f. The transfer of oxygen and the avoidance of Hoc shear during peak loading conditions greatly limits the designer's options while balancing the costs of the bioreaetor and the final settler. This is necessary, however, because of the nature of activated sludge oxygen transfer devices and because the effluent from a CMAS bioreaetor passes directly to the settler. More latitude with respect to floe shear can be gained in the design by adding a tlocculation chamber prior to the settler, as discussed in Section 10.2.5. or by adding equalization prior to the CMAS system, as discussed above.
This section has focussed on CMAS design as a means for presenting the basic-factors that must be considered in the design of any activated sludge system. Like CMAS, EAAS usually has both the biomass and the oxygen requirement distributed uniformly throughout the bioreaetor. Consequently, the design procedure for an
EAAS system is essentially the same. The only difference is that due to the long SRTs typically used with the EAAS process, floe shear is not usually a factor limiting the bioreactor size. Rather, the major issue is to maintain MLSS in suspension in an economical manner.
10.3.4 Conventional Activated Sludge, High-Purity Oxygen Activated Sludge, and Selector Activated Sludge—Systems with Uniform Mixed Liquor Suspended Solids Concentrations, but Variations in Oxygen Requirements
The basic approach to the design of all other activated sludge systems is the same as that presented in Section 10.3.3, although CMAS is the only variation for which that approach can be used directly. However, because of guiding principle No. 4 in Table 9.1, much of the information can be used for the other variations, with appropriate modifications. The activated sludge variations for which the least modification is required are those which have uniform MLSS concentrations throughout. These are conventional activated sludge (CAS), high-purity oxygen activated sludge (HPOAS), and selector activated sludge (SAS). As discussed in Sections 7.2 and 10.1.2, CAS and HPOAS can both be considered to behave as a number of completely mixed tanks in series. In HPOAS the separate tanks are real, since the bioreactor is staged by partitioning it, whereas in CAS the tanks are imaginary, representing the residence time distribution of the bioreactor. For purposes of design and analysis, however, both can be considered to be made of equal size tanks in series. Examination of the simulations presented in Figure 7.6 shows that the variation in the MLSS concentration from tank to tank is insignificant, justifying the assumption of uniform concentration throughout the system. An SAS system generally is designed with a series of small completely mixed tanks preceding the main bioreactor, as shown in Figure 10.6. As indicated in Section 10.1.2, the main bioreactor may be one large completely mixed basin, or it may be like CAS, in which case it can be considered to behave as a number of tanks in series. In either case, however, the SAS system can be modeled for design and analysis as several unequal tanks in series, but with uniform MLSS concentration throughout.
For all of these systems, decisions about the design SRT are made in the same way as previously discussed. Moreover, the mass of MLSS in the system, (XM , • V)sv^„„ can be calculated directly with Eq. 9.11 and the total steady-state oxygen requirement can be estimated with Eqs. 9.13 and 10.16. The total transient-state oxygen requirement can be estimated with the techniques discussed in Section 10.3.2 and illustrated in Example 10.3.3.5 for a CMAS system. The added level of complexity arises from the need to distribute that oxygen requirement appropriately throughout the reactor system, and to size the oxygen transfer system in a corresponding manner.
The need to spatially distribute the oxygen requirement in CAS and HPOAS systems can be seen by examining Figure 7.5, which shows both the steady-state and range of diurnal oxygen requirements in such systems. There it can be seen that the variation from the first to the last tank is large, particularly when diurnal loading variations are imposed on the system. Similar variations will occur in SAS systems, with peak requirements occurring in the selector. Failure to properly account for such spatial variations will lead to poor performance and/or an uneconomic system. Simulation is the most accurate way to predict the variation in oxygen requirement, and its use is encouraged, but sufficient information for simulation is often unavailable, requiring approximations. Thus, before we consider the design approach for these systems, we need to consider how to approximate the required spatial oxygen distribution.
Approximate Technique for Spatially Distributing Oxygen Requirements. The spatial distribution of the oxygen requirement requires consideration of the different events contributing to oxygen utilization and the rates at which they occur. First consider a system operating at steady-state with a uniform loading. Utilization of readily biodegradable substrate is very rapid and will usually be complete even in systems with very short SRTs. It is the major contributor to the high utilization rate in tank one of Figure 7.5. Utilization of slowly biodegradable substrate, on the other hand, is slower because it is limited by the rate of hydrolysis reactions. However, we saw in Figure 9.3 that it is often complete in systems with SRTs as short as two days. This suggests that most slowly biodegradable substrate will have been used in the first third to one-half of a tanks-in-series system, depending on the system SRT. Its use contributed to a substantial portion of the oxygen consumption in the first two tanks in Figure 7.5. Biomass decay, on the other hand, is a very slow reaction that occurs at a constant rate throughout the entire activated sludge process because its rate is driven solely by the biomass concentration, which can be considered to be uniform, as discussed earlier. It contributed the base rate seen in tanks four and five of Figure 7.5, and the same contribution also occurred in all of the preceding tanks. Finally, nitrification is also a slow process, but faster than decay. Furthermore, its rate is driven by the ammonia-N concentration, which is soluble, and attains a maximum at moderate ammonia-N concentrations. Consequently, nitrification will occur at its maximal rate in the first few tanks in the system, but often will be completed before the last tank is reached, depending on the system SRT. In Figure 7.5, nitrification contributed to oxygen consumption primarily in the first three tanks.
