GPS-X Technical Reference

123 Suspended Growth Models
The entries within the table are the stoichiometric parameters or relations, designated vij,
used in defining the net process rate for a component. These parameters define the mass
relationships between components and are defined in the lower-left hand corner of the
table. If a process does not directly affect a component's rate, then the table cell will be
empty (the entry is assumed to be zero in this case).
The net reaction rate of a component, designated r i, is the sum of all the process rates,
which cause a change in the mass of that component. The expression used to determine
the net rate is listed in the table in the row labeled "Observed Conversion Rates".
When the model is presented in matrix format, this equation has a simple visual
interpretation. To determine the net rate of change for a component, first identify the
column of the component of interest and move down that column until you find a table
cell containing an entry. Multiply the table cell entry by the process rate shown in the
rightmost column. The sum of these individual process rates is the net reaction rate. Do
likewise for all remaining rows in the column, which contain stoichiometric parameters.
Stoichiometric Parameters
Two common stoichiometric parameters appearing in the biological models in this
chapter are derived from their respective chemical equations. First is the nitrogen to
oxygen stoichiometric parameter used in the denitrification equation. A value of 2.86 is
used throughout the GPS-X libraries for all the biological models. This value is derived
from the molecular ratios shown in the equation below:
Equation 6.32
The ratio of mass oxygen produced per mass nitrogen gas produced is 160:56 or 2.86:1.
Similarly, the stoichiometric ratio of oxygen to nitrogen required for nitrification (used in
many of the biological models) has a value of 4.57 which is derived from the molecular
ratios shown in the equation below:
Equation 6.33
The ratio of mass of oxygen to mass of nitrogen is 64:14 or 4.57:1.
Since these stoichiometric ratios cannot change, the values are hard-coded in GPS-X
without user access.
GPS-X Technical Reference