GPS-X Technical Reference

149 Suspended Growth Models
Algebraic pH Solver
In the MANTIS2 model, the pH is estimated in all the unit processes. The dissociation
reactions for acid and bases are much faster than the other processes used in the model,
therefore, algebraic form of equations are chosen over the differential form for expressing
acid/base dissociation reactions. The pH in each unit process is estimated by solving a set
of algebraic equations which include a charge balance equation along with the
equilibrium equations for each ionic species. Although temperature dependency of the
dissociation constant is considered in the model, ionic activity corrections are not applied
in the equations. For a typical wastewater, the ionic activity correction may not be
important, but at higher ionic concentrations the model pH values should be used with
appropriate caution.
The MANTIS2 model uses the dissociation equations for: carbonic acid (diprotonic),
phosphoric acid (triprotonic), ammonium (monoprotonic), acetic acid (monoprotonic),
propionic acid (monoprotonic), nitrous acid (monoprotonic). The charge balance equation
to solve for [H + ] concentration is prepared using the ionized species of weak acid/base
and strong anions (NO 3- , sana) and strong cations (Ca 2+ , Mg 2+ , K + , scat).
PRE-FERMENTER MODEL (PREFERMENTER)
The pre-fermentation model implemented in GPS-X is based on the Munch et al. model (1999),
which is a mechanistic model aiming at describing the effect of design and operating parameters
on the rate of VFAs production. The pre-fermenter model is found in the CSTR object only.
A general reaction pathway is shown in Figure 6-9.
Figure 6-9 - General Reaction Pathway
GPS-X Technical Reference