The sand filtration process is normally comprised of a clarification chain including other unit operations which precede filtration in the treatment sequence and can not be conceived of completely independent of the filtration stage. The conventional treatment scheme consists of coagulation-flocculation-settling followed by filtration. When the preceding process, in this case flocculation and/or settling, becomes insufficient, subsequent rapid filtration can be used to ensure a high quality of the effluent treated. However, this action is achieved at the expense of the evolution of filter head loss. Problems in washing and cleanliness of the mass may arise. Filtration is often viewed as serving as a coagulant flocculator. This is referred to as flocculation-filtration. The presence of thin, highly electronegative colloids (e.g., activated carbons) introduced in the form of powder in the settling phase may be a problem for the quality of the settled effluent. The carbon particles, which are smaller than 50 fim, penetrate deeply into the sand filter beds. They may rapidly provoke leakage of rapid filters. The same holds for small colloids other than activated carbon. Activated silica, which may have a favorable or an unfavorable effect on filtration, is composed of ionized micella formed by polysilicic acid-sodium polysilicate. This become negatively charged colloidal micella. The behavior of activated silicas depends on the conditions of neutralization and the grade of the silicate used in the preparation of the material. Activated silica is a coagulant aid that contributes to coalescence of the particles. Hence, it brings about an improvement in the quality of settled or filtrated water, depending on the point at which it is introduced. Preconditioning of the sand surface of filters by adding polyelectrolytes is an alternative use of sand filters as a coagulator-flocculator. In the treatment of drinking water the method depends on the limitations of these products in foodstuffs. The addition of polyphosphates to a water being subjected to coagulation usually has a negative effect; specifically the breaking of the agglomeration velocity of the particles during flocculation will occur in sand filtration. The addition of polyphosphates simultaneously with phosphates can be of value in controlling corrosion. This sometimes makes it possible to avoid serious calcium carbonate precipitation at the surface of filter grains when handling alkaline water. The application concerns very rapidly incrusting water while maintaining high hardness in solution. The addition of polyphosphates involves deeper penetration of matter into the filter mass. Hence, the breaking of flocculation obtained by the action of polyphosphates enables the thinner matters to penetrate the filters more deeply. These products favor the "in-depth effects" of the filter beds. Their use necessitates carefully checking that they are harmless from a hygienic point of view. The depth penetration of material in coagulation-filtration is almost opposite to the concept of using the filter as a screen. Precipitation initiated by germs plays a significant role. Empirical relations are normally relied on in the design of filters as a function of the penetration in depth of coagulated material. The concentration of those residual matters in filtered water (Cf) depends on several factors: the linear infiltration rate (vf), the effective size of the filter medium (ES), the porosity of the filter medium (e), the final loss of head of the filter bed (Ah), the depth of penetration of the coagulated matter (/), the concentration of the particles in suspension in the water to be filtered (C0), and the water height (H). The following generalized relation is often found among the filtration engineer's notes.
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 counterflow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters.
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