diagram shown on Fig. 6-4. Although Fig 6-4 was developed for water treatment applications. it has been found to apply reasonably well to most wastewater applications, with minor variations. As shown on Fig 6-4, the approximate regions in which the different phenomena associated with particle removal in conventional sedimentation and filtration processes are operative are plotted as a function of the alum dose and the ph of the treated effluent after alum has been added. For example. optimum particle removal sweep floc occurs in the pH range of 7 to 8 with an alum dose of 20 to 60 mg/L Fig. 6-4 Typical operating ranges for alum coagulation Generally. for many wastewater effluents that have high pH values(e, g. 7.3 to 8.5 extremely low alum dosages. Because the characteristics of wastewater will vary from treatment plant to treatment plant, bench-scale and pilot-plant tests must be conducted to establish the appropriate chemical Importance of Initial Chemical Mixing with Metal Salts. Perhaps the least appreciated fact about chemical addition of metal salts is the importance of the rapid initial mixing of the chemicals with the wastewater to be treated. They found that the rate-limiting step in the coagulation process was the time required for the colloidal transport step brought about by Brownian motion(ie, perikinetic flocculation) which was estimated to be on the order of 1.5 to 3. 3 10-s. Clearly, based on the literature and actual field evaluations, the instantaneous rapid and intense mixing of metal salts is of critical importance, especially where the metal salts are to be used as coagulants to lower the surface charge of the colloidal particles. It should be noted that although achieving extremely low mixing times in large treatment plants is often difficult, low mixing times can be achieved by using multiple mixers 6-3 Chemical Precipitation For Improved Plant Performance Chemical precipitation. as noted previously, involves the addition of chemicals to alter the physical state of dissolved and suspended solids and facilitate their removal by sedimentation. In the past, chemical precipitation was often used to enhance the degree of Tss and BOD removal: (1)where there were seasonal variations in the concentration of the wastewater(such as in cannery wastewater), (2)where an intermediate degree of treatment was required, and (3)as an aid to the sedimentation process. Since about 1970. the need to provide more complete removal of the organic compounds and nutrients(nitrogen and hosphorus) contained in wastewater has brought about renewed interest in chemical precipitation. In current practice, chemical precipitation is used (1) as a means of improving the performance of primary (2)as a basic step in the independent physical-chemical treatment of wastewater.(3)for Aside from the determination of the required chemical dosages, the principal design considerations related facilities and the selection and design of the chemical storage, feeding. piping and control systems essing to the use of chemical precipitation involve the analysis and design of the necessary sludge pro Chemical Reactions in Wastewater Precipitation Applications Over the years a number of different substances have been used as precipitants. The degree of clarification obtained depends on the quantity of chemicals used and the care with which the process is controlled. It is possible by chemical precipitation to obtain a clear effluent, substantially free from matter in suspension or in the colloidal state. The chemicals added to wastewater interact with substances that are either normally present in the wastewater or added for this purpose. The most common chemicals are listed in Table 6-2. The reactions involved with(1)alum,(2) lime, (3)ferrous sulfate(copperas)and lime, (4) ferric chloride, (5)ferric chloride and lime, and(6) ferric sulfate and lime are considered in the following discussion(Metcalf Eddy, 1935) Tab. 6-2 Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater treatment Chemical Formule Form Percent 17(A2O3 Al2SO314H° 594 17A2O Aluminum chloride 133.3 Liquid 63-73。sco 8599 Ferric chloride 162.2 20 (Fe) Ferric sulfate 515 Granular Ferrous sulfate Granular Sodium aluminate No Al,O. 163.9 1006-7 diagram shown on Fig. 6-4. Although Fig. 6-4 was developed for water treatment applications, it has been found to apply reasonably well to most wastewater applications, with minor variations. As shown on Fig. 6-4, the approximate regions in which the different phenomena associated with particle removal in conventional sedimentation and filtration processes are operative are plotted as a function of the alum dose and the pH of the treated effluent after alum has been added. For example, optimum particle removal by sweep floc occurs in the pH range of 7 to 8 with an alum dose of 20 to 60 mg/L. Fig. 6-4 Typical operating ranges for alum coagulation Generally, for many wastewater effluents that have high pH values (e.g., 7.3 to 8.5), low alum dosages in the range of 5 to 10 mg/L will not be effective. With proper pH control it is possible to operate with extremely low alum dosages. Because the characteristics of wastewater will vary from treatment plant to treatment plant, bench-scale and pilot-plant tests must be conducted to establish the appropriate chemical dosages. Importance of Initial Chemical Mixing with Metal Salts. Perhaps the least appreciated fact about chemical addition of metal salts is the importance of the rapid initial mixing of the chemicals with the wastewater to be treated.They found that the rate-limiting step in the coagulation process was the time required for the colloidal transport step brought about by Brownian motion (i.e., perikinetic flocculation) which was estimated to be on the order of 1.5 to 3.3×10-3 s. Clearly, based on the literature and actual field evaluations, the instantaneous rapid and intense mixing of metal salts is of critical importance, especially where the metal salts are to be used as coagulants to lower the surface charge of the colloidal particles. It should be noted that although achieving extremely low mixing times in large treatment plants is often difficult, low mixing times can be achieved by using multiple mixers. 6-3 Chemical Precipitation For Improved Plant Performance Chemical precipitation, as noted previously, involves the addition of chemicals to alter the physical state of dissolved and suspended solids and facilitate their removal by sedimentation. In the past, chemical precipitation was often used to enhance the degree of TSS and BOD removal: (1) where there were seasonal variations in the concentration of the wastewater (such as in cannery wastewater), (2) where an intermediate degree of treatment was required, and (3) as an aid to the sedimentation process. Since about 1970, the need to provide more complete removal of the organic compounds and nutrients (nitrogen and phosphorus) contained in wastewater has brought about renewed interest in chemical precipitation. In current practice, chemical precipitation is used (1) as a means of improving the performance of primary settling facilities, (2) as a basic step in the independent physical-chemical treatment of wastewater, (3) for the removal of phosphorus, and (4) for the removal of heavy metals. Aside from the determination of the required chemical dosages, the principal design considerations related to the use of chemical precipitation involve the analysis and design of the necessary sludge processing facilities, and the selection and design of the chemical storage, feeding, piping, and control systems. Chemical Reactions in Wastewater Precipitation Applications Over the years a number of different substances have been used as precipitants. The degree of clarification obtained depends on the quantity of chemicals used and the care with which the process is controlled. It is possible by chemical precipitation to obtain a clear effluent, substantially free from matter in suspension or in the colloidal state. The chemicals added to wastewater interact with substances that are either normally present in the wastewater or added for this purpose. The most common chemicals are listed in Table 6-2. The reactions involved with (1) alum, (2) lime, (3) ferrous sulfate (copperas) and lime, (4) ferric chloride, (5) ferric chloride and lime, and (6) ferric sulfate and lime are considered in the following discussion (Metcalf & Eddy, 1935). Tab. 6-2 Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater treatment