GPS-X Technical Reference

71 Influent Models
CODFractions
CODStates
This model requires an input of total COD, total TKN, total phosphorus (in CNPlib),
state variables (the state variables are normally zero except for ammonia, soluble
ortho-phosphate and alkalinity) and several stoichiometric fractions. From these inputs,
the complete set of state variables, composite variables and nutrient fractions are
calculated (Figure 5-18 and Figure 5-19).
The calculation of the nitrogen and phosphorus (CNPlib) state variables and fractions is
complicated in the codfractions influent models. This is because some of the nitrogen
and phosphorus in the influent is associated with the organic state
variables.(i.e. N content of active biomass). Therefore, the model must adjust itself
depending on the applicable composite variable model and the current organic states. As
with the other influent models, these nutrient fractions are specified in this model.
However, in the event that a mass balance is not achievable with the user input data, the
codfractions model will recalculate these nutrient fractions to force the mass balance. It
is recommended that Influent Advisor be used to set-up and understand these
calculations.
This model has the advantage that each of the calculations is based on the total COD,
total TKN, total phosphorus (CNPlib), soluble ortho-phosphate (CNPlib) and ammonia
inputs. Therefore, (as often is available in practice) a series of these data over a period of
time can be read in from a data file and the influent state variables will vary with the
load. This is in contrast to other influent models in which some state variables are input
directly and will not automatically vary with a load change.
This model works similarly to the codfractions influent model, however all COD input
fractions are set as a fraction of total COD. This allows users to specify total COD, TKN
and ammonia as their main characterization inputs. Soluble inert COD (si), readily
biodegradable substrate (ss, sf or slf), particulate inert material (xi), unbiodegradable cell
products (xu) and biomass concentrations (xbh, xba and xbp) are specified via fractions of
total COD, as shown in Figure 5-9.
GPS-X Technical Reference