Yobs value will vary depending on whether the substrate is defined as BOD, bCoD, or COD Observed volatile suspended solids yield values, based on BOD, are illustrated on Fig. 7-5 The observed yield decreases as the Srt is increased due to biomass loss by more endogenous respiration. The yield is lower with increasing temperature as a result of a higher endogenous respiration rate at higher temperature. The yield is higher when no primary treatment is used, as more nbvss remains in the influent wastewater. The total mass of dry solids wasted/day includes TSS and not just VSS. The TSSI includes the VSS plus inorganic solids Oxygen Requirements. The oxygen required for the biodegradation of carbonaceous material determined from a mass balance using the bcod concentration of the wastewater treated and the amount of biomass wasted from the system per day. If all of the bCod were oxidized to COz, H20, and NH3, the oxygen demand would equal the bCOD concentration. However, bacteria oxidize a portion of the bCoD to provide energy and use the remaining portion of the bCod for cell growth. Oxygen is also consumed for endogenous respiration, and the amount will depend on the system SRT. For a given SrT, a mas balance on the system can be done where the bCod removal equals the oxygen used plus the biomass VSS remaining in terms of an oxygen equivalent. The oxygen requirements for BOd removal without imary treatment and nitrification can be computed. As an approximation, for BOD removal only, the oxygen requirement will vary from 0.90 to 1. 3 kg O2/kg BOD removed for SRTs of 5 to 20 d, respectively(WEF, 1998) NOx is the amount of Tkn oxidized to nitrate. A nitrogen mass balance for the system that accounts for the influent TKN, nitrogen removed for biomass synthesis, and unoxidized effluent nitrogen is done to determine NOx. The nitrogen mass balance is based on the assumption of 0. 12 g N/g biomass(CsH7NO2 for biomass) Nutrient Requirements. If a biological system is to function properly, nutrients must be available in adequate amounts. Using the formula CsH7NO2, for the composition of cell biomass, about 12. 4 percent by weight of nitrogen will be required. The phosphorus requirement is usually assumed to be about one-fifth of the nitrogen value. These are typical values, not fixed quantities, because it has been shown that the percentage distribution of nitrogen and phosphorus in cell tissue varies with the system SRT and environmental conditions. The amount of nutrients required can be estimated based on the daily biomass production rate. It should be noted that nutrient limitations can occur when the concentrations of nitrogen and phosphorus are in the range of 0. 1 to 0.3 mg/L. As a general role, for SRT values greater than 7 d, about 5 g nitrogen and 1 g phosphorus will be required per 100 g of Bod to provide an excess of nutrients Other Chemical Requirements. In addition to the nutrient requirements, alkalinity is a major chemical requirement needed for nitrification. The amount of alkalinity required for nitrification, taking into account cell growth, is about 7.07 g CaCO3/g NH4-N. In addition to the alkalinity required for nitrification dditional alkalinity must be available to maintain the ph in the range from 6.8 to 7.4. Typically the amount of residual alkalinity required to maintain pH near a neutral point(ie, pH= 7) is between 70 and 80 mg/L as CaCO3 Mixed-Liquor Settling Characteristics. Clarifier design must provide adequate clarification of the effluent and solids thickening for the activated-sludge solids. In the design of installations where sludge characteristics are not known, data from other installations must be assumed or experience of the designer with similar suspended growth processes must be utilized Two commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volume index(SVi)and the zone settling rate(WEF, 1998 ). The Svi is the volume of l g of sludge after 30 min of settling. The Svi is determined by placing a mixed-liquor sample in a I-to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample Mlss concentration. For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 min in a l-L cylinder would have an SV of 100 mL/g. A value of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typically associated with filamentous growth( Parker et al., 2001) Because the SVI test is empirical, it is subject to significant errors. For ample, if sludge with a concentration of 10,000 mg/L did not settle all after 30 min the svi value would be 100. To avoid erroneous results and to allow for a meaningful comparison of svI results for different sludges, the diluted SVI(DS VI) test has been used (Jenkins et al., 1993). In Jenkins's analysis, the sludge sample is diluted with process effluent until the settled volume after 30 min is 250 mL/L or less. The standard SVI test is then followed with this sample 7-77-7 Yobs value will vary depending on whether the substrate is defined