Activated-sludge reactor operating conditions (low DO, low F/M, and complete-mix operation) wastewater characteristics variations in flowrate clearly have an effect on the Variations in composition development of filamentous populations. One of the kinetic features of filamentous Hapticity that relates to these conditions is that Nutrient content they are very competitive at low Nature of waste components substrate concentrations whether it be organic substrates, DO, or nutrients Thus, lightly loaded complete-mix Short circuiting (aeration tanks and clarifiers) activated-sludge systems or low DO imited return sludge pumping capacity provide an environment more ssve dissolved oxygen favorable to filamentous bacteria Insuficient nutrients than to the desired floc - formins Low F/M bacteria Insufficient soluble BOD Filamentous bacteria such as Beggiatoa and Thiothrix grow well on hydrogen sulfide and reduced substrates, respectively, that would be found in septic wastewaters (Wanner, 1994). When the influent wastewater contains fermentation products such as volatile fatty acids and reduced sulfur compounds(sulfides and thiosulfate), Thiothrix can proliferate. Prechlorination of the wastewaters has been done in some cases to prevent their growth. Besides causing bulking problems activated-sludge systems, Beggiatoa and Thiothrix can create problems in fixed-film systems, including trickling filters and rotating biological contactors In the control of bulking, where a number of variables are possible causes, a checklist of items to investigate is valuable. The following items are recommended:(1)wastewater characteristics, (2 dissolved oxygen content, (3) process loading, (4) return and waste sludge pumping rates, (5)internal plant overloading, and(6) clarifier operation One of the first steps to be taken when sludge settlor characteristics change is to view the mixed liquor under the microscope to determine what type of microbial growth changes or floc structure changes can be related to the development of bulking sludge. A reasonable quality phase-contrast microscope with magnification up to 1000 times(oil immersion)is necessary to view the filamentous bacteria structure and size Wastewater Characteristics. The nature of the components found in wastewater or the absence of certain components, such as trace elements, can lead to the development of a bulked sludge. If it is known that industrial wastes are being introduced into the system either intermittently or continuously, the quantity of nitrogen and phosphorus in the wastewater should be checked first, because limitations of both or either are known to favor bulking. Nutrient deficiency is a classic problem in the treatment of industrial wastewaters containing high levels of carbonaceous BOD. Wide fluctuations in pH are also known to be detrimental in plants of conventional design. Variations in organic waste loads due to batch-type operations can also lead to bulking and should be checked Dissolved Oxygen Concentration. Limited dissolved oxygen has been noted more frequently than any other cause of bulking. If the problem is due to limited oxygen, it can usually be confirmed by operating the aeration equipment at full capacity by decreasing the system SrT, if possible, to reduce the oxygen demand. The aeration -equipment should have adequate capacity to maintain at least 2 mg/L of dissolved oxygen in the aeration tank under normal loading conditions. If 2 mg/L of oxygen cannot be maintained, installation of improvements to the existing aeration system may be required Process Loading/Reactor Configuration. The aeration Sr should be checked to make sure that it is within the range of generally accepted values. In many cases, complete-mix systems with long SRIs and sequent low F/M ratios experience filamentous growths. In such systems, the filamentous organisms are more competitive for substrate. Laboratory research and full-scale investigations have led to activated-sludge design configurations that provide conditions favoring the dominance of floc-forming bacteria over filamentous organisms (Jenkins et al., 1993). Reactors in series with various types of environmental conditions, i.e., aerobic, anoxic, and anaerobic, are generally used to augment or replace a complete-mix reactor. The series configurations are called selector processes because they provide conditions that cause selection of floc-forming bacteria in lieu of filamentous organisms as the dominant population 7-127-12 Activated-sludge reactor operating conditions (low DO, low F/M, and complete-mix operation) clearly have an effect on the development of filamentous populations. One of the kinetic features of filamentous organisms that relates to these conditions is that they are very competitive at low substrate concentrations whether it be organic substrates, DO, or nutrients. Thus, lightly loaded complete-mix activated-sludge systems or low DO (<0.5 mg/L) operating conditions provide an environment more favorable to filamentous bacteria than to the desired floc-forming bacteria. Filamentous bacteria such as Beggiatoa and Thiothrix grow well on hydrogen sulfide and reduced substrates, respectively, that would be found in septic wastewaters (Wanner, 1994). When the influent wastewater contains fermentation products such as volatile fatty acids and reduced sulfur compounds (sulfides and thiosulfate), Thiothrix can proliferate. Prechlorination of the wastewaters has been done in some cases to prevent their growth. Besides causing bulking problems in activated-sludge systems, Beggiatoa and Thiothrix can create problems in fixed-film systems, including trickling filters and rotating biological contactors. In the control of bulking, where a number of variables are possible causes, a checklist of items to investigate is valuable. The following items are recommended: (1) wastewater characteristics, (2) dissolved oxygen content, (3) process loading, (4) return and waste sludge pumping rates, (5) internal plant overloading, and (6) clarifier operation. One of the first steps to be taken when sludge settling characteristics change is to view the mixed liquor under the microscope to determine what type of microbial growth changes or floc structure changes can be related to the development of bulking sludge. A reasonable quality phase-contrast microscope with magnification up to 1000 times (oil immersion) is necessary to view the filamentous bacteria structure and size. Wastewater Characteristics. The nature of the components found in wastewater or the absence of certain components, such as trace elements, can lead to the development of a bulked sludge. If it is known that industrial wastes are being introduced into the system either intermittently or continuously, the quantity of nitrogen and phosphorus in the wastewater should be checked first, because limitations of both or either are known to favor bulking. Nutrient deficiency is a classic problem in the treatment of industrial wastewaters containing high levels of carbonaceous BOD. Wide fluctuations in pH are also known to be detrimental in plants of conventional design. Variations in organic waste loads due to batch-type operations can also lead to bulking and should be checked. Dissolved Oxygen Concentration. Limited dissolved oxygen has been noted more frequently than any other cause of bulking. If the problem is due to limited oxygen, it can usually be confirmed by operating the aeration equipment at full capacity by decreasing the system SRT, if possible, to reduce the oxygen demand. The aeration-equipment should have adequate capacity to maintain at least 2 mg/L of dissolved oxygen in the aeration tank under normal loading conditions. If 2 mg/L of oxygen cannot be maintained, installation of improvements to the existing aeration system may be required. Process Loading/Reactor Configuration. The aeration SRT should be checked to make sure that it is within the range of generally accepted values. In many cases, complete-mix systems with long SRTs and subsequent low F/M ratios experience filamentous growths. In such systems, the filamentous organisms are more competitive for substrate. Laboratory research and full-scale investigations have led to activated-sludge design configurations that provide conditions favoring the dominance of floc-forming bacteria over filamentous organisms (Jenkins et al., 1993). Reactors in series with various types of environmental conditions, i.e., aerobic, anoxic, and anaerobic, are generally used to augment or replace a complete-mix reactor. The series configurations are called selector processes because they provide conditions that cause selection of floc-forming bacteria in lieu of filamentous organisms as the dominant population