Granular media filtration

Granular media filtration is used for treating aqueous waste streams. The filter media consists of a bed of granular particles (typically sand or sand with anthracite or coal). The anthracite has adsorptive characteristics and hence can be beneficial in removing some biological and chemical contaminants in the wastewater. This material may also be substituted for activated charcoal. 

The filter application is typically applied to handling streams containing less than 100 to 200 mg/Liter suspended solids, depending on the required effluent level. Increased-suspended solids loading reduces the need for frequent backwashing. The suspended solids concentration of the filtered liquid depends on the particle size distribution, but typically, granular media filters are capable of producing a 

One of the reasons why it is important to remove suspended solids in water is that the particles can act as a source of food and housing for bacteria. Not only does this make microbiological control much harder but, high bacteria levels increase the fouling of distribution lines and especially heat transfer equipment that receive processed waters (for example, in one s household hot water heater). The removal of suspended contaminants enables chemical treatments to be at their primary jobs of scale and corrosion prevention and microbial control. 

A typical physical-chemical treatment system incorporates three dual medial (sand anthracite) filters connected in parallel in its treatment train. The major maintenance consideration with granular medial filtration is the handling of the backwash. The backwash will generally contain a high concentration of contaminants and require subsequent treatment. 

Granular media filters are usually preceeded by sedimentation in order to reduce the suspended solids load on the filter. Granular media filtration can also be installed ahead of biological or activated carbon treatment units to reduce the suspended solids load and in the case of activated carbon to minimize plugging of the carbon columns. 

Granular media filtration is only marginally effective in treating colloidal size particles in suspensions. Usually these particles can be made larger by flocculation although this will reduce run length. In cases where it is not possible to flocculate such particles (as in the case of many oil/water emulsions), other techniques such as ultrafiltration may be nessesary. 

Filtration is an effective means of removing low levels of solids firom wastes provided the solids content does not vary greatly and the filter is backwashed at appropriate intervals. The operation can te easily integrated with other treatment steps, and further, is well suited to mobile treatment systems as well as on-site or fixed installations. 

Precipitation and flocculation can be integrated into more complex treatment systems. The performance and reliability of these processes depends greatly on the variability of the composititMi of the waste being treated. Chemical addition must be determined using laboratory tests and must be adjusted with compositional changes of the waste being treated or poor performance will result. 

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MONITORING OF ALGAE REMOVAL BY GRANULAR MEDIA FILTRATION... 

Capital cost savings may be possible because of smaller land and plant building requirements. Treatment costs may be slightly less than conventional coagulation/sedimentahon/granular media filtration when filterable solids are low (11-13), although sedimentation would not usually be needed for such high-quality source waters. 

SPILL CLEAN-UP the concentration of asbestos fibers in drinking water can be removed by granular media filtration asbestos cement pipes can be coated with a chemical precipitate to avoid the release of fibers Ifom dissolution and leaching effects. 

In the domain below a mass concentration of about 50 mg/L and Zxv < 30 /xm, granular-media filtration without flocculation as a pretreatment step (direct filtration) easily satisfies the performance criteria for a 90% removal. Although removal efficiency appears to be a minimum in the size range 0.2 to 2 /xm (17,24), a, proper selection of media size and depth will ensure satisfactory removals, provided that particulates are destabilized (17). Filters will tolerate mass concentrations higher than 50 mg/L if particulate density and shear strength are increased. 

Figure 7. Comparison of size frequency distributions effect of flocculation preceding granular-media filtration. (0) Filter influent, with flocculation (O) filter influent without flocculation ( , filter effluents.

An additional potential application of particulate counting is process control and monitoring. With the improvement in aqueous particle counters, on-line measurement of number concentrations and size distributions for particulates larger than 1-2 pm is now feasible. Both feedforward and feedback process control applications can be envisioned. Feed-forward control could be used to estimate the coagulant chemical requirements needed for particle destabilization based on measurement of particle count and estimation of particulate surface area (32). Feedback control possibilities include control of the particle size distribution entering a filter, control of chemical dosing prior to granular-media filtration, and control of filter operation. 

For most particulates above 1 /xm in natural waters and wastewaters, the power-law coeflBcient appears to be greater than 3. Therefore, adequate removal of the particulate fraction by sedimentation or flotation requires a reduction in p by, for example, coagulation/flocculation, which shifts the major portion of particulate surface area and mass into size classes above about 30 /xm. If granular-media filtration is used as the particulate separation process, only particulate destabilization may be necessary to achieve desired removals. 

They can also form in deep bed filtration, in which a dispersion is passed through a bed of granular material such as fine sand (this is also known as depth filtration, granular media filtration or slow sand filtration). 

Either Rotary, vacuum filtration, air filtration, belt filtration, granular media filtration, baghouse filtration, electrostatic precipitation... 

Granular media filtration Activated carbon filtration Membrane filtration Deaeration-Decarbonation Chemical oxidation Disinfection Electrocoagulation Ion exchange (IX)... 

Figure 7.2.12. Three models of granular media filtration mechanism, (a) UBEs, of capillary tube collectors and limiting particle trajectory of a particle in a capillary, (b) UBEs of spherical collectors and limiting particle trajectory around a spherical collector, (c) UBEs of constricted tube collectors and limiting particle trajectory in a constricted tube. (After Tien (1989).)...

Waste Type Handled. Pressurized and gravity-fed granular media filtration systems are used for waste streams containing suspended solids. Vacuum, belt press, and pressure chamber filtration processes are used primarily to dewater sludges. 

Select either granular media or membrane filtration. For source water turbidity of 20 NTU or higher consider a combination of single-stage coarse granular media filtration followed by MF or UF membrane filtration. 

Footprint Membrane technologies are very space efficient as compared to granular media filtration. The smaller footprint benefits of membrane filtration are usually of greater importance when upgrading existing water treatment plants of limited site area availability or where the cost of new land acquisition is significant. 

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