Internal Plant Overloading. To avoid internal plant overloading recycle loads should be controlled so they are not returned to the plant flow during times of peak hydraulic and organic loading Examples of recycle loads are centrate or filtrate from sludge dewatering operations and supernatant from sludge digesters Clarifier Operation. The operating characteristics of the clarifier may also affect characteristics. Poor settling is often a problem in center-feed circular tanks where sludge is removed fro the tank directly under the point where the mixed liquor enters. Sludge may actually be retained in the tank for many hours rather than the desired 30 min and cause localized septic conditions. If this is the case then the design is at fault and changes must be made in the inlet feed well and sludge withdrawal Temporary Control Measures. In an emergency situation or while the aforementioned factors are being investigated, chlorine and hydrogen peroxide may be used to provide temporary help. Chlorination of return sludge has been practiced quite extensively as a means of controlling bulking A typical design for a low(5 to 10 h)I system uses 0.002 to 0.008 kg of chlorine per kg mLSS. d (Jenkins et al., 1993) Although chlorination is effective in controlling bulking caused by filamentous growths, it is ineffective when bulking is due to light floc containing bound water. Chlorination normally results in the production of a turbid effluent until such time as the sludge is free of the filamentous forms. chlorination of a nitrifying sludge will also produce a turbid effluent because of the death of the nitrifying organisms. The use of chlorine also raises issues about the formation of trihalomethanes and other compounds with potential health and environmental effects. Hydrogen peroxide has also been used in the control of filamentous organisms in bulking sludge. Dosage of hydrogen peroxide and treatment time depend on the extent of the filamentous development ising Sludge. Occasionally, sludge that has good settling characteristics will be observed to rise or float to the surface after a relatively short settling period. The most common cause of this phenomenon is denitrification, in which nitrites and nitrates in the wastewater are converted to nitrogen gas. As nitrogen gas is formed in the sludge layer, much of it is trapped in the sludge mass. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids and the presence of more floating sludge on the secondary clarifier surface. Rising sludge is common in short SRT systems, where the temperature encourages the initiation of nitrification, and the mixed liquor is very active due to the low sludge age Rising sludge problems may be overcome by()increasing the return activated-sludge withdrawal rate from the clarifier to reduce the detention time of the sludge in the clarifier, (2) decreasing the rate of flow of aeration liquor into the offending clarifier if the sludge depth cannot be reduced by increasing the return activated-sludge withdrawal rate, (3) where possible, increasing the speed of the sludge-collecting mechanism in the settling tanks, and (4)decreasing the srt to bring the activated sludge out of nitrification. For warm climates where it is very difficult to operate at a low enough SrT to limit nitrification, an anoxic/aerobic process is preferred to denitrification to prevent rising sludge and to improve sludge settling characteristics Nocardia Foam. Two bacteria genera, Nocardia and Microthrix parvicella, are associated with extensive foaming in activated-sludge processes. These organisms have hydrophobic cell surfaces and attach to air bubbles, where they stabilize the bubbles to cause foam. The organisms can be found at high concentrations in the foam above the mixed liquor. Both types of bacteria can be identified under microscopic examination. Nocardia has a filamentous structure. and the filaments are very short and are ontained within the floc particles Microthrix parvicella has thin filaments extending from the floc particles. Foaming on an activated-sludge basin and a microscopic view of Nocardia are shown on Fig 7-9 Fig. 7-9 The foam is thick. has a brown color. and can build up in thickness of 0.5 (b)microscopic observation of gram-stained Nocardia filaments The foam production can occur with both diffused and mechanical aeration but is more pronounced with diffused aeration and with higher air flowrates. problems of nocardia foaming in the activated sludge can 7-137-13 Internal Plant Overloading. To avoid internal plant overloading, recycle loads should be controlled so they are not returned to the plant flow during times of peak hydraulic and organic loading. Examples of recycle loads are centrate or filtrate from sludge dewatering operations and supernatant from sludge digesters. Clarifier Operation. The operating characteristics of the clarifier may also affect sludge settling characteristics. Poor settling is often a problem in center-feed circular tanks where sludge is removed from the tank directly under the point where the mixed liquor enters. Sludge may actually be retained in the tank for many hours rather than the desired 30 min and cause localized septic conditions. If this is the case, then the design is at fault, and changes must be made in the inlet feed well and sludge withdrawal equipment. Temporary Control Measures. In an emergency situation or while the aforementioned factors are being investigated, chlorine and hydrogen peroxide may be used to provide temporary help. Chlorination of return sludge has been practiced quite extensively as a means of controlling bulking. A typical design for a low (5 to 10 h) τ system uses 0.002 to 0.008 kg of chlorine per kg MLSS.d (Jenkins et al., 1993). Although chlorination is effective in controlling bulking caused by filamentous growths, it is ineffective when bulking is due to light floc containing bound water. Chlorination normally results in the production of a turbid effluent until such time as the sludge is free of the filamentous forms. Chlorination of a nitrifying sludge will also produce a turbid effluent because of the death of the nitrifying organisms. The use of chlorine also raises issues about the formation of trihalomethanes and other compounds with potential health and environmental effects. Hydrogen peroxide has also been used in the control of filamentous organisms in bulking sludge. Dosage of hydrogen peroxide and treatment time depend on the extent of the filamentous development. Rising Sludge. Occasionally, sludge that has good settling characteristics will be observed to rise or float to the surface after a relatively short settling period. The most common cause of this phenomenon is denitrification, in which nitrites and nitrates in the wastewater are converted to nitrogen gas. As nitrogen gas is formed in the sludge layer, much of it is trapped in the sludge mass. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids and the presence of more floating sludge on the secondary clarifier surface. Rising sludge is common in short SRT systems, where the temperature encourages the initiation of nitrification, and the mixed liquor is very active due to the low sludge age. Rising sludge problems may be overcome by (1) increasing the return activated-sludge withdrawal rate from the clarifier to reduce the detention time of the sludge in the clarifier, (2) decreasing the rate of flow of aeration liquor into the offending clarifier if the sludge depth cannot be reduced by increasing the return activated-sludge withdrawal rate, (3) where possible, increasing the speed of the sludge-collecting mechanism in the settling tanks, and (4) decreasing the SRT to bring the activated sludge out of nitrification. For warm climates where it is very difficult to operate at a low enough SRT to limit nitrification, an anoxic/aerobic process is preferred to denitrification to prevent rising sludge and to improve sludge settling characteristics. Nocardia Foam. Two bacteria genera, Nocardia and Microthrix parvicella, are associated with extensive foaming in activated-sludge processes. These organisms have hydrophobic cell surfaces and attach to air bubbles, where they stabilize the bubbles to cause foam. The organisms can be found at high concentrations in the foam above the mixed liquor. Both types of bacteria can be identified under microscopic examination. Nocardia has a filamentous structure, and the filaments are very short and are contained within the floc particles. Microthrix parvicella has thin filaments extending from the floc particles. Foaming on an activated-sludge basin and a microscopic view of Nocardia are shown on Fig. 7-9. The foam is thick, has a brown color, and can build up in thickness of 0.5 to 1 m. The foam production can occur with both diffused and mechanical aeration but is more pronounced with diffused aeration and with higher air flowrates. Problems of Nocardia foaming in the activated sludge can Fig. 7-9 Nocardia foam: (a)example of foam on an aeration tank; (b)microscopic observation of gram-stained Nocardia filaments