ABSTRACT
Components 6-11 are all soluble, and with the exception of Si, which is inert, are the constituents upon which the biomass acts. The presence of S1 in the matrix is simply to remind us that wastewaters contain nonbiodegradable soluble COD which passes through the bioreactor unaffected by biological activity (see Section 5.2.1). Ss is readily biodegradable substrate, which is removed by growth of heterotrophic biomass under aerobic or anoxic conditions and is generated by hydrolysis of slowly biodegradable organic matter. Its concentration in the wastewater entering a bioreactor must be determined experimentally, and the procedures for doing so are discussed in Chapter 8. Component 8 is oxygen, which is removed by aerobic growth of heterotrophic and autotrophic bacteria. The stoichiometric term for oxygen associated with heterotroph growth is the same as that in Table 5.1, but the term associated with autotroph growth contains the factor 4.57. That factor is required because ammonia is the substrate for autotrophic nitrifying bacteria and its concentration in the matrix (SNH, component 10) is expressed as nitrogen, whereas oxygen is expressed as COD. Furthermore, YA has units of mg of biomass COD formed per mg
of nitrogen converted. Since the stoichiometric expression for oxygen in process 3 (autotrophic growth) comes from a COD-based stoichiometric equation, a factor must be included for the COD equivalents of ammonia-Nin order to have consistent units. As discussed previously, the oxidation state of nitrogen is changed from - III to + V as nitrifiers form nitrate from ammonia. The amount of oxygen required to accept the electrons removed during this oxidation is 4.57 g OJg N, as indicated in Table 3.1. Thus, that factor must be included. Unlike the model in Table 5.1, which used the traditional approach to decay, no oxygen utilization is associated directly with biomass loss in Table 6.1 because it is modeled with the lysis:regrowth approach. Rather, the oxygen utilization associated with biomass loss occurs because of the use of readily biodegradable substrate generated by hydrolysis of the slowly biodegradable substrate formed by death and lysis. Component 9, SNo, is nitrate-N. It is formed by aerobic growth of autotrophic bacteria and is lost as it serves as the electron acceptor for anoxic growth of heterotrophic bacteria. In the latter role, the oxidation state of the nitrogen is changed from + V to zero and the factor 2.86 appearing in the stoichiometric coefficient represents the oxygen equivalence of this change in units of g COD/g N, as shown in Table 3.1. Examination of column 10 shows that ammonia-N is involved in several reactions. Since ammonia is the preferred form of nitrogen for biomass growth, the term -iN;xB is included in rows 13 to represent the amount of nitrogen incorporated into new biomass. No provision is made in this model for reduction of nitrate-N to ammonia-N for incorporation into biomass in the event insufficient ammonia is present. This restriction should be recognized. Other models11 '33 allow use of nitrate-N for biomass synthesis. The second stoichiometric coefficient in column 10 for aerobic growth of autotrophic bacteria represents the use of ammonia as a substrate and is analogous to the coefficients used for readily biodegradable substrate ( column 7) removal by heterotrophic biomass in rows 1 and 2. Ammonia is formed by ammonification of soluble organic nitrogen, SNs, which is the last nitrogen based soluble constituent. It, in turn, is formed by hydrolysis of particulate organic nitrogen.
