The impact of the number of CSTRs in series on the performance of CAD is illustrated in Figure 12.14. This figure was developed for the situation depicted in

SRT, days

SRT, days

Figure 12.14 Effects of SRT and number of equivalent CSTRs in series on the percent VSS destruction and SOUR in a conventional aerobic digester. The curves are theoretical and were generated from Eqs. 12.11 and 12.12 using the same kinetics and solids characteristics used to generate Figures 5.10 and 12.10.

SRT, hrs

Figure 12.14 Effects of SRT and number of equivalent CSTRs in series on the percent VSS destruction and SOUR in a conventional aerobic digester. The curves are theoretical and were generated from Eqs. 12.11 and 12.12 using the same kinetics and solids characteristics used to generate Figures 5.10 and 12.10.

Figures 5.10 and 12.10, except that the bioreactor was divided into one to four equal sized compartments. Performance is significantly improved by configuring the bioreactor as two CSTRs in series rather than as a single CSTR, allowing a given degree of stabilization to be achieved at a lower SRT. However, compared to the two CSTR system, less improvement is obtained by going to three and four CSTR systems. Similarly, the pathogen destruction efficiency of an aerobic digester is improved by configuring it as a series of CSTRs. This has been clearly demonstrated for the ATAD process where two CSTRs in series are typically used, with significant improvements in bioreactor performance.

A CSTRs-in-series configuration can be obtained in several ways. Consider, for example, the intermittent feed CAD system illustrated in Figure 12.5a. If more than one bioreactor is available, they can be operated in an alternating fashion in which one is fed and decanted for a period of time while another is off-line to allow digestion and pathogen inactivation to proceed. After digested solids are removed from the second bioreactor, feed is then directed to it while the first is taken off-line for further reaction. The advantages of CSTRs in series can be achieved with the continuous feed process by splitting the bioreactor into two compartments and directing the underflow from the settler to the second one. This approach is necessary because, as discussed in Section 7.2.2, the recycle of solids around the entire system would make it completely mixed with respect to biomass, which would make it behave like a single CSTR. Consequently, care must be taken to ensure that plug-flow type conditions are truly achieved with regard to the flow of solids through the bioreactor. Because the solids concentration in the first compartment will be less than that in the second, the volume of the first compartment should be larger to fully gain the benefits of the tanks-in-series configuration.

12.3 PROCESS DESIGN 12.3.1 Overview

The design of an aerobic digester can be accomplished by application of the principles presented in this and previous chapters. The decisions that must be made include:

• Selection of the process option, i.e., whether the system will be CAD, A/AD, or ATAD. In addition, a decision must be made as to whether operation will be intermittent or continuous.

• Selection of the bioreactor configuration, i.e, whether it will be a single CSTR or a series of CSTRs.

• Selection of the bioreactor feed solids concentration and physical reactor configuration. Both of these factors affect the heat balance for the bioreactor and determine whether significant autoheating will occur. If significant auto-heating is expected, a heat balance should be performed to estimate the bioreactor operating temperature. Procedures for performing a heat balance are available elsewhere." 41

• Selection of the bioreactor SRT. This is done on the basis of the desired percent VSS destruction or SOUR to be achieved, using equations like 12.4, 12.6, 12.11, or 12.12. Alternatively, graphical information like that in Figure 12.11 can be used.

• Determination of the required bioreactor volume. This determination is based on the selected SRT, influent flow rate, and desired bioreactor solids concentration.

• Calculation of the oxygen requirement. The power input required to meet the oxygen requirement can then be calculated using the procedures presented in Section 10.2.5.

• Determination of the power input required to achieve adequate mixing and to maintain solids in suspension. This can be done with Eq. 12.10 or with the procedures presented in Section 10.2.5. Just as in activated sludge design, the power required for mixing must be compared to the power required for oxygen transfer, and the larger of the two provided.

• Evaluation of the need for supplemental alkalinity for pH control As discussed in Section 12.1.2, nitrification of released ammonia-N will result in destruction of alkalinity if CAD is used. If the amount of alkalinity available is insufficient, the pH in the digester will drop, reducing the rate at which digestion occurs. Consequently, the amount of alkalinity available should be compared to the amount of alkalinity likely to be destroyed to determine the need for supplementation.

Several procedures can be used to perform the necessary process calculations. Among them are those based on empirical correlations, those using batch data in simple models, and those using the simplified model of Chapter 5 or IAWQ ASM No. 1. The basic approaches are similar to those used to design activated sludge and biological nutrient removal (BNR) systems. Consequently, all of the procedures presented in those chapters will not be repeated here. Rather, just the unique points will be emphasized.

12.3.2 Design from Empirical Correlations

Empirical correlations such as Figure 12.11 can be used to select a design temper-ature-SRT product. Then estimation of the digester operating temperature allows direct calculation of the required SRT. If the solids are to be thickened prior to digestion, the SRT is equal to the HRT, allowing the bioreactor volume to be calculated from the influent solids flow rate using the definition of HRT, as given by Eq. 4.15. If solids are to be thickened during digestion, in either the intermittent or the continuous process, then the VSS concentration in the digester is given by:

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