Many of the factors that affect the performance of aerobic digestion have already been identified. This section summarizes the significant factors and quantifies their effects.
Figure 5.10 illustrates the effect of SRT on the performance of a conventional aerobic digester of the type shown in Figure 12.5b, as simulated with the simple model of Chapter 5. There it can be seen that the total solids concentration decreases as the SRT of the digester is increased, which is to be expected. Perhaps more importantly, however, the figure demonstrates that the buildup of biomass debris in the system limits the percent solids destruction that can be achieved. In this case about 53% solids destruction is achieved at an SRT of 1000 hrs, or about 42 days, as shown in Figure 12.10. At that SRT, debris accounts for about 83% of the remaining solids, suggesting that little additional solids destruction will occur, even if the SRT is extended greatly. This can also be seen from the slope of the VSS destruction curve in Figure 12.10. This characteristic should be kept in mind when selecting the SRT for an aerobic digester. A point may quickly be reached at which further increases in SRT will have minimal effect, making expenditures for additional tank volume questionable. That point will depend on the nature of the influent solids, their bio-degradability, and the temperature of the system.
Figure 5.10 also shows the effect of SRT on the oxygen requirement of the digester and that curve has been converted into the SOUR, as shown in Figure 12.10. Because the oxygen requirement curve in Figure 5.10 does not consider nitrification of the released ammonia-N, neither does the SOUR curve in Figure 12.10. Should the pH and other factors allow nitrifiers to grow, the SOUR would be 40% greater than shown. Like the VSS destruction curve, the SOUR curve quickly reaches a point where further increases in SRT have little effect. Just how rapidly that occurs also depends on the nature of the influent solids, their biodegradability, and the temperature of the system. This, too, must be considered when an SRT is being
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