Chemical kinetics microbial population

Now we consider situations in which transformation of the organic compound of interest does not cause growth of the microbial population. This may apply in many engineered laboratory and field situations (e.g., Semprini, 1997 Kim and Hao, 1999 Rittmann and McCarty, 2001). The rate of chemical removal in such cases may be controlled by the speed with which an enzyme catalyzes the chemical s structural change (e.g., steps 2, 3 and 4 in Fig. 17.1). This situation has been referred to as co-metabolism, when the relevant enzyme, intended to catalyze transformations of natural substances, also catalyzes the degradation of xenobiotic compounds due to its imperfect substrate specificity (Horvath, 1972 Alexander, 1981). Although the term, co-metabolism, may be used too broadly (Wackett, 1996), in this section we only consider instances in which enzyme-compound interactions limit the overall substrate s removal. Since enzyme-mediated kinetics were characterized long ago by Michaelis and Menten (Nelson and Cox, 2000), we will refer to such situations as Michaelis-Menten cases. 

Fig. 2 Deviations from first-order kinetics of the inactivation of microbial populations by treatment with chemical agents due to (A) an initial lag in the rate of killing or (B) a decrease in the rate of killing with time of exposure.

Thus, the simplest quantitative model for biodegradation in a surface water is one in which the dissolved organic chemical concentration is significantly less than Ks, such that Eq. [2-71b] applies, and cell density X is assumed constant. The change in chemical concentration with time, Eq. [2-72], is then proportional to the chemical concentration, and first-order kinetics may be applied. Many rate constants have been published for surface waters (Table 2-7). Note that Vmax, Ks, and X are not individually measured their effects are lumped into a single empirical rate constant. Because degradation rates are highly dependent on the nature and abundance of the microbial population present in the surface water at the time the experiment was... 

Somewhat similar to enzyme kinetics, but definitely not the same, is the area of microbial kinetics. Here one is concerned with reactions between entities that may not be of the same level of organization (i.e., not atom-atom, atom-molecule, etc,). Indeed, microbial kinetics is more concerned with interactions between populations of living organisms, and some of the problems seem more akin to population dynamics than to chemical kinetics. In fact, in much of the discussion below we are concerned with population-changing processes. First, we need to define a few terms. 

Kinetics of microbial growth In the case of a sudden contamination of an aquifer by a chemical compound, the buildup of the appropriate population of microorganisms able to decompose the compound may be a slow and complicated process. Such situations have to be simultaneously analyzed by field observations and models, but the latter may become rather complicated. 

Microbial kinetics14 26 can be separated in four distinct levels at the molecular or enzyme, the macromolecular or cell component, cellular, and population level. Because each level has its own unique characteristics, different kinetic treatments are needed. Moreover, the environment in which these reactions take place also affects the kinetics. For example, reactions at the molecular/ enzyme level involve enzyme-catalyzed reactions. When these reactions occur in solution, their kinetic behavior is similar to that of homogeneous catalyzed chemical reactions as described in Chapter 31. However, when enzymes are attached to inert solid supports or contained within a solid cell... 

The death of a single microbial cell is a biochemical process (or series of processes) the entrapment of individual microbial cells in or on filters is due to physical forces. These effects on individual cells are peculiar to individual sterilization processes. On the other hand, the effects of inactivating processes and filtering processes on populations of microbial cells are sufficiently similar to be described by one general form—exponential death. Exponential kinetics are typical of first-order chemical reactions. For inactivation this can be attributed to cell death arising from some reaction that causes irreparable damage to a molecule or molecules essential for continuing viability. 

The Michaelis-Menten kinetics model, illustrated for a lake in Example 2.20, may also be applied to a flowing stream in which the microorganisms are attached to the surfaces of the charmel, have a relatively steady cell density, and are exposed to the full chemical concentration in the stream (Cohen et al., 1995 Kim et al., 1995). Microorganisms attached to solid surfaces form biofilms, as populations of attached microbes accumulate on top of one another, building up a layer of microbes embedded in an extracellular matrix which they secrete. Within biofilms, the microbial cell density X corresponds to the number of attached microorganisms divided by the volume of the biofilm. In wastewater treatment engineering, a biofilm is often referred to as attached growth. A biofilm may also be called a bacterial... 

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