Enmeshment in a Precipitate

If certain metal salts are added to water or wastewater in sufficient amounts, rapid formation of precipitates will occur. Colloids may serve as condensation nuclei for these precipitates or may become enmeshed as the precipitates settle. Removal of colloids in this manner is frequently referred to as sweep-floc coagulation. Several characteristics that distinguish sweep-floc coagulation from double layer compression and adsorption have been reported.

1. An inverse relationship exists between the optimum coagulant dosage and the concentration of colloids to be removed. At low colloid concentrations a large excess of coagulant is required to produce a large amount of precipitate that will enmesh the relatively few colloidal particles as it settles. At high colloid concentrations, coagulation will occur at a lower chemical dosage because the colloids serve as nuclei to enhance precipitate formation.
2. Optimum coagulation conditions do not correspond to a minimum zeta potential but depends on pH depending on solubility-pH relationship for that coagulant.