installations, phosphorus levels equal to or less than 0. 02 mg/L have been achieved in the filtered effluent. To achieve such low levels of phosphorus removal, the backwash water from the second filter which contains small particles and residual coagulant is recycled to the first filter to improve floc formation within the first-stage filter and the influent to waste ratio Comparison of Chemical Phosphorus Removal Processes The advantages and disadvantages of the removal of phosphorus by the addition of chemicals at various points in a treatment system are summarized in Table 6-4. It is recommended that each alternative point of application be evaluated carefully Tab. 6-4 Advantages and disadvantages of chemical addition in various sections of a treatment plant for phosphorus removal Level of tre Disadvantage Applicable to most plants; increased Least efficient use of metal; polymer may be BOD and suspended solids removal west degree of metal leakage: to dewater than primary sludge lime recovery demonstrated Secondary Lowest cost; lower chemical dosag than primary: improved stability of with low-alkalinity wastewaters, a pH contre ed sludge; polymer not system may be necessary: cannot use lime percentage of volatile solids, reducing the Advanced- Lowest phosphorus effuent; most Highest copital cost; highest metal leakage precipitation efficient metal use; lime recovery Estimation of Sludge Quantities from Phosphorus Precipitation The additional BOD and TSS removals afforded by chemical addition to primary treatment may also solve overloading problems on downstream biological systems, or may allow seasonal or vear-round nitrification. depending on biological system designs. The BOD removal in the primary sedimentation operation is on the order of 50 to 60 percent at a pH of 9.5. The amount of primary sludge will also Increase significantly 6-5 Chemical Precipitation For Removal Of Heavy Metals And dissolved Inorganic Substances The technologies available for the removal of heavy metals from wastewater include chemical precipitation, carbon adsorption, ion exchange, and reverse osmosis. Of these technologies, chemical precipitation is most commonly employed for most of the metals. C (OH and sulfide(S-) Carbonate(CO32) has also been used in some special removed separately or coprecipitated with phosphorus Precipitation Reactions Metals of interest include arsenic (As). barium(Ba). cadmium(Cd). copper(Cu). mercury(Hg). nickel Se) and zinc(Zn). Most of these metals can be precipitated as hydroxides or sulfides Solubility products for free metal concentrations in equilibrium with hydroxide and sulfide precipitates are reported in Table 6-5. In wastewater treatment facilities, metals are precipitated most commonly as metal hydroxides through the addition of lime or caustic to a pH of minimum solubility. However, several of these compounds are, as discussed previously, amphoteric (ie, capable of either accepting or donating a proton) and exhibit a point of minimum solubility. The minimum effluent concentration levels that can be achieved in the chemical precipitation of heavy metals are reported in Table 6-6. In practice, the minimum achievable residual metal concentrations will also depend on the nature and concentration of the organic ewater as well as the temperature. Because of the many uncertainties associated with the precipitation of metals, laboratory bench-scale or lot-plant testing should be conducted Tab. 6-5 Solubility products for free metal ion Cadmium hydroxide Cd(OH)+Cd++2OH 13.93 concentrations in equilibrium with hydroxides Cadmium sulide and sulfides Chromium hydroxide 1oHh2++Cr3++30H CulOH) +-+Cu2++2OH- Copper sulfide cu54Cu2+52- Iron(H) hydroxide Fe(OH), Fe2++2OH Iron(U)sulfide FeS + Fe2.+S2 Pb(OH) +,Pb2.+2OH- 14.93 Lead sulfide PbS +, Pb2++52- 28.15 Hg(OH) e Hg*+ 2OH Nickel hydroxide Ni(OH)++Ni2++2OH Nickel sulfide 2 6- Silver hydroxide AgOH + Ag+ OH- 1493 Silver sulfide Zinc hydroxide Zn(OH) + Zn2++2OH Zine sulfide Zns + 2Ag'+S6-13 installations, phosphorus levels equal to or less than 0.02 mg/L have been achieved in the filtered effluent. To achieve such low levels of phosphorus removal, the backwash water from the second filter which contains small particles and residual coagulant is recycled to the first filter to improve floc formation within the first-stage filter and the influent to waste ratio. Comparison of Chemical Phosphorus Removal Processes The advantages and disadvantages of the removal of phosphorus by the addition of chemicals at various points in a treatment system are summarized in Table 6-4. It isrecommended that each alternative point of application be evaluated carefully. Tab. 6-4 Advantages and disadvantages of chemical addition in various sections of a treatment plant for phosphorus removal Estimation of Sludge Quantities from Phosphorus Precipitation The additional BOD and TSS removals afforded by chemical addition to primary treatment may also solve overloading problems on downstream biological systems, or may allow seasonal or year-round nitrification, depending on biological system designs. The BOD removal in the primary sedimentation operation is on the order of 50 to 60 percent at a pH of 9.5. The amount of primary sludge will also increase significantly. 6-5 Chemical Precipitation For Removal Of Heavy Metals And Dissolved Inorganic Substances The technologies available for the removal of heavy metals from wastewater include chemical precipitation, carbon adsorption, ion exchange, and reverse osmosis. Of these technologies, chemical precipitation is most commonly employed for most of the metals. Common precipitants include hydroxide (OH) and sulfide (S2- ). Carbonate (CO3 2- ) has also been used in some special cases. Metal may be removed separately or coprecipitated with phosphorus. Precipitation Reactions Metals of interest include arsenic (As), barium (Ba), cadmium (Cd), copper (Cu), mercury (Hg), nickel (Ni), selenium (Se), and zinc (Zn). Most of these metals can be precipitated as hydroxides or sulfides. Solubility products for free metal concentrations in equilibrium with hydroxide and sulfide precipitates are reported in Table 6-5. In wastewater treatment facilities, metals are precipitated most commonly as metal hydroxides through the addition of lime or caustic to a pH of minimum solubility. However, several of these compounds are, as discussed previously, amphoteric (i.e., capable of either accepting or donating a proton) and exhibit a point of minimum solubility. The minimum effluent concentration levels that can be achieved in the chemical precipitation of heavy metals are reported in Table 6-6. In practice, the minimum achievable residual metal concentrations will also depend on the nature and concentration of the organic matter in the wastewater as well as the temperature. Because of the many uncertainties associated with the precipitation of metals, laboratory bench-scale or pilot-plant testing should be conducted. Tab. 6-5 Solubility products for free metal ion concentrations in equilibrium with hydroxides and sulfides