as BOD, bCOD, or COD. Observed volatile suspended solids yield values, based on BOD, are illustrated on Fig. 7-5. The observed yield decreases as the SRT is increased due to biomass loss by more endogenous respiration. The yield is lower with increasing temperature as a result of a higher endogenous respiration rate at higher temperature. The yield is higher when no primary treatment is used, as more nbVSS remains in the influent wastewater.The total mass of dry solids wasted/day includes TSS and not just VSS. The TSSI includes the VSS plus inorganic solids. Oxygen Requirements. The oxygen required for the biodegradation of carbonaceous material is determined from a mass balance using the bCOD concentration of the wastewater treated and the amount of biomass wasted from the system per day. If all of the bCOD were oxidized to CO2, H20, and NH3, the oxygen demand would equal the bCOD concentration. However, bacteria oxidize a portion of the bCOD to provide energy and use the remaining portion of the bCOD for cell growth. Oxygen is also consumed for endogenous respiration, and the amount will depend on the system SRT. For a given SRT, a mass balance on the system can be done where the bCOD removal equals the oxygen used plus the biomass VSS remaining in terms of an oxygen equivalent. The oxygen requirements for BOD removal without nitrification can be computed. As an approximation, for BOD removal only, the oxygen requirement will vary from 0.90 to 1.3 kg O2/kg BOD removed for SRTs of 5 to 20 d, respectively (WEF, 1998). NOx is the amount of TKN oxidized to nitrate. A nitrogen mass balance for the system that accounts for the influent TKN, nitrogen removed for biomass synthesis, and unoxidized effluent nitrogen is done to determine NOx. The nitrogen mass balance is based on the assumption of 0.12 g N/g biomass (C5H7NO2 for biomass). Nutrient Requirements. If a biological system is to function properly, nutrients must be available in adequate amounts. Using the formula C5H7NO2, for the composition of cell biomass, about 12.4 percent by weight of nitrogen will be required. The phosphorus requirement is usually assumed to be about one-fifth of the nitrogen value. These are typical values, not fixed quantities, because it has been shown that the percentage distribution of nitrogen and phosphorus in cell tissue varies with the system SRT and environmental conditions. The amount of nutrients required can be estimated based on the daily biomass production rate. It should be noted that nutrient limitations can occur when the concentrations of nitrogen and phosphorus are in the range of 0.1 to 0.3 mg/L. As a general role, for SRT values greater than 7 d, about 5 g nitrogen and 1 g phosphorus will be required per 100 g of BOD to provide an excess of nutrients. Other Chemical Requirements. In addition to the nutrient requirements, alkalinity is a major chemical requirement needed for nitrification. The amount of alkalinity required for nitrification, taking into account cell growth, is about 7.07 g CaCO3/g NH4-N. In addition to the alkalinity required for nitrification, additional alkalinity must be available to maintain the pH in the range from 6.8 to 7.4. Typically the amount of residual alkalinity required to maintain pH near a neutral point (i.e., pH ≈ 7) is between 70 and 80 mg/L as CaCO3. Mixed-Liquor Settling Characteristics. Clarifier design must provide adequate clarification of the effluent and solids thickening for the activated-sludge solids. In the design of installations where sludge characteristics are not known, data from other installations must be assumed or experience of the designer with similar suspended growth processes must be utilized. Two commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volume index (SVI) and the zone settling rate (WEF, 1998). The SVI is the volume of 1 g of sludge after 30 min of settling. The SVI is determined by placing a mixed-liquor sample in a 1- to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample MLSS concentration. For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 min in a 1-L cylinder would have an SVI of 100 mL/g. A value of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typically associated with filamentous growth (Parker et al., 2001 ). Because the SVI test is empirical, it is subject to significant errors. For example, if sludge with a concentration of 10,000 mg/L did not settle at all after 30 min, the SVI value would be 100. To avoid erroneous results and to allow for a meaningful comparison of SVI results for different sludges, the diluted SVI (DSVI) test has been used (Jenkins et al., 1993). In Jenkins's analysis, the sludge sample is diluted with process effluent until the settled volume after 30 min is 250 mL/L or less. The standard SVI test is then followed with this sample. Fig. 7-5 Net solids production vs. solids retention time(SRT) and temperature:(a)with primary treatment and (b)without primary treatment