FIG. 7.32.1 Precipitation of chemical pollutants by pH adjustment.

Adding lime to municipal wastewater elevates the pH and causes insolubilization of calcium phosphate, calcium carbonate, and magnesium hydroxide. The hydroxide precipitated at an elevated pH acts as a coagulant, destabilizing and enmeshing colloidal pollutants. The chemistry of lime treatment is generally described by water softening reactions. However, the presence of dissolved organic matter and condensed phosphates in municipal wastewater causes some interference with water softening reactions, such as the precipitation of calcium carbonate. Figure 7.32.2 shows the solubility of the total and calcium hardness ions in a typical municipal wastewater.

Polyelectrolytes in suspensions become attached to two or more particles and provide bridging between them.

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