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COAGULATION-FLOCCULATION Clarification is a tertiary treatment including the phases of coagulation, flocculation and sedimentation in special basins through the addition of metallic (Fe and Al) salts, with or without lime or synthetic coagulant. These substances promote the aggregation of particles favouring the solid-liquid separation, that occur during sedimentation or filtering phases. The reference doses are 20-30 ppm of coagulant with 0.3-0.5 ppm of polyelectrolyte. The flocculation is a mixing unit of treatment used after coagulant addition to leave flocculate the particles destabilized during the coagulation phase and to favour sedimentation or filtering. The formation of aggregated particles is achieved by a slow mixing that favours the collision and the aggregation between the particles. The aggregated particles are removed by sedimentation basins or filtration units (direct filtration). In the following Table, the percentages of removal obtained with a process of coagulation with final sedimentation, using various types of coagulants are listed. As regards the removal of the viruses this process guarantees a percentage of removal of 99%. The wastewater reclaimed by coagulation and sedimentation or direct filtration, followed by a phase of disinfection, can be reused for most of the reuse applications. Coagulation-sedimentation typical constituent removals* (Source: adapted
from WPCF, 1989)
*
Values given in terms of percent removal from secondary effluent |