Alum. When alum is added to wastewater containing calcium and magnesium bicarbonate alkalinity, a precipitate of aluminum hydroxide will form. The insoluble aluminum hydroxide is a gelatinous floc that settles slowly through the wastewater sweeping out sus reaction is exactly analogous when magnesium bicarbonate is substituted for the calcium salt Lime. A sufficient quantity of lime must therefore be added to combine with all the free carbonic acid and with the carbonic acid of the bicarbonates(half-bound carbonic acid) to produce calcium carbonate Much more lime is generally required when it is used alone than when sulfate of iron is also used where industrial wastes introduce mineral acids or acid salts into the wastewater Ferrous Sulfate and time. In most cases, ferrous sulfate cannot be used alone as a precipitant because lime must be added at the same time to form a precipitate. The formation of ferric hydroxide is dependent on the presence of dissolved oxygen, and, as a result, ferrous sulfate is not used commonly in wastewater Enhanced Removal of Suspended Solids in Primary Sedimentation he degree of clarification obtained when chemicals are added to untreated wastewater depends on the quantity of chemicals used, mixing times, and the care with which the process is monitored and controlled With chemical precipitation. it is possible to remove 80 to 90 percent of the total suspended solids (tss Comparable removal values for well-designed and well-operated primary sedimentation tanks without the ddition of chemicals are 50 to 70 percent of the TSS. 25 to 40 percent of the BOD, and 25 to 75 percent of the bacteria. Because of the variable characteristics of wastewater, the required chemical dosages nould be determined from bench-or pilot-scale tests Independent Physical-Chemical Treatment In some localities, industrial wastes have rendered municipal wastewater difficult to treat by biological means. In such situations. physical-chemical treatment may be an alternative approach. This method of treatment has met with limited success because of its lack of consistency in meeting discharge h costs for chemicals, handling and disposal of the great voli of sludge resulting from the addition of chemicals. and numerous operating problems. Based on typical performance results of full-scale plants using activated carbon, the activated-carbon columns removed only 50 to 60 percent of the applied total BOD, and the plants did not meet consistently the effluent standards for secondary treatment. In some instances, substantial process modifications have been required to reduce the operating problems and meet performance requirements, or the process has been replaced by biological treatment rare. Physical-chemical treatment is used more extensively for the treatment o Depending on the treatment objectives, the required chemical dosages and application rates should be determined from bench-or pilot-scale tests A flow diagram for the physical-chemical treatment of untreated wastewater is presented on Fig 6-5 first-stage prec pH adjustment by recarbonation(if required), the wastewater granular-medium filter to remove any residual floc and then through carbon columns to remove dissolved organic compounds. The filter is shown as optional, but its use is recommended to reduce the blinding and Caton treated effluent from the carbon column is usually chlorinated before discharge to the Centere Snubber receiving waters. cathan washin Fig. 6-5 Typical flow diagram ofan independent physical-chemical treatment pl resulting from chemical precipitation is one of the greatest difficulties associated with chemical treatment. Sludge is produced in great volume from most chen recipitation operations often reaching 0. 5 percent of the volume of wastewater treated when lime is us 6-4 Chemical Precipitation For Phosphorus Removal The removal of phosphorus from wastewater involves the incorporation of phosphate into TSS and the6-8 Alum. When alum is added to wastewater containing calcium and magnesium bicarbonate alkalinity, a precipitate of aluminum hydroxide will form. The insoluble aluminum hydroxide is a gelatinous floc that settles slowly through the wastewater, sweeping out suspended material and producing other changes. The reaction is exactly analogous when magnesium bicarbonate is substituted for the calcium salt. If less than this amount of alkalinity is available, it must be added. Lime is commonly used for this purpose when necessary, but it is seldom required in the treatment of wastewater. Lime. A sufficient quantity of lime must therefore be added to combine with all the free carbonic acid and with the carbonic acid of the bicarbonates (half-bound carbonic acid) to produce calcium carbonate. Much more lime is generally required when it is used alone than when sulfate of iron is also used where industrial wastes introduce mineral acids or acid salts into the wastewater. Ferrous Sulfate and time. In most cases, ferrous sulfate cannot be used alone as a precipitant because lime must be added at the same time to form a precipitate. The formation of ferric hydroxide is dependent on the presence of dissolved oxygen, and, as a result, ferrous sulfate is not used commonly in wastewater. Enhanced Removal of Suspended Solids in Primary Sedimentation The degree of clarification obtained when chemicals are added to untreated wastewater depends on the quantity of chemicals used, mixing times, and the care with which the process is monitored and controlled. With chemical precipitation, it is possible to remove 80 to 90 percent of the total suspended solids (TSS) including some colloidal particles, 50 to 80 percent of the BOD, and 80 to 90 percent of the bacteria. Comparable removal values for well-designed and well-operated primary sedimentation tanks without the addition of chemicals are 50 to 70 percent of the TSS, 25 to 40 percent of the BOD, and 25 to 75 percent of the bacteria. Because of the variable characteristics of wastewater, the required chemical dosages should be determined from bench- or pilot-scale tests.. Independent Physical-Chemical Treatment In some localities, industrial wastes have rendered municipal wastewater difficult to treat by biological means. In such situations, physical-chemical treatment may be an alternative approach. This method of treatment has met with limited success because of its lack of consistency in meeting discharge requirements, high costs for chemicals, handling and disposal of the great volumes of sludge resulting from the addition of chemicals, and numerous operating problems. Based on typical performance results of full-scale plants using activated carbon, the activated-carbon columns removed only 50 to 60 percent of the applied total BOD, and the plants did not meet consistently the effluent standards for secondary treatment. In some instances, substantial process modifications have been required to reduce the operating problems and meet performance requirements, or the process has been replaced by biological treatment. Because of these reasons, new applications of physical-chemical treatment for municipal wastewater are rare. Physical-chemical treatment is used more extensively for the treatment of industrial wastewater. Depending on the treatment objectives, the required chemical dosages and application rates should be determined from bench- or pilot-scale tests. A flow diagram for the physical-chemical treatment of untreated wastewater is presented on Fig. 6-5. As shown, after first-stage precipitation and pH adjustment by recarbonation (if required), the wastewater is passed through a granular-medium filter to remove any residual floc and then through carbon columns to remove dissolved organic compounds. The filter is shown as optional, but its use is recommended to reduce the blinding and headloss buildup in the carbon columns. The treated effluent from the carbon column is usually chlorinated before discharge to the receiving waters. Fig. 6-5 Typical flow diagram of an independent physical-chemical treatment plant The handling and disposal of the sludge resulting from chemical precipitation is one of the greatest difficulties associated with chemical treatment. Sludge is produced in great volume from most chemical precipitation operations, often reaching 0.5 percent of the volume of wastewater treated when lime is used. 6-4 Chemical Precipitation For Phosphorus Removal The removal of phosphorus from wastewater involves the incorporation of phosphate into TSS and the