Third reactor, 3 g COD/g mlss.d The F/M ratio is calculated for the first reactor using the volume and mlss concentration at that and the influent wastewater flowrate and COd concentration The f/M value shown for the second includes the volume of the first and second reactor and the applied loading as the product of the flowrate and COD concentration Ibertson(1987)recommended a similar approach based on a BOD F/M loading of 3 to 5 g BOD/g MLSS in the first reactor, with the second and third reactors being equal to and twice the first reactor volume, respectively. Albertson further notes that if the loading to the first reactor is too high(F/M>8 g BOD/g MLSS. d), a viscous, nonfilamentous-type bulking can develop. The kinetic concept of a high F/M selector suggests that it be aerobic, and high DO concentrations are needed to maintain an aer obic floc (6 to 8 mg/L). In many cases, such high DO concentrations are not practical or provided, and the staged selector design(described above) is operated at a low to zero DO concentration so that a metabolic selector mechanism is involved A sequencing batch reactor(SBR) can also act as a very effective high F/M selector, depending on the wastewater strength and feeding strategy. For high-strength wastewaters with a relatively large fraction of the SBr volume occupied by the influent wastewater, a high initial F/M ratio can occur. The subsequent reaction by the batch process is equal to that for a plug-flow reactor. Metabolic-Based Selector. With biological nutrient-removal processes, improved sludge-settling characteristics, and, in many cases, minimal filamentous bacteria growth has been observed. The anoxic or anaerobic metabolic conditions used in these processes favor growth of the floc-forming bacteria. The filamentous bacteria cannot use nitrate or nitrite for an electron acceptor, thus yielding a significant advantage to denitrifying floc-forming bacteria. Similarly, the filamentous bacteria do not store polyphosphates and thus cannot consume acetate in the anaerobic contact zone in biologica phosphorus-removal designs, giving an advantage for substrate uptake and growth to the phosphorus-storing bacteria. In some wastewater-treatment facilities (Seattle and San Francisco,for example), an anaerobic selector has been used before the aeration tank in low SrT activated-sludge systems designed for BOD removal, even though phosphorus removal is not required Where nitrification is used and phosphorus removal is not required, anoxic selectors(either the stage high F/M gradient or the single-stage designs) have been used. For the high F/M anoxic or selectors, the resultant mixed-liquor SvI may be in the range of 65 to 90 mL/g, and for single-tank anoxic selectors, SVI values in the range of 100 to 120 mL/g are more commonly obtained The use of selector designs in activated sludge is more common because of the many advantages derived from the minimal investment in a relatively small reactor volume. By improving sludge settling, the activated-sludge treatment capacity may be increased, as higher MLSS concentrations are usually possible The hydraulic capacity of the secondary clarifiers is also increased 7-4 Processes for bod removal and nitrification Process design considerations For BOD removal and nitrification processes, the rbCOD concentration is important for evaluating the oxygen demand profiles for plug-flow, staged, and batch-fed processes. The effect of nb VSS concentration in the influent will be significant in process sludge production and aeration volume requirements In the following paragraphs, three activated-sludge process design examples are provided to demonstrate application of these fundamental principles to BOD removal and nitrification processes omplete-Mix Activated-Sludge Process In a typical complete-mix activated-sludge(CMAS) process, effluent from the primary sedimentation tank and recycled return activated sludge are introduced typically at several points in the reactor. Because the tank contents are thoroughly mixed, the organic load, oxygen demand, and substrate concentration are uniform throughout the entire aeration tank and the f/m ratio is low Care should be taken to assure that the CMAS reactor is well mixed and that influent feed and effluent with-drawn points are selected to prevent short-circuiting of untreated or partially treated wastewater. The complete-mix reactor is usually configured in square, rectangular, or round shapes. Tank dimensions 7-157-15 . Third reactor, 3 g COD/g MLSS.d The F/M ratio is calculated for the first reactor using the volume and MLSS concentration at that reactor and the influent wastewater flowrate and COD concentration. The F/M value shown for the second reactor includes the volume of the first and second reactor and the applied loading as the product of the influent flowrate and COD concentration. Albertson (1987) recommended a similar approach based on a BOD F/M loading of 3 to 5 g BOD/g MLSS in the first reactor, with the second and third reactors being equal to and twice the first reactor volume, respectively. Albertson further notes that if the loading to the first reactor is too high (F/M > 8 g BOD/g MLSS.d), a viscous, nonfilamentous-type bulking can develop. The kinetic concept of a high F/M selector suggests that it be aerobic, and high DO concentrations are needed to maintain an aerobic floc (>6 to 8 mg/L). In many cases, such high DO concentrations are not practical or provided, and the staged selector design (described above) is operated at a low to zero DO concentration so that a metabolic selector mechanism is involved. A sequencing batch reactor (SBR) can also act as a very effective high F/M selector, depending on the wastewater strength and feeding strategy. For high-strength wastewaters with a relatively large fraction of the SBR volume occupied by the influent wastewater, a high initial F/M ratio can occur. The subsequent reaction by the batch process is equal to that for a plug-flow reactor. Metabolic-Based Selector. With biological nutrient-removal processes, improved sludge-settling characteristics, and, in many cases, minimal filamentous bacteria growth has been observed. The anoxic or anaerobic metabolic conditions used in these processes favor growth of the floc-forming bacteria. The filamentous bacteria cannot use nitrate or nitrite for an electron acceptor, thus yielding a significant advantage to denitrifying floc-forming bacteria. Similarly, the filamentous bacteria do not store polyphosphates and thus cannot consume acetate in the anaerobic contact zone in biological phosphorus-removal designs, giving an advantage for substrate uptake and growth to the phosphorus-storing bacteria. In some wastewater-treatment facilities (Seattle and San Francisco, for example), an anaerobic selector has been used before the aeration tank in low SRT activated-sludge systems designed for BOD removal, even though phosphorus removal is not required. Where nitrification is used and phosphorus removal is not required, anoxic selectors (either the staged high F/M gradient or the single-stage designs) have been used. For the high F/M anoxic or anaerobic selectors, the resultant mixed-liquor SVI may be in the range of 65 to 90 mL/g, and for single-tank anoxic selectors, SVI values in the range of 100 to 120 mL/g are more commonly obtained. The use of selector designs in activated sludge is more common because of the many advantages derived from the minimal investment in a relatively small reactor volume. By improving sludge settling, the activated-sludge treatment capacity may be increased, as higher MLSS concentrations are usually possible. The hydraulic capacity of the secondary clarifiers is also increased. 7-4 Processes for BOD Removal and Nitrification Process Design Considerations For BOD removal and nitrification processes, the rbCOD concentration is important for evaluating the oxygen demand profiles for plug-flow, staged, and batch-fed processes. The effect of nbVSS concentration in the influent will be significant in process sludge production and aeration volume requirements. In the following paragraphs, three activated-sludge process design examples are provided to demonstrate application of these fundamental principles to BOD removal and nitrification processes. Complete-Mix Activated-Sludge Process In a typical complete-mix activated-sludge (CMAS) process, effluent from the primary sedimentation tank and recycled return activated sludge are introduced typically at several points in the reactor. Because the tank contents are thoroughly mixed, the organic load, oxygen demand, and substrate concentration are uniform throughout the entire aeration tank and the F/M ratio is low. Care should be taken to assure that the CMAS reactor is well mixed and that influent feed and effluent with-drawn points are selected to prevent short-circuiting of untreated or partially treated wastewater. The complete-mix reactor is usually configured in square, rectangular, or round shapes. Tank dimensions depend mainly on the size, type, and mixing pattern of the aeration equipment