polyphosphorus are transformed into orthophosphorus) usually results in the best removal. Some additional nitrogen removal occurs because of better settling but essentially no ammonia is removed nt reduce BOD loadings to the nitrification can rtant features of adding metal salts and polvmers at different points in the treatment process are discussed in this section. Metal Salt Addition to Primary Sedimentation Tanks. When aluminum or iron salts are added to untreated wastewater, they react with the soluble orthophosphate to produce a precipitate. Organic particles. The insolubilized phosphorus, as well as considerable quantities of BOD and TSS, are removed rom the system as primary sludge. Adequate initial mixing and flocculation are necessary upstream primary facilities. whether separate basins are provided or existing facilities are modified to provide these functions. Polymer addition may be required to aid in settling In low-alkal init waters. the addition of a cessary to keep pH in the 5 to 7 range. Alum generally is applied in a molar ratio in Metal Salt Addition to Secondary Treatment. Metal salts can be added to the untreated wastewater, in the activated-sludge aeration tank, or the final clarifier influent channel. In trickling filter systems. the salts are added to the untreated wastewater or to the filter effluent. Multipoint additions have also been exchange, and agglomeration, and removed from the process with either the primary or secondary sludges. or both. Theoretically the minimum solubility of AlPO4 occurs at about pH 6.3. and that of FePO4 occurs at about pH 5.3: however practical applications have yielded good phosphorus removal anywhere in the nge of pH 6.5 to 7.0 which is compatible with most biological The use of ferrous salts is limited because they produce low orus levels only at high pH values. In low-alkalinity waters. either sodium aluminate and alum or ferric plus lime, or both can be used to maintain the ph higher than 5.5. Improved settling and lower effluent BOD result from chemical addition, particularly if polymer is also added to the final clarifier. Dosages generally fall in the range of a l to 3 metal ion-phosphorus molar ratio. Metal Salt and Polymer Addition to Secondary Clarifiers. In certain cases, such as trickling filtration and extended aeration activated-sludge processes. solids may loculate and settle well in the econdary clarifier. This settling problem may become acute in plants that are overloaded The addition of aluminum or iron salts will cause the precipitation of metallic hydroxides or phosphates, or both d iron salts, along with certain organic polymers can also be used to coagulate colloidal particles and to improve removals on filters. The resultant coagulated colloids and precipitates will settle readily in the secondary clarifier, reducing the Tss in the effluent and effecting phosphorus removal Dosages of aluminum and iron salts usually fall in the range of 1 to 3 metal ion/phosphorus on a molar ratio basis if the residual phosphorus in the secondary effluent is greater than 0.5 mg/L. To achieve significantly higher Polymers may be added(l) to the mixing zone of a highly mixed or preceding a static or dvnamic mixer. or(3)to an aerated channel. Althor times of 10 to 30 seconds have been used for polymers. shorter mixing times are favored. Polymers should not be subjected to insufficient or excessive mixing, as noted previously, because the process efficiency will diminish resulting in poor settling and thickening characteristics Phosphorus removal Using Lime The use of lime for phosphorus removal is declining because of(1)the substantial increase in the mass of sludge to be handled compared to metal salts and (2) the operation and maintenance problems associated with the handling, storage. and feeding of lime. When lime is used, the principal variables controlling the dosage are the degree of removal required and the alkalinity of the wastewater. The operating dosage must usually be determined by onsite testing. Lime has been used customarily either as a precipitant in the primary sedimentation tanks or following secondary treatment clarification Although lime recalcination lowers chemical costs. it is a feasible alternative only for large plants. Where a lime recovery system is required for a cost-effective operation, which converts the calcium carbonate in the sludge to lime by heating to 980c. the carbon dioxide from is process or other onsite stack gas (containing 10 to 15 percent carbon dioxide)is generally used as the source of recarbonation for pH adiustment of the wastewater. Lime Addition to Primary Sedimentation Tanks. Both low and high lime treatment can be used to precipitate a portion of the phosphorus (usually about 65 to 80 percent). When lime is used, both the calcium and the hydroxide react with the orthophosphorus to form an insoluble hydroxyapatite [Cas(OH)(PO4)3]. A residual phosphorus level of 1.0 mg/L can be achieved with the addition of effluent 6-116-11 polyphosphorus are transformed into orthophosphorus) usually results in the best removal. Some additional nitrogen removal occurs because of better settling, but essentially no ammonia is removed unless chemical additions to primary treatment reduce BOD loadings to the point where nitrification