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Figure 9.7 Net yield as a function of SRT and temperature for domestic wastewater (a) with primary treatment and (b) without primary treatment. (From Design of Municipal Wastewater Treatment Plants, Manual of Practice No. 8, Water Environment Federation. 1992. Copyrighl © Water Environment Federation; reprinted with permission.)

solid (VSS)/mg five-day biochemical oxygen demand (BOD,). Actually, any unit system could be used to report Y„, as long as it is consistent with the methods used to measure XM and the substrate. The interconversion between various unit systems was discussed in Section 8.7. We have elected to use either COD or total suspended solid (TSS) units as the measure of MLSS in this book. Activated sludges typically have a volatile solids content of approximately 75%, so the XM • V values calculated with Eq. 9.2 can be converted to a TSS basis by dividing by 0.75.

The utility of Eq. 9.2 is that it can be used to estimate the mass of MLSS that will be present in the bioreactor, regardless of its configuration. The distribution of those solids will depend on the system configuration, as illustrated in Chapter 7 through simulation.

The net yield can also be used to calculate other information needed for a preliminary process design, such as the excess solids wastage rate, which is needed for design of the solids handling and disposal system. In Chapter 5, we derive Eq. 5.31 which expresses the excess biomass wastage rate in terms of Y,,,,,,... By analogy to the development of Eq. 9.2, an expression can be developed from which the solids wastage rate can be calculated during preliminary design:

This equation has particular significance because it has another important use. If an existing treatment facility is to be upgraded, then information will be available in the plant records on the amount of solids wasted daily, as well as the quantity and strength of the wastewater. Such information allows Y„ to be calculated from the historical records with Eq. 9.3. It can then be used in Eq. 9.2 to estimate XM • V for use in the design.

As discussed in Section 9.3.3, another factor that must be considered during design is the electron acceptor requirement. The basic COD balance states that the amount of electron acceptor required is equal to the biodegradable COD entering a bioreactor minus the COD of the solids wasted from the bioreactor. This is what led to Eq. 5.35, which gives the oxygen requirement in a simple CSTR receiving a soluble substrate. By analogy to the development of the previous preliminary design equations, an equation can be developed that relates the oxygen requirement to the flow and waste load in a manner similar to Eq. 9.3:

In this case, a new empirical coefficient, Yn,, the process oxygen stoichiometric coefficient, has been defined, the units of which must be consistent with the units on Ss(, and XM). Since BOD, is a commonly used measure of biodegradable substrate concentration in practice, Y,,: is commonly expressed as nig O./mg BOD,. For domestic wastewater, the value of Y,i: is known as a function of the SRT, as illustrated in Figure 9.8.M Two things are of note about this figure. First, the values of Yn: exceed 1.0. This follows directly from the fact that BODs is not a measure of all of the electrons available in a substrate, as discussed in Section 8.6. Second, Y„. increases as the SRT is increased in a manner consistent with Figure 5.6, illustrating that the empirical relationship depicted by Eq. 9.4 conforms to the fundamental principles developed earlier. For an existing facility Eq. 9.4 can be rearranged to calculate the process oxygen stoichiometric coefficient associated with a given waste load. Procedures for measuring the process oxygen requirement of such a facility are described elsewhere. u

The concept embodied in the COD mass balance and reflected in Eq. 5.35 can be generalized to any electron acceptor by expanding the substrate term to include slowly biodegradable substrate, as was done with Eq. 9.4, and incorporating an

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