC. The distribution by MW of organic carbon after sand filtration is as follows >5000 daltons, 17 (Gl) 1000-5000 daltons, 29 (G3) and <1000 daltons, 54 (G5). 

Rapid Sand Filtration. Pyrochromatograms of fractions Gl, G3, and G5 collected from sand-filtered water are presented in Figures 7-9. 

Fig. 1 describes a typical rapid sand filter. Note that hydraulic grading of the sand occurs in the backwash cycle, with the very finest sand forming the top of the bed. Filtration occurs in the top few inches. Once a suspended solid passes the top layer, larger and larger voids occur, and the chance of passing through the filter is greatly increased. 

Cooling water makeup should contain less than 5 ppm SS. If the level of suspended solids is considerably higher, say over 20 ppm SS, pretreatment by the use of polyelectrolytes or filtration is recommended. If the SS level is perhaps 5 to 10 ppm or if there is air-blown dust entering a cooling system, a sidestream sand filter or self-cleaning filter will be of benefit. In cases of high SS, chemical polymeric dispersants may be suitable as a total replacement for the sidestream filter but preferably will be used in tandem with a filter. 

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