Microbial death phase

From the data reported above, it is clear that the semi-continuous system is much more efficient than the batch one. If the main cause of microbial death is the CO2 concentration in the liquid phase, in the flux process high gas concentrations are reached faster, as the bubbling enhances and favours the mass transport of CO2 from the gas- to the liquid phase. In the semi-continuous process, a total sterilization is obtained with an operating pressure of 74 bar and an exposure time of 10 minutes, while in a batch process, 60 minutes are needed with an operating pressure of 200 bars. Tables 9.10-4 and 9.10-5 give results for other experiments run with the semi-continuous system. 

The model does not include the death phase, but industrially the amount of nutrient is chosen to terminate the microbial growth before the onset of the death phase, anyway. The cell growth is initiated by inoculation with cells from a stationary nutrient exhausted culture, i.e., when Ca = Cr = G and M = D. Numerical integration of (15.2-5 to 15.2-8) leads to results for a batch culture given in Fig. 1.5.2-1. The model shows a lag phase, an exponential growth phase, a change in the cell composition and a stationary phase with a relatively small number of cells. 

Microbial growth is generally described in terms of cell numbers, although an increase in the mass of the cell population also usually occurs. In laboratory culture, bacteria exhibit a growth curve that can be divided into four main phases lag exponential stationary and death (Fig. 5.45). 

A cycle for arsenic in a stratified lake is shown in Fig. 4. The reactions include transfers from solution to solid phases, conversions from one oxidation state to another, and ligand exchanges. Some of the processes are chemical, some occur through microbial mediation, and some can occur either way. Fish and plants enter the cycle by concentrating arsenic, especially trimethylarsine. Upon death the organisms settle to the bottom where the arsenic is removed to the sediments or recycled depending on the physical and chemical conditions [6]. 

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