Magnesium

Lime treatment for phosphorus precipitation and SS removal from raw and biologically treated municipal waste-water has become an accepted sanitary engineering unit process. The chemistry of lime treatment related to colloidal suspension destabilization is not a well-developed technology. However, certain observations based on laboratory and full-scale plant operating results can be made.

Adding hydrated lime [Ca(OH)2] to wastewater simply increases the solution pH. At increasing pH levels, calcium phosphate, calcium carbonate, and magnesium hydroxide are insoluble as shown in Figure 7.34.6. The phosphate and carbonate precipitates undergo some agglomerative and adsorptive interactions with soluble and colloidal pollutants.

However, the coagulant species of significance is the magnesium hydroxide precipitate. Thus, wastewater treatment facilities must adjust the pH upward to a value that causes magnesium precipitation. This pH level varies with different wastewaters. For example, the lower the initial magnesium concentrations, the higher the pH required to precipitate the amount of magnesium necessary for destabilization. The incremental precipitation of the magnesium coagulant is shown as Mg2+ in Figure 7.34.6.

Two factors are significant in the lime treatment of wastewater. The first is the excessive amount of chemical sludge (precipitates) produced. Depending on the amount of Ca2+, HCO 3, and PO| present, chemical sludge production ranges from 4500 to 6500 lb per MG. Second, a variety of precipitates are formed. These precipitates range in character from colloidal hydroxylapatite [Ca5OH-(PO4) 3] to dense CaCO3 to voluminous Mg(OH)2 floc. The proportions of these precipitates and the originally present SS determine the net floc settling rate, concentration of settled sludge, and sludge dewatering properties.

SOLUBLE ION LIME

CONCENTRATION DOSAGE

SOLUBLE ION LIME

CONCENTRATION DOSAGE

INCREASING pH —

FIG. 7.34.6 Soluble ion distribution at increasing pH values.

INCREASING pH —

FIG. 7.34.6 Soluble ion distribution at increasing pH values.

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