Biodegradable COD Cone., mg/L

Figure 9.2 Typical operating ranges for aerobic/anoxic and anaerobic suspended growth biochemical operations. (Adapted from Half).

Figure 9.2. These include the wastewater temperature, flow rate, and composition, and they will be discussed in greater detail in Chapters 10, 11, and 13. Nevertheless, Figure 9.2 can be used for preliminary screening of biological wastewater treatment options.

9.3.2 Solids Retention Time

As illustrated in Part II of this book, the SRT exerts a dominant effect on the capabilities and performance of a biochemical operation. For example, it affects the types of microorganisms that can grow in a bioreactor, as well as their activity, thereby determining effluent quality. Because of the multiple effects associated with SRT, many factors must be considered during its selection. In fact, it is seldom possible to select the SRT based on a single criterion, such as effluent substrate concentration. The range of typical SRT values is already known for many applications, and often an appropriate SRT can be selected based on experience. In this section we will consider such situations.

Before addressing appropriate SRT values, it should be emphasized that the selected SRT must always exceed the minimum SRT associated with the microorganisms responsible for a particular required biochemical transformation. As discussed in Section 5.1.3, the minimum SRT is the value below which a particular group of microorganisms is unable to grow in a suspended growth bioreactor. As expressed in Eq. 5.16, it is a function of the influent concentration of the limiting substrate for the microorganisms of interest and the kinetic parameters describing their growth on that substrate. The kinetic parameter that exerts the most pronounced effect is |i. Since the |i value for heterotrophs growing on readily biodegradable substrate is high, the minimum SRT for them is very low. In contrast, because the (1 value for autotrophic nitrifying bacteria is very low, the minimum SRT associated with them may be quite high. The same may be true for heterotrophs growing on xenobiotic chemicals. If the SRT is maintained at a value less than the minimum SRT for the subject bacteria, they will be wasted from the bioreactor faster than they grow and a stable population will not develop. In other words, washout occurs, as discussed in Section 5.1.3. Conversely, if the operating SRT exceeds the minimum SRT, then the subject bacteria will be able to grow in the process and the reaction will occur. However, as seen in Chapters 5 and 7, the degree of conversion will depend on the operating SRT and the bioreactor configuration, and both must be chosen to meet effluent quality goals. This generally requires the operating SRT to be well above the minimum SRT. The ratio of the operating SRT to the minimum SRT is called the safety factor. To ensure that their washout does not occur, the safety factor for the most slowly growing microorganisms required in a bioreactor should always exceed 1.5, although larger values may be required in some circumstances. Furthermore, larger values may result when other factors control the choice of the SRT. Factors affecting the choice of the SRT for various named biochemical operations will be discussed in subsequent chapters. Only a brief overview is given here.

Aerobic/Anoxic Systems. Figure 9.3 illustrates the ranges of operating SRTs over which various events will occur in aerobic/anoxic systems. Because the ranges

] Removal of Soluble Biogenic Organic Matter I i Solubilization and Metabolism of Particulate Organic Matter

Degradation of Xenobiotic

Compounds [

Stabilization of Biomass [

|_| Flocculation for Domestic Wastewater

I I Flocculation for Industrial Wastewater

(Temperature Dependent. See Figure 9.4)

] Nitrification

^ Phosphorus Removal

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