Use can be made of the generalizations in the preceding paragraph for partitioning the total oxygen requirement into its component parts. Consider first the heterotrophic oxygen requirement. Rearrangement of Eq. 9.13, after neglecting Ss, allows the oxygen requirement to be divided into two component parts, that associated with biomass synthesis and that caused by decay:
The first term on the right side of the equation is the oxygen used for synthesis of new biomass, whereas the second term is the oxygen utilization for biomass decay. The synthesis term can be further subdivided into oxygen utilization for biomass synthesis from readily biodegradable substrate, and oxygen utilization for biomass synthesis from slowly biodegradable substrate:
RO„ = F(Ss„ + Xs„)(l - YiUi()Xu.i) + F(Ss„ + Xs„)
RO„ = F(Ss„ + Xs„)(l - YiUi()Xu.i) + F(Ss„ + Xs„)
This subdivision is desirable because of the differences in the rates of utilization of the two substrate types, as discussed above. The oxygen utilization due to decay of biomass may be given by one term because the type of substrate from which biomass was grown has no effect on its decay rate:
These component oxygen utilization terms can be used to determine the profile of heterotrophic oxygen utilization through a CAS, HPOAS, or SAS system. The distribution of oxygen utilization for decay is the easiest. Because biomass is distributed evenly throughout these systems and because the rate of decay is proportional to the biomass concentration, the rate of oxygen utilization due to decay is the same throughout the system. As a result, the mass of oxygen required for decay in any tank, i, of a multitank system is just:
where V, is the volume of tank i and V, is the total system volume.
The distribution of the oxygen requirement for the utilization of slowly biodegradable substrate is less exact and is dependent on an approximation. Let xs be the SRT at which slowly biodegradable substrate utilization would be essentially complete in a CMAS system. If we then recognize that biomass is uniformly distributed in a tanks-in-series system, and neglect any kinetic benefits to hydrolysis of having a tanks-in-series configuration, then we can approximate the fraction of the system volume within which biomass synthesis occurs on slowly biodegradable substrate, fy.xs, as:
We saw earlier that slowly biodegradable substrate can be fully utilized at SRTs as short as two days. If we considered that figure to be applicable in a system with an SRT of four days, we might expect oxygen utilization for synthesis of biomass from slowly biodegradable substrate to occur in the first half of the system. Furthermore, if the system could be characterized as being equivalent to five tanks in series, we would expect 40% of that oxygen utilization to occur in each of the first two tanks and 20% in the third. None would occur in the last two tanks. Alternatively, if the system behaved like three tanks in series, we might expect 67% of the utilization to occur in the first tank, 33% in the second, and none in the third. While this technique is crude, it at least provides a means to approximate where oxygen utilization is likely to occur.
Distribution of the oxygen requirement for biomass synthesis from readily biodegradable substrate requires computation of the volume of a fictitious completely mixed bioreactor, Vt., that receives the influent stream and the RAS flow, and reduces the substrate concentration to a desired level. The oxygen requirement would then be apportioned to the initial fraction of the activated sludge bioreactor that contained an equivalent volume. If we let SSF be the desired readily biodegradable substrate concentration in the fictitious completely mixed bioreactor, then it follows from
Monod kinetics, Eq. 3.36, that the specific growth rate of the biomass in that reactor must be:
A mass balance on readily biodegradable substrate in that reactor, neglecting the contribution of hydrolysis of slowly biodegradable substrate, gives:
Y„,[F(Ss„ - Ssi) - F,(Ss„ - Ss)] /in,Q. M-m , =-. v Y--(10.29)
where Ss is the readily biodegradable substrate concentration in the effluent from the entire activated sludge process and XM ,, is the MLSS concentration in the fictitious bioreactor. Because the composition of the MLSS in any activated sludge system is the same throughout, the active fraction in the MLSS of the fictitious bioreactor will be the same as the active fraction in the activated sludge system under consideration, which is governed bv the system SRT. Making use of this fact, and assuming that F,(SM - SJ << F(Sso - SM), that Ss, << Ssu, and that Ss << (Ss<) + Xso), it can be shown that:
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