also lead to foaming in anaerobic and aerobic digesters that receive the waste-activated sludge. Nocardia growth is common where surface scum is trapped in either the aeration basin or secondary clarifiers Aeration basins that are baffled with flow from one cell to the next occurring under the baffles, instead of over the top, encourage Nocardia growth and foam collection Methods that can be used to control Nocardia include(1)avoiding trapping foam in the secondar treatment process,(2) avoiding the recycle of skimmings into the secondary treatment process, and (3) oSing chlorine spray on the surface of the Nocardia foam. The use of a selector design may help to scourage Nocardia foaming, but significant foaming has been observed with anoxic/aerobic processes The addition of a small concentration of cationic polymer has controlling Nocardia foaming(Shao et al., 1997) The presence of Nocardia has also been associated with the presence of Nocardia-Microthrix with fats and edible oils in wastewater. Reducing the oil and grease content from discharges to the collection system from restaurants, truck stops, and meatpacking facilities by effective degreasing Activated-Sludge selector Processes In the above discussion, problems caused by ed sudge to nuisance microorganisms in activated sludge were bacteria on sludge settling characteristics and the potential of sludge bulking when the filamentous bacteria are present in high numbers. Prior to the 19705 configurations:(a)/aerobic;(b)high F/; ilamentous bulking was considered nevitable consequence of activated-sludge treatment, but work by Chudoba et al(1973)with staged versus complete-mix activated-sludge reactors led to the concept that reactor configuration designs, now termed selectors, could be used to control filamentous bulking and improve sludge-settling characteristics The concept of a selector is the use of a specific bioreactor design that favors the growth of floc-forming acteria instead of filamentous bacteria to provide an activated sludge with better settling and thickening properties. The high substrate concentration in the selector favors the growth of nonfilamentous A selector is a small tank (20 to 60 min contact time)or a series of tanks in which the incom ing wastewater is mixed with return sludge under aerobic, anoxic, and anaerobic conditions. Various types of selectors are shown on Fig. 7-10. The selector reactor precedes the activated-sludge aeration tank and may be designed as a separate reaction stage for a complete-mix reactor(see Fig. 7-10a)or as individual compartments in a plug-flow system(see Fig. 7-10b and c). Sequencing batch reactors may also be perated to employ the selector concept. The goal in the selector is to have most of the rbCOD consumed by the floc-forming bacteria Because the particulate degradable Cod decomposes at a much slower rate and will be present in the aeration tank, the rbCoD must be utilized for the benefit of the floc-forming bacteria. Selector designs are based on either kinetic or metabolic mechanisms. The kinetics-based selector designs are called high F/M selectors, and the metabolic-based selectors are either anoxic or anaerobic processes Kinetics-Based Selector. Selector designs based on biokinetic mechanisms provide for reactor substrate concentrations that result in faster substrate uptake by the floc-forming bacteria. While filamentous bacteria are more efficient for substrate utilization at low substrate concentrations, the floc-forming bacteria have higher growth rates at high soluble substrate concentrations. A series of reactors at relatively low t values(minutes)is used to provide high soluble substrate concentrations, in contrast to feeding influent wastewater to aeration tanks with t values on the order of hours for three reactors in series the following COD F/M ratios, based on the: influent to flowrate and COD concentration, are recommended First reactor, 12 g COD/g MLSS. d Second reactor, 6 g COD/g MLSS.d 7-147-14 also lead to foaming in anaerobic and aerobic digesters that receive the waste-activated sludge. Nocardia growth is common where surface scum is trapped in either the aeration basin or secondary clarifiers. Aeration basins that are baffled with flow from one cell to the next occurring under the baffles, instead of over the top, encourage Nocardia growth and foam collection. Methods that can be used to control Nocardia include (1) avoiding trapping foam in the secondary treatment process, (2) avoiding the recycle of skimmings into the secondary treatment process, and (3) using chlorine spray on the surface of the Nocardia foam. The use of a selector design may help to discourage Nocardia foaming, but significant foaming has been observed with anoxic/aerobic processes. The addition of a small concentration of cationic polymer has been used with some success for controlling Nocardia foaming (Shao et al., 1997). The presence of Nocardia has also been associated with the presence of Nocardia-Microthrix with fats and edible oils in wastewater. Reducing the oil and grease content from discharges to the collection system from restaurants, truck stops, and meatpacking facilities by effective degreasing processes can help control potential Nocardia problems. Activated-Sludge Selector Processes In the above discussion, problems caused by nuisance microorganisms in activated sludge were presented, including the effect of filamentous bacteria on sludge settling characteristics and the potential of sludge bulking when the filamentous bacteria are present in high numbers. Prior to the 1970s, f ilamentous bulking was considered an i nevitable consequence of activated-sludge treatment, but work by Chudoba et al. (1973) with staged versus complete-mix activated-sludge reactors led to the concept that reactor configuration designs, now termed selectors, could be used to control filamentous bulking and improve sludge-settling characteristics. The concept of a selector is the use of a specific bioreactor design that favors the growth of floc-forming bacteria instead of filamentous bacteria to provide an activated sludge with better settling and thickening properties. The high substrate concentration in the selector favors the growth of nonfilamentous organisms. A selector is a small tank (20 to 60 min contact time) or a series of tanks in which the incoming wastewater is mixed with return sludge under aerobic, anoxic, and anaerobic conditions. Various types of selectors are shown on Fig. 7-10. The selector reactor precedes the activated-sludge aeration tank and may be designed as a separate reaction stage for a complete-mix reactor (see Fig. 7-10a) or as individual compartments in a plug-flow system (see Fig. 7-10b and c). Sequencing batch reactors may also be operated to employ the selector concept. The goal in the selector is to have most of the rbCOD consumed by the floc-forming bacteria. Because the particulate degradable COD decomposes at a much slower rate and will be present in the aeration tank, the rbCOD must be utilized for the benefit of the floc-forming bacteria. Selector designs are based on either kinetic or metabolic mechanisms. The kinetics-based selector designs are called high F/M selectors, and the metabolic-based selectors are either anoxic or anaerobic processes. Kinetics-Based Selector. Selector designs based on biokinetic mechanisms provide for reactor substrate concentrations that result in faster substrate uptake by the floc-forming bacteria. While filamentous bacteria are more efficient for substrate utilization at low substrate concentrations, the floc-forming bacteria have higher growth rates at high soluble substrate concentrations. A series of reactors at relatively low τ values (minutes) is used to provide high soluble substrate concentrations, in contrast to feeding influent wastewater to aeration tanks with τ values on the order of hours. For three reactors in series, the following COD F/M ratios, based on the: influent to flowrate and COD concentration, are recommended. . First reactor, 12 g COD/g MLSS.d . Second reactor, 6 g COD/g MLSS.d Fig. 7-10 Typical selector configurations: (a)anaerobic/aerobic; (b)high F/M; (c)anoxic selector