can occur. A number of the important features of adding metal salts and polymers at different points in the treatment process are discussed in this section. Metal Salt Addition to Primary Sedimentation Tanks. When aluminum or iron salts are added to untreated wastewater, they react with the soluble orthophosphate to produce a precipitate. Organic phosphorus and polyphosphate are removed by more complex reactions and by adsorption onto floc particles. The insolubilized phosphorus, as well as considerable quantities of BOD and TSS, are removed from the system as primary sludge. Adequate initial mixing and flocculation are necessary upstream of primary facilities, whether separate basins are provided or existing facilities are modified to provide these functions. Polymer addition may be required to aid in settling. In low-alkalinity waters, the addition of a base is sometimes necessary to keep pH in the 5 to 7 range. Alum generally is applied in a molar ratio in the range of a 1.4 to 2.5 mole Al/mole P. Metal Salt Addition to Secondary Treatment. Metal salts can be added to the untreated wastewater, in the activated-sludge aeration tank, or the final clarifier influent channel. In trickling filter systems, the salts are added to the untreated wastewater or to the filter effluent. Multipoint additions have also been used. Phosphorus is removed from the liquid phase through a combination of precipitation, adsorption, exchange, and agglomeration, and removed from the process with either the primary or secondary sludges, or both. Theoretically, the minimum solubility of AlPO4 occurs at about pH 6.3, and that of FePO4 occurs at about pH 5.3; however, practical applications have yielded good phosphorus removal anywhere in the range of pH 6.5 to 7.0, which is compatible with most biological treatment processes. The use of ferrous salts is limited because they produce low phosphorus levels only at high pH values. In low-alkalinity waters, either sodium aluminate and alum or ferric plus lime, or both, can be used to maintain the pH higher than 5.5. Improved settling and lower effluent BOD result from chemical addition, particularly if polymer is also added to the final clarifier. Dosages generally fall in the range of a 1 to 3 metal ion-phosphorus molar ratio. Metal Salt and Polymer Addition to Secondary Clarifiers. In certain cases, such as trickling filtration and extended aeration activated-sludge processes, solids may not flocculate and settle well in the secondary clarifier. This settling problem may become acute in plants that are overloaded. The addition of aluminum or iron salts will cause the precipitation of metallic hydroxides or phosphates, or both. Aluminum and iron salts, along with certain organic polymers, can also be used to coagulate colloidal particles and to improve removals on filters. The resultant coagulated colloids and precipitates will settle readily in the secondary clarifier, reducing the TSS in the effluent and effecting phosphorus removal. Dosages of aluminum and iron salts usually fall in the range of 1 to 3 metal ion/phosphorus on a molar ratio basis if the residual phosphorus in the secondary effluent is greater than 0.5 mg/L. To achieve phosphorus levels below 0.5 mg/L, significantly higher metal salt dosages and filtration will be required. Polymers may be added (1) to the mixing zone of a highly mixed or internally recirculated clarifier, (2) preceding a static or dynamic mixer, or (3) to an aerated channel. Although mixing times of 10 to 30 seconds have been used for polymers, shorter mixing times are favored . Polymers should not be subjected to insufficient or excessive mixing, as noted previously, because the process efficiency will diminish, resulting in poor settling and thickening characteristics. Phosphorus Removal Using Lime The use of lime for phosphorus removal is declining because of (1) the substantial increase in the mass of sludge to be handled compared to metal salts and (2) the operation and maintenance problems associated with the handling, storage, and feeding of lime. When lime is used, the principal variables controlling the dosage are the degree of removal required and the alkalinity of the wastewater. The operating dosage must usually be determined by onsite testing. Lime has been used customarily either as a precipitant in the primary sedimentation tanks or following secondary treatment clarification. Although lime recalcination lowers chemical costs, it is a feasible alternative only for large plants. Where a lime recovery system is required for a cost-effective operation, it includes a thermal regeneration facility, which converts the calcium carbonate in the sludge to lime by heating to 980。C. The carbon dioxide from this process or other onsite stack gas (containing 10 to 15 percent carbon dioxide) is generally used as the source of recarbonation for pH adjustment of the wastewater. Lime Addition to Primary Sedimentation Tanks. Both low and high lime treatment can be used to precipitate a portion of the phosphorus (usually about 65 to 80 percent). When lime is used, both the calcium and the hydroxide react with the orthophosphorus to form an insoluble hydroxyapatite [Ca5(OH)(PO4)3]. A residual phosphorus level of 1.0 mg/L can be achieved with the addition of effluent