GPS-X Technical Reference

Digestion Models 280
Methane
The biological generation of methane can be expressed in terms of bacterial growth rates
and yield coefficients. The model assumes that the solubility of methane is negligible and
all methane generated is immediately transferred to the gas phase:
Equation 10.8
where:
vm
= volume of the liquid phase in the digester
Carbon Dioxide
The mass transfer of carbon dioxide between the liquid and gas phases is calculated using
the standard two-film gas transfer equation.
Equation 10.9
where:
klac2o2 = mass transfer coefficient for CO 2
co2sat
= saturation concentration of CO 2 in the liquid phase, and
H2co3=sco2t-hco3-co2
The pH model within the basic digester model calculates bicarbonate (hco3) and
carbonate (co2). The concentration of dissolved CO 2 in the liquid phase at equilibrium
(co2sat) is calculated using Henry's law:
Equation 10.10
where:
henryco2 = Henry’s law constant for CO 2
gco2
= partial pressure of CO 2 in the gas phase
GPS-X Technical Reference