iltration facilities to which chemicals can be added. In the high lime system, sufficient lime is added to raise the ph to about 11. After precipitation, the effluent must be recarbonated before biological treatment In activated-sludge systems, the ph of the primary effluent should not exceed 9.5 or 10; higher pH values can result in biological process upsets. In the trickling filter process. the carbon dioxide generated during treatment is usually sufficient to lower the pH without recarbonation. The dosage for low lime treatment is usually in the range of 75 to 250 mg/L as Ca(OH)2 at pH values of 8.5 to 9.5. In low lime systems, however, the conditions required for precipitation are more specialized; the Ca/Mgt mole ratio is <5/1 Lime Addition Following Secondary Treatment. Lime can be added to the waste stream af biological treatment to reduce the level of phosphorus and TSs. Single-stage process and two-stage process flow diagrams for lime addition are shown on Fig. 6-7. On Fig. 6-7a, a single-stage lime precipitation process is used for the treatment of secondary effluent In the first-stage clarifier of the wO-stage process shown on Fig. 6-7b, sufficient lime is added to raise the ph above ll to precipitate the soluble phosphorus as basic calcium phosphate(apatite). The calcium carbonate precipitate formed in the process acts as a coagulant for TSS removal. An example of a large lime precipitation unit is shown on Fig. 6-8. The excess soluble calcium is removed in the second-stage clarifier as a calcium carbonate precipitate by adding carbon dioxide gas to reduce the ph to about 10. Generally, there is a second injection of carbon dioxide to the second-stage effluent to reduce the formation of scale. To remove the residual levels of tss he secondary clarifier effluent is passed through a multimedia filter or a membrane filter. Care should be taken to limit excess calcium in the filter feed to ensure cementing of the filter media will not occur Wanlawwnter Recycled lime sposal Fig. 6-7 Typical lime treatment process flow diagram for phosphorus removal:(a)single-stage system; (b)two-stage system Fig. 6-8 Phosphorus Removal with Effluent Filtration Depending on the quality of the settled secondary effluent, chemical addition has been used to impro microfiltration before the performance of effluent filters. Chemical addition has also been used to achieve specific treatment objectives including the removal of specific contaminants such as phosphorus, metal ions, and humic substances. The removal of phosphorus by chemical addition to the two-stage filtration process has very effective for the removal of phosphorus. Based on the performance data from full-scale 6-126-12 filtration facilities to which chemicals can be added. In the high lime system, sufficient lime is added to raise the pH to about 11. After precipitation, the effluent must be recarbonated before biological treatment. In activated-sludge systems, the pH of the primary effluent should not exceed 9.5 or 10; higher pH values can result in biological process upsets. In the trickling filter process, the carbon dioxide generated during treatment is usually sufficient to lower the pH without recarbonation. The dosage for low lime treatment is usually in the range of 75 to 250 mg/L as Ca(OH)2 at pH values of 8.5 to 9.5. In low lime systems, however, the conditions required for precipitation are more specialized; the Ca2+ /Mg2+ mole ratio is ≤5/1. Lime Addition Following Secondary Treatment. Lime can be added to the waste stream after biological treatment to reduce the level of phosphorus and TSS. Single-stage process and two-stage process flow diagrams for lime addition are shown on Fig. 6-7. On Fig. 6-7a, a single-stage lime precipitation process is used for the treatment of secondary effluent. In the first-stage clarifier of the two-stage process shown on Fig. 6-7b, sufficient lime is added to raise the pH above 11 to precipitate the soluble phosphorus as basic calcium phosphate (apatite). The calcium carbonate precipitate formed in the process acts as a coagulant for TSS removal. An example of a large lime precipitation unit is shown on Fig. 6-8. The excess soluble calcium is removed in the second-stage clarifier as a calcium carbonate precipitate by adding carbon dioxide gas to reduce the pH to about 10. Generally, there is a second injection of carbon dioxide to the second-stage effluent to reduce the formation of scale. To remove the residual levels of TSS and phosphorus, the secondary clarifier effluent is passed through a multimedia filter or a membrane filter. Care should be taken to limit excess calcium in the filter feed to ensure cementing of the filter media will not occur. Fig. 6-7 Typical lime treatment process flow diagram for phosphorus removal: (a)single-stage system; (b)two-stage system Phosphorus Removal with Effluent Filtration Depending on the quality of the settled secondary effluent, chemical addition has been used to improve the performance of effluent filters. Chemical addition has also been used to achieve specific treatment objectives including the removal of specific contaminants such as phosphorus, metal ions, and humic substances. The removal of phosphorus by chemical addition to the contact filtration process is used in many parts of the country to remove phosphorus from wastewater treatment plant effluents which are discharged to sensitive water bodies. A two-stage filtration process has proved to be very effective for the removal of phosphorus. Based on the performance data from full-scale Fig. 6-8