The hydraulic performances required of the sand with slow filters are inferior to those for rapid filters. In the case of slow filters, one can use fine sand, since the average filtration velocity that is usually necessary lies in the range 2 to 5 m/day.
In slow filtration, much of the effect is obtained by the formation of a filtration layer, including the substances that are extracted from the water. At the early stages of the operation, these substances contain microorganisms able to effect, beyond the filtration, biochemical degradation of the organic matter. This effect also depends on the total surface of the grains forming the filter material. The probability of contact between the undesirable constituents of the water and the surface of the filter medium increases in proportion to the size of the total surfac«
The actual diameter of the sands used during slow filtration typically lies between 0.15 and 0.35 mm. It is not necessary to use a ganged sand. The minimum thickness of the layer necessary for slow filtration is 0.3 to 0.4m, and the most efficient filtration thickness typically is at 2 to 3 cm.
The actual requirements for the sand in slow filtration are chemical in nature. Purity and the absence of undesirable matters are more important than grain-size distribution in the filtration process. On the other hand, the performance of rapid filters requires sands with quite a higher precise grain size. In the case of rapid filtration, the need for hydraulic performances is greater than in slow nitration. This means that the grain-size distribution of the medium is of prime concern in
In wastewater treatment plants, the purity of the sand media used must be examined regularly. In addition, both the head loss of the filter beds and an analysis of the wash water during the operation of washing the filters must be checked regularly. Special attention must also be granted to the formation of agglomerates. The presence of agglomerates is indicative of insufficient washing and the possible formation of undesirable microbiological development zones within the filter bed.
The primary mechanisms that control the operation of sand filtration are:
• Mass attraction, or the effect of van der Waals forces
Straining action consists of intercepting particles that are larger than the free interstices left between the filtering sand grains. Assuming spherical grains, an evaluation of the interstitial size is made on the basis of the grains' diameter (specific diameter), taking into account the degree of nonhomogeneity of the grains.
Porosity constitutes a important criterion in a description based on straining. Porosity is determined by the formula VL/VC, in which V c is the total or apparent volume limitated by the filter wall and VL 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 bed making up the active filter. Biochemical transformations occur in this layer as well, which are necessary to make slow filters efficient as filters with biological activity.
Filtration occurs correctly only after buildup of the sand mass. This formation includes a "swelling" of the grains and, thus, of the total mass volume, with a corresponding reduction in porosity. The increases and swellings are a result of the formation of deposits clinging to the empty zones between grains.
The porosity of a filter mass is an important factor. This property is best defined by experiment. A general rule of thumb is that for masses with the effective size greater than 0.4 - 0.5 mm and a specific maximum diameter below 1.2 mm the porosity is generally between 40 and 55 % of the total volume of the filter mass. Layers with spherical grains are less porous than those with angular material.
The second important mechanism in filtration is that of settling. From Stoke's law of laminar particle settling, the settling velocity of a particle is given by :
g v volumetric mass density of the water volumetric mass density of the particles in suspension diameter of the particles
In sedimentation zones the flow conditions are laminar. A place is available for the settling of sludges contained in the water to be filtered.
Although the total inner surface that is available for the formation of deposits in a filter sand bed is important, only a part of this is available in the laminar flow zones that promote the formation of deposits. Usually material with a volumetric mass slightly higher than that of water is eliminated by sedimentation during filtration. Such matter could be, for example, organic granules or particles of low density. In contrast, colloidal material of inorganic origin-sludge or clay, for instance—with a diameter of 1 - 10 ¿¿m is only partially eliminated by this process, in which case the settling velocities in regard to the free surface become insufficient for sedimentation.
The trajectory followed by water in a filter mass it is not linear. Water is forced to follow the outlines of the grains that delineate the interstices. These changes in direction are also imposed on particles in suspension being transported by the water. This effect leads to the evacuation of particles in the dead flow zones. Centrifugal action is obtained by inertial force during flow, so the particles with the highest volumetric mass are rejected preferentially.
Diffusion filtration is another contributor to the process of sand filtration. Diffusion in this case is that of Brownian motion obtained by thermal agitation forces. This compliments the mechanism in sand filtration. Diffusion increases the contact probability between the particles themselves as well as between the latter and the filter mass. This effect occurs both in water in motion and in stagnant water, and is quite important in the mechanisms of agglomeration of particles (e.g., flocculation).
The next mechanism to consider is the mass attraction between particles which is due to van der Waals forces. These are universal forces contributing to the transport and fixation mechanism of matter. The greater the inner surface of the filters, the higher is the probability of attractive action. Van der Waals forces imply short molecular distances, and generally play a minor role in the filtration process. Moreover, they decrease very quickly when the distance between supports and particles increases. Nevertheless, the indirect effects, which are able to provoke an agglomeration of particles and, thus, a kind of flocculation, are not to be neglected and may become predominant in the case of flocculation-filtration, or more generally in the case of filtration by flocculation. Electrostatic and electrocinetic effects are also factors contributing to the filtration process. Filter sand has a negative electrostatic charge. Microsand in suspension presents an electrophoretic mobility. The value of the electrophoretic mobility, or of the corresponding zeta potential, depends on the pH of the surrounding medium. Usually a coagulation aid is used to condition the surface of microsand. In filtration without using coagulant aids, other mechanisms may condition the mass more or less successfully. For instance, the formation of deposits of organic matter can modify the electrical properties of the filtering sand surfaces. These modifications promote the fixation of particles by electrokinetic and electrostatic processes, especially coagulation. Also, the addition of a neutral or indifferent electrolyte tends to reduce the surface potential of the filtering sand by compression of the double electric layer. This is based on the principles of electrostatic coagulation. The sand, as the carrier of a negative charge spread over the surface of the filter according to the model of the double layer, will be able to fix the electropositive particles more exhaustively. This has a favorable effect on the efficiency of filtration of precipitated carbonates or of floes of iron or aluminum hydroxide-oxide. Optimal adherence is obtained at the isoelectric point of the filtrated material. In contrast, organic colloidal particle carriers of a negative charge such as bacteria are repulsed by the electrostatic mechanism in a filter with a fresh filter mass. In this case, the negative charges of the sand itself appear unchanged. With a filter that is conditioned in advance, there are sufficient positively charged sites to make it possible to obtain an electrochemical fixation of the negative colloids.
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