subsequent removal of those solids. Phosphorus can be incorporated into either biological solids(e.g, microorganisms) or chemical precipitates. The topics to be considered include (I)the chemistry of phosphate precipitation, (2)strategies for phosphorous removal, (3) phosphorus removal using metal salts and polymers, and (4) phosphorus removal using lime Chemistry of Phosphate Precipitation The chemical precipitation of phosphorus is brought about by the addition of the salts of multivalent metal ions that form precipitates of sparingly soluble phosphates. The multivalent metal ions used most commonly are calcium [ Ca(l)) aluminum [Al(llD), and iron Fe(ll) Polymers have been used effectively in conjunction with alum and lime as flocculant aids. Because the chemistry of phosphate recipitation with calcium is quite different than witl and iron. the two different precipitation are considered separately in the following discussion. Phosphate Precipitation with Calcium. Calcium is usually added in the form of lime Ca(OH). From the equations presented previously, it will be noted that te alkal unity to precipitate CaCO3. As the ph value of the waste t 10. excess calcium ions will then react with the phosphate to precipitate hydroxvlapa CaRol PO4)6(OH)2. Because of the reaction of lime with the alkalinity of the wastewater. the quantity of lime required will. in i pr the wastewater. The quantity of lime required to precipitate the phosphorus in wastewater is typically about 1. 4 to 1.5 times the total alkalinity expressed as CaCo3. Because a high pH value is required to precipitate phosphate, coprecipitation is usually not feasible. When lime is added to raw wastewater or to econdary effluent pH adiustment is usually required before subsequent treatment Recarbonation with carbon dioxide(CO,) is used to lower the pH vi Phosphate Precipitation with Aluminum and Iron. In the case of alum and iron I mole will precipitate I mole of phosphate; however, these reactions are deceptively simple and must be considered in light of the many competing reactions and their associated equilibrium constants, and the effects of alkalinity. pH. trace elements, and ligands found in wastewater. Because of the many competing reactions cannot be used to estimate the required chemical dosages directly. Therefore, dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests, especially if polymer used. Pure metal phosphates are precipitated within the shaded area, and mixed complex polynuclear species are formed outside toward higher and lower pH values Strategies for Phosphorus Removal The precipitation of phosphorus from wastewater can occur in a number of different locations within a process flow diagram(see Fig 6-6). The general locations where phosphorus can be removed may be classified as(1) pre-precipitation, (2)coprecipitation, and (3) postprecipitation 696-9 subsequent removal of those solids. Phosphorus can be incorporated into either biological solids (e.g., microorganisms) or chemical precipitates. The topics to be considered include (1) the chemistry of phosphate precipitation, (2) strategies for phosphorous removal, (3) phosphorus removal using metal salts and polymers, and (4) phosphorus removal using lime. Chemistry of Phosphate Precipitation The chemical precipitation of phosphorus is brought about by the addition of the salts of multivalent metal ions that form precipitates of sparingly soluble phosphates. The multivalent metal ions used most commonly are calcium [Ca(II)], aluminum [Al(III)], and iron [Fe(III)]. Polymers have been used effectively in conjunction with alum and lime as flocculant aids. Because the chemistry of phosphate precipitation with calcium is quite different than with aluminum and iron, the two different types of precipitation are considered separately in the following discussion. Phosphate Precipitation with Calcium. Calcium is usually added in the form of lime Ca(OH)2. From the equations presented previously, it will be noted that when lime is added to water it reacts with the natural bicarbonate alkalinity to precipitate CaCO3. As the pH value of the wastewater increases beyond about 10, excess calcium ions will then react with the phosphate to precipitate hydroxylapatite Ca10(PO4)6(OH)2. Because of the reaction of lime with the alkalinity of the wastewater, the quantity of lime required will, in general, be independent of the amount of phosphate present and will depend primarily on the alkalinity of the wastewater. The quantity of lime required to precipitate the phosphorus in wastewater is typically about 1.4 to 1.5 times the total alkalinity expressed as CaCO3. Because a high pH value is required to precipitate phosphate, coprecipitation is usually not feasible. When lime is added to raw wastewater or to secondary effluent, pH adjustment is usually required before subsequent treatment or disposal. Recarbonation with carbon dioxide (CO2) is used to lower the pH value. Phosphate Precipitation with Aluminum and Iron. In the case of alum and iron, 1 mole will precipitate 1 mole of phosphate; however, these reactions are deceptively simple and must be considered in light of the many competing reactions and their associated equilibrium constants, and the effects of alkalinity, pH, trace elements, and ligands found in wastewater. Because of the many competing reactions cannot be used to estimate the required chemical dosages directly. Therefore, dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests, especially if polymers are used. Pure metal phosphates are precipitated within the shaded area, and mixed complex polynuclear species are formed outside toward higher and lower pH values. Strategies for Phosphorus Removal The precipitation of phosphorus from wastewater can occur in a number of different locations within a process flow diagram (see Fig. 6-6). The general locations where phosphorus can be removed may be classified as (1) pre-precipitation, (2) coprecipitation, and (3) postprecipitation