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 bed is contained within a basin and is supported by an underdrain system which allows the filtered liquid to be drawn off while retaining the filter media in place. As water containing suspended solids passes through the bed of filter medium, the particles become trapped on top of, and within, the bed. The filtration rate is reduced at a constant pressure unless an increase in the amount of pressure is applied to force the wastewater through the filter bed. In order to prevent plugging of the upper surface and uppermost depth of the bed, the filter is backflushed at high velocity to dislodge the filtered particles. The backwash water contains high concentrations of solids and is sent to further treatment steps within the watsewater treatment plant.

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. \

filtered liquid with a suspended solids concentration as low as 1 to as high as 10 mg/Liter. Large flow variations will affect the effluent's quality.

Granular media filters are usually preceded 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 lengths. 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 must be considered. Figure 2 illustrates a common sand filter that most people are familiar with in swimming pool applications. Such systems rely on very fine sand media that can typically remove suspended particles about 0.5 ftm in size. Filtration is an effective means of removing low levels of solids from wastes provided the solids concentration does not vary greatly and the filter is backwashed at appropriate intervals during the filtration cycle. The operation can be easily integrated with other treatment steps, and further, is well suited to mobile treatment systems as well as on-site or fixed installations. In short, sand filtration technologies, although simple, are quite versatile in meeting treatment challenges.

High Efficiency 0,13mm/.5 microns

High Efficiency 0,13mm/.5 microns

Figure 2. A simple sand filtration unit

A typical multi-media sand filtration unit is shown in Figure 3. In this configuration a coarse layer of media is used to reduce the contaminant loading to the final layer. This allows multimedia filters to use finer media. Such units generally remove suspended solids down to about 15 /xm, and they require large volumes of water to properly remove contaminant that is trapped deep within the bed. Often manufactures of these types of systems claim 90 % removal of 0.5 /xm particles and larger. This can be a misleading statement as quite often only about 5 % of the 0.5 /¿m particles will be removed. Grouping the 0.5 /xm particles with much larger particles allows the claim to be met by removing a few large volume particles from the tower sump, even though the vast majority of fine particles remain to foul heat exchange equipment.

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.

Multimedia Depth 0.55mm/15 microns

Figure 3. Mulimedia sand filter.

In this application, the operations of precipitation and flocculation play important roles. Precipitation is a physiochemical process whereby some, or all, of a substance in solution is transformed into a solid phase. It is based on alteration of the chemical equilibrium relationships affecting the solubility of inorganic species. Removal of metals as hydroxides and sulfides is the most common precipitation application in wastewater treatment. Lime or sodium sulfide is added to the wastewater in a rapid mixing tank along with flocculating agents. The wastewater flows to a flocculation chamber in which adequate mixing and retention time is provided for agglomeration of precipitate particles. Agglomerated particles are then separated from the liquid phase by settling in a sedimentation chamber, and/or by other physical processes such as filtration.

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.

Gravity Sand Filters
Figure 4. Cutaway view of a rapid sand filter.
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