Practical Applications

The inorganic coagulant doses required to achieve colloid destabilization depend on the mixing and the liquid-solids separation method.

Coagulant Mixing

The hydrodynamics of inorganic coagulant-wastewater mixing are important. The rapid reaction of aluminum and ferric iron and the fact that soluble kinetic intermediates, the effective coagulating species, are adsorbed on colloid surfaces necessitate rapid, intense dispersion of coagulants into the wastewater. Inadequate mixing causes localized pH and metal ion concentrations, which require increased coagulant dosages to achieve colloid destabilization. Wastewater treatment facilities must add pH adjusting chemicals, e.g., Ca(OH)2, and have them completely reacted before coagulant dispersion to assure the proper pH coagulation level. In addition, they must consider seasonal variations in the wastewater temperature. As the temperature decreases, they must increase the mixing energy to maintain the level of mixing intensity.

Liquid-Solids Separation

Definitive inorganic coagulant dosage is required to effect colloidal suspension destabilization. However, in the practical application of coagulation-flocculation to remove colloidal material from wastewater, the inorganic coagulant dose required depends on the method of flocculation and the liquid-solids separation employed.

Figure 7.34.7 shows the condition that exists when the unit process serves to coagulate and clarify biologically treated wastewater. The inorganic coagulant dose required to coagulate the colloidal material prior to removal by granular media filtration, e.g., coal-sand filters, is one-half to one-sixth that required for removal by mechanical floc-culation followed by gravity settling. The reason for this phenomenon is that the deep-bed filter is an efficient floc-culation device (bringing destabilized colloids together). On the other hand, gravity settling requires an excess of metal hydroxide precipitate (above that required for desta-bilization) to produce a settleable floc.

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