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and considerably more complex, including 19 components and 19 process rate equations that require 22 stoichiometric coefficients and 42 kinetic parameters.1'1 Because of its size, ASM No. 2 will not be described in detail herein. However, because we use it in Chapter 7, some of its major characteristics will be presented.

Some processes that were explicitly modeled in ASM No. 1 were simplified in ASM No. 2 in order to minimize its size. For example, processes 6 and 8 in Table 6.1, ammonification of soluble organic nitrogen and hydrolysis of particulate organic nitrogen, were eliminated. Their functions were made implicit by assuming that they occurred in stoichiometric proportion to soluble substrate removal and hydrolysis of slowly biodegradable organic matter. This accomplished the same thing as ASM No.

1, but with fewer process rate expressions. Organic phosphorus conversion to soluble phosphate was handled in a similar manner.

The events occurring under anaerobic conditions are quite different in the two models. ASM No. 1 assumed that growth and hydrolysis stopped under anaerobic conditions, although microbial death and lysis continued. This was adequate for the processes ASM No. 1 depicts, but is entirely inadequate for biological phosphorus removal. Consequently, ASM No. 2 includes fermentation, uptake of acetate for formation of PHB and other polyhydroxyalkanoic acids (PHAs), and release of soluble phosphate from hydrolysis of polyphosphate. The inclusion of fermentation required the partitioning of readily biodegradable substrate into two components, readily fermentable substrate and fermentation products, represented by acetate. Ac-

etate is produced from readily fermentable substrate under anaerobic conditions and is taken up by the PAOs, as depicted by Eq. 3.82, forming PHB, as given by Eq. 3.83. Under anoxic conditions, i.e., when nitrate-N is present as an electron acceptor, fermentation decreases and the common heterotrophic biomass competes with the PAOs for acetate.

The scope of activities of the common heterotrophic bacteria was expanded. Under anaerobic conditions, they ferment readily fermentable substrate, producing acetate, but they cannot grow. They can only grow under aerobic and anoxic conditions, and can use both readily fermentable substrate and acetate for that purpose. Because heterotrophic growth cannot occur under anaerobic conditions, ASM No. 2 is not capable of modeling a totally anaerobic system. It can only mimic the performance of an anaerobic zone in a system with aerobic and anoxic zones.

Our knowledge of PAOs is still evolving. As a consequence, several simplifying assumptions were made in ASM No. 2 with respect to their growth. For example, they are assumed to grow only under aerobic conditions and can use only stored PHB as a growth substrate, as indicated in Eq. 3.85. They are assumed to be unable to use nitrate-N as an electron acceptor or to use any other electron donor, either stored in the cell or in the medium. These are reasonable assumptions, but exceptions to them are known to exist/'Thus, as we gain additional knowledge, it is very likely that ASM No. 2 will undergo revision. Nevertheless, even in its initial form it is a very powerful tool that allows engineers to explore the complex microbial events occurring in biological phosphorus removal systems.

6.1.5 Application of International Association on Water Quality Activated Sludge Models

Activated sludge models No. 1 and No. 2 are considerably more complex than the one used in Chapter 5 (Table 5.1). As a consequence, it is impossible to attain analytical solutions for the concentrations of the various constituents in a bioreactor, as was done in Chapter 5. Rather, matrix solutions and numerical techniques must be used, depending on the complexity of the system under study. Several organizations have developed computer codes for solving the simultaneous mass balance equations for the constituents in the models, allowing their application to a variety of bioreactor configurations. One such code is SSSP,7 which was developed for implementation of ASM No. 1 on microcomputers. It is menu driven and may be used for both steady-state and dynamic simulations. It was used to perform the simulations for single CSTRs in this chapter and for multiple bioreactor systems in Chapter 7. Another is ASIM,15 which implements both models, as well as several others. It was used for the some of the simulations in Chapter 7. Table 6.4 lists several computer codes that are available for using both IAWQ activated sludge models. In addition, Dold1" has developed a code that extends ASM No. 1 to include phosphorus removal, but it differs somewhat from ASM No. 2.

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