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0 0 0.2 0 4 0 6 0 8 1.0 1 2 1 4 1 6 Recycle Ratio, a

Figure 10.19 Effect of the biomass recycle ratio on the fraction of MLSS in the last tank of a SFAS system, as calculated with Eq. 10.43.

culated with Eq. 10.32. As seen in Figure 7.11, the extent of nitrification in SFAS is essentially the same as in a CMAS system. However, the ammonia-N concentration in the early tanks will be lower, as shown in Figure 7.15. Thus, to be conservative. Eq. 10.32, which assumed that the residual ammonia-N concentration was negligible, should be used without modification. Because the influent to type of SFAS system considered here is distributed evenly among the tanks, all of the synthesis oxygen requirements should be apportioned evenly as well. If some other influent flow distribution were used, the synthesis oxygen requirements should be apportioned in the same manner as the flow. The oxygen requirement due to heterotrophic decay can be calculated with Eq. 10.25, while the requirement for autotrophic decay can be calculated with Eq. 10.33. Both should be apportioned to each tank in accordance with the fraction of the MESS in each as given by Eq. 10.42. Although the recycle ratio will influence that distribution, the effect will be relatively small so the distribution can be made on the basis of the most commonly used ratio. Because the MLSS concentration decreases from tank to tank down the chain, so will this oxygen requirement. The transient-state oxygen requirement can be distributed in a similar manner by using the approaches developed for CMAS systems and considering the distribution of influent to all tanks.

For equal influent flow distribution to all tanks, the first tank will have the highest oxygen requirement, and thus it is used to determine the minimum acceptable bioreactor volume based on oxygen transfer. This can be calculated with Eq. 10.5. Even though the last tank will have a somewhat lower oxygen requirement than the first, it should be used to calculate the lower limit based on floe shear because its effluent goes to the final settler. That limit can be calculated with Eq. 10.4. Both calculations should be made on the basis of the summer time oxygen requirement, with cither the steady- or transient-state requirement being used, depending upon the nature of the influent flow. The last tank will have the lowest oxygen requirement, and it should be used to determine the maximum acceptable bioreactor volume. The upper limit on the size of last tank can be calculated with Eq. 10.3 by using the minimum steady-state or transient-state oxygen requirement (as needed) as calculated for winter operating conditions.

Once the limits on tank volume are known, they can be used with the mass of MLSS in the last tank to establish an acceptable range of MLSS concentrations for that tank. The mass of MLSS in the last tank is just the fraction of biomass in that tank multiplied by (XMJ • V)s,Ml„, as calculated with Eq. 9.11 for winter conditions. Since the MLSS concentration in the last tank is the same as the concentration entering the final settler, various concentrations within the allowable range can be investigated for their effects on final settler size and operation. Once one is selected, it, in turn, establishes the total system volume, just as in the other designs.

Sometimes it may be desirable to alter the configuration of an existing activated sludge system to that of SFAS. In that situation, the planned change must be evaluated to be sure that it can be made while still accomplishing the treatment objectives. The steps for doing that are similar to those for designing a SFAS system, except that the system volume and number of equivalent tanks-in-series are known, thereby fixing the volume of each tank. Thus, the major questions are whether the desired effluent quality can be met with the existing system SRT and whether the needed amount of oxygen can be transferred to each tank while meeting the con straints on mixing energy input and oxygen transfer. The steps outlined above can be followed to answer those questions.

Design of Contact Stabilization Activated Sludge Systems. A schematic diagram of a CSAS system is shown in Figure 7.19. There it can be seen that the influent enters the contact tank, from which it flows directly to the final settler. The biomass recycle from the settler flows to the stabilization basin where additional reactions can occur before the biomass is returned to the contact tank. As far as organic substrates are concerned, removal from the wastewater undergoing treatment must occur in the contact tank, where the majority of the soluble substrate is metabolized. Particulate organic matter is entrapped in the mixed liquor, with its degradation occurring in both the contact and stabilization tanks. As discussed in Section 7.4, one characteristic of CSAS is that partial nitrification can occur, with the extent depending on both the system SRT and the fraction of the biomass in the contact tank. CSAS systems are seldom designed specifically to achieve partial nitrification, and thus the design approach presented here is based on biodegradable COD removal. However, because some nitrification will occur if the proper environmental conditions are achieved, its occurrence must be considered or the estimation of the amount of oxygen required in the system will be incorrect. While it is possible to derive solvable analytical equations for organic substrate removal with a few simplifying assumptions that are unlikely to be violated, the same is not true for nitrification. Only approximations can be achieved. This means that the only truly accurate way to estimate the degree of nitrification and the distribution of the associated oxygen requirement is through simulation with a model like ASM No. 1 or No. 2. However, because hand calculations are very useful during preliminary design we will present an approach for using them to evaluate the degree of nitrification likely to occur. Their approximate nature should be recognized, however, and appropriate caution should be exercised in their use.

The first task in the design of a CSAS process is the selection of the system SRT, which requires the consideration of many factors as discussed previously. After that, the relative amount of biomass in the two tanks must be selected to ensure that the desired effluent quality is attained. This, in turn, requires selection of the biomass recycle ratio and the fraction of the system volume allocated to each tank. Because system performance depends on both of those factors, as illustrated in Figures 7.247.27, CSAS systems should not be built with separate vessels for the contact and stabilization tanks because such a design fixes their relative volumes. Rather, both should be in the same vessel, with a non-load-bearing, curtain wall between them. This will allow their relative sizes to be changed as circumstances require.

Selection of the fraction of biomass in the contact tank requires the following. The heterotrophic specific growth rate in the contact tank, |i.H (, must be consistent with the desired effluent readily biodegradable substrate concentration. Thus, as in SFAS design, Eq. 10.28 may be used to calculate |x,u by setting Ssl. equal to the desired effluent substrate concentration. The kinetic parameters should be those for the coldest expected operating condition since it will control. Following the same logic used to derive Eqs. 10.29 and 10.30, it can be shown that the specific growth rate in the contact tank is given by:

when it is assumed that no utilization of slowly biodegradable substrate occurs. In Eq. 10.44, Vc is the volume of the contact tank and XM , ( is the MLSS concentration in it. Since the value of jx,,.t- as calculated with Eq. 10.44 must be less than or equal to the value of (jlh < associated with the desired effluent substrate concentration as calculated with Eq. 10.28, then:

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