The model shows that hydrogen ion concentration (pH) controls
the acetic acid (HAc) degradation (mol of decomposed
HAc/mol of initial HAc), high H+ concentration leads to low
decomposition. The modelled equilibrium pH value of approximately
4 (cf. pKaof HAc), however, are low in comparison
to empirical observations (pH 7–8). This phenomenon results
from the initial substrate (acetic acid). The pH value does
not affect the methane yield (mol of produced CH4/mol of
HAc decomposed) but it does change the solubility of carbon
dioxide.
The model also shows that the solubility/vaporisation of carbon
dioxide has an influence on the overall enthalpy change
in the model system. The vaporisation of the carbon dioxide
requires energy, which can be sufficient to make an exothermic
system endothermic. The thermodynamic equilibrium model,
therefore, suggests that injection of carbon dioxide into the
system would not affect methane conversion, or acetic acid
degradation significantly. However, gas collection from the
vapour phase may result in better degradation of acetic acid, but
will require the supply of much more external energy (thermal
energy) to the system.
This equilibrium model indicates the possibility of interpreting
the behaviour of bioprocesses that proceed close to
equilibrium and discriminating behaviour from “true” inhibition,
which occurs when processes are far from equilibrium.
Such knowledge can be used to inform the development of future
bio-kinetic models of anaerobic digestion process.