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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