GPS-X Technical Reference

25 Modelling Fundamentals
Vesilind zone settling parameters – These two parameters give the settling
velocity of the sludge in the hindered settling zone (exponential portion of the
curve). They can be determined through a series of column tests (Vesilind, 1968).
Flocculant settling parameter – If all the above settling parameters are known,
then this one is generally easy to estimate by fitting the simulated effluent
suspended solids simulations to observed data.
Alternatively, settling velocity model parameters can be estimated using a time-series of
influent and effluent (overflow and underflow) suspended solids. The non-linear
parameter optimization procedure available in GPS-X can be used effectively in this
particular case.
A Typical Calibration Event
In an ideal case all the physical, operational and influent parameters are known for the
given wastewater treatment plant, while some of the most important kinetic,
stoichiometric and settling parameters are experimentally determined. In such a case the
modeller estimates the missing parameters using defaults at the beginning, then
modifying those which need adjustment and observing the response of several system
output variables.
It is possible to start with a steady-state calibration, i.e., taking the dry weather days from
a daily log of the treatment plant and optimizing for the average of these values.
Averages, which contain high flow periods (typical monthly or yearly averages), should
not be used for steady-state calibration.
Dynamic calibration should follow with typical diurnal data or selected high disturbance
(storm flow) events. The larger the scale of the disturbance between reasonable limits, the
more sensitive the calibration procedure will be. Hydraulic shocks are usually ideal for
settler calibrations, while diurnal data, process start-up, or recovery is better for
calibration of organic degradation and nitrification.
One fully documented event gives reasonable confidence for the given conditions (flow,
temperature, influent composition, etc.). If the model is to be used under varying
conditions, the above procedure has to be repeated accordingly (i.e., winter, summer, dry
weather, wet weather, etc.). Verification means simulating a dynamic event with a given
calibrated set of parameters, without modifying those, and finding reasonable accordance
of simulated data with the measurements.
GPS-X Technical Reference