Chemical mixing and flocculation or solids contact are important mechanical steps in the overall coagulation process. Application of the processes to wastewater generally follows standard practices and employs basic equipment. Chemical mixing thoroughly disperses coagulants or their hydrolysis products so the maximum possible portion of influent colloidal and fine supracolloidal solids are absorbed and destabilized. Flocculation or solids contact processes increase the natural rate of contacts between particles. This makes it possible, within reasonable detention periods, for destabilized colloidal and fine supracolloidal solids to aggregate into particles large enough for effective separation by gravity processes or media filtration.
These processes depend on fluid shear for coagulant dispersal and for promoting particle contacts. Shear is most commonly introduced by mechanical mixing equipment. In certain solids contact processes shear results from fluid passage upward through a blanket of previously settled particles. Some designs have utilized shear resulting from energy losses in pumps or at ports and baffles.
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