Lime Precipitation Of Phosphorus

Figure 7.31.2 shows a phosphate precipitation system, which is a modification of the primary settling step in a primary-secondary treatment plant. Raw wastewater is mixed with lime and flocculated to precipitate the phosphate in the form of an insoluble calcium salt. Other chemicals, such as ferric chloride, can also be used in place of lime.

The process shown in Figure 7.31.2 recirculates lime sludges for economy of lime usage. A wastewater treatment facility can also reclaim lime from the sludge by incinerating the solids. This process provides between 80 and 95% phosphate removal together with substantial BOD removal. Lime tends to soften the water, and the large amounts of lime sludge generated present greater sludge handling problems.

FIG. 7.31.2 Lime precipitation of phosphorus.
FIG. 7.31.3 Ferrous precipitation of phosphate.

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