Multiple Microbial Activities in a Single Continuous Stirred Tank Reactor

In Chapter 5, we investigated the growth of aerobic heterotrophic bacteria in a single continuous stirred tank reactor (CSTR) receiving a soluble substrate. Through development of a simple model we saw that the SRT is an important determinant of bioreactor performance because it is related to the specific growth rate of biomass at steady-state. We also saw that there is a minimum SRT below which biomass growth cannot occur, as well as a minimum substrate concentration that can be achieved no matter how large the SRT. Finally, we saw how stoichiometry can be applied to determine the amount of electron acceptor required and the amount of excess biomass produced. All of these characteristics are of fundamental importance and apply to all types of biomass, both heterotrophic and autotrophic, in all types of environments, whether aerobic, anoxic, or anaerobic. Thus, even though the concepts in Chapter 5 are developed in the context of aerobic growth of heterotrophs, they are broadly applicable.

In spite of the broad utility of the concepts, the model developed in Chapter 5 has two characteristics that restrict its applicability in many wastewater treatment situations. One is that it is limited to soluble, readily biodegradable substrates, whereas most wastewaters contain particulate contaminants and soluble constituents of large molecular weight that must be reduced in size before they can be taken into the bacteria for biodégradation. If a model is to accurately depict the response of bioreactors receiving such wastewaters, it must include hydrolysis reactions. The other is that the biomass is assumed to be in a constant biochemical environment with no limitation by the electron acceptor. In many systems, however, limitations or alterations in the supply of electron acceptor cause shifts between aerobic and anoxic conditions, with short periods of anaerobiosis as well, and during these shifts the concentration of the electron acceptor may be limiting. Therefore, it would be desirable for a model to handle such situations.

In order to encourage practicing engineers to use modeling more extensively during the analysis of alternative wastewater treatment systems, in 1983 the International Association on Water Quality (IAWQ) [formerly the International Association on Water Pollution Research and Control (IAWPRC)] appointed a task group to review models for suspended growth cultures and to produce one capable of depicting the performance of wastewater treatment systems receiving both soluble and particulate substrates in which organic substrate removal, nitrification, and denitri-fication were all occurring. In other words, they were to consider most of the pro cesses discussed in Section 2.4. They completed their task in 1986 and submitted a report to 1AWQ which was published in 1987,l71H outlining the major features of activated sludge model (ASM) No. 1. The task group was influenced by the published work of many researchers, but that of Marais and colleagues at the University of Cape Town in South Africa had a major impact on their thinking. A summary of much of the South African work can be found elsewhere.12 Because ASM No. 1 is the result of the deliberations of several researchers with diverse opinions, and because it is capable of mimicking the performance of pilot'2 and full' scale systems, it will be adopted herein for investigating more fully the performance of suspended growth bioreactors. In this chapter, ASM No. 1 will be used to illustrate the impact in a single CSTR of the processes and events not covered in Chapter 5 and in Chapter 7 it will be used to investigate the performance of multiple bioreactor systems.

Because of the success of ASM No. 1, the IAWQ task group on mathematical modeling was reconstituted and asked to produce a consensus model capable of mimicking the performance of systems capable of performing organic substrate removal, nitrification, denitrification, and phosphorus removal. This was a complicated task because of the complexity of biological phosphorus removal and the evolving nature of our understanding of it. Nevertheless, they were successful, releasing their report in 1995,'" calling the new model ASM No. 2. Use will be made of the model in Chapter 7, but it will not be explained in the same detail as ASM No. 1 because of the large number of components and processes involved. However, the major rate expressions associated with phosphorus removal in the model were presented in Section 3.7.

Modeling is now used extensively in biological wastewater treatment, in large part because of the success of ASM No. 1. Similar concepts have been applied to develop descriptive models for anaerobic wastewater treatment processes.1" ' Space does not permit their investigation here, but the reader is encouraged to consult the primary literature concerning them.

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