CXA has the units of g-biomass g-initial-dry-solids-1, these units typically being written as g-biomass g-IDS-1.

Unlike the other methods of expressing biomass concentration, these measures will only change in response to changes in the amount of biomass. In the absence of growth, they will not change as a result of changes in either moisture content or dry solids content. Therefore they will be referred to as "absolute biomass concentrations". Concentrations expressed in the manner shown in Eqs. (14.1) and (14.2) will be referred to as "relative biomass concentrations". There are other absolute measures of biomass:

• the amount of biomass per gram of inert support material, which can be used in some cases where an inert support matrix is impregnated with nutrients;

• the absolute amount of biomass within the bioreactor;

• the amount of biomass per unit volume of the substrate bed. Note that this is only an absolute biomass concentration in those cases in which the bed volume does not change significantly during the process. Biomass per unit volume is typically not used to express biomass concentration in laboratory-scale experiments, but biomass concentrations may be expressed in this manner within mathematical models of bioreactors.

In this book we will use the symbol X to represent either the absolute mass of biomass in the bioreactor or the mass of biomass per m3 of substrate bed. Other symbols will be used to represent a concentration based on a denominator that does not change, such as grams initial dry solid, or grams of inert support material.

14.3.4 Which Set of Units Is Best to Use for Expressing the Biomass?

It is probably best, in the kinetic studies undertaken in the laboratory, to express the biomass concentration on an absolute basis. This is because key phenomena that will be included in the bioreactor model (such as the production of waste metabolic heat, the consumption of O2, and the production of CO2) depend directly on the absolute amount of biomass.

However, as will become obvious in the following section, absolute biomass contents have not always been used in growth profiles reported in the literature (Viccini et al. 2001). This must be kept in mind when analyzing kinetic profiles taken from the literature. In any case, it is not difficult to convert between absolute and relative concentrations if the yield and maintenance coefficients are known. A method of doing this is presented in Chap. 16.2.

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