Will a certain food provide energy when utilized by microorganisms? If the answer is yes, then the food will be eaten; and if it is in a wastewater, the wastewater will be cleaned up. The answer to this question can now be quantified by the combination of the concept of enthalpy and entropy. This combination is summed up in a term called free energy. Free energy G is defined as
Because H is an energy content and S is a wasted energy, G represents the useful energy (or, alternatively, the maximum energy) the fuel can provide after the wasteful
energy (represented by S) has been subtracted from the energy content. Thus, the term free energy.
Biological processes are carried out at a given constant temperature as well as constant pressure. Thus, differentiating the free-energy equation at constant temperature, dG = dH - TdS = dH - Qrev (15.9)
Note: In order for G to be a maximum (i.e., to be a free energy), Q must be the Qrev as depicted in the equation.
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