sludge has remained in the system. SrT is the most critical parameter for activated-sludge design as Srt affects the treatment process performance, aeration tank volume, sludge production, and oxygen requirements. For BOD removal, SRT values may range from 3 to 5 d, depending on the mixed-liquor temperature. At 18 to 25 C an SRT value close to 3 d is desired where only BOD removal is required. To limit nitrification, some activated-sludge plants have been operated at SRT values of l d or less. At 10C SRI values of 5 to 6 d are common for BOD removal only. Temperature and other factors that affect Srt in various treatment applications are summarized in Table 7-2 SRT ranges for activated-sludge Removal of soluble BOD in domes -2 d Develop flocculent biomass for treating dustrial wastewater 3-18 Temperature/ compounds Biological phosphorus removal Stabilization of activated sludge 20-40 Degradation of xenobiotic compounds For nitrification design, a safety factor is used to increase the Srt above that calculated based on nitrification kinetics and the required effluent NH4-N concentration. A factor of safety is used for two reasons:(1)to allow flexibility for operational variations in controlling the Srt, and (2) to provide for dditional nitrifying bacteria to handle peak TKN loadings. The influent TKN concentration and mass loading can vary throughout the day(a peak to average tKn loading of 1.3 to 1.5 is not unusual, depending on plant size)and can also be affected by return flows from digested and dewatered biosolids processing. By increasing the design SRT, the inventory of nitrifying bacteria is increased to meet the NH4-N concentration at the peak load so that the effluent NH4-N concentration requirement is achieved Food to Microorganism Ratio. A process parameter commonly used to characterize process designs and operating conditions is the food to microorganism(biomass)ratio(F/M). Typical values for the BOD F/M ratio reported in the literature vary from 0.04 g substrate/g biomass. d for extended aeration processes to 1.0 g/g d for high rate processes. The BOd F/M ratio is usually evaluated for systems that were designed based on SrT to provide a reference point to previous activated -sludge design and operating Volumetric Organic Loading Rote. The volumetric organic loading rate is defined as the amount of BOD or COD apport from 0.3 to more than 3. 0. Higher volumetric organic loadings generally result in plied to the aeration tank volume per day, Organic loadings, expressed in kg BOd o COD/md, may higher required oxygen transfer rates per unit volume for the aeration system prediction of sludge production for the activated-sludge process. Sludge wil ty depends on the Sludge Production. The design of the sludge-handling and disposal/reuse facil accumulate in the activated-sludge process if it cannot be processed fast enough by an undersized sludge-handling facility Eventually, the sludge inventory capacity of the activated-sludge sy stem will be exceeded and excess solids will exit in the secondary clarifier effluent, potentially violating discharge limits. The sludge production relative to the amount of Bod removed also affects the aeration tank size Two methods are used to determine sludge production. The first method is based on an estimate of ar bserved sludge production yield from published data from similar facilities, and the second is based on actual activated-sludge process design in 000 which wastewate characterization is done and the various sources actIo are considered and accounted for. For given wastewater. the 04060811.5234567101520304050 Figure 8-7 Net solids production vs. solids retention time (SRn) and temperoture: (o) with primary treatment and (b)without primary treame7-6 sludge has remained in the system. SRT is the most critical parameter for activated-sludge design as SRT affects the treatment process performance, aeration tank volume, sludge production, and oxygen requirements. For BOD removal, SRT values may range from 3 to 5 d, depending on the mixed-liquor temperature. At 18 to 25。C an SRT value close to 3 d is desired where only BOD removal is required. To limit nitrification, some activated-sludge plants have been operated at SRT values of 1 d or less. At 10。 C, SRT values of 5 to 6 d are common for BOD removal only. Temperature and other factors that affect SRT in various treatment applications are summarized in Table 7-2. For nitrification design, a safety factor is used to increase the SRT above that calculated based on nitrification kinetics and the required effluent NH4-N concentration. A factor of safety is used for two reasons: (1) to allow flexibility for operational variations in controlling the SRT, and (2) to provide for additional nitrifying bacteria to handle peak TKN loadings. The influent TKN concentration and mass loading can vary throughout the day (a peak to average TKN loading of 1.3 to 1.5 is not unusual, depending on plant size) and can also be affected by return flows from digested and dewatered biosolids processing. By increasing the design SRT, the inventory of nitrifying bacteria is increased to meet the NH4-N concentration at the peak load so that the effluent NH4-N concentration requirement is achieved. Food to Microorganism Ratio. A process parameter commonly used to characterize process designs and operating conditions is the food to microorganism (biomass) ratio (F/M). Typical values for the BOD F/M ratio reported in the literature vary from 0.04 g substrate/g biomass.d for extended aeration processes to 1.0 g/g.d for high rate processes. The BOD F/M ratio is usually evaluated for systems that were designed based on SRT to provide a reference point to previous activated-sludge design and operating performance. Volumetric Organic Loading Rote. The volumetric organic loading rate is defined as the amount of BOD or COD applied to the aeration tank volume per day, Organic loadings, expressed in kg BOD or COD/m3 .d, may vary from 0.3 to more than 3.0. Higher volumetric organic loadings generally result in higher required oxygen transfer rates per unit volume for the aeration system. Sludge Production. The design of the sludge-handling and disposal/reuse facility depends on the prediction of sludge production for the activated-sludge process. Sludge will accumulate in the activated-sludge process if it cannot be processed fast enough by an undersized sludge-handling facility. Eventually, the sludge inventory capacity of the activated-sludge system will be exceeded and excess solids will exit in the secondary clarifier effluent, potentially violating discharge limits. The sludge production relative to the amount of BOD removed also affects the aeration tank size. Two methods are used to determine sludge production. The first method is based on an estimate of an observed sludge production yield from published data from similar facilities, and the second is based on the actual activated-sludge process design in which wastewater characterization is done and the various sources of sludge production are considered and accounted for. For a given wastewater, the Tab. 7-2 Typical minimum SRT ranges for activated-sludge treatment