Alum And Ferric Precipitation Of Phosphate

Figure 7.31.4 shows a process in which metal salts (alum or ferric) added to the secondary effluent produce phosphate precipitates that are trapped in the granular media filter. Periodic backwashing of the filter yields a waste phosphorus sludge that is uniformly mixed in an equalization tank. This sludge is then recycled back to the primary settling basin in proportion to the plant influent flow rate. The waste sludge adsorbs more phosphorus and thereby reduces the phosphate load on the filter.

Because of the reduced phosphorus concentration in the filter influent, chemical costs can be reduced by as much as 30%. Flow- and phosphorus-monitoring instruments can be used to automate this process. Laboratory and pilot-plant operations have produced effluent phosphorus concentrations of less than 1 mg/l.

Figure 7.31.5 and Table 7.31.1 describe the components and performance of two additional wastewater treatment systems. In system A for the 10-mgd flow shown, two 34-ft diameter by 10-ft high granular media filters are required primarily for SS removal, but they can also remove phosphorus if precipitating chemicals are added. Additional BOD and COD removal is provided by five 20ft diameter by 20-ft high carbon columns plus one standby column of the same size. Granular carbon can be regenerated in a furnace. Furnaces can also recalcine the lime sludge (burn the CaCO3 into CaO).

In System B, chemical coagulation takes place in two 90-ft diameter by 17-ft sidewall depth columns, which provide high-rate solids-contact treatment and inorganic phosphorus removal.

FIG. 7.31.4 Alum or ferric precipitation of phosphate.

FIG. 7.31.5 Granular filtration, carbon adsorption, and chemical coagulation units. Chemicals can be added to achieve greater BOD, SS, and phosphorus removal.

FIG. 7.31.4 Alum or ferric precipitation of phosphate.

FIG. 7.31.5 Granular filtration, carbon adsorption, and chemical coagulation units. Chemicals can be added to achieve greater BOD, SS, and phosphorus removal.

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