Figure 2.4 Effect of temperature on the true growth yield, Y. The units on Y are mg biomass formed per mg substrate COD removed. (From R. E. Muck and C. P. L. Grady, Jr.. Temperature effects on microbial growth in CSTR's. Journal of the Environmental Engineering Division, ASCE 100:1147-1163, 1974. Reprinted by permission of the American Society of Civil Engineers ).

O Mixed Culture49 □ A. aerogenes77 - v C. utilid6

may influence Y is the composition of the microbial community. When it is heterogeneous, the waste products from one species serve as growth factors for another, thereby converting a seemingly minimal medium into a complex one. Consequently, it might be anticipated that the yields from mixed microbial cultures would be slightly higher than those from pure cultures growing on the same medium. A comparison of the two revealed this to be the case.:K

Constancy of Y in Biochemical Operations. Biochemical operations use mixed microbial communities to treat wastewaters containing mixtures of substrates. Thus, it is apparent that Y will depend on both the character of the wastewater and the particular community that develops on it. It is important that this variability be recognized by engineers designing biochemical operations, because then the estimated yield values will be interpreted in an appropriate way. As seen in Chapter 3, similar conclusions can be reached about the kinetic parameters associated with biochemical operations. This means that designers must utilize considerable judgement and allow for uncertainty. This situation does not prevent generalities from being made, however. For example, examination of a large number of yield values indicates that Y will generally lie within the range of 0.48-0.72 mg biomass COD formed per mg substrate COD utilized for aerobic heterotrophs degrading carbohydrates."' Under similar conditions, Y values for growth on a number of xenobiotic compounds, including substituted phenols, benzenes, and phthalate esters, lay within the range of 0.20-0.00 mg biomass COD formed per mg substrate COD removed.1" One study"' reported the range of yield values for autotrophs to be from 0.06 to 0.35 mg biomass COD per mg nitrogen oxidized, with values for Nitrobacter being lower than those for Nitrosomonas. Likewise, another study"" reported the Y value for Nitrobacter to be 0.12 mg biomass COD per mg nitrogen oxidized and the value for Nitrosomonas to be 0.47. Although ranges such as these provide the engineer with an idea of the magnitudes to be expected, designs should only be based on estimates of Y obtained from laboratory and pilot-scale studies of the particular waste to be treated.

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