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Figure 10.11 Combinations of process loading factors and aeration basin dissolved oxygen concentrations where bulking and nonbulking sludges occur in CMAS system. (From J. C. Palm, D. Jenkins, and D. S. Parker, Relationship between organic loading, dissolved oxygen concentration and sludge settleability in the completely-mixed activated sludge process. Journal, Water Pollution Control Federation 52:2484-2506, 1980. Copyright © Water Environment Federation. Reprinted with permission.)

Substrate Concentration, mg/L

Figure 10.12 Comparison of the growth kinetics of Hoc forming and filamentous bacteria.

Substrate Concentration, mg/L

Figure 10.12 Comparison of the growth kinetics of Hoc forming and filamentous bacteria.

ments and the floe formers will out-compete the filaments. If, on the other hand, the substrate concentration is S_> the specific growth rate of the filaments is higher than that of the floe formers and the filaments will out-compete the floe formers. This illustrates the characteristics of an environment that favors the growth of filamentous bacteria: the substrate must be supplied continuously in a manner that results in a low concentration. Continuous substrate supply is required so that biomass growth can occur. The residual concentration must be low to provide a competitive advantage for the filamentous organism. Said simply, filamentous organisms are good scavengers; they consume substrates more efficiently than floe-forming bacteria. Consequently, filaments proliferate under conditions that favor scavenging organisms. Such conditions typically occur in CMAS and sometimes occur in CAS and SFAS.

One approach for controlling the growth of filamentous bacteria with a high affinity for readily biodegradable organic matter is to reconfigure the bioreactor to create a substrate concentration gradient. This is the concept of kinetic selection, mentioned in Section 10.1.2. The goal is to produce a substrate concentration at the inlet to the bioreactor that favors the growth of floe-forming bacteria at the expense of the filamentous bacteria, as illustrated in Figure 10.12 by the concentration S,. This can be accomplished by providing highly plug-flow conditions within the bioreactor. As illustrated in Figure 10.13, " the SVI of activated sludge is influenced by the residence time distribution in the bioreactor as characterized by the equivalent number of tanks in series. SVIs are generally low for bioreactors with a flow pattern characterized as five tanks in series or more. In a process with a low equivalent number of tanks in series, the residual readily biodegradable substrate concentration in the first tank will be relatively low, similar to S, in Figure 10.12. This low concentration favors the growth of filamentous bacteria. As the number of equivalent tanks in series is increased, the readily biodegradable substrate concentration in the first tank increases until it approaches S,. which favors the growth of floc-forming bacteria